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Surprising observation - real or misinterpreted?

eddantes

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Hello all,

As a speaker building newb, I am trying to develop the skills to make a good speaker. I built a turnturntable with degree markers for taking measurements and was just trying to run through a quick measurement in my cramped basement to practice. I measured my M22 (horizontal only) with a small window to try to eliminate the reflections in my cramped basement (500-20K).

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Comparing the orange measurement with ASR Spinorama I think it looks good from around 2K and up. Under that seems like I have don't have enough resolution. Still the observation isn't about my measurements. It's about a little experiment I ran - applying an F26 felt "cut-out" to the tweeter (double sided tape) and remeasuring.

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I was expecting some improvements with difraction. And I think I did get some. See below:

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But what I was surprised by are the improvements in directivity. It seems this felt functions a bit like a wave guide. Stock M22 has a directivity issue as can be seen below

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I believe I did see that in my measurements (unless I am misunderstanding things):

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But here's the surprise once the felt was added:

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Not only was each measurement angle smoother in FR, but as response became narrower (as expected) it appears directivity was somewhat corrected.

Am I reading that right? Is that what I am seeing or am I reading into this something that doesn't exist?
 
Assuming your measurements are solid, it looks like a real effect to me.

Not an experienced speaker-builder myself either, but maybe the diffraction and directivity effects are one and the same. If there's a significant diffraction that would also tend to cause the sound to scatter differently in aggregate...

I think you can figure out the diffraction points by measuring the distances to the edges from the drivers - do any of those dimensions correspond to the wavelengths where you saw a change in the measurements?
 
I'm no expert, but my guess is that the reduction in diffraction will probably be offset by a decrease in the dispersion from that tweeter.
 
Not only was each measurement angle smoother in FR, but as response became narrower (as expected) it appears directivity was somewhat corrected.
I would rather say ”changed”. As there is now a dip on axis around 2.5kHz now, this results in the ”pinch” moving down. Whether that combined with the narrowing is better or worse, I do not know.
1736074236164.png


But the peaks/unevenness off axis indeed have been ”corrected”.
When I bought my first audio gear in the 70s there were British speakers (Spendor? Rogers?) that had felt around the tweeter on the baffle (while having a protruding collar around the baffle :facepalm: ).
The felt was used as some kind of band aid, after digging the tweeter into a pit for some reason (having an old school grill in this case) the diffraction is so awful that the felt has to help out.
I always wondered why hardly anybody uses foam or felt to control directivity/diffraction in speakers.
Seems to be a powerful measure.
 
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Dunlavy routinely used felt around their tweeters and even midrange. Here's a thread dedicated to Dunlavy speakers:
 
The felt would damp (or even eliminate) edge diffraction and baffle surface reflection effects and thus smoothens FR.
 
I'm no expert, but my guess is that the reduction in diffraction will probably be offset by a decrease in the dispersion from that tweeter.
Me neither. However, using my layman's logic, absorption in the felt converts sound waves into heat instead of allowing them to propagate into the room, which makes your assumption seem correct to me. ;)
In any case, using felt around tweeters or even midrange drivers is nothing new. In fact, it’s more of a traditional approach. Probably decades ago, analysis and measurement equipment wasn’t as advanced, and manufacturing high-quality waveguides was likely quite expensive if even possible.

That doesn't rule out the possibility that some speakers might still benefit from it on a case-by-case basis today.
 
I would rather say ”changed”. As there is now a dip on axis around 2.5kHz now, this results in the ”pinch” moving down. Whether that combined with the narrowing is better or worse, I do not know.
View attachment 418787

But the peaks/unevenness off axis indeed have been ”corrected”.
When I bought my first audio gear in the 70s there were British speakers (Spendor? Rogers?) that had felt around the tweeter on the baffle (while having a protruding collar around the baffle :facepalm: ).
The felt was used as some kind of band aid, after digging the tweeter into a pit for some reason (having an old school grill in this case) the diffraction is so awful that the felt has to help out.
I always wondered why hardly anybody uses foam or felt to control directivity/diffraction in speakers.
Seems to be a powerful measure.
You're right. There is a pinch. But squinting at the bundle of traces it still does seem a more uniform progression. I'll make a custom felt pad for the 22 and will attempt a full spin, once the weather warms up. The goal here was to get to a place where I can generate my own CEA2034 chart. I think then we will see if this is beneficial. Subjectively - I can't really tell.
 
Could it be that not all felts are equal? Wilson audio also uses felt, but it's used much more sparingly. Theirs might also be less dense; more porous, which might also affect what its diffraction and absorbtion properties are? So likely using different felts would have different results, requiring different felt thickness, position, design, etc.?


iu


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Also that your felt could act like a mini horn, or rather has small absorption and diffraction effects given its proximity to the tweeter?
 
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