การวัดในงาน Audio โดยมากมักจะแสดงในหน่วย Decibels.
เนื่องมาจาก Audio เป็นสัญญาณที่มีความแตกต่างของระดับสัญญาณที่กว้างมาก. อย่างเช่น ความต่างของเสียงที่เกิดจากแสดงดนตรี Rock อาจจะมีความต่างกันเป็นล้านเท่ากับเสียงที่เกิดจากการพลิกหน้าหนังสือ
ช่วงของระดับสัญญาณที่กว้างมากนี้ยากที่จะพูดถึงแบบ Scale เชิงเส้น ( Liner Scale ).
Decibel เป็นหน่วยเชิงทวีคูณหรือเชิงกำลัง ( Logarithm Scale ) ซึ่งย่อย่านความต่างของระดับสัญญาณที่กว้างมาก ๆ
ลงมา ซึ่งง่ายต่อการเข้าใจมากกว่า. โดยการใช้อัตราส่วนของสัญญาณที่เพิ่มขึ้นในอัตราสิบเท่า จะเท่ากับความต่าง 1 Decibel.
หูของมนุษย์ได้ยินความต่างของระดับเสียงในช่วงที่กว้างมากเช่นเดียวกับพื้น ฐานของ Logarithm. ดังนั้นการวัดในหน่วย Decibel Scale จึงมีความถูกต้องและเหมาะสมที่จะนำมาใช้มากกว่า Liner Scale.
dBm ( Decibel milli Watt ) คือ มาตรฐานหน่วยวัด Voltage ของสัญญาณขณะมี Load ( อ้างอิงที่ 1 mW ) เมื่อวัดค่า dBm จะต้องบอกถึงค่า Impedance ของ load ที่ใช้ในการวัดด้วย. อย่างเช่น ในด้าน Audio ส่วนใหญ่เราจะใช้ Load ที่มีค่า Impedance 600 Ω หรือ 150 Ω. ส่วนในงานด้าน Video, Radio Frequency และ Transmission Line มักจะใช้ค่า Impedance 50 Ω, 75 Ω และ 300 Ω.
ตัวอย่างเช่น 0 dBm ที่คิดจากค่าอ้างอิงที่ 1 mW ที่ Load Impedance 600 Ω หรือเท่ากับ 0.775 Vrms. ซึ่งหาได้จาก V=√(P x Z)= √ (1mW x 600 Ω) = 0.77459 V นั่นเอง.
งานในด้าน Video, Radio Frequency และ Transmission Line นั้น อุปกรณ์มักถูกออกแบบมาให้มี Input Impedance และ Output Impedance ที่คงที่ในย่านความถี่ที่ใช้งาน ที่เป็นเช่นนี้เพราะที่ความถี่สูง Input Impedance, Output Impedance และ Impedance ของสายส่งมีผลกับ SWR (Standing Wave Ratio ) ซึ่งมีผลต่อการส่งผ่านสัญญาณ. โดยทั่วไปอุปกรณ์เกี่ยวกับงานด้านนี้จะบอกคุณลักษณะของ Gain หรือ Loss มาในรูปของ dBm.
ส่วนงานด้าน Audio นั้นเป็นที่รู้กันว่าโดยมากเราจะใช้ dBm ที่คิดจากค่าอ้างอิงที่ 1 mW ที่ Load Impedance 600 Ω. แต่ในปัจจุบันอุปกรณ์ในด้าน Audio นั้นถูกออกแบบขึ้นมาให้มี Input Impedance มากกว่า 600 Ω ( ส่วนใหญ่มี Input Impedance มากกว่า 10 KΩ ) และมี Output Impedance น้อยกว่า 600 Ω ( ส่วนใหญ่มี Output Impedance อยู่ระหว่าง 50 Ω ถึง 600 Ω ) ซึ่งสามารถทำให้นำไปใช้งานได้ง่าย
โดยที่ Input Impedance และ Output Impedance ไม่ค่อยมีผลกระทบกับสัญญาณเสียง เนื่องจากสัญญาณเสียงอยู่ในช่วงความถี่ต่ำ คือ 20 Hz ถึง 20 KHz. นอกจากเราจะส่งสัญญาณ 20 KHz ผ่านสายยาวเกินกว่า 1200 เมตรเท่านั้น Impedance Matching จึงจะมีผล.
dBu ( Decibel unloaded) หรือdBv คือ มาตรฐานหน่วยวัด Voltage ของ สัญญาณ อ้างอิงที่ 0 dBu ที่คิดจากค่าอ้างอิงที่ 0.775 Vrms. ที่ไม่คำนึงว่าจะมี Load หรือไม่ ซึ่งถ้าในขณะที่มี Load Impedance 600 Ω ต่ออยู่ 0 dBu จะเท่ากับ 0 dBm นั่นเอง
dBV ( Decibel Volt) คือ มาตรฐานหน่วยวัด Voltage ของ สัญญาณ อ้างอิงที่ 0 dBV ที่คิดจากค่าอ้างอิงที่ 1 Vrms.
dBuV ( Decibel MicroVolt) คือ มาตรฐานหน่วยวัด Voltage ของ สัญญาณ อ้างอิงที่ 0 dBuV ที่คิดจากค่าอ้างอิงที่ 1 uVrms. ส่วนใหญ่จะใช้วัดสัญญาณที่มีขนาดเล็กมาก ส่วนใหญ่ใช้ในการวัดความแรงของสัญญาณในย่าน Radio Frequency. เช่น RF ของวิทยุระบบ FM ควรมีความแรงมากกว่า 40 dBuV.
