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What Causes Off-Axis Nulls?
What Causes Off-Axis Nulls?
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Old 9th September 2019, 08:56 PM   #1
Patrick Bateman is offline Patrick Bateman  United States
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Default What Causes Off-Axis Nulls?

On another thread, we were discussing the Behringer B2031P. It segued into a discussion about vertical nulls, and I thought it might be interesting to illustrate why they happen.

90% of you already know how this works, but I like making diagrams to help myself understand things. So here goes...

Click the image to open in full size.

This is a Behringer B2031P. It has an 8" woofer, a 1" tweeter, and the vertical spacing between woofer and tweeter is about 8". (20cm.)

Click the image to open in full size.

Using XDir, we can see that this geometry is going to lead to some deep nulls at 30 degrees off axis. Why is that?

Click the image to open in full size.

Here's why.

When listening on-axis, we are equidistant from the woofer and tweeter. At 30 degrees off axis (vertically) there's a big null. The null occurs because there's a pathlength difference of 10cm between the woofer and the tweeter. 1700Hz is 20cm long; therefore a pathlength difference of 10cm creates a big fat null, because the two are 180 degrees out-of-phase.

Hope that makes sense!

If so, let's talk about beaming:

Click the image to open in full size.

Here's the polar response from a typical 8" woofer. This is a Tymphany P22WO03.

At 30 degree of axis, at 1700Hz, the woofer is only "down" about 1dB from it's on axis response. This is because an 8" woofer has a diaphragm that measures about 6.5" in diameter. So the woofer isn't even beaming very much until it reaches about 2khz. (2khz is 6.5" long.)

Nulls occur when two sources are out-of-phase. If two sources are playing 90dB and they're perfectly out-of-phase, you get an output of 0dB. If one source is playing 90dB and the other is playing 89dB, the results won't be much different. There will still be a null, it just won't be as deep.

This is a situation where a waveguide can come in handy, because a carefully optimized waveguide can play about -6dB off axis. Basically it's a lot easier to reduce the off-axis energy of the *tweeter* not the woofer. The beaming of the woofer is largely determined by it's diameter.

In a situation like we see with the Behringer pictured here, there's a few options:

1) lower the crossover point. This is easier said than done; there aren't many 1" tweeters that can handle a xover point of 1khz.

2) The Behringer is largely a clone of a Genelec design from a couple decades back. The newer Genelec models simply moved the tweeter closer to the woofer. This works nicely.

Click the image to open in full size.
In the measured vertical response of the Behringer, we can see that they simply lived with the off-axis null. It is very prominent at 1700Hz. Thanks to dantheman for the measurements : Great Waveguide List

Last edited by Patrick Bateman; 9th September 2019 at 08:59 PM.
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