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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

End of Topic 4
Created: 18-Aug-2009