ZilchLab said:
I took the w Foam curves and made them red. I set the background of the no Foam to transparent so that when i laid the No Foam over the Foam, and differences caused by the foam would peak out from underneath as reddish bits.
Some bits will be a bit fuzzy, the 2 charts were not the same size so i had to fix that.
dave
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I don't think I see the difference, except the high end attenuation. And more hi attenuation off axis.
"The concept here is that the foam will attenuate the HOM more than the fundamental mode because the HOM travel a longer distance."
Not that I see. It's attenuating all right, but I don't suspect we are to conclude that HOMs are contributing that much to the VHF response.
"The foam plug will also attenuate the mouth reflections, thus reducing a major cause of linear distortion."
I'm not seeing that, either, to any significant degree....
Robh3606 said:"I don't think I see the difference, except the high end attenuation. And more hi attenuation off axis."
I see the same thing you do.
Rob
I have an open cell plugged 18 Sound horn here and that's what I see - attenuation toward the top and faster drop off of axis.
Listening to it (eqed to closely match in response and level) compared to the non foam horn there is something else going on in it sounds a little more layed back - like a typical dome tweeter. - boring-
I use this with a midrange horn where the mid horn goes 250 to 1200. The problem with the foam treble horn is it is attenuated below the level of my midrange. It also looses the dynamic SNAP of the non-foam horn and does not have the output capability.
The foamed horn sounds like a dome tweeter.
The 18 Sound horns do not "honk" with or without foam.
I don't like the foam -now if you were using a lesser midrange like a big heavy 10, 12, 15" woofer (my midrange is 108 db/w/m) then it might be a better match. There is no way a direct radiator woofer can keep up with a good compression driver/horn. I gave up on that a long time ago.
TrueSound said:
That's a pretty good description of it, definitely. It sounds like a dome tweeter, but with directivity and more headroom.
This afternoon I was cleaning the house, and had to literally put down what I was doing, and walk downstairs to listen to a song that came on.* The sound can stop you in your tracks.
* I was playing a mix of tracks on the PC
TrueSound said:
Did you put any foam in the mid horn? What ppi is the foam? I used 30 ppi in both but I had to pad down the mid, either way. I got no significant SPL losses on either by applying 30ppi.
You might find the hi end "snap" is there if you pad down the mid.
But it may be the "snap" is HOMs.
But this is getting away from the topic - How to measure HOMs.
Panomaniac wrote:
Probably not. More likely, you are seeing exactly what Geddes expects you to see.
HOMs are a higher frequency phenomenon, so more attenuation at high end should be expected. No?
Think of it this way, in the first inch or so, the higher frequency HOMs will diffract. This means some of the wave will reflect not just on an angle of incidence like we might see a bullet ricochet off a wall, but will sort of spray backwards and across the waveguide as well as forward in the guide. The higher the frequency, the more likely these diffraction products will reflect several times across the the narrowest part of the waveguide The chances these products will continue along the guide on axis are mighty slim, I should think. And they will radiate from the mouth delayed with respect to the on-axis waves (whichever axis is under examination).
So attenuation by foam of non-zero axis high frequencies should be greater.
But since the os waveguide has a falling off axis power response anyway, I think its rather difficult to judge exactly what's going on.
I think the difference between impulse responses with and without foam is far more likely to show a diminishment of time based distortion.
Probably not. More likely, you are seeing exactly what Geddes expects you to see.
HOMs are a higher frequency phenomenon, so more attenuation at high end should be expected. No?
Think of it this way, in the first inch or so, the higher frequency HOMs will diffract. This means some of the wave will reflect not just on an angle of incidence like we might see a bullet ricochet off a wall, but will sort of spray backwards and across the waveguide as well as forward in the guide. The higher the frequency, the more likely these diffraction products will reflect several times across the the narrowest part of the waveguide The chances these products will continue along the guide on axis are mighty slim, I should think. And they will radiate from the mouth delayed with respect to the on-axis waves (whichever axis is under examination).
