Loudspeaker Facts:
Directivity as a Design Issue
Page 3
First page - "Throw" Away Lines
Second page - The Light Analogy
Now let's talk about a loudspeaker again
Enough analogies, let's look at a single loudspeaker system in a computer model to get an idea of what this looks like. This is a single box, not a cluster, which would be even more complicated. In these images we're using color to map loudness so that red is the loudest sound, and blue is the quietest, and green is in between.
This is a loudspeaker modeled at 250Hz. Note that there is a roughly circular coverage area near the ceiling and an ellipsoid on the floor. The balcony is much lower in level, as we saw with the light model above. To get the balcony loud enough would make the level on the floor and ceiling very high indeed.
At 500Hz we are seeing the coverage pattern tighten up, just as we described above in the light analogy. This speaker is obviously pointed at the balcony.
At 2,000Hz, the horn in this system has a tight coverage pattern, and it illuminates the balcony quite well. There is still some spill onto the ceiling and floor area, but only in some select areas.
Note that the 250Hz coverage had its highest level near the loudspeaker and on the floor below it, while the highest 500Hz level was in the balcony or at least the rear half of the room. The 2,000Hz coverage was almost entirely directed into the balcony. So what would people hear in all these areas? They would hear a difference in the balance of the loudspeaker's frequency response caused by variations in the directivity becuase you can't adjust the frequency balance to be the same in all areas. If you add in more speakers without taking into account this variation in directivity and off-axis response, you will have tremendous difficulty with "hot-spots" and a low feedback threshold.
Have another look at that animated directivity balloon of the real full range packaged array type loudspeaker. (65kB image)
Now add in room acoustics...
The room acoustics play havoc with this as well, if the room reverb time is excessive or has an uneven spectrum, the problems generated by the variations in directivity will even further complicate the problems shown here. If the low frequency reverb time is excessive, the omnidirectional nature of the bass loudspeaker will excite the reverberant field to a greater extent, increasing the likelihood of low frequency feedback.
Is it impossible to put speakers in a room?
No it isn't impossible, but it requires some thoughtful analysis to get it to work right. It may not always be possible to use a single cluster to get the uniformity of coverage you want. To maintain a reasonable feedback threshold you may require additional loudspeakers in the back of a room, or larger more directional loudspeakers in the front of a room. When designing a loudspeaker cluster it is especially important to consider the directivity at all usable frequencies to prevent surprises when the system is turned on. The room and the speaker system form part of the signal chain, and no sound system can be designed without due consideration for the characteristics of both.
Knowing what to believe
You may recognize some problems that you are experiencing with your church or theatre sound system from this description. Where the ceiling is low and the room is deep, these problems are exaggerated. You may have inappropriate loudspeaker products for your application. We can offer our professional design services in designing or redesigning loudspeaker systems to optimize the coverage and frequency response in your facility.
First page - "Throw" Away Lines
Second Page - The Light Analogy
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