ขอบคุณที่มา http://www.soundkrub.com/
วันอาทิตย์ที่ 28 กุมภาพันธ์ พ.ศ. 2553
Damping factor
ค่า damping factor คือ อะไร
Damping factor เอาแบบง่ายๆนะ มันก็คือ ค่า วัดอัตราส่วน ของค่า impedance ของลำโพง ต่อค่า impedance output ของ power amp ว่ามีค่ามากน้อยเท่าไหร่ ค่านี้ยิ่งมากยิ่งดี
เวลาเราส่งสัญญานเสียงจาก power amp ไปขับ ลำโพง ให้มันสั่นสร้างเป็นเสียงให้เราได้ยิน เมื่อเราหยุดสัญญาน แล้ว แต่ ลำโพงมันไม่สามารถหยุด สั่นได้ทันที เหมือนสัญญาน ก็มันมีมวล มีน้ำหนัก มันก็ต้องค่อยๆหยุด ทีนี้ไอ้การที่ค่อยๆหยุดนี่แหละ หมายความว่า สัญญานเสียงน่ะหมดไปแล้ว แต่ตัวลำโพงเองยังสร้างเสียงอยู่ (ก็มันยังสั่นอยู่) เอาง่ายๆว่า ค่า damping factor นี่คือ พลังงานที่จะไปหยุดลำโพง ให้หยุดสั่นได้ตามสัญญานเสียง
เอาเป็นว่าเป็นค่า หยุดลำโพงไม่ให้สั่นเกินที่เราสั่งแล้วกัน
ซึ่งจะมีผลมากในความถี่ต่ำๆ เพราะต้องใช้พลังงานเยอะ ในการสร้าง ใช้ woofer คัวใหญ่ ความถี่สูง ใช้พลังงานไม่มาก ก็หยุดได้สบายครับ ดังนั้น
ถ้า damping factor ของ power amp ดีๆ เราจะได้
1 Low distortion ก็มันหยุดได้ใกล้เคียงตามสั่งนี่ครับ
2 Low noise (Hiss)
3 Flat frequency respond แปลว่าตอบสนองต่อความถี่ ได้ราบเรียบเหมือนจริง
ขอบคุณที่มา http://www.soundkrub.com
Damping factor เอาแบบง่ายๆนะ มันก็คือ ค่า วัดอัตราส่วน ของค่า impedance ของลำโพง ต่อค่า impedance output ของ power amp ว่ามีค่ามากน้อยเท่าไหร่ ค่านี้ยิ่งมากยิ่งดี
เวลาเราส่งสัญญานเสียงจาก power amp ไปขับ ลำโพง ให้มันสั่นสร้างเป็นเสียงให้เราได้ยิน เมื่อเราหยุดสัญญาน แล้ว แต่ ลำโพงมันไม่สามารถหยุด สั่นได้ทันที เหมือนสัญญาน ก็มันมีมวล มีน้ำหนัก มันก็ต้องค่อยๆหยุด ทีนี้ไอ้การที่ค่อยๆหยุดนี่แหละ หมายความว่า สัญญานเสียงน่ะหมดไปแล้ว แต่ตัวลำโพงเองยังสร้างเสียงอยู่ (ก็มันยังสั่นอยู่) เอาง่ายๆว่า ค่า damping factor นี่คือ พลังงานที่จะไปหยุดลำโพง ให้หยุดสั่นได้ตามสัญญานเสียง
เอาเป็นว่าเป็นค่า หยุดลำโพงไม่ให้สั่นเกินที่เราสั่งแล้วกัน
ซึ่งจะมีผลมากในความถี่ต่ำๆ เพราะต้องใช้พลังงานเยอะ ในการสร้าง ใช้ woofer คัวใหญ่ ความถี่สูง ใช้พลังงานไม่มาก ก็หยุดได้สบายครับ ดังนั้น
ถ้า damping factor ของ power amp ดีๆ เราจะได้
1 Low distortion ก็มันหยุดได้ใกล้เคียงตามสั่งนี่ครับ
2 Low noise (Hiss)
3 Flat frequency respond แปลว่าตอบสนองต่อความถี่ ได้ราบเรียบเหมือนจริง
ขอบคุณที่มา http://www.soundkrub.com
Image Resolution, Format, Aspect Ratio
Image Formats
format / aspect ratio comparison: 16:9 video format, 16:10 PC format, 4:3 'old' TV and PC format
ขอบคุณที่มา http://www.bnoack.com/
format / aspect ratio comparison: 16:9 video format, 16:10 PC format, 4:3 'old' TV and PC format
ขอบคุณที่มา http://www.bnoack.com/
Acoustical terms
Hearing damage is specified from 84 dB SPL+ for 4hrs or more from continuous industrial machine noise. This specification may vary. Time is halved for each 3dB increase (87dB/2hrs) (90dB/1hr) etc. Some people (not all) with hearing damage caused by loud noise, suffer hearing sensitivity loss in the 1K to 3K Hz range only, regardless of what frequencies caused the damage. Normally our hearing is maximally sensitive in the 1K to 3K Hz range. Its this range that enables us to interpret intelligibility in speech. When communicating with someone who suffers loss of sensitivity in this range, it is necessary to speak to them slowly, not loudly.
Reverberant noise of city streets, work places and recreational venues is often in excess of what is safe to experience. The reverberant noise from people talking loudly in restaurants and bars with hard ceilings can exceed 90 dB SPL. Excessive room reverberation can hold noise at a constant level similar to machine noise. It is simply cheaper to make buildings with hard reverberant surfaces. High-powered sound systems are often blamed but not always the problem. The transient peaks of music can be held at a continuous level by reverberation easily adding another 20dB to 30dB more sound energy. Litigation for hearing damage will hopefully bring about social change to force venues and public spaces to be designed more acoustically absorbent.
Without quiet environments in which music can be enjoyed, our way of life can find no order. It is relatively simple to create acoustically absorbent environments. It only takes the will to do so.
The 1st commandment 'know thy Critical Distance'
Acoustic terms and calculations
Absorption coef α = noise absorbed by a material, frequency dependant. Specified from 0 to 1
(fully reflective is 0 = 0% absorption) (0.5 = 50% absorption) (1 = 100% absorption)
Absorption coef = average absorption of room.
Acoustical Masking is any sound of 6dB+ that masks others of similar frequency.
Anechoic: is 100% acoustically absorbent room.
Critical Distance is distance from source where direct and reverberant sound is equal.
Critical Distance Dc = 0.14/√QR (Q = directivity factor 1 of sound source. R = room constant)
Directivity factor Q1 = sound dispersing spherically. Q2 = sound dispersing hemi-spherically. etc
Directivity Index is directivity factor expressed in dB. eg. hemispherical dispersion Q2 = +3dB
Echo: is sound reflected back from 10 meters or more, heard as distinct repeat.
Inverse square law (-6dB/2D) As direct sound doubles in distance, energy diminishes to 1/4
Mean Free Path 4V/S Average distance of reflections. (V volume. S surface) of room.
Mean Free Time Average time of reflections, calculated from mean free path.
Path length is the distance of walls and ceiling from which the sound is reflected back.
Reverberation is sound reflected back from less than 10 meters, not heard as distinct repeat.
Room Constant R = S /1 - small number = reverberant. Large number = absorbent.
RT60 is time reverberation diminishes to - 60dB (1/1,000,000) measured at each 1/3 octave.
