14 November 2010

Sound absorption

(Extracted from :  http://www.lenardaudio.com/education/04_acoustics_3.html)

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.


Acoustic absorption

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

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
stringed instruments

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)

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.

Speaker position

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

Ceiling absorption

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

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