Cabinet Edge Diffraction....

[SWL]

I just learned of this recently and I have some ongoing experiments with some F-11 acoustical felt.

 

It seems that in the research that I've done so far that the majority of the articles I've come across deal with 'non horn' type speakers. I'm wondering if the horn owners here have any comments/opinions/experience in dealing with diffraction specifically with horns that are mounted in a cabinet.

 

I've never heard of this before (just goes to show how little I know) but I'm finding it very interesting. The experimenting I've done so far has been mostly discouraging but I have found a couple techniques that actually have improved the sound....center imaging in particular. I have a couple more things I want to try.

 

In the meantime, what's your opinion on this?

 

Thanks,

-Scott

[moray james]

Reflections form the horn mouth are perhaps one of the major issue with horns. If you damp inside the mouth edge and things get better then you have found an obvious reflection. I prefer to diffract or reflect problem frequencies inside of the horn or inside the compression driver. Horns on a flat baffle will sometimes have reflections from the baffle around the mouth so damping there can be useful or you can simply remove the baffle and mount the horn(s)on an open frame on top of the cabinet this then makes for easy time alignment of horns. Best regards Moray James

[dtel]

OK Scott, here's an opinion.....I have no idea, I am not one of the technical people around here. 

 

But if I got the right idea of what you saying with edge diffraction if when the sound wave leaves the driver or horn what happens to the wave ? Does it hit a cabinet or something else and even how it leaves the horn itself as a possibly way of changing the sound. If this is what your describing your right and it definitely affects horns also.

 

I remember Roy Delgado talking about a bubble as he calls it, the bubble leaves the driver/horn shaped by the design of the horn and other things. It can definitely can be affected which changes the sound. I asked because I was/am considering putting wood trim around the 402 horn just as decoration. From the response I got it was as if I was considering changing the whole design of the horn, it can affect everything.  He knew I was not one of the technical people there and answered so I could understand, he said think of it a an actual bubble leaving the horn, every thing including the mouth of the horn effects it and everything it bounces off after that.

 

Sorry I can't give any technical help, it's just not me, but you are on to something, it's as important as the rest of the horn design and every change can and will effect something down the line of that bubble.  

[SWL]

Reflections form the horn mouth are perhaps one of the major issue with horns

 

'Reflection' and 'diffraction' are two different things...no?

[moray james]

reflection bounce off a surface and the wave is redirected, when a wave is diffracted it is scattered over some angle. Scattering may or may not be uniform in frequency (usually not).

Problematic reflections in a horn are those which reflect from the mouth back into the diaphragm where cancellation occurs. Best regards moray James.

Edited January 15, 2015 by moray james

[SWL]

http://www.audiokarma.org/forums/showthread.php?t=421717&page=3

 

So without having to retype everything this is what I've got going on so far starting with post #36.

Edited January 15, 2015 by SWL

[SWL]

Basically, any F-11 that I've applied to the front baffle has had a negative affect on the sound. The worst being when the felt was applied around the tweeter and even worse yet in the throat of the tweeter.

 

One exception was when I applied a 1/8" strip (1/8" thick-1/2" wide) on the top mounting flange of the mid-horn in between the mid and the tweet.

 

I've been reading that placing the felt on various places on the front baffle prevents the sound from diffracting at the cabinet edge in addition to other places on the front baffle. (screws, adjacent drivers etc.) I've yet to see anyone place the felt on the OUTSIDE of the front baffle/cabinet edge. Since I've been striking out with almost everything I've tried on the front baffle I figured I'd give it a try since I've got a yard and half of F-11 sitting here doing nothing. Results yet to be determined but I'm not hearing any of the detrimental factors I was hearing with the felt applied to the front baffle. Those factors being that the felt was deadening the sound, taking away detail and fidelity. Not good.

Edited January 15, 2015 by SWL

[moray james]

You will find that the flat surfaces of the F-11 acoustical felt that you are using is much more reflective at mid and high frequencies than is the edge of the felt. How the felt is oriented will depend upon the desired effect. Built up layers of felt used so that the absorbing surface is that of the absorbent edge will enhance absorption dramatically. Obviously the thickness of the material will impact absorbency as frequency is decreased. Best regards Moray James.

[Don Richard]

Diffraction in horns is made much worse if the horn's mouth is undersized, and most are. Putting absorbent material inside a horn not designed for that is iffy at best. IMO, putting absorption on the room walls near the horn's mouth will make a more positive difference. Have someone hold a mirror on the wall near the horn's mouth and slide it away from the speaker until the horn's mouth can be seen from the listening position. That's the location where you want to put the absorbent material.