So attenuation by foam of non-zero axis high frequencies should be greater.
But since the os waveguide has a falling off axis power response anyway, I think its rather difficult to judge exactly what's going on.
I think the difference between impulse responses with and without foam is far more likely to show a diminishment of time based distortion.
FrankWW said:
I have one mid horn almost done with the foam.
The 18 Sound (1200 Hz up, B&C DE250 driver) horn has the foam. 30 ppi that has been recommended here. This stuff is hard to work with.
It is triamped with level controls so it's easy to match levels. I had to turn the mid horn down (the whole channel but the treble horn actually) to match the more efficient non foam channel in response. Then balance the channels
The foamed horns needs the lower range padded down more because of the loss in response in the upper range due to the foam.
"But it may be the "snap" is HOMs"
Yes that's why I started the thread. How do I know if HOM is effecting the sound in that matter without a way to measure them?
If it is then I actually like HOM. I know it's the foam but think it's because of the attenuation both on and off axis.
LOL - we know how to measure OHMs, Andrew.
Well I am glad not to be blind or missing something. But agree that maybe the FR plot ins't the best way to see the HOMs. Impulse, then? Or IM distortion?
And how much of that attenuated engery in the high end is HOM?
The concept of what the foam is suppoded to do is pretty clear, but how can you "see" it?
BTW Truesound, what are you running for a mid? Big Altec or JBL driver with phenolic diaphragm? Or one of the Yamahas?
Well I am glad not to be blind or missing something. But agree that maybe the FR plot ins't the best way to see the HOMs. Impulse, then? Or IM distortion?
And how much of that attenuated engery in the high end is HOM?
The concept of what the foam is suppoded to do is pretty clear, but how can you "see" it?
BTW Truesound, what are you running for a mid? Big Altec or JBL driver with phenolic diaphragm? Or one of the Yamahas?
TrueSound said:
Hi,
You can't make comparisons like this if you are not re-eq'ing the system to have the same response as before. Even level matching will not get you there - you need to account for the attenuation at high frequencies. Otherwise you cannot say if you are hearing the change in frequency response or the effect of the foam (well, one effect of the foam is the change in response, but that's not what is being discussed here). It's not completely clear from your post if you did this or not, so I thought I'd mention it.
ZilchLab said:
To get more of an idea of what the foam is doing, we should probably be testing it with a horn that's known to honk. We all know that sound and some horns are worse than others. It doesn't really matter (to me) if that's HOM or mouth diffraction or whatever if (big if) the foam can improve it. Usually horns that sound like that have a ragged impedance curve and, if the foam is doing some good, we should be able to see the impedance curve smooth out independent of the frequency response. If it did, that would indicate there's more going on than just a lowpass filter. So who has a honky horn and some foam to give it a try?
Y'all see my problem here, now.
I ask for evidence of HOMs, and I am sent to the Geddes ALMA presentation.
What I see is we stick in a foam plug, and it does something, and what it does might easily elicit, "AHHH, it sounds different, perhaps better, even."
I see what everyone else sees, apparently, and it is not evidence of HOMs without overlaying it with a bunch of assumptions.
It should be reasonably easy to document what the foam does outside of this context. JBL used it for L100 (and other) grilles 40 years ago.
No, I am NOT going to stuff a 511A with open cell reticulated foam, but an H91 might be a candidate for de-honking....
I ask for evidence of HOMs, and I am sent to the Geddes ALMA presentation.
What I see is we stick in a foam plug, and it does something, and what it does might easily elicit, "AHHH, it sounds different, perhaps better, even."
I see what everyone else sees, apparently, and it is not evidence of HOMs without overlaying it with a bunch of assumptions.
It should be reasonably easy to document what the foam does outside of this context. JBL used it for L100 (and other) grilles 40 years ago.
No, I am NOT going to stuff a 511A with open cell reticulated foam, but an H91 might be a candidate for de-honking....
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