Solution for beaming?

Re: Solution for beaming?

John,

Depending on your crossover point, the dual 15" design (assuming the standard vertical arrangement) exhibits several problems.
  1. The physical size of the woofer itself begins to cause beaming at a relatively low frequency, depending on your desired dispersion you may be limited here.
  2. The physical arrangement of the two 15s vertically means you are going to have polar pattern asymmetry, as they are several wavelengths long at the usual XO points.
  3. Above about 800Hz (give or take, depending on design) the 15" begins to have internal reflection problems. Sound is emitted from the voice coil, travels along the cone, and is partly damped at the surround. The whole cone doesn't move at the same rate, and the wave propagating to the surround isn't perfectly absorbed, so the energy reflects from the surround and travels back down the cone to the voice coil where it interferes and causes time smear with your intended radiation. For this reason using a 15" woofer above about 800Hz usually results in a less desirable midrange character due to time smear from this re-emission.

The device you mention, essentially a simple phase plug, can help with only 1/3 of your problem.
 
Re: Solution for beaming?

Bennett Prescott said:
John,

Depending on your crossover point, the dual 15" design (assuming the standard vertical arrangement) exhibits several problems.
  1. The physical size of the woofer itself begins to cause beaming at a relatively low frequency, depending on your desired dispersion you may be limited here.
  2. The physical arrangement of the two 15s vertically means you are going to have polar pattern asymmetry, as they are several wavelengths long at the usual XO points.
  3. Above about 800Hz (give or take, depending on design) the 15" begins to have internal reflection problems. Sound is emitted from the voice coil, travels along the cone, and is partly damped at the surround. The whole cone doesn't move at the same rate, and the wave propagating to the surround isn't perfectly absorbed, so the energy reflects from the surround and travels back down the cone to the voice coil where it interferes and causes time smear with your intended radiation. For this reason using a 15" woofer above about 800Hz usually results in a less desirable midrange character due to time smear from this re-emission.

The device you mention, essentially a simple phase plug, can help with only 1/3 of your problem.
Click to expand...

Bennett.....

What if any improvement might you get if you simply shelve off the lower 15 @ 400hz like EV did with the T-252?
 
Re: Solution for beaming?

Dick Rees said:
What if any improvement might you get if you simply shelve off the lower 15 @ 400hz like EV did with the T-252?
Click to expand...

Well, you'll help problems 1 and 3, for sure. However, unless you're Super Damn Clever In The Time Domain™ you're going to be adding the phase lag to that woofer of, presumably, a 2nd order low pass. So at frequencies much below 400Hz it will be delayed, and then around 400Hz you'll have an interesting phase relationship between the two woofers. So you sort of trade predictable beaming for an strange polar and frequency response, which is going to interact in a totally new and exciting way in the vertical versus the horizontal. Also, that phase lag is going to drag the LF lobe down, so your box will have a LF polar that doesn't match the HF polar's overall dispersion angle.
 
Craig Hauber said:
So what your saying is that the dual-15 design is flawed and there is nothing you can do to save it?
Click to expand...

Not really. What I am saying is that most dual 15" in a vertical line boxes are lower priced. Lower prices boxes can't afford high end compression drivers, which would let you run that crossover point down. To behave properly, even using clever design and filtering to make the asymmetric LF pattern match the HF pattern, you need a low crossover point.

I'm sure that even given its limitations the dual 15" design can work well for most audience shapes and sizes that people want to use it for. However, placing a homemade phase plug in front of each woofer is not going to adequately address the shortcomings of that design for the cases where it doesn't work satisfactorily.
 
Re: Solution for beaming?

Wouldn't an all pass filter provide the same response in the time domain, just not provide the filter, for the uncrossed over woofer? I think I remember reading a paper from Charlie Hughes on this topic.
 
Re: Solution for beaming?

Bennett Prescott said:
John,

Depending on your crossover point, the dual 15" design (assuming the standard vertical arrangement) exhibits several problems.
  1. The physical size of the woofer itself begins to cause beaming at a relatively low frequency, depending on your desired dispersion you may be limited here.
  2. The physical arrangement of the two 15s vertically means you are going to have polar pattern asymmetry, as they are several wavelengths long at the usual XO points.
  3. Above about 800Hz (give or take, depending on design) the 15" begins to have internal reflection problems. Sound is emitted from the voice coil, travels along the cone, and is partly damped at the surround. The whole cone doesn't move at the same rate, and the wave propagating to the surround isn't perfectly absorbed, so the energy reflects from the surround and travels back down the cone to the voice coil where it interferes and causes time smear with your intended radiation. For this reason using a 15" woofer above about 800Hz usually results in a less desirable midrange character due to time smear from this re-emission.