RT60 Metric = 0.16/S Imperial = 0.05/S (S surface area. average absorption) of room.
Standing Waves are bass wavelengths cancelled or increased, reflected from walls or ceiling.
Sabin absorption = to 1 square ft of open window.
Sabin Metric absorption = to 1 square meter of open window.
Sabin of person is approx 0.5 Sabin.
Sound transmission class specification of noise reduction through building material
Wavelength (Greek letter symbol Lambda) lambda = velocity of sound / Frequency
ขอบคุณที่มา http://www.lenardaudio.com/
Reverberant noise of city streets, work places and recreational venues is often in excess of what is safe to experience. The reverberant noise from people talking loudly in restaurants and bars with hard ceilings can exceed 90 dB SPL. Excessive room reverberation can hold noise at a constant level similar to machine noise. It is simply cheaper to make buildings with hard reverberant surfaces. High-powered sound systems are often blamed but not always the problem. The transient peaks of music can be held at a continuous level by reverberation easily adding another 20dB to 30dB more sound energy. Litigation for hearing damage will hopefully bring about social change to force venues and public spaces to be designed more acoustically absorbent.
Without quiet environments in which music can be enjoyed, our way of life can find no order. It is relatively simple to create acoustically absorbent environments. It only takes the will to do so.
The 1st commandment 'know thy Critical Distance'
Acoustic terms and calculations
Absorption coef α = noise absorbed by a material, frequency dependant. Specified from 0 to 1
(fully reflective is 0 = 0% absorption) (0.5 = 50% absorption) (1 = 100% absorption)
Absorption coef = average absorption of room.
Acoustical Masking is any sound of 6dB+ that masks others of similar frequency.
Anechoic: is 100% acoustically absorbent room.
Critical Distance is distance from source where direct and reverberant sound is equal.
Critical Distance Dc = 0.14/√QR (Q = directivity factor 1 of sound source. R = room constant)
Directivity factor Q1 = sound dispersing spherically. Q2 = sound dispersing hemi-spherically. etc
Directivity Index is directivity factor expressed in dB. eg. hemispherical dispersion Q2 = +3dB
Echo: is sound reflected back from 10 meters or more, heard as distinct repeat.
Inverse square law (-6dB/2D) As direct sound doubles in distance, energy diminishes to 1/4
Mean Free Path 4V/S Average distance of reflections. (V volume. S surface) of room.
Mean Free Time Average time of reflections, calculated from mean free path.
Path length is the distance of walls and ceiling from which the sound is reflected back.
Reverberation is sound reflected back from less than 10 meters, not heard as distinct repeat.
Room Constant R = S /1 - small number = reverberant. Large number = absorbent.
RT60 is time reverberation diminishes to - 60dB (1/1,000,000) measured at each 1/3 octave.
RT60 Metric = 0.16/S Imperial = 0.05/S (S surface area. average absorption) of room.
Standing Waves are bass wavelengths cancelled or increased, reflected from walls or ceiling.
Sabin absorption = to 1 square ft of open window.
Sabin Metric absorption = to 1 square meter of open window.
Sabin of person is approx 0.5 Sabin.
Sound transmission class specification of noise reduction through building material
Wavelength (Greek letter symbol Lambda) lambda = velocity of sound / Frequency
ขอบคุณที่มา http://www.lenardaudio.com/
Sound absorption
Acoustical absorption of furnishing and curtain fabrics against walls readily absorb high frequencies but have limited absorption at low frequencies. The further curtain fabrics are placed away from walls, the better the absorption is to include lower frequencies. The amount of sound energy absorbed depends on type of material, weight and pleating width. Rock wool (fibreglass) has the highest absorption capacity, converting molecular air movement to heat (at molecular level). Fibreglass consists of minute razor sharp fibres that are irritant and need to be contained within fabric.
Brick, stone, concrete, reflect all sound. Timber, gyprock, steel, reflect most high frequencies and a % low frequency is absorbed by the wall. The remaining low frequency energy that is not reflected or absorbed passes through the wall. Nothing can be done about sound that passes through a wall. Bass frequencies are the most difficult to absorb.
The 1/4 wave-length rule. Acoustical absorbent material must be placed away from walls and ceiling at a distance of 1/4 wavelength of the lowest frequency to be absorbed. This will include all higher frequencies if the absorbent material is soft furnishing or fibreglass. Please note that the ceiling should also be included. Understandably this will slightly reduce the physical size of the room. Acoustically the room will sound and feel LARGER. Also an acoustic absorbent environment is relaxing and calming.
Bass trap refers to distance the absorbent material is from a wall to include absorbing bass frequencies. Lowest frequency absorbed is governed by the material being at a distance of 1/4 wavelength from a wall. Recording studios can have fabric up to 6ft / 2meters from walls. At 1/4 wavelength the molecular air movement is maximum, and is converted to heat by the absorbent material. The remaining sound that gets through the absorbent material is reflected back from the wall and again absorbed by the absorbent material.
Standing Waves are bass frequencies reflected back from walls and ceiling. The reflected bass interferes with the new incoming bass frequencies, causing cancellations at different points throughout the room. Each bass note will behave differently and the cancelled points will be in different positions. Moving speakers or listening position does not solve the problem. The only solution is to insure that the room is 100% absorbent at all bass frequencies. Standing waves also refer to how a string behaves on a musical instrument. There are excellent descriptions of standing waves on other web sites which include animation. Right mouse click to open in new window and allow time to download animation. While waiting to download continue reading.
room standing waves
www.kettering.edu/~drussell/demos.html
www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-basics.htm
stringed instruments
www.id.mind.net/~zona/mstm/physics/waves/standingWaves/standingWaves1/StandingWaves1.html
Panel Absorbers consist of large sheets of plywood formed into complex architectural shapes. The panels can break up standing waves, deflect high frequencies and resonate to absorb bass energy. The formulas governing their behaviour are complex and the outcome is unpredictable and unknown until constructed. Almost without exception they require time consuming trial and error modifications to get them to work as predicted. There are only a few acoustical architects that have mastered them. The below formula gives an approximation only.
fres. = √60/md (fres = frequency of max absorption) (m = panel mass Kg/m2 (d = depth of air space in meters)
www.primacoustic.com/indexstudio.htm
Anechoic chamber is 100% absorbant at all frequencies. No sound can enter or escape from the room and is 100% silent. The closest we can experience this is in an open field, forest or desert on a perfectly still night. Simply described as free field. No sound is reflected or returned. Everyone should experience being in an anechoic chamber or spend time a silent free field to attain a reference. Surprising how different and revealing a sound system actually sounds and therapeutically humbling a reality change can be.
