Audio Engineering

Room Acoustics 861

The complete formula for calculating the complete series of modes is as follows:

F

nx

Lx

ny

Ly

nz

Lz

 172

⎛^222

⎝

⎜⎜

⎜

⎞

⎠

⎟⎟

⎟⎟

⎛

⎝

⎜⎜

⎜⎜

⎞

⎠

⎟⎟

⎟⎟

⎛

⎝

⎜⎜

⎜

⎞

⎠

⎟⎟

⎟⎟

⎡

⎣

⎢⎢

⎢

⎢

⎤

⎦

⎥

⎥

⎥

1

2

.

At low frequencies, the density of room modes is low, causing each to stand out and
consequently be more audible. However, at higher frequencies, the density becomes very
much greater, forming a continuous spectrum, that is, at any given frequency, a number
of modes will occur, which counteract each other. Room modes therefore generally cause
problems at low or lower midfrequencies (typically up to 250–500 Hz). While this is
primarily due to the low modal density at these frequencies, it is also exacerbated by the
general lack of sound absorption that occurs at low frequencies in most rooms.

After determining that the room/studio dimensions are appropriate to ensure low
coincidence of modal frequencies, control of room modes is brought about by providing
appropriate absorption to damp down the resonances.

29.3.3 Absorber Performance

Sound absorbers effectively fall into four categories:

1. High and mediumfrequency dissipative porous absorbers.


2. Low-frequency panel or membrane absorbers.

5

10

15

20

25

30

35

40

45

10 20 30 40 50 60 70 80 90 100

Frequency (Hz)

13.5

12

10.5

9

7.5

6

4.5

3

Meters

Distance between room surfaces (feet)

Figure 29.9 : Axial mode frequency versus room dimension.