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Old 1st December 2012, 07:42 AM   #11
Bon is offline Bon  Australia
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Originally Posted by theaudiophile View Post
I want to see if I can get an agreement from the experts here about this. I was having a dispute with a professor of physics about the use of wool to control reflections from the tweeter. He tells me there is no reflection since there is no wave with angle of incidence at pi/2 radians.

I can clearly hear the positive effects of wool on the baffle. Its also intuitively clear that it must be soaking up some of those nasty waves which are sliding along the baffle that should not really be there.

What do you guys think?

If there are no reflections whats the point of applying wool? Why does it work?
The phenomenon is referred to as grazing ray propagation. It is real and wool felt exerts a measurable reduction on its effect.
The scattered acoustic boundary wave generated by grazing incidence at a slightly rough rigid surface | Browse - Journal of the Acoustical Society of America
"A new theory describing sound scatter from a low roughness rigid surface [I. Tolstoy, ’’The scattering of spherical pulses by slightly rough surfaces,’’ J. Acoust. Soc. Am. 66, 1135–1144 (1979)] has predicted that at near‐grazing incidence a boundary wave will be formed in the fluid above the surface, and that at sufficient ranges the amplitude of this scattered boundary wave will exceed that of the direct wave. A model experiment has been conducted using a point source and receiver embedded in a rough plane surface constructed of close‐packed rigid hemispherical bosses and the prediction of the amplitude of the boundary wave has been fully confirmed. In addition, the experiment has revealed that the coherent scattered boundary wave, which at lowest frequencies or ranges leads the incident wave by 90°, becomes more nearly in phase with the incident spherical wave as frequency or range are increased, thereby further strengthening the signal at the rough surface relative to smooth surface propagation."
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Old 1st December 2012, 08:56 AM   #12
Rudolf is offline Rudolf  Germany
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Originally Posted by Bon View Post
The phenomenon is referred to as grazing ray propagation. It is real and wool felt exerts a measurable reduction on its effect.
The scattered acoustic boundary wave generated by grazing incidence at a slightly rough rigid surface | Browse - Journal of the Acoustical Society of America
What I can read in the abstract makes me quite sure they were researching waves in water travelling along a hard surface (metal?). Considering the very different physical conditions between water/metal(?) and air/felt I wouldn't dare to make any comparisons.

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Old 1st December 2012, 12:56 PM   #13
Bon is offline Bon  Australia
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Originally Posted by Rudolf View Post
What I can read in the abstract makes me quite sure they were researching waves in water travelling along a hard surface (metal?). Considering the very different physical conditions between water/metal(?) and air/felt I wouldn't dare to make any comparisons.

OK then how about
Cutting Corners | Stereophile.com
Keith Howard is a loudspeaker reviewer I have a lot of respect for. He has probably forgotten more than most of us will ever know.
There are numerous studies of the contributions of grazing incident waves to the far field sound. For example
The radiation of Sound from Surfaces at Grazing Angles of Incidence ...
http://researchbank.rmit.edu.au/eser.../Pavasovic.pdf
All I am saying is the effects of grazing rays are is detrimental to the performance of an idealised baffle mounted driver. It is a judgement call whether the perceived effects are sufficiently noticeable to attempt to address by strategies such as felt damping. It is not a huge expenditure of effort in a typical diy build. Even if the baffle has diffraction controlled edges, I have always been concerned about the effect of the grazing rays when they reach the typical grille frame. Two solutions are to dispense with grilles or build a wool felt insert inside the grille as Avalon do with their "cloaking device".
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Old 1st December 2012, 01:26 PM   #14
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Originally Posted by Rudolf View Post
What I can read in the abstract makes me quite sure they were researching waves in water travelling along a hard surface (metal?). Considering the very different physical conditions between water/metal(?) and air/felt I wouldn't dare to make any comparisons.

I don't agree. The reference to 'waves' also applies to sound in air. Just picture a sine 'wave'.
As far as modeling of acoustics and boundries is concerned, what applies to air can also apply to water for that matter. It's just a different fluid medium with different physical constants (i.e. density, viscosity, reynolds numbers, etc..).
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Old 1st December 2012, 02:35 PM   #15
Rudolf is offline Rudolf  Germany
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Originally Posted by Bon View Post
All I am saying is the effects of grazing rays are is detrimental to the performance of an idealised baffle mounted driver. It is a judgement call whether the perceived effects are sufficiently noticeable to attempt to address by strategies such as felt damping.
Bon,
if we have a tweeter mounted flush in a baffle. What would be the incident angle of the sound traveling from the tweeter along the baffle surface? The thread opener did not ask about edge diffraction of any sort.

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Old 1st December 2012, 03:39 PM   #16
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Originally Posted by Rudolf View Post
Bon,
if we have a tweeter mounted flush in a baffle. What would be the incident angle of the sound traveling from the tweeter along the baffle surface? The thread opener did not ask about edge diffraction of any sort.

I don't trust what you tell me any more Rudolf. It's a good thing I started this thread to expose the truth. Why cant we all agree about this?
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Old 1st December 2012, 03:43 PM   #17
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Considering the very different physical conditions between water/metal(?) and air/felt I wouldn't dare to make any comparisons.

Why not? You were happy to make comparisons about sound waves and water waves when you explained diffraction.
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Old 1st December 2012, 04:55 PM   #18
system7 is offline system7  United Kingdom
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Most tweeters are DESIGNED to work best on a flat baffle. Preferably a large one and the driver rebated flush. That's why I don't lose much sleep over it.

However, we HAVE discovered here that felt on the baffle will reduce the overall SPL from the tweeter. Makes sense. It absorbs some energy at some frequencies.

It raises as many issues as it solves. Felt rings do odd beaming things. Uniform felt is more predictable, but again will cause some oddities as it gets thicker. I'm sticking to rigid baffles personally. But don't let me stop you unearthing the TRUTH, whatever it may be!
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Old 1st December 2012, 06:05 PM   #19
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What's to debate here? We know baffle diffraction exists, we know that felt coverings can have an affect on it. Debating whether felt should be used requires a specific example and lots of measurements, and of course listening. The argument referred to in the OP is surely a confusion between reflection and diffraction, making the professor correct.
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Old 1st December 2012, 06:37 PM   #20
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If you look at post 11 its gets more confusing. And system7 how does the felt reduce the output from the tweeter?
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