Recording studio control rooms often have walls and ceiling slope outward and upward, away from the speakers and screen. Absolutely no sound should reflect from the rear wall. For amplified performance including cinema's, all walls and ceiling, yes ceiling, should be as close to 100% absorbent as possible at all frequencies (free field).
Echo and excessive reverberation destroys intelligibility and enjoyment for the audience. Absolutely no echo must be allowed to be reflected from the back wall to the stage. The further away from the stage performance the more acoustically absorbent the room should become.
Perfect room
For live acoustic performance the stage walls and ceiling can have a small % of controlled acoustic reflection to enhance the performance. Only from the stage. Acoustic path lengths must be as short as practical. An exaggeration of short acoustic path lengths is a bathroom. Long acoustic path lengths are echoes (churches) and cause difficulty for musicians to play in time.
Sound system placement. Facing speakers directly forward adds excessive reflection from walls, and further reduces intelligibility. Many roadie sound engineers incorrectly mix in mono, in front of one speaker stack facing forward.
The speaker system should be turned inward to improve directivity, and minimise wall reflection. The angle that speakers could be turned inward can only be approximated by academic calculation. The most suited angle has to be found by trial and error. Wherever possible mixing should be from the centre, in stereo, where sound from left and right speakers intersects and at a distance no further back than where direct sound from the speakers is equal to the reflected reverberant energy of the room (Critical Distance).
www.genelec.com/support/flushmount.php
The above picture is to bring attention to the importance of acoustical absorption of ceilings. Many cinema complexes provide acoustical absorption on walls, but forget about ceilings. Below is the address of a company that supplies and consults on acoustical absorption, with many excellent pictures of applications as above.
www.acousticalsurfaces.com
Architectural Acoustics
The information on this site is not meant to substitute academic text books. The aim is to prioritize the order of information to enable good acoustic design, often omitted in academic text.
An excellent referred text is 'Sound System Engineering' by Don and Carolyn Davis.
A Weighting sound measurement is non-linear and scaled in reference to our subjective hearing at low level. Our hearing is very sensitive at low level at the higher frequencies 500Hz to 4KHz, and less sensitive at bass frequencies. A weighting is used for noise measurement of office, work-place, and external traffic environment. A weighting is not appropriate for music and entertainment venues.
C Weighting sound measurement is flat and therefore the correct method of measurement for music and entertainment venues. At higher power (music) our hearing tends to be even at all frequencies especially bass.
Note Building noise specifications are referenced to A Weighting sound measurement, and often limited to frequencies within voice range (250, 500, 1000 and 2000 Hz). Many architects have failed to fully understand the difference between A and C weighting specifications when designing entertainment venues. Bass energy is the most difficult to control, and the least understood, and therefore the largest problem in litigation issues of noise pollution.
(1) Stopping sound The only way to stop all sound from entering or escaping a room is to construct double brick walls, double sealed ceilings, double sealed doors etc. This is approached from the theory of 2 rooms, one within the other with an air gap in between. This is justified by recording, radio and TV studios, but is not economical practical for most homes and venues. The closer to achieving this the better with double-glazed windows, solid timber doors, sealing air gaps, multiple baffled air conditioning etc.
The above table shows approx attenuation -dB of reducing sound getting through a building material. Increasing the thickness of a building material x 2 increases attenuation by approx -6dB. Building materials are specified with Sound Transmission Class (STC) and Noise Reduction Coefficient (NRC). Education of STC and NRC is available on many building material suppliers web sites, including building construction details.
STC and NRC only refer to isolation in speech frequencies (250, 500, 1000 and 2000 Hz) and provide no information of a materials ability to reduce low frequency noise, eg. bass in music etc.
www.stcratings.com
(2) Absorbing sound within a room is essential. But internal absorption has only limited ability to reduce sound that passes through walls. Absorbing sound that has been created inside the room limits reverberation therefore reducing overall sound energy. Absorbing the majority of sound before it strikes the first wall, reduces sound reflected to other walls. Again, this can only indirectly help reduce some sound getting through walls.
The above table shows approx absorption of a material as a ratio. It can be seen that a plywood wall absorbs bass but reflects hi frequencies. Plywood and many other low weight building materials can act as low frequency resonant absorbers as described above in Panel absorbers. Increasing the thickness of a building material x 2 increases attenuation by approx -6dB.
Absorption coef. α = sound absorbed by a material as ratio 0 to 1. Frequency dependant.
Absorption coef. = average absorption of a room as ratio 0 to 1 Frequency dependant.
(fully reflective is 0 = 0% absorption) (0.5 = 50% absorption) (1 = 100% absorption)
Air attenuation / 100 meters (300ft) is approx -3dB/octave from approx 1K Hz dependant on humidity and temp.
www.acoustics.com
(3) Understanding dB for sound absorption.
3dB = x 2 power change we only hear as a bit less or bit more as loud.
10dB = x 10 power change we only hear as double or half as loud.
α 0.5 absorbs 50% sound energy, and 50% reflected.
50% = -3dB, only heard as a bit less as loud to the ear.
α 0.9 absorbs 90% sound energy, and 10% reflected.
90% absorption = -10dB approx only heard as 1/2 as loud to the ear.
Hypothetical example (not calculating for distance of walls) the sound would have to be reflected 6 times through an acoustical absorbent material of α 0.9 for it to be reduced to -60dB RT60.
Simply put, to reduce the amount of echo and reverberation by 1/2 to our hearing the amount of acoustical absorption required may be x 10 greater than one would have assumed.
(4) Reverberation path-lengths First is the direct sound striking a wall.
A % is reflected, a % is absorbed, a % gets through the wall.
The reflected sound then strikes another wall.
A % is reflected, a % is absorbed, a % gets through the wall.
The reflected sound then strikes another wall.
A % is reflected, a % is absorbed, a % gets through the wall. so on and so on.
Sound may have to be reflected many times through the absorbent material on walls to be reduced to RT60 -60dB 1/1,000,000 of its original energy. A larger room 2 x surface area with same absorbent material will have 1/2 the RT60. But a larger room has longer path lengths. If the path-lengths are 20 meters (60ft) or greater the reflections will be heard as distinct repeats (echoes). Reverberation is bad but echoes are infinitely worse. The larger the room the greater the absorbent coef of the material on the walls and ceiling will need be, to insure zero echoes.
