Handbook for Sound Engineers

134 Chapter 6

compared to a 95.3 Hz mode in the nonrectangular
room. Fig. 6-10 shows the 0,3,0 mode at 102.9 Hz of a
rectangular room contrasted to a 103.9 Hz resonance in
the nonrectangular room. These pressure distribution
diagrams of Fig. 6-10 give an excellent appreciation of
the distortion of the sound field by extreme skewing of
room surfaces.
When the shape of the room is irregular, as in Fig.
6-10, the modal pressure pattern is also irregular. The
number of modes per frequency band in the irregular
room is about the same as the regular room because it is
determined principally by the volume of the room rather
than its shape. Instead of axial, tangential, and oblique
modes characteristic of the rectangular room, the reso-
nances of the nonrectangular room all have the char-
acter of 3D (obliquelike) modes. This has been
demonstrated by measuring decay rates and finding less
fluctuation from mode to mode. Note that the modes did
not go away and that there was not a significant change
in the frequency of the modes nor in the distribution of
the modes relative to frequency. What changed was the

distribution of the modes in the physical space. The

benefits of asymmetrical, nonrectangular designs must

be measured against the drawbacks as we shall see later

on in this chapter.

6.6 Summation of Modal Effect

Room modes determine the performance of small rooms

below fl. The following criteria should be applied when

evaluating room ratios or dimensions in terms of modal

distribution. When considering axial modes, there

should be no modes within 5 Hz of each other, and no

mode should be greater than 20 Hz from another. Since

the modal bandwidth in small rooms is approximately

5 Hz, any modes that are within 5 Hz of each other will

effectively merge into one. Modes that are isolated by

more than 20 Hz will not have masking from any other

modes nearby and will likely stand out. Obviously there

should not be any double or triple modes. Some criteria

Figure 6-10. A comparison of calculated 2D sound fields in rectangular and nonrectangular rooms having the same areas.
After Reference 8.

A. The 1,0,0 mode of the rectangular room (34.3 Hz)

compared to the nonrectangular room (31.6 Hz). B. The 3,1,0 mode of the rectangular room (81.1 Hz)compared to the nonrectangular room (85.5 Hz).

C. The 4,0,0 mode in the rectangular room (98 Hz)

compared to the nonrectangular room (95.3 Hz).

D. The 0,3,0 mode (102.9 Hz) contrasted to the

nonrectangular room (103.9 Hz).