7-4 Behavior of Sound

What is intensity?

We have learned that the sound intensity tells us the amount of energy that is being transmitted.

    Intensity = Energy / (Time X Area)
    Intensity = Power / Area

The unit for intensity is decibels (db).

The diagram above shows us how the intensity is transmitted from the source of sound.

What is reflection?

We have learned that sound waves will reflect over the surfaces it comes in contact with. A sound wave will continue to be reflected by a surface until the angle of reflection increases the angle of incidence. Furthermore, surfaces with a concave shape will focus the sound more than a flat surface (which is why you want to be careful to avoid too many protrusion on a wall).

Regular reflection occurs if the surface dimension is large compared to the wavelength of the sound. Performers and audience members expect a degree of reflection to occur so that there is a rich blend of sounds throughout the hall.

What is diffusion?

Diffusion occurs when the dimensions of a surface equals the sound's wavelengths. During diffusion, sound in scattered (via reflection) in all directions.

What is diffraction?

Diffraction occurs when the dimensions of a surface is more than the sound's wavelengths. During diffraction, sound waves will change direction in order to pass through or around a barrier.

What is absorption?

Sound waves will be absorbed by the air and the surface it comes in contact with. The larger the area of a surface, the more sound energy that will be absorbed. (This is why sound waves will be absorbed more in a hall fill with people than a rehearsal with no audience members present.)

    Total Absorption = a1s1 + a2s2 + a3s3 ...
    where a is the absorbtivity and s is the area of the surface in square meters

Absorption Coefficients for Various Materials

Material	125 Hz	250 Hz	500 Hz	1000 Hz	2000 Hz	4000 Hz
Air, per 100 m^3, relative humidity 50%	-	-	-	0.30	0.90	2.40
Concrete block, painted	0.01	0.05	0.06	0.07	0.09	0.08
Glass, window	0.35	0.25	0.18	0.12	0.07	0.04
Plaster, on lath	0.14	0.10	0.06	0.05	0.04	0.03
Plaster, on concrete block	0.12	0.09	0.07	0.05	0.05	0.04
Plaster, on brick	0.01	0.02	0.02	0.03	0.04	0.05
Plywood, paneling	0.28	0.22	0.17	0.09	0.10	0.11
Concrete block, unpainted	0.36	0.44	0.31	0.29	0.39	0.25
Drapery, lightweight	0.03	0.04	0.11	0.17	0.24	0.35
Drapery, heavyweight	0.14	0.35	0.55	0.72	0.70	0.65
Terrazzo floor	0.01	0.01	0.02	0.02	0.02	0.02
Marble floor	0.01	0.01	0.01	0.01	0.02	0.02
Wood floor	0.15	0.11	0.10	0.07	0.06	0.07
Carpet, on concrete	0.02	0.06	0.14	0.37	0.60	0.65
Carpet, on pad	0.08	0.24	0.57	0.69	0.71	0.73
Carpet, indoor-outdoor	0.01	0.05	0.10	0.20	0.45	0.65
Acoustical tile, suspended	0.76	0.93	0.83	0.99	0.99	0.94
Upholstered furniture, fabric	0.19	0.37	0.56	0.67	0.61	0.59
Upholstered furniture, leather	0.44	0.54	0.60	0.62	0.58	0.50

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