Calculations for designing rooms with the appropriate acoustical absorption must include the subjective loudness of how we hear sound. Our ears expand when it is quite to hear detail and contract when loud. Many architects make errors by not including calculations for the subjective hearing experience of loudness variation and loss of intelligibility and annoyance caused by echo and reverberation. The result is that most entertainment venues, work environments and homes have less acoustical absorption than required, or at worst, no acoustical absorption at all.
Repeat. A room that is larger requires more absorbent material with a higher absorbent coef.
(5) Room Constant R is a modified ratio number representing direct to reverberant sound. The R number is academic and has no significance on its own but is used for making further calculations. An example is Critical distance and Articulation index. Room constant calculation is not always needed because a simple listening test can achieve most results required just as accurately. However, understanding the principles behind Room Constant and Room Loss is important.
Room constant calculation assumes a position from the sound source from where the inverse square law applies. In most cases this is at 1 meter. 3ft.
(a) In theory a 100% reverberant room the critical distance would be close to the sound source. The ratio between direct and reverberant sound would be close to 1:1.
Room constant R = small number approx 1.
(b) In theory a 100% absorbent room, the critical distance would be at the walls. The reverberation would be close to 0. The ratio between direct and reverberant sound would be very large.
Room constant R = large number, similar to surface area of room S.
Room Constant: R = S /1 - . small number = reverberant. Large number = absorbent.
(S = surface area of room) ( = average absorption coef) Frequency dependant.
The drawings above and below are simplified to give a basic understanding of the principles described. No matter what calculations of room acoustics are being looked at, always keep the knowing of 'Critical Distance' as priority. The outcome objective is for the Critical Distance to be as far as possible from the sound source at all frequencies. Maintain this as the primary objective and you will never become lost.
The more reverberant the room is the closer the Critical Distance.
The more absorbent the room is the further the Critical Distance.
T60 is measurement of time reverberation diminishes to one millionth (-60dB).
(a) Assume curtain material in cinema has absorbent coef 0.9 (90%) at hi-frequencies.
Reverberation time short T60 = 1/10 sec (100 milli-sec). Hi-frequencies sound clear.
Critical distance is further away.
(b) Assume same curtain material in cinema has absorbent coef 0.1 (10%) at bass frequencies.
Reverberation time long T60 = 1.5 sec Bass sounds muddled.
Critical distance is close to sound system.
(6) Regulation and Litigation Many entertainment venues are in suburbs where noise regulations are strict. Complying with regulations by driving at the speed limit, may be acceptable on the road. But saving $ in building construction by doing the least possible to comply with noise regulations is risky. Heavy metal, rap and techno is offensive to the majority of the conservative population, regardless of how far below the regularity noise level the music is heard at especially bass.
Many venues complying with noise regulations have still been closed down, sometimes resulting in successful litigation against architects by venue owners. Architects have a legal responsibility to correctly advise venue owners of the regulatory and social non-acceptance of noise pollution.
(7) Calculations and Testing Procedures Calculations for designing a studio or entertainment venue must always contain the knowing of Critical distance at all frequencies, as priority. The absolute rule is that form (visual) must follow function (sound).
The Sydney Opera House is without doubt one of the the worlds worst example of an acoustical environment that had been only conceived from a visual design. This resulted from the initial conditions locked in by visual design, 'form following function' in the wrong order. As a visual tourist attraction it is sucessful but as an entertainment venue it is government subsitised.
www.keithyates.com/glossary.htm
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Brick, stone, concrete, reflect all sound. Timber, gyprock, steel, reflect most high frequencies and a % low frequency is absorbed by the wall. The remaining low frequency energy that is not reflected or absorbed passes through the wall. Nothing can be done about sound that passes through a wall. Bass frequencies are the most difficult to absorb.
The 1/4 wave-length rule. Acoustical absorbent material must be placed away from walls and ceiling at a distance of 1/4 wavelength of the lowest frequency to be absorbed. This will include all higher frequencies if the absorbent material is soft furnishing or fibreglass. Please note that the ceiling should also be included. Understandably this will slightly reduce the physical size of the room. Acoustically the room will sound and feel LARGER. Also an acoustic absorbent environment is relaxing and calming.
Bass trap refers to distance the absorbent material is from a wall to include absorbing bass frequencies. Lowest frequency absorbed is governed by the material being at a distance of 1/4 wavelength from a wall. Recording studios can have fabric up to 6ft / 2meters from walls. At 1/4 wavelength the molecular air movement is maximum, and is converted to heat by the absorbent material. The remaining sound that gets through the absorbent material is reflected back from the wall and again absorbed by the absorbent material.
Standing Waves are bass frequencies reflected back from walls and ceiling. The reflected bass interferes with the new incoming bass frequencies, causing cancellations at different points throughout the room. Each bass note will behave differently and the cancelled points will be in different positions. Moving speakers or listening position does not solve the problem. The only solution is to insure that the room is 100% absorbent at all bass frequencies. Standing waves also refer to how a string behaves on a musical instrument. There are excellent descriptions of standing waves on other web sites which include animation. Right mouse click to open in new window and allow time to download animation. While waiting to download continue reading.
room standing waves
www.kettering.edu/~drussell/demos.html
www.isvr.soton.ac.uk/SPCG/Tutorial/Tutorial/Tutorial_files/Web-basics.htm
stringed instruments
www.id.mind.net/~zona/mstm/physics/waves/standingWaves/standingWaves1/StandingWaves1.html
Panel Absorbers consist of large sheets of plywood formed into complex architectural shapes. The panels can break up standing waves, deflect high frequencies and resonate to absorb bass energy. The formulas governing their behaviour are complex and the outcome is unpredictable and unknown until constructed. Almost without exception they require time consuming trial and error modifications to get them to work as predicted. There are only a few acoustical architects that have mastered them. The below formula gives an approximation only.
fres. = √60/md (fres = frequency of max absorption) (m = panel mass Kg/m2 (d = depth of air space in meters)
www.primacoustic.com/indexstudio.htm
Anechoic chamber is 100% absorbant at all frequencies. No sound can enter or escape from the room and is 100% silent. The closest we can experience this is in an open field, forest or desert on a perfectly still night. Simply described as free field. No sound is reflected or returned. Everyone should experience being in an anechoic chamber or spend time a silent free field to attain a reference. Surprising how different and revealing a sound system actually sounds and therapeutically humbling a reality change can be.
