Page 27 - Winter 2008

surrounding the device. Based on these measurements, man- ufacturers publish the directional properties in octave or third octave bands, expressed as the on-axis sensitivity (the sound pressure level at 1 meter for a 1 watt electrical input), and the change in level in directions other than on-axis. Angular increments of 5 or 10 degrees are most commonly used. Programs interpolate level values between the meas- urement points. Directivity data, given in standard unen- crypted ASCII files, are the most useful since it allows the raw data to be examined directly.
When loudspeakers are arranged in lines or clusters, they should be treated as individual sources by the model. When the sound fields from two loudspeakers overlap, there is an interaction due to the differences in phase, which creates local increases and decreases in the received signal. In these cases the signals from each loudspeaker must be combined by taking both their level and relative phase into considera- tion. Direct field levels should be calculated in this manner when the sound pressure levels of overlapping sound fields are nearly the same.
It is also important to use the sound power levels of indi- vidual loudspeakers to calculate the reverberant field contri- bution, since the phase relationships are not maintained at large distances or after many reflections. Programs that bun- dle groups of loudspeakers together and treat them as one source, do not accurately calculate their reverberant field contribution since the reverberant field sound power level is underestimated.
Coverage
Calculating the direct sound level at each receiver is one of the main objectives of a computer model. Direct field data can be presented numerically as sound pressure levels at each receiver location, or can be displayed graphically. The numeric approach has the advantage of allowing adjustment of the gain of individual loudspeakers to achieve the most
even coverage. A calculation of the standard deviation of the direct field level within the intended coverage area is a useful measure of evenness (usually plus or minus 2 dB). Programs should, at a minimum, calculate levels in the 500 Hz, 1 kHz, and 2 kHz bands. The 500 Hz band is particularly important for feedback control since loudspeakers tend to be less direc- tional in this band.
The final design in the Mount Saint Mary’s Chapel is shown in Fig. 3, a drawing of the floor plan with the loud- speakers superimposed. Normally distributed loudspeakers are positioned at an elevation approximately equal to their spacing but preferably not farther than 20-25 feet above the listener. In this design they were pointed straight down, but that is not always necessary. Even where the ceilings are slant- ed, distributed loudspeakers can be used, although the spac- ing depends on the pitch of the ceiling. In this church com- puter calculations yielded standard deviations of 0.8, 1.1, and 1.4 dB in the 500 Hz, 1 kHz, and 2 kHz bands, which is satis- factory.
Intelligibility—Liveness
Even coverage is not the only goal. Don Davis once observed that even coverage can be obtained by pointing all the loudspeakers at the ceiling. The next concern in the design is intelligibility, clearly the sound should be intelligi- ble, and this requires pointing the loudspeakers at the listen- ers. One of the traditional measures of intelligibility is the number of consonant-vowel-consonant syllables misunder- stood. In early studies listening tests were carried out using a group of spoken words in a neutral carrier sentence. The intelligibility was expressed in terms of badness, that is, loss of intelligibility, instead of goodness. Intelligibility metrics were studied at Bell Labs in the 1920’s and 30’s using a single source and receiver in listening tests.
Later the intelligibility was expressed in terms of bad- ness, that is, loss of intelligibility in a measure called Liveness.
Fig. 3. Floor plan with loudspeaker layout.
Sound System Design 25