Baffle Diffraction

[picowallspeaker]

What is the difference between a stationary sound and a single sound? Because sounds evolve in space and time. In ambient the sound has a decay... It is not like the light that travels at...the speed of light, but much slower. There are many attributes that can get lost by keeping the sound-light analogy

 

[wonderfulaudio]

There are many attributes that can get lost by keeping the sound-light analogy

Reflection and mirror terms are being used with respect to sound, not light.

 

[picowallspeaker]

And how do the sound waves and the "light" waves propagate ?

 

[33Polkhigh]

They are very similar and yet fundamentally different. Both can be thought of as creating image sources. In a reflection the virtual image is exactly that of a mirror - some distance behind the reflector. On the other hand, diffraction results from a boundary change and it too creates a virtual image source, but positioned exactly at the diffraction point (or line or whatever.) Hence diffraction will always be much closer in time to the direct sound than a reflection will. A very significant difference.

Some people say that the material the soundwave reflects off of has an effect. So sound bouncing off of ceramic tile or glass might carry some resonances from the material?

If one can suspend judgment on the dubious design and decor consequences of this idea, might a lot of edge diffraction be reduced if, instead of the apparently little effect a felt ring has, instead do a variation on the "towel trick" used with success on horns (but to reduce the cavity resonance), as follows: fabricate what I'd call a hood (cowl?) around the driver(s), sort of a partial funnel or torus around it. This would, I would think, greatly reduce any waves to the side and to nearby diffraction-prone edges.

Something like this can be used to help absorb sound on the baffle.

https://www.ebay.com/itm/nuLOOM-Han...ug-2-x-3-2-x-3-/352474200949?oid=302695842764

The best commonly available sound absorbing material is rockwool. One could glue pieces to the baffle and hide it all under a grill. It doesn't degrade over time like foam either. The lower the frequency the harder it is to absorb in general. A low frequency wave will often find a way around the absorber.

 

[norman bates]

I surround my tweet with 1/2" f10 wool.

Grade F10 Pressed Wool Felt Sheet, White, Meets SAE J314, 1/2" Thickness, 12" Width, 12" Length: Felt Raw Materials: Amazon.com: Industrial & Scientific

Maybe this is the same.
F10 Felt Sheets and Strips - Felt - Grainger Industrial Supply

Here is a review of diffraction b gone.
DIFFRACTIONBEGONE REVIEW

And lastly an article "diffraction doesn't have to be a problem". It shows graphs for the meter readers even.
Diffraction Doesn't Have to be a Problem

Subjectively it adds sharpness. I've played with foam around horn mouths, it helps.

 

[Soldermizer]

This is what I've used for two years...

For two years I've been happy with a pair of "parts or repair" Yorkville Unity U15 that I run active. Among other easy mods, I stuffed the horns with polyfill, green thing is air filter to hold it in. Towl cowl is a horn trick. I have limited measurement ability, so can't claim "better or worse." I have bought some 30 PPI foam to make a proper plug a la Geddes.


Appearance is clearly not a priority. I have four cats and the "rat fur" serves as a good scratching post. Be appalled if you like, but these replaced some Bose 901, so surely this is an upgrade!

 

[gedlee]

Some people say that the material the soundwave reflects off of has an effect. So sound bouncing off of ceramic tile or glass might carry some resonances from the material?

The answer to that question could fill a book. And did! "Sound Absorption Technology" by Uno Ingard. But what you think of as solid reflection, like a mirror, is going to be more accurate than not. Extreme effects require extreme divergence from flat and rigid.

 

[andy2]

I don't think sound absorbing materials on the baffle will do. It's just too easy not to mention visually unfit for human. I think you just have to be careful with your baffle design geometrically. There is really no other way. Diffraction is caused by the shape of the baffle therefore the only way to fix it is to fix the shape of the baffle.

 

[norman bates]

No harm in trying it.

 

[Bill Coltrane]

I don't think sound absorbing materials on the baffle will do. It's just too easy not to mention visually unfit for human. I think you just have to be careful with your baffle design geometrically. There is really no other way. Diffraction is caused by the shape of the baffle therefore the only way to fix it is to fix the shape of the baffle.

Isn't diffraction caused by a change in acoustic impedance?
If so, felt could work.

 

[keyser]

Diffraction is mostly an imaging problem as it does NOT have much effect on colouration.

