THAM15 - a compact 15" tapped horn - Page 9 - diyAudio
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Old 24th January 2011, 12:02 AM   #81
Djim is offline Djim  Netherlands
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Quote:
Originally Posted by Brian Steele View Post
I'd like to suggest that perhaps some sort of closed pipe or hemholtz resonator might be a predictable and effective way of dealing with the notch in the THAM15 design.
That may prove the RESULT, but not the CAUSE. (sorry Brian)
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Old 24th January 2011, 12:24 AM   #82
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Originally Posted by jbell View Post
Here's an example. I know that If I put in a reflector in red where shown, it'll cut my overall response, and especially my 100hz area response.
If you're putting in the reflector exactly as shown, it's going to narrow the horn's path at that point. Not by much, but perhaps the narrowing is what's making the difference. I think that to avoid the constriction, the reflector needs to be a bit smaller so that the shortest distance from the edge of the internal panel to the reflector is somewhere between the distances of the edge of the internal panel to the two adjacent side walls. In any case, as we're talking about a bass horn, it's not really required.


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Another case... a solid rear brace along the long back of that design cuts 40hz by 2db. why? more bracing should be better, and louder -- less loss through panel flex. Hollow bracing or even my 2 braces as designed in the ss15 reduce, but don't eliminate that rear panel flex. But it's louder than a solid brace.
What's the thickness of the solid brace? Perhaps the horn is sensitive to cross-sectional changes at that point.


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That particular cabinet does a couple things that I don't have answers to... but again... I am just happy with the results.
...and that's probably what's most important .
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Old 24th January 2011, 12:32 AM   #83
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Originally Posted by Djim View Post
That may prove the RESULT, but not the CAUSE. (sorry Brian)
Good try, but the effect of pipe and Helmholtz resonators is well-documented and in line in theory. The use of "reflectors" whose effectiveness is based on the assumption that "standing waves" occur in horns below 1/2L however, is not supported by any theory .
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Old 24th January 2011, 12:56 AM   #84
Djim is offline Djim  Netherlands
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Brian I hope you understand I'm looking for the same thing, get the theory correct behind this...

It’s not that I don’t believe in the Helmholtz resonators as a practical solution, I certainly do! But it is still fixing the problem afterwards instead of fixing the problem itself (that's why we try to find the right theory to treat problems in future on a predictable way)

Again Jbell says his “reflector” panel influence the area of 100 Hz. I see the length of the back panel is 29inch high. 29inch is = ¼ wavelengths of 116Hz. Is this again a coincidence?
Or can we really influence a signal in a horn with "reflectors" (sorry have no better word yet) at ¼ wavelengths between two parallel walls?
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Old 24th January 2011, 01:12 AM   #85
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Originally Posted by Djim View Post
But it is still fixing the problem afterwards instead of fixing the problem itself (that's why we try to find the right theory to treat problems in future on a predictable way)
The theory in this case may actually give us a predictable means of fixing the problem, something we can test with modeling first before cutting wood. The more we can predict with modeling, the less need for bonfires .


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Originally Posted by Djim View Post
Again Jbell says his “reflector” panel influence the area of 100 Hz. I see the length of the back panel is 29inch high. 29inch is = ¼ wavelengths of 116Hz. Is this again a coincidence?
It's likely coincidence. A FR and an IR test before and after the addition of the reflector may shed some light. I've pointed out though that the reflector that JBell indicated in his image would actually restrict the horn at that point - perhaps that is what is contributing to the 2dB loss at 100 Hz.


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Originally Posted by Djim View Post
Or can we really influence a signal in a horn with "reflectors" (sorry have no better word yet) at ¼ wavelengths between two parallel walls?
I'm under the impression that reflectors in a normal horn are usually put there to improve midrange response. I don't think those are applicable to tapped horns, which have a passband that lies in the bass region.
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Old 24th January 2011, 02:01 AM   #86
Djim is offline Djim  Netherlands
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Brian I’m not talking standing waves here...Altough... At the beginning and at the end of a standing wave it’s a zero point.. This means an amplification of the signal, right? Now, we see dips which means cancellation – out of fase.

Sanders himself seem to have influenced his Tham at 170Hz with reflectors. Jbell has given 2 examples of similar type. That it happens all around ¼ wavelength is not possible because of your theory of standing waves which is correct. But I’m talking about dips – out of fase signals. Standing waves are resulting in peaks not dips as you proved!

Now go back to my example in room acoustics of measuring at a ¼ wavelength in a full wavelength standing wave. You will see that the signal of a standing wave, that is reflected from a wall, at ¼ of its full wavelength is out of fase (look for the chapter in you books about nodes and anti-nodes)

So what theory tells you, you can’t influence a signal within a horn at ¼ wavelength???
And whitch of your theories says that anywhere in a horn, where two parallel surfaces ‘see’ each other, can't reflect a signal out of fase?


This also means you are right, you can’t influence a full wavelength with a reflector but you can prevent parallel walls that develop ¼ wavelength – out of fase – signals by the use of reflectors… And no we are not talking about standing waves we are talking about cancellation as the result of ¼ wavelength reflected by walls...


I'm not saying I'm right but I haven't seen any other explanation.... (yet)

Last edited by Djim; 24th January 2011 at 02:05 AM.
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Old 24th January 2011, 02:15 AM   #87
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Standing waves are resulting in peaks not dips as you proved!
Not correct. Check my graph. Look at what's happening at 700 Hz. That's 2*F for that particular horn/pipe geometry. And there's a null that's not predicted by HornResp.

And of course there's still the second issue to consider - the size of those reflectors - IMO they are simply too small to make any significant difference at low frequencies, reflection-wise. If they're making any difference, it's likely along what Danley and others have suggested - they're reducing the volume of the horn.

Last edited by Brian Steele; 24th January 2011 at 02:18 AM.
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Old 24th January 2011, 02:24 AM   #88
Djim is offline Djim  Netherlands
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See, you don’t get my point. Tell me if I’m wrong but any standing wave at it’s zero point can’t be out of fase. It is causing an out of fase signal as soon the standing wave is shifted from the original signal towards a ¼ of its full wavelength.

Secondly, The reflector doesn’t reflect the wavelength because its to small. It prevents the wall surface to develop a ¼ wavelength signal…
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Old 24th January 2011, 02:54 AM   #89
jbell is offline jbell  United States
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I gave you 2 examples.... I have more.... and not just from me.

I also played with reflectors on my big cabinet, and the cube... so I can give you #4, and #5 as well. and guess what fraction of a wave they all line up on?
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Old 24th January 2011, 02:56 AM   #90
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Originally Posted by Djim View Post
See, you don’t get my point. Tell me if I’m wrong but any standing wave at it’s zero point can’t be out of fase. It is causing an out of fase signal as soon the standing wave is shifted from the original signal towards a ¼ of its full wavelength.
You are trying to bend an observation about the effect on a signal source 1/4L from a wall with what's going on in a horn. IMO the only time that could possibly apply is if you're talking about the distance from the horn's DRIVER to one of the horn's walls.

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Secondly, The reflector doesn’t reflect the wavelength because its to small. It prevents the wall surface to develop a ¼ wavelength signal…
For the wavelengths we're talking about, I don't think it makes any difference.
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