The device you mention, essentially a simple phase plug, can help with only 1/3 of your problem.
Click to expand...

Thanks for reply Bennett. That is the type info I needed to see. Now to digest..............
 
Re: Solution for beaming?

David Sturzenbecher said:
Wouldn't an all pass filter provide the same response in the time domain, just not provide the filter, for the uncrossed over woofer? I think I remember reading a paper from Charlie Hughes on this topic.
Click to expand...

Sure, but now we're talking about tri-amping a 2x 15" box and processing it with DSP that has all pass filters available. It's not very easy to do an all pass filter, especially at a low frequency, in a passive crossover! I suppose, were I to do it, I would bi-amp the box, and use a 2nd order passive between the horn and the middle 15", and use a very beefy HF driver.
 
Bennett Prescott said:
I suppose, were I to do it, I would bi-amp the box, and use a 2nd order passive between the horn and the middle 15", and use a very beefy HF driver.
Click to expand...

:)
Bingo

P.s.: I pushed a concept a bit further and used 1st order crossover for HF ... Acoustical crossover point turned out to become a point below 600 Hz. 'Sadly' it took DE1050 to get away with it

AlesD wirelessly
 
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Re: Solution for beaming?

john lutz said:
Speaker Directivity Modifier :: TGP Webzine

Wonder if this would be useful for the problem I have with my DIY 15x2 cabs? Seems it's always a poor trade off between off axis roll off of highs vs how low I dare push the horns.
Click to expand...
John,

The problem with the linked Speaker Directivity modifier is it does little in the frequency range where the 15" beaming is a problem, above 800 to 1500 Hz or so.
Above 2K it is effective, but also reduces overall level by around 10 dB, the "highs" will only sound half as loud, and would require 10 times the power to be at the level they were before attenuation.

A simple solution to widen dispersion is to reduce the width of the speaker opening to around the wavelength of the upper range that you want to be -6 dB at 45 degree off axis.

Below is a picture of a dual 10" using this technique, I played with the opening width until the 10" matched the HF horn at the acoustical crossover point of 1500 Hz.
Another advantage of covering the sides of the speaker is it creates an acoustical high pass filter, getting rid of some of the problems Bennett mentions in point #3 in post #3. In the case of the dual 10", the crossover was much simpler because cone breakup above 2K was acoustically reduced in level.

As a side, the 2x10" has been reduced to a single 10" in large part because of what he mentioned in point #2 in post #3, and how difficult it is to make a good passive 2.5 way crossover that does not exhibit the problems Bennett mentions in post #5.
That said, there are passive 2.5 way implementations that work quite well, the UREI 2x15" with a coax top speaker comes to mind.

Art
 

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Re: Solution for beaming?

Art Welter said:
John,

The problem with the linked Speaker Directivity modifier is it does little in the frequency range where the 15" beaming is a problem, above 800 to 1500 Hz or so.
Above 2K it is effective, but also reduces overall level by around 10 dB, the "highs" will only sound half as loud, and would require 10 times the power to be at the level they were before attenuation.

A simple solution to widen dispersion is to reduce the width of the speaker opening to around the wavelength of the upper range that you want to be -6 dB at 45 degree off axis.

Below is a picture of a dual 10" using this technique, I played with the opening width until the 10" matched the HF horn at the acoustical crossover point of 1500 Hz.
Another advantage of covering the sides of the speaker is it creates an acoustical high pass filter, getting rid of some of the problems Bennett mentions in point #3 in post #3. In the case of the dual 10", the crossover was much simpler because cone breakup above 2K was acoustically reduced in level.

As a side, the 2x10" has been reduced to a single 10" in large part because of what he mentioned in point #2 in post #3, and how difficult it is to make a good passive 2.5 way crossover that does not exhibit the problems Bennett mentions in post #5.
That said, there are passive 2.5 way implementations that work quite well, the UREI 2x15" with a coax top speaker comes to mind.

Art
Click to expand...

Thanks Art, for sharing that info. Gives me something more to play with. I was actually thinking on the same track but hadn't yet searched out the details. I remembered a lot of old style boxes using that build but didn't fully understand why.
 
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