Uhm... yes it does?


Yes it does.


Worst case is a point source in the middle of a circle. You'll get one big comb-filter. You bet it causes coloration. Moreover, it leads to a different comb at different angles. Practical speakers have less of an edge diffraction problem, but it's a real issue.

Edge diffraction affects coloration, imaging and directivity.

 

[scottjoplin]

No harm in trying it.

Better yet, it works

 

[bentoronto]

Really? Are you not listening. There is no doubt that we can hear diffraction. That's not even in question. The issue comes down to timing and levels.

You ripped my comment out of context. Not nice. For sure, if severe enough, likely you can hear the difference in tone colour between a speaker with anti-diffraction treatment and the one without using white noise or trick test tones.

But if we are talking about the felt ring FR plot referred to earlier, if you had a speaker with that felt anti-diffraction ring around the tweeter being compared blind to the same speaker without the ring, could anybody say "A has the ring and B doesn't" on music or white noise? Or even, "A sounds better"... which is what this thread is all about?

B.

 

[andy2]

Isn't diffraction caused by a change in acoustic impedance? If so, felt could work.

That's an interesting terminology and I may have heard it first mentioned with respect to diffraction but I think what you meant. Absorbing materials may work to a degree but not all frequency range. Some may work at higher frequency but it depends on which freq. affected by diffraction since each different baffle design may have different diffraction signature. So you may have to find the righ absorbing material for that freq. For me, visually it's just a no go.

I think it's best to fix the root cause which is in the baffle design.

 

[gedlee]

Uhm... yes it does?


Yes it does.


Edge diffraction affects coloration, imaging and directivity.

I did not say it had no effect on coloration, but it is imaging that my research suggests is the greatest effect. And I do not believe that diffraction would have a significant effect on polar response except in the extreme cases like you suggest. I should hope that one is not trying to make diffraction dominate, and as it gets smaller and smaller its effect on polar response becomes minimal, a little more so on coloration, but on imaging it remains significant even at extremes of correction. You have to consider the level of effects as we try to minimize them. Otherwise you just keep chasing things that don't make a difference.

For sure, if severe enough, likely you can hear the difference in tone colour between a speaker with anti-diffraction treatment and the one without using white noise or trick test tones.

But if we are talking about the felt ring FR plot referred to earlier, if you had a speaker with that felt anti-diffraction ring around the tweeter being compared blind to the same speaker without the ring, could anybody say "A has the ring and B doesn't" on music or white noise? Or even, "A sounds better"... which is what this thread is all about?

B.

As I said above, I am talking about imaging and how diffraction degrades it, but you keep talking about coloration. Either we talk about the same things or why bother to talk at all. And if you keep narrowing the discussion down (especially to the classic catch-all "Does it sound better to me?") to the point where you can "win" the argument, then there is no point in continuing.

 

[keyser]

When we're talking about how diffraction affects imaging, I wonder if it's the temporal issues that degrade imaging, or the amplitude response variations with small lateral head-movements.

 

[andy2]

temporal issues that degrade imaging, or the amplitude response variations

I must think it's both since our brain uses both to locate sound.

 

[Eva]

In listening environments with walls and ceiling, lots of inconsistent radiation off-axis (as from comb-filtering or standing waves) adds a lot of load to the regions of the brain that relate sound with 3D space. That can take as much brain power as protecting a lie.

 

[gedlee]

When we're talking about how diffraction affects imaging, I wonder if it's the temporal issues that degrade imaging, or the amplitude response variations with small lateral head-movements.

I'd say that it is dominantly timing, and here is how I know.

Lidia and I just recently completed a study of room reflections on image localization (these results are preliminary). The results indicated that a random noise signal had no effect on image location or blur (ability to localize in precise directions), but there was significant coloration of the signal. On the other hand with transient impulse signals there was a significant (TBD) directional shift toward the reflection and a significant increase in the blur of the image. Ipsi-lateral and contra-lateral reflections were significantly different. These results - for the impulses only - depended on frequency with the lower frequencies being less affected. We used headphones so head movement was not a factor.

I believe that this clearly indicates that we localize on the transients in a signal and that it is the higher frequency transients that carry the most information. Reflections do affect coloration, but that was not the focus of the study.

 

[PMA]

How about to test this outdoors, without room reflections. Big garden or so, the only reflection is ground surface. Listening results are quite interesting outdoors.