Recording studio control rooms often have walls and ceiling slope outward and upward, away from the speakers and screen. Absolutely no sound should reflect from the rear wall. For amplified performance including cinema's, all walls and ceiling, yes ceiling, should be as close to 100% absorbent as possible at all frequencies (free field).
Echo and excessive reverberation destroys intelligibility and enjoyment for the audience. Absolutely no echo must be allowed to be reflected from the back wall to the stage. The further away from the stage performance the more acoustically absorbent the room should become.
Perfect room
For live acoustic performance the stage walls and ceiling can have a small % of controlled acoustic reflection to enhance the performance. Only from the stage. Acoustic path lengths must be as short as practical. An exaggeration of short acoustic path lengths is a bathroom. Long acoustic path lengths are echoes (churches) and cause difficulty for musicians to play in time.
Sound system placement. Facing speakers directly forward adds excessive reflection from walls, and further reduces intelligibility. Many roadie sound engineers incorrectly mix in mono, in front of one speaker stack facing forward.
The speaker system should be turned inward to improve directivity, and minimise wall reflection. The angle that speakers could be turned inward can only be approximated by academic calculation. The most suited angle has to be found by trial and error. Wherever possible mixing should be from the centre, in stereo, where sound from left and right speakers intersects and at a distance no further back than where direct sound from the speakers is equal to the reflected reverberant energy of the room (Critical Distance).
www.genelec.com/support/flushmount.php
The above picture is to bring attention to the importance of acoustical absorption of ceilings. Many cinema complexes provide acoustical absorption on walls, but forget about ceilings. Below is the address of a company that supplies and consults on acoustical absorption, with many excellent pictures of applications as above.
www.acousticalsurfaces.com
Architectural Acoustics
The information on this site is not meant to substitute academic text books. The aim is to prioritize the order of information to enable good acoustic design, often omitted in academic text.
An excellent referred text is 'Sound System Engineering' by Don and Carolyn Davis.
A Weighting sound measurement is non-linear and scaled in reference to our subjective hearing at low level. Our hearing is very sensitive at low level at the higher frequencies 500Hz to 4KHz, and less sensitive at bass frequencies. A weighting is used for noise measurement of office, work-place, and external traffic environment. A weighting is not appropriate for music and entertainment venues.
C Weighting sound measurement is flat and therefore the correct method of measurement for music and entertainment venues. At higher power (music) our hearing tends to be even at all frequencies especially bass.
Note Building noise specifications are referenced to A Weighting sound measurement, and often limited to frequencies within voice range (250, 500, 1000 and 2000 Hz). Many architects have failed to fully understand the difference between A and C weighting specifications when designing entertainment venues. Bass energy is the most difficult to control, and the least understood, and therefore the largest problem in litigation issues of noise pollution.
(1) Stopping sound The only way to stop all sound from entering or escaping a room is to construct double brick walls, double sealed ceilings, double sealed doors etc. This is approached from the theory of 2 rooms, one within the other with an air gap in between. This is justified by recording, radio and TV studios, but is not economical practical for most homes and venues. The closer to achieving this the better with double-glazed windows, solid timber doors, sealing air gaps, multiple baffled air conditioning etc.
The above table shows approx attenuation -dB of reducing sound getting through a building material. Increasing the thickness of a building material x 2 increases attenuation by approx -6dB. Building materials are specified with Sound Transmission Class (STC) and Noise Reduction Coefficient (NRC). Education of STC and NRC is available on many building material suppliers web sites, including building construction details.
STC and NRC only refer to isolation in speech frequencies (250, 500, 1000 and 2000 Hz) and provide no information of a materials ability to reduce low frequency noise, eg. bass in music etc.
www.stcratings.com
(2) Absorbing sound within a room is essential. But internal absorption has only limited ability to reduce sound that passes through walls. Absorbing sound that has been created inside the room limits reverberation therefore reducing overall sound energy. Absorbing the majority of sound before it strikes the first wall, reduces sound reflected to other walls. Again, this can only indirectly help reduce some sound getting through walls.
The above table shows approx absorption of a material as a ratio. It can be seen that a plywood wall absorbs bass but reflects hi frequencies. Plywood and many other low weight building materials can act as low frequency resonant absorbers as described above in Panel absorbers. Increasing the thickness of a building material x 2 increases attenuation by approx -6dB.
Absorption coef. α = sound absorbed by a material as ratio 0 to 1. Frequency dependant.
Absorption coef. = average absorption of a room as ratio 0 to 1 Frequency dependant.
(fully reflective is 0 = 0% absorption) (0.5 = 50% absorption) (1 = 100% absorption)
Air attenuation / 100 meters (300ft) is approx -3dB/octave from approx 1K Hz dependant on humidity and temp.
www.acoustics.com
(3) Understanding dB for sound absorption.
3dB = x 2 power change we only hear as a bit less or bit more as loud.
10dB = x 10 power change we only hear as double or half as loud.
α 0.5 absorbs 50% sound energy, and 50% reflected.
50% = -3dB, only heard as a bit less as loud to the ear.
α 0.9 absorbs 90% sound energy, and 10% reflected.
90% absorption = -10dB approx only heard as 1/2 as loud to the ear.
Hypothetical example (not calculating for distance of walls) the sound would have to be reflected 6 times through an acoustical absorbent material of α 0.9 for it to be reduced to -60dB RT60.
Simply put, to reduce the amount of echo and reverberation by 1/2 to our hearing the amount of acoustical absorption required may be x 10 greater than one would have assumed.
(4) Reverberation path-lengths First is the direct sound striking a wall.
A % is reflected, a % is absorbed, a % gets through the wall.
The reflected sound then strikes another wall.
A % is reflected, a % is absorbed, a % gets through the wall.
The reflected sound then strikes another wall.
A % is reflected, a % is absorbed, a % gets through the wall. so on and so on.
Sound may have to be reflected many times through the absorbent material on walls to be reduced to RT60 -60dB 1/1,000,000 of its original energy. A larger room 2 x surface area with same absorbent material will have 1/2 the RT60. But a larger room has longer path lengths. If the path-lengths are 20 meters (60ft) or greater the reflections will be heard as distinct repeats (echoes). Reverberation is bad but echoes are infinitely worse. The larger the room the greater the absorbent coef of the material on the walls and ceiling will need be, to insure zero echoes.
Calculations for designing rooms with the appropriate acoustical absorption must include the subjective loudness of how we hear sound. Our ears expand when it is quite to hear detail and contract when loud. Many architects make errors by not including calculations for the subjective hearing experience of loudness variation and loss of intelligibility and annoyance caused by echo and reverberation. The result is that most entertainment venues, work environments and homes have less acoustical absorption than required, or at worst, no acoustical absorption at all.