Edited January 16, 2015 by Don Richard

[SWL]

Results yet to be determined but I'm not hearing any of the detrimental factors I was hearing with the felt applied to the front baffle.

 

I experimented with this further last night and I could not hear any effect on the sound this way....guess that's why nobody has commented on felt being applied to the outside edge of the front baffle. Don't know until you try, right?

[SWL]

You will find that the flat surfaces of the F-11 acoustical felt that you are using is much more reflective at mid and high frequencies than is the edge of the felt.

 

Ok, so if I'm understanding you correctly....the 1/8" thick felt that I cut into 1/2" wide strips should be applied up on it's edge rather than laying flat on the baffle? I can try that. I'll probably double up the 1/8" strips to make mounting easier.....doubling them up makes them measure real close to 1/2" thick.

 

Hopefully this does make it more reflective because the other way REALLY deadened the sound....big time. When you are applying the felt in the mouth of the horn are you covering all of it or just certain spots? I covered the whole inside and couldn't get it out of there fast enough.

 

I may even try the ping pong ball thing today....which has me even more curious as to why would you want to add an object that adds more diffraction. What does it do? Improve imaging, etc.? Deal with a harsh sounding horn?

 

...just trying to understand.

[SWL]

Diffraction in horns is made much worse if the horn's mouth is undersized, and most are. Putting absorbent material inside a horn not designed for that is iffy at best. IMO, putting absorption on the room walls near the horn's mouth will make a more positive difference. Have someone hold a mirror on the wall near the horn's mouth and slide it away from the speaker until the horn's mouth can be seen from the listening position. That's the location where you want to put the absorbent material.

Thanks Don. Yes, I have my reflection points treated as well as some random broadband absorption. Bass traps as well....next on the list is to experiment with diffusion.

[SWL]

OK Scott, here's an opinion.....I have no idea, I am not one of the technical people around here.

Thanks dtel. I'm in the same boat since my knowledge of electronics is very limited. I'm more into experimenting and letting my ears decide the final outcome. Of course common sense and physics cannot be ignored which is why I turn to these forums for advice/knowledge.

[Ski Bum]

I'm just a curious hobbyist myself, but it's my understanding that diffraction from cabinet edges and such is somewhat sidestepped by horns due to their directivity; their controlled pattern doesn't allow as much interaction as direct radiators on a 180 deg. wave guide would, or to a much less degree.  But some horn folks go the extra mile to make sure diffraction is minimized (e.g. Gedlee speakers, with the waveguides and front baffles molded to form a seamless integral piece, with beveled edges to boot).

 

Speaking of Gedlee, isn't the open cell foam plug in the waveguide specifically to quell reflections within the horn?  Has anyone tried that approach on their Klipsch?  If ping pong balls are in consideration, this should be tried too. 

[moray james]

You will find that the flat surfaces of the F-11 acoustical felt that you are using is much more reflective at mid and high frequencies than is the edge of the felt.

 

Ok, so if I'm understanding you correctly....the 1/8" thick felt that I cut into 1/2" wide strips should be applied up on it's edge rather than laying flat on the baffle? I can try that. I'll probably double up the 1/8" strips to make mounting easier.....doubling them up makes them measure real close to 1/2" thick.

 

Hopefully this does make it more reflective because the other way REALLY deadened the sound....big time. When you are applying the felt in the mouth of the horn are you covering all of it or just certain spots? I covered the whole inside and couldn't get it out of there fast enough.

 

I may even try the ping pong ball thing today....which has me even more curious as to why would you want to add an object that adds more diffraction. What does it do? Improve imaging, etc.? Deal with a harsh sounding horn?

 

...just trying to understand.

Yes layering up the felt with minimal application of adhesive to the layers and then stand the felt layers up on edge absorption is much improved and so too is bandwidth, the taller the felt the lower the absorption frequency. Obviously this is not always a viable application technique.

You don't want to felt dam the interior of the horn. With correct measurement of reflections direct application to the physical point of reflection can be made but you need very expensive gear to make those measurements. Trial and error based upon experience are practical. I don't put felt in side horns anymore.

The ball inserted into the horn is there to play pool with reflections which otherwise make it back to the diaphragm As I said at higher frequencies the waves simply diffract around the ball as it it were not there. I use a 38 mm dia. ball in the K79 tweeter and a 40 mm ball in the K701 mid horn as well as in the KLF and CF horns. Position varies with the horn. Simple to experiment with give it a try but be prepared to move it around to find the optimum location. I do all my positioning by ear. Best regards Moray James.

