beaming ?

[vince]

Hello, I have finished my baffle for my new WMTMW speaker. I have the Scan-Speak 95000 one inch tweeter and two Peerless three and a quarter inch mids, this is the middle of surr. to mid of surr. I am electronicly cross at 24db per octave using Marchand gear. My question is this, I have heard in the forum at times about "beaming", can someone explain a little? I need to determine the cross point so I can make the purchase from Marchand for the plug in module's. I will get a few on either side of the cross point I come up with. Just want to limit beaming problems if possible.
Thanks
Vince

[Anthony]

Put simply, beaming is where you ask a driver to play too high. There are other issues with playing too high, such as breakup distortion, but that's another discussion.

I forget where the actual cuts are, but as the wavelength of the sound approaches the width of the driver the sound tends to beam more. when this happens, you get full output on-axis, but then very rapid lowering of volume as you move off axis. If you toe in your speakers and sit exactly in the sweet spot, you may never notice beaming at all.

But in the real world, you have people sitting off axis, or the speakers fire straight ahead, so this is a problem. With the beaming, a nice natural sounding speaker might sound hollow or dull at certain frequencies because those are being beamed and you aren't on the axis. This balance will change based on driver size and position (both speaker and seating position). Also, if it's bad enough, you may have one ear hearing the full signal and the other attenuated, which needless to say, sounds weird.

So beaming is to be avoided. Thankfully, it's easy to avoid if you just don't push the drivers to play too high. I don't have my reference books handy, so hopefully someone else will chime in with some actual figures.

I've crossed 3" speakers at 3 Khz with no beaming problems and crossed 8" speakers at 500 Hz with no beaming. When I crossed those same 8" speakers at 1kHz you could hear the beaming effect.

Good luck.

[Bill Fitzmaurice]

vince wrote:

Hello, I have finished my baffle for my new WMTMW speaker. I have the Scan-Speak 95000 one inch tweeter and two Peerless three and a quarter inch mids, this is the middle of surr. to mid of surr. I am electronicly cross at 24db per octave using Marchand gear. My question is this, I have heard in the forum at times about "beaming", can someone explain a little? I need to determine the cross point so I can make the purchase from Marchand for the plug in module's. I will get a few on either side of the cross point I come up with. Just want to limit beaming problems if possible.
Thanks
Vince

What's more important is the excessively lobed response you'll get from the woofers and mids if they are crossed too high. Ideally they should be crossed no higher than where one wavelength equals their center to center distance. Where beaming is concerned ideally the actual cone diameter should be no more than 1 wavelength where it crosses over. But there's a fair amount of wiggle room between what's ideal and what's sufficient.

[vince]

Thanks guys, I wish I would have thought of this sooner, but I think I can make it work. The distance from center of Scan Speak donme to center of Peerless mid is exactly five inches. I will do now some calculations and see what I can come up with concerning diameter of mid and center to ceter situation. Again thanks for the input!
Vince

[JMB]

Okay, it won't allow me to post a link. There is a website called the FRD Consortium, where you can find the Asymmetrical Response Pattern Estimator (ARPE). This will graphically provide you with what you want (you need excel or perhaps open office to run this).

Jay