Repeat. A room that is larger requires more absorbent material with a higher absorbent coef.
(5) Room Constant R is a modified ratio number representing direct to reverberant sound. The R number is academic and has no significance on its own but is used for making further calculations. An example is Critical distance and Articulation index. Room constant calculation is not always needed because a simple listening test can achieve most results required just as accurately. However, understanding the principles behind Room Constant and Room Loss is important.
Room constant calculation assumes a position from the sound source from where the inverse square law applies. In most cases this is at 1 meter. 3ft.
(a) In theory a 100% reverberant room the critical distance would be close to the sound source. The ratio between direct and reverberant sound would be close to 1:1.
Room constant R = small number approx 1.
(b) In theory a 100% absorbent room, the critical distance would be at the walls. The reverberation would be close to 0. The ratio between direct and reverberant sound would be very large.
Room constant R = large number, similar to surface area of room S.
Room Constant: R = S /1 - . small number = reverberant. Large number = absorbent.
(S = surface area of room) ( = average absorption coef) Frequency dependant.
The drawings above and below are simplified to give a basic understanding of the principles described. No matter what calculations of room acoustics are being looked at, always keep the knowing of 'Critical Distance' as priority. The outcome objective is for the Critical Distance to be as far as possible from the sound source at all frequencies. Maintain this as the primary objective and you will never become lost.
The more reverberant the room is the closer the Critical Distance.
The more absorbent the room is the further the Critical Distance.
T60 is measurement of time reverberation diminishes to one millionth (-60dB).
(a) Assume curtain material in cinema has absorbent coef 0.9 (90%) at hi-frequencies.
Reverberation time short T60 = 1/10 sec (100 milli-sec). Hi-frequencies sound clear.
Critical distance is further away.
(b) Assume same curtain material in cinema has absorbent coef 0.1 (10%) at bass frequencies.
Reverberation time long T60 = 1.5 sec Bass sounds muddled.
Critical distance is close to sound system.
(6) Regulation and Litigation Many entertainment venues are in suburbs where noise regulations are strict. Complying with regulations by driving at the speed limit, may be acceptable on the road. But saving $ in building construction by doing the least possible to comply with noise regulations is risky. Heavy metal, rap and techno is offensive to the majority of the conservative population, regardless of how far below the regularity noise level the music is heard at especially bass.
Many venues complying with noise regulations have still been closed down, sometimes resulting in successful litigation against architects by venue owners. Architects have a legal responsibility to correctly advise venue owners of the regulatory and social non-acceptance of noise pollution.
(7) Calculations and Testing Procedures Calculations for designing a studio or entertainment venue must always contain the knowing of Critical distance at all frequencies, as priority. The absolute rule is that form (visual) must follow function (sound).
The Sydney Opera House is without doubt one of the the worlds worst example of an acoustical environment that had been only conceived from a visual design. This resulted from the initial conditions locked in by visual design, 'form following function' in the wrong order. As a visual tourist attraction it is sucessful but as an entertainment venue it is government subsitised.
www.keithyates.com/glossary.htm
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Critical distance
"Know thy Critical Distance" is the 1st commandment of acoustics. Critical Distance is the distance from the sound source where the direct and reverberant sound energies become equal. The more reverberant a room is, the closer the Critical Distance is to the sound source. The more absorbent a room is, the further the Critical Distance is from the sound source. (Critical Distance is different at all frequencies).
For good acoustic design the Critical Distance should be as far as possible from the sound source and the resultant reverberation minimal and even at all frequencies. Direct sound from the speaker system diminishes in level as a function of the distance (inverse square law) whereas reverberation constantly spreads throughout the room. Because there is new incoming sound from the speakers reverberation keeps building up until the new incoming sound equals the sound absorbed (steady-state).
When the reverberant sound becomes 12dB or greater than the direct sound all intelligibility is lost. The simplest way to find 'Critical Distance' is to play compressed pop music through the sound system. Begin with one speaker (left or right). Walk back and forth around the room, and you will be surprised how easy it is to identify the critical distance. Repeat the exercise with the other speaker, then both speakers. Its surprising how accurate our ears are when compared with acoustic measurement microphones.
* The more reverberant the room is the closer the Critical Distance.
* The more absorbent the room is the further the Critical Distance.
* Near field or Direct field is inside the Critical Distance.
* Far field or Reverberant field is outside the Critical Distance.
Critical Distance: Dc = 0.14/√QR (Q = directivity factor 1 of sound source. R = room constant)
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For good acoustic design the Critical Distance should be as far as possible from the sound source and the resultant reverberation minimal and even at all frequencies. Direct sound from the speaker system diminishes in level as a function of the distance (inverse square law) whereas reverberation constantly spreads throughout the room. Because there is new incoming sound from the speakers reverberation keeps building up until the new incoming sound equals the sound absorbed (steady-state).
When the reverberant sound becomes 12dB or greater than the direct sound all intelligibility is lost. The simplest way to find 'Critical Distance' is to play compressed pop music through the sound system. Begin with one speaker (left or right). Walk back and forth around the room, and you will be surprised how easy it is to identify the critical distance. Repeat the exercise with the other speaker, then both speakers. Its surprising how accurate our ears are when compared with acoustic measurement microphones.
* The more reverberant the room is the closer the Critical Distance.
* The more absorbent the room is the further the Critical Distance.
* Near field or Direct field is inside the Critical Distance.
* Far field or Reverberant field is outside the Critical Distance.
Critical Distance: Dc = 0.14/√QR (Q = directivity factor 1 of sound source. R = room constant)
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Acoustic principles
Auditoriums well designed give acoustical directivity from stage to audience with a fine balance of reverberation to enhance performance. When recording, musicians select the right reverberation for the music. Once recorded extra room reverberation detracts from the music. The perfect listening room for recorded music is 100% absorbent at all frequencies (free field).
Reverberation is sound reflecting off the floor, walls and ceiling and builds up to a percentage of the direct sound and is different at all frequencies. Some rooms are absorbent at high frequencies, but reverberant at low frequencies and vice versa. As we move closer to the speakers the direct sound gets louder and clearer. As we move back from the speakers direct sound diminishes (inverse square law) but the reverberant sound remains constant and limits intelligibility.
RT60 is time in seconds for reverberation to diminish to - 60dB (1/1,000,000). With practice this test can be approximated with a single hand-clap, in a quite room, as in the above graph. But with continuous sound (music) reverberation builds up and remains at a constant level.