[moray james]

I'm just a curious hobbyist myself, but it's my understanding that diffraction from cabinet edges and such is somewhat sidestepped by horns due to their directivity; their controlled pattern doesn't allow as much interaction as direct radiators on a 180 deg. wave guide would, or to a much less degree.  But some horn folks go the extra mile to make sure diffraction is minimized (e.g. Gedlee speakers, with the waveguides and front baffles molded to form a seamless integral piece, with beveled edges to boot).

 

Speaking of Gedlee, isn't the open cell foam plug in the waveguide specifically to quell reflections within the horn?  Has anyone tried that approach on their Klipsch?  If ping pong balls are in consideration, this should be tried too. 

Horns do have a great deal of directivity but do not forget that they lose that pattern control at low frequencies and that is why larger horns are better than smaller horns. I have been using a combination of balls and open cell foam in cone speakers and in my horns for many years. I also use dynamat and acoustical felt in my compression drivers. Best regards Moray James.

[cradeldorf]

Most of the problem has already been compensated for in the crossovers. That's how the big boys do it.

Putting 1/4 x 1" peel and stick weatherstripping foam on the inside of my woofer spokes was the best mod I ever did.

Edited January 21, 2015 by cradeldorf

[SWL]

Putting 1/4 x 1" peel and stick weatherstripping foam on the inside of my woofer spokes was the best mod I ever did.

 

What did it do.....and how does that relate to cabinet diffraction?

[Chris A]

Understanding small-room acoustics and what goals that you're trying to achieve clearly isn't a well-known endeavor, including the sub-category of "baffle diffraction". 

 

First, it is critical to understand your listening space (height, width and length), other near-field reflectors close to the loudspeakers, such as electronic equipment, racks, other loudspeakers, furniture, proximity to walls, toe-in, etc.  A picture usually helps a great deal.

 

Second, knowing the exact loudspeaker type(s) that you use is important, especially the midrange horn: what is its mouth size (height x width) and its lowest crossover frequency.

 

Third, knowing the RT60 of your room as a function of frequency is also important.  Putting absorbent material around the edges of a midrange horn mouth (top, bottom, and sides) or on top of the loudspeaker cabinet, or on the front of the loudspeaker baffle--all these things depend on the room itself--in terms of "should you use any?", "how much?", "where?", "what acoustic absorbancy properties should the material have?".  All of these considerations are important. 

For the KLF-30 midrange horn, it crosses to the woofers at 825 Hz (G# that is 1 1/2 octaves above middle C on the piano keyboard) which is where you'll be having nearfield reflection and diffraction issues.  The woofers are vertically aligned on the front baffle.  At 825 Hz, a quarter wavelength is about 4 inches, so the two woofers will be interacting with each other to form fairly pronounced lobes in the vertical direction with each other and the midrange horn at ~825 Hz.  The question is: "what does placing absorbent material around the woofers and midrange horn on the front and side baffles do?"  My guess is that you'll hear more improvement in sound quality if you simply move everything away from the loudspeakers (about 3 feet minimum from the midrange horn mouth) and experiment with toe-in.  Placing material on top of the loudspeakers, and placing absorbent carpet down in from of the loudspeaker (if you presently have hard floors)--these things will affect the nearfield sound radiation pattern as much as putting a little absorbent material on or around the front baffle.

 

Chris

Edited January 21, 2015 by Chris A

[pzannucci]

I'm just a curious hobbyist myself, but it's my understanding that diffraction from cabinet edges and such is somewhat sidestepped by horns due to their directivity; their controlled pattern doesn't allow as much interaction as direct radiators on a 180 deg. wave guide would, or to a much less degree.  But some horn folks go the extra mile to make sure diffraction is minimized (e.g. Gedlee speakers, with the waveguides and front baffles molded to form a seamless integral piece, with beveled edges to boot).

 

Speaking of Gedlee, isn't the open cell foam plug in the waveguide specifically to quell reflections within the horn?  Has anyone tried that approach on their Klipsch?  If ping pong balls are in consideration, this should be tried too. 

Yes, cabinet diffraction is mostly with cone and dome drivers though exists to a much lesser degree with horns.  Some work on foam around the mouth to cut down on beaming and diffraction was shown in http://www.excelsior-audio.com/Publications/QTWaveguide/QTWaveguide_WhitePaper.pdf

 

As Chris said, there are also issues with multiple woofers that may be mitigated with some dampening material.  Unfortunately the real change is to lower the crossover point which can't adequately be done with the mid horn or position the woofers differently.