Handbook for Sound Engineers

132 Chapter 6

However 282 Hz is also a problem frequency because it
is separated by more than 20 Hz on either side.

6.4 Criteria for Evaluating Room Modes

So far we have shown that there are a few general

guidelines for designing small rooms with good distri-

bution of room modes. We know that if two or more

modes occupy the same frequency or are bunched up

and isolated from neighbors, we are immediately

warned of potential coloration problems. Over the

years, a number of authors have suggested techniques

for the assessment of room mores and methods for

predicting the low frequency response of rooms based

on the distributions of room modes. Most notably Bolt^7 ,

Gilford,^26 Louden,^25 Bonello,^3 and D’Antonio^27 have all

suggested criteria. Possibly the most widely used

criteria is that suggested by Bonello.

Bonello’s number one criterion is to plot the number

of modes (all the modes, axial, tangential, and oblique)

in octave bands against frequency and to examine the

resulting plot to see if the curve increases monotonically

(i.e., if each octave has more modes than the

preceding one or, at least, an equal number). His

number two criterion is to examine the modal frequen-

cies to make sure there are no coincident modes, or, at

least, if there are coincident modes, there should be five

Figure 6-8. The application of Bonello's criterion 1 to the 23 ft × 17 ft × 9 ft room. From AcousticX.

Table 6-7. Axial Modes for a Room 18 Ft × 13 Ft
×9Ft

Frequency Spacing Frequency Spacing

31.39 219.72 2.41

43.46 12.07 251.11 31.39

62.78 19.32 251.11 0.00

62.78 0.00 260.77 9.66

86.92 24.15 282.50 21.73

94.17 7.24 304.23 21.73

125.56 31.39 313.89 9.66

125.56 0.00 313.89 0.00

130.38 4.83 345.28 31.39

156.94 26.56 347.69 2.41

173.85 16.90 376.67 28.97

188.33 14.49 376.67 0.00

188.33 0.00 391.15 14.49

217.31 28.97

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