Note: Changing the loudness of music does not change the reverb time T60.
RT60 Metric = 0.16/S Imperial = 0.05/S (S surface area. average absorption) of room.
Path-lengths refer to the distance of walls and ceiling, characterising the reverberation. Stage walls and ceiling can have limited controlled acoustic reflection to enhance the performance. Only from the stage. Acoustic path lengths must be as short as practical. An exaggeration of short acoustic path-lengths is a bathroom. Long acoustic path-lengths are echoes (churches) and cause difficulty for musicians to play in time. Different reverberation path-lengths suit different music. No single reverberation path-length suits all music.
Sound travels at 344 meters / sec (1ft / milli-sec) approx. Walls or ceiling that are 10 meters 30ft from the sound source, can reflect sound back to the musicians. The total distance being 20 meters 60ft, which is 30 milli-sec delay. Sound reflected back from 10 meters and greater cause difficulty for musicians especially when playing modern percussive music.
Echo is heard as distinct repeat, 100 milli-seconds (1/10 sec) or greater, from walls and ceiling with path-lengths greater than 15 meters (45ft) apart. Echoes cause difficulty for musicians to play in time and destroy intelligibility. Only the egos of deity's suit the excessive echo and reverberation of churches for pipe organs and choirs to sing their praise.
Auditorium Design. Many auditoriums are visually beautiful but reverberant nightmares. Auditoriums and Concert halls evolved before electronic sound re-enforcement was available. Mozart hated the excessive echo and reverberation of many large concert halls, which restricted his music. Mozart often preferred to perform outside. Historically reverberation was used to increase sound level to the audience. But the cost of increasing sound level by reverberation is at the loss of intelligibility. However a small amount of short path-length reverberation can beautifully enhance a performance. There is no one single path-length of reverberation to suit all music. The larger the concert hall, the longer the acoustic path-lengths (echoes) and the slower the music has to be to retain intelligibility.
The above auditorium is typical of current design. The majority of concert halls of approx 2,000 seating have an average reverberation time RT of 1.5 seconds. But auditoriums can now be anechoic designed . Free of reverberation and echo. Amplification with computer management skilfully applied can provide the ideal sound level with correct reverberation, perfectly tailored for any performance imaginable. However the understanding to manage auditorium acoustics requires a specialised field of study combining music, architectural and electro-acoustics. Formal education in this field has yet to evolve. Greek and Roman architecture demonstrated exceptional skill of sound and reverberation management in the designing of some amphitheatres.
The original designers of amphitheatres had an understanding of 'critical distance'. As we move closer to the performance the direct sound gets louder and clearer. As we move away from the performance direct sound diminishes (inverse square law) whereas reverberation can remain constant. Some amphitheatres have the unique capacity to maintain a constant 'critical distance' providing an even balance of direct and reverberant sound throughout the theatre. Without understanding 'Critical Distance' all other knowledge on acoustics has no meaning.
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Reverberation is sound reflecting off the floor, walls and ceiling and builds up to a percentage of the direct sound and is different at all frequencies. Some rooms are absorbent at high frequencies, but reverberant at low frequencies and vice versa. As we move closer to the speakers the direct sound gets louder and clearer. As we move back from the speakers direct sound diminishes (inverse square law) but the reverberant sound remains constant and limits intelligibility.
RT60 is time in seconds for reverberation to diminish to - 60dB (1/1,000,000). With practice this test can be approximated with a single hand-clap, in a quite room, as in the above graph. But with continuous sound (music) reverberation builds up and remains at a constant level.
Note: Changing the loudness of music does not change the reverb time T60.
RT60 Metric = 0.16/S Imperial = 0.05/S (S surface area. average absorption) of room.
Path-lengths refer to the distance of walls and ceiling, characterising the reverberation. Stage walls and ceiling can have limited controlled acoustic reflection to enhance the performance. Only from the stage. Acoustic path lengths must be as short as practical. An exaggeration of short acoustic path-lengths is a bathroom. Long acoustic path-lengths are echoes (churches) and cause difficulty for musicians to play in time. Different reverberation path-lengths suit different music. No single reverberation path-length suits all music.
Sound travels at 344 meters / sec (1ft / milli-sec) approx. Walls or ceiling that are 10 meters 30ft from the sound source, can reflect sound back to the musicians. The total distance being 20 meters 60ft, which is 30 milli-sec delay. Sound reflected back from 10 meters and greater cause difficulty for musicians especially when playing modern percussive music.
Echo is heard as distinct repeat, 100 milli-seconds (1/10 sec) or greater, from walls and ceiling with path-lengths greater than 15 meters (45ft) apart. Echoes cause difficulty for musicians to play in time and destroy intelligibility. Only the egos of deity's suit the excessive echo and reverberation of churches for pipe organs and choirs to sing their praise.
Auditorium Design. Many auditoriums are visually beautiful but reverberant nightmares. Auditoriums and Concert halls evolved before electronic sound re-enforcement was available. Mozart hated the excessive echo and reverberation of many large concert halls, which restricted his music. Mozart often preferred to perform outside. Historically reverberation was used to increase sound level to the audience. But the cost of increasing sound level by reverberation is at the loss of intelligibility. However a small amount of short path-length reverberation can beautifully enhance a performance. There is no one single path-length of reverberation to suit all music. The larger the concert hall, the longer the acoustic path-lengths (echoes) and the slower the music has to be to retain intelligibility.
The above auditorium is typical of current design. The majority of concert halls of approx 2,000 seating have an average reverberation time RT of 1.5 seconds. But auditoriums can now be anechoic designed . Free of reverberation and echo. Amplification with computer management skilfully applied can provide the ideal sound level with correct reverberation, perfectly tailored for any performance imaginable. However the understanding to manage auditorium acoustics requires a specialised field of study combining music, architectural and electro-acoustics. Formal education in this field has yet to evolve. Greek and Roman architecture demonstrated exceptional skill of sound and reverberation management in the designing of some amphitheatres.
The original designers of amphitheatres had an understanding of 'critical distance'. As we move closer to the performance the direct sound gets louder and clearer. As we move away from the performance direct sound diminishes (inverse square law) whereas reverberation can remain constant. Some amphitheatres have the unique capacity to maintain a constant 'critical distance' providing an even balance of direct and reverberant sound throughout the theatre. Without understanding 'Critical Distance' all other knowledge on acoustics has no meaning.
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