I found this thread:
Speaker Directivity - The Gear Page
regarding controlling dispersion in larger diameter woofers, like 12" and 15".
The idea comes from a Jay Mitchell, whom I do not know personally.
Potentially infringing material removed by moderation.
Speaker Directivity - The Gear Page
regarding controlling dispersion in larger diameter woofers, like 12" and 15".
The idea comes from a Jay Mitchell, whom I do not know personally.
Potentially infringing material removed by moderation. Isn't this simply a way of converting an open U-frame speaker to a rear-ported resistance box, i.e. a cardioid-like speaker?
Could be done with open cell PU foam, however I have my doubts about performance with high volume displacements. Area of the "port" should be large enough to ensure linear behaviour of the foam. Other materials may be better suited. Keep in mind guitar cabinets have different requirements than hi-fi speakers.
Could be done with open cell PU foam, however I have my doubts about performance with high volume displacements. Area of the "port" should be large enough to ensure linear behaviour of the foam. Other materials may be better suited. Keep in mind guitar cabinets have different requirements than hi-fi speakers.
The foam goes on front of the driver, so I'm not sure how it would become a "rear-ported resistance box". I don't know what that is anyway.
The NRC for the foam I ordered is not known, so it would be probably better to specify a particular NRC curve to obtain optimum results. The problem I found is that the NRC rated foams are not flat sheet, or at least I couldn't find any.
It's true that guitar speakers are open baffle types, but the principles of beaming at high frequencies is no different as far as I know. I'm just learning about this technique so bear with me here.
Okay, get the idea. I misinterpreted it at first and associated it with this: DIY archive of Kimmo Saunisto (see "Cardioid bass").
If what he says is true (about measured results), it may be interesting for hi-fi indeed. The ring probably attenuates and low-passes the sound so the radiating area at high frequencies is reduced. It reminds me of the woofer mountings of newer Emerald Physics speakers and also of a compression driver. Large radiating area, small aperture, in this case realized by the "doughnut" ring.
If what he says is true (about measured results), it may be interesting for hi-fi indeed. The ring probably attenuates and low-passes the sound so the radiating area at high frequencies is reduced. It reminds me of the woofer mountings of newer Emerald Physics speakers and also of a compression driver. Large radiating area, small aperture, in this case realized by the "doughnut" ring.
He states that forcing the driver to have a smaller effective radiating area at higher frequencies makes the dispersion angle greater at those higher frequencies. Of course, this is not a light switch, where it's "on" or "off", and I'm sure there could be some very cool refinements to this technique if I had the right measuring gear (and knew how to use it!). For example, we could specify a particular NRC curve, which typically for acoustic foam goes up with frequency, but the shape of the curve changes based on the thickness and other properties of the foam. Also, the ideal NRC curve could be matched to the driver's response. Even further, the 3" hole in the middle is just his application, but what if a different size hole is better? What if the hole size should be designed to match the tweeter or midrange size? Should the thickness of the foam vary over the diameter as well, effectively changing the NRC curve? So many questions...
Of course, what I'm talking about is matching the dispersion of the lower frequency driver to that of the higher frequency driver at the crossover frequency. That's the sticky wicket, I think. If this trick works, then it's ok to use a 12" (or 10" or 15") driver in a simple two way loudspeaker.
Another cool feature of this modification is that the virtual 3" driver that is formed by the hole will be surrounded by acoustically absorptive foam so there should be very little if any diffraction at the higher frequencies.
The foam donut is a good idea for a guitar speaker which is used full-range, as it gives wider dispersion in the upper "ice pick" 4K range.
A single front loaded 12 inch cone has an approximate -6 dB 60 degree beamwidth at 2.5K, and -6 dB 30 degree beamwidth at 4K.
Good two way designs using 12" or 15" cross over at a frequency that the HF horn dispersion matches the LF driver dispersion, around 800 to 1200 Hz for a 90 degree 6 dB down point. Some PA speakers cross as high as 2K to protect the HF driver, but large woofers generally have bad breakup modes above 2K, so dispersion that high is a lesser problem than sound quality.
