Audio Engineering

108 Chapter 3

Table 3.5 : Absorption of Seats and Audiencea

Materials 125 Hz 250 Hz 500 Hz 1 kHz 2 kHz 4 kHz

Audience, seated, depending on

spacing and upholstery of seats

2.5–4.0 3.5–5.0 4.0–5.5 4.5–6.5 5.0–7.0 4.5–7.0

Seats

Heavily upholstered with fabric 1.5–3.5 3.5–4.5 4.0–5.0 4.0–5.5 3.5–5.5 3.5–4.5

Heavily upholstered with leather,

plastic, etc.

2.5–3.5 3.0–4.5 3.0–4.0 2.0–4.0 1.5–4.0 1.0–3.0

Lightly upholstered with leather,

plastic, etc.

1.5–2.0

Wood veneer, no upholstery 0.15 0.20 0.25 0.30 0.50 0.50

Wood pews

No cushions, per 18-in. length 0.40

Cushioned, per 18-in. length 1.8–2.3

a Values given are in sabins per person or unit of seating.

3.13.1 The Mean Free Path (MFP)

The mean free path is the average distance between refl ections in a space. For our sample
space:

MFP

V

S







4

4

500 000

42 500

47

,

,

⎛

⎝

⎜⎜

⎜

⎞

⎠

⎟⎟

⎟⎟

ft.

If a sound is generated in the sample space, part of it will travel directly to a listener and
undergo inverse-square-law level change on its way. Some more of it will arrive after
having traveled fi rst to some refl ecting surface, and still more will fi nally arrive having
undergone several successive refl ections (each 47 ft apart on the average). Each of these
signals will have had more attenuation at some frequencies than at others because of
divergence, absorption, refl ection, refraction, diffraction, etc.

We can look at this situation in a different manner. Each sound made will have traveled
4.5 s 1130 ft/s, or 5085 ft. Since the mean free path is 47 ft, then we can assume each