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horn surfaces [message #17280] Fri, 31 December 2004 10:48 Go to next message
Floyd Andrews is currently offline  Floyd Andrews
Messages: 31
Registered: May 2009
Baron
Do the interior surfaces of my horns have to be smooth, or can I spray them with a textured paint such as "Soundcoat" or a similar vibration damping surface finish?

Re: horn surfaces [message #17281 is a reply to message #17280] Fri, 31 December 2004 12:30 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)
It depends on the frequency, and surface irregularities are a matter of scale. At low bass frequencies, even a very rough stair-step curve for a flare wall surface is fine. Bass wavelengths are measured in dozens of feet, so deviations on the surface can be several inches and not matter. But at 20kHz, the wavelength is about an inch and a half, so I would rather have a pretty smooth tweeter flare.

Thanks, Wayne! (nt) [message #17296 is a reply to message #17281] Mon, 03 January 2005 18:33 Go to previous messageGo to next message
Floyd Andrews is currently offline  Floyd Andrews
Messages: 31
Registered: May 2009
Baron
NT

Re: horn surfaces [message #17311 is a reply to message #17281] Sat, 08 January 2005 13:19 Go to previous messageGo to next message
Earl Geddes is currently offline  Earl Geddes
Messages: 220
Registered: May 2009
Master
But Wayne, you have to admit that there is no data for this position. I have often thought about the effect of rough walls and cannot conclude that it is possitive or negative. As my new speakers how I do the heresy of putting foam in the waveguide!!! It works though, and there is a very good reason for it. Some things are not always what they seem.

Re: horn surfaces [message #17316 is a reply to message #17311] Sat, 08 January 2005 18:04 Go to previous messageGo to next message
Adrian Mack is currently offline  Adrian Mack
Messages: 568
Registered: May 2009
Illuminati (1st Degree)
Im sure what Wayne is referring to is possible unwanted phase changes caused by irregularities in the surface of a rough horn wall, causing frequency response abberations.

As to foam in the waveguide, the foam would be absorbing the high frequencies (depending on how high you use them), and changing the frequency response more or less to your liking. In designing a horn you generally assume smooth wall's as it would not be easy to accurately model horn surfaces with random rough walls. On that note rough walls could be used to ones benefit but would probably take a lot of measurements and trial and error, and theres usually better ways to fix a problem such as rough frequency response than this.

Re: horn surfaces [message #17319 is a reply to message #17311] Sun, 09 January 2005 03:31 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)
You know Earl, you may be right about the absense of data to support a claim that a rough-walled horn is equal in performance to a smooth-walled horn used at low frequency.

But it does stand to reason.

I think though, that there is some indirect evidence of this fact. Folded horns use straight sections, each one expanding a little more than the one preceding it. This only approximates an exponential flare, yet the performance is similar to what is expected from a purely exponential flare.

The conclusion is easy to draw that the coarseness of the curve has little effect because it is small in relation to wavelength.

Wavelenghth can be scaled down to treble frequencies and horn flares with rough sand-grain texture become similar irregularities. Wavelengths are large compared to the grain of the horn surface, so my assumption is that it doesn't matter in the passband. But I guess a real test might be worth doing, to sand one smooth and compare the difference.

Re: horn surfaces [message #17320 is a reply to message #17316] Sun, 09 January 2005 03:40 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)
I think if surface aberrations are small enough, they won't matter. But if they're large relative to wavelength, then they will alter wave propogation and have an effect.

Where there are places in a horn that flare angle changes rapidly, diffraction causes a change of wave propogation. This can be at the throat, the mouth or anywhere in between. If the diffraction causes two wavefronts to be generated and to co-mingle, there can be interactions and possibly destructive interference which cause response irregularities. Even without interactions, rapid changes in directionality cause uneven response due to the fact that the driver's total energy is focused more when DI is high and spread out more when it drops. At the mouth, dispersion control is ultimately lost at some low frequency as the sound bends around it.

There are a variety of methods used to combat these kinds of irregularities, and to reduce anomalies in dispersion and resoponse. Some use radiused transitions. Some use absorbent foam at transitions. Some use hybrid horn shapes. These things are used in an attempt to reduce ripples in response and directivity and other anomalies like pattern flip and astigmatism.


Re: horn surfaces [message #17322 is a reply to message #17319] Sun, 09 January 2005 12:35 Go to previous messageGo to next message
Earl Geddes is currently offline  Earl Geddes
Messages: 220
Registered: May 2009
Master
Wayne

Agreed to your post. IF the wall roughness is small relative to the wavelength then it will not be much of an effect.

I was thinking more about how one could use a deliberate roughness at the wall to shape the wavefront by creating a boundary layer effect. from my book it is obviuos that one wants the wavefront to diminish at the edges of the waveguide.

Also, please remember that I do not use waveguides or horns at LF. They need to be too big to do any real good. Sure they offer some gain, but remember that I always use big high performance drivers for directional reasons and so gain is not really very important to me. (True unless I am doing a pro sound system, but then the waveguides can be big.) I use waveguides strictly for their directivity qualities, not for their loading ability. SO when I think about horns or waveguides I never think about low frequencies. In essence, if the wavelength is bigger than the waveguide dimensions then the waveguide is no longer a directional control device and I don't really consider it.



Re: horn surfaces [message #17323 is a reply to message #17316] Sun, 09 January 2005 12:41 Go to previous messageGo to next message
Earl Geddes is currently offline  Earl Geddes
Messages: 220
Registered: May 2009
Master
You will have to be clearer on what you mean by "phase changes caused by irregularities"

Of coarse the foam absorbs high frequencies, but ALL CD devices have to be EQ anyways so I just EQ in this loss. No problem.

I believe that I could model he rough walls. I know that I can model, and have done so, an absorptive wall, as well as an absorbtive internal medium. A rough wall just have a different kind of boundary condition.

You will also have to be more specific about "theres usually better ways to fix a problem such as rough frequency response than this". I am not new to waveguide design and usually know all of the techniques involved, but I am not sure what you are refering to here. If you are refering to the use of the foam, I strongly disagree that there are better ways. If there were then I would use them and not have to "invent" the better solution.




Re: horn surfaces [message #17325 is a reply to message #17322] Sun, 09 January 2005 12:58 Go to previous messageGo to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 18787
Registered: January 2001
Illuminati (33rd Degree)
Agreed, 100%.

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