At any rate, the foam donut is a good idea for those that are using large speakers full range or crossed high enough where HF beaming is a concern.
Art Welter
Larger full range drivers rely on the HF beaming to keep the on axis frequency response somewhat flat. Widening the dispersion would result in a severe on axis drop in HF, the speaker would sound "dull".
Jay claims a loss of 1dB on axis for his particular foam, which is from Mcmaster-Carr.
Thanks. The tweeter I have is recommended to crossover at 2.5kHz. Resonance appears to be about 1800Hz. The woofer appears to breakup starting at about 1800Hz. I'm using 24dB active filters, so maybe I can cross this tweeter over lower, since I believe B&C specify crossover points based on 12dB passive filters. Otherwise, I have to buy different drivers.
I've had good results using foam and felt to induce some modest directivity changes/improve horn termination. It'll very much be an experimental process, but is very worth pursuing.
Originally Posted by weltersys
Larger full range drivers rely on the HF beaming to keep the on axis frequency response somewhat flat. Widening the dispersion would result in a severe on axis drop in HF, the speaker would sound "dull".
At any rate, a 3" hole should give wide dispersion to around 4500 Hz, which is about the top end for most electric guitar sound.
To have wide dispersion at 16 K would require the hole to be reduced to around 1", I think the level would drop a lot more than 1dB up high.
Certainly would be worth trying, anybody could test the concept using an old t-shirt instead of foam, so it could be done cheaply.
This whole discussion would be more appropriate in the full-range forum, as multi-way system crossovers are usually designed with the beam width of the drivers at the crossover frequency in mind.
Art
Larger full range drivers rely on the HF beaming to keep the on axis frequency response somewhat flat. Widening the dispersion would result in a severe on axis drop in HF, the speaker would sound "dull".
That may be true, without actually seeing the results I'm somewhat doubtful.
At any rate, a 3" hole should give wide dispersion to around 4500 Hz, which is about the top end for most electric guitar sound.
To have wide dispersion at 16 K would require the hole to be reduced to around 1", I think the level would drop a lot more than 1dB up high.
Certainly would be worth trying, anybody could test the concept using an old t-shirt instead of foam, so it could be done cheaply.
This whole discussion would be more appropriate in the full-range forum, as multi-way system crossovers are usually designed with the beam width of the drivers at the crossover frequency in mind.
Art
Well, my problem is related to building a 2 way with the drivers I have on hand, so that's what led me to this solution and my posting in this section. I agree that for full range application it may be better to discuss it there.
So, for my application, a 3" hole appears to be more than adequate to control dispersion up to a crossover point the tweeter can handle. The horn tweeter has controlled dispersion of some kind, I don't recall right now.
So, for my application, a 3" hole appears to be more than adequate to control dispersion up to a crossover point the tweeter can handle. The horn tweeter has controlled dispersion of some kind, I don't recall right now.
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You could add a midrange driver if the tweeter and woofer sound poor in the 2K range.
Regardless of midrange sound quality and dispersion, intermodulation distortion starts making its "gargly" sound on a woofer played over a wide range (above 1500 Hz or so) if driven near or above Xmax.
Thanks! I'll have to read up on intermodulation distortion in woofers. Maybe there's a foam for that too? 🙄 Since this 12PE32 driver has a sensitivity of 101dB, and with the foam probably about 100dB, I doubt I'll ever exceed xmax... but I'll read up anyway.
Adding a midrange is possible, but not desirable since I only have a 2 way active crossover and I want to keep the overall loudspeaker smaller rather than larger.
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Here's the spl response from B&C. It looks to me that the cone starts to break up above 2kHz, after the dip. So, maybe cross over max at 1.8kHz? I've also attached the measured response of the tweeter with the horn. I just held the mic in front of the horn a couple of feet away. This is with a RTA set to pink noise set to peak hold. It's probably not that accurate, but it's what I did. I'll have to build a test box for the woofer and measure that also.
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