Master Handbook of Acoustics by Everest F. Alton & Pohlmann Ken

Author:Everest, F. Alton & Pohlmann, Ken [Everest, F. Alton]
Language: eng
Format: mobi
Publisher: McGraw-Hill Education
Published: 2009-05-30T16:00:00+00:00

The Bonello Criterion

In reviewing various preferred room ratios, we have observed that choosing the right ratio of dimensions is crucial in obtaining good modal response. For example, the ratio of any two dimensions should not be a whole number or close to a whole number. Bonello suggests a method for determining the acoustical desirability of the proportions of rectangular rooms. He divides the low end of the audible spectrum into bands ⅓-octave wide and considers the number of modes in each band below 200 Hz. The ⅓-octave bands are chosen because they approximate the critical bands of the human ear.

To meet Bonello’s criterion, each ⅓ octave should have more modes than the preceding one, or at least the same number. Modal coincidences are not tolerated unless there are at least 5 modes in that band.

How does a 15.4 × 12.8 × 10 ft room qualify by this criterion? Figure 13-22 shows that it passes this test. The plot climbs steadily upward with no downward anomalies. The horizontal section at 40 Hz is allowed. This suggests good modal response.

Although many authors have suggested various room dimension ratios for optimal modal response, it is important to remember that there is no ideal ratio of dimensions. Furthermore, the quest for a perfect ratio is an impossible one. In real rooms, the structural integrity of the room is not uniform at low frequencies; with a given sound source location, the various modes are not excited equally; and a seated listener can only hear a few of the modes. Modal response is a genuine problem, but predicting the response with certainty using generalized assumptions is very difficult. In other words, working from general guidelines and recommendations, the modal response of each room must be considered on a case-by-case basis.

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