How is HOM measured?

[Pano]

Sure, I know that the foam is not the same as the screen - but it made you think, yeah?

And just how different IS the face of the foam from the face of the screen? Can it create similar problems?

 

[Sheldon]

I won't be taking mine out any time soon. The foam absorbs reflections the screen has the potential make more. All I know is I have about .020 between my diaphrams and my phase plugs. I am not taking any chances of some damn bug messing up the works.

I don't know if the bug screens cause any problems or not. But you don't need both. The screen can't substitute for the foam, as the application requires depth. But the foam can substitute for the screen to prevent foreign matter from entering the driver.

Sheldon

 

[mige0]

Sure, I know that the foam is not the same as the screen - but it made you think, yeah?

And just how different IS the face of the foam from the face of the screen? Can it create similar problems?

Haven't got the right grade of foam yet - so I'm pretty light on own experience.
My guess would be that it *is* a valid approach - but most certainly will turn out the same way as why I left the path of boxed speakers.

Not my cup of coffe any more.

You simply can't do closed vernted ... speakers without stuffing.
Only OB can do without - and horns.

What I found from the sparse subjective descriptions that are more than the repetition of Earls arguments (lowest possible HOM ever = must be better) - I certainly could link my own audition on that:

More tending to the well behaved characteristic of a direct radiating speaker but also "The thrill has gone".

Same balancing act as in boxed speakers IMO - you have to dampen down unwanted cavity resonance and trade in "live feeling" - plus you *always* (at least as for me) get the sound colored in a very specific way - that isn't really what I'm after.


Broadly spoken - have you ever seen a world class singer stuffing his mouth with foam to get rid of her / his "vioce chord / throat HOM" ?

Same for microphones - I'm talking about their "sound" not how correct they are (specs).
The bug screen and foam and very light fabrics needed there makes all the difference - not necessarily the capsue and electronics alone - its an *art* that not each and every brand has the skills to get the best balance of.



Michael

 

[gedlee]

I don't know if the bug screens cause any problems or not. But you don't need both. The screen can't substitute for the foam, as the application requires depth. But the foam can substitute for the screen to prevent foreign matter from entering the driver.

Sheldon

Hi Sheldon

I remove the screens in all of my speakers and recommend everyone else do likewise. In my measurements the foam attenuates at almost exactly the same amount as the bug screen meaning that swapping the foam for the bug screen is a wash as far as loss is concerned. But because the foam is very widely dispersed throughout the waveguide it is far more effective on some types of waves than others. The flat screen will have its greatest effect on the zero mode, while the foam plug will have its greatest effect on the HOMs. This was all looked into in some depth years ago. It was not left to chance!!

 

[Wayne Parham]

DBT - Audibility of HOM

There are some essential problems in terms of using a listening test to make a determination about these matters.

I'd say stick to the objective empirical measurements, the engineering simulations and the mathematical descriptions as far as determining their existence.

In terms of "better or worse" that is a can of big fat juicy worms. Any one of us can make that determination WRT their own system, or their own preferences, but that proves not a thing...

I'm not talking about anything subjective, and I don't suggest opinions about quality be asked of the listeners. The only thing I would want to know would be whether they could hear a difference or not. After several tests, a pattern would emerge of what conditions (loudspeakers, horn types, etc.) and what SPL was required before the majority of people could detect HOM.

My question is, what is the threshold of audibility. I'm not talking about subjective values, what is better or worse. I'm trying to learn what level people can hear. That's why I'd suggest a DBT would be useful.

The question for me is, can people reliably detect HOM, at what levels and in what situations. In what devices can HOM be detected? At what SPL?

Start with a single standard horn, a round OS profile if you want. Try it with and without the foam. Try it at 70dB, 80dB and 90dB. Identify whether or not the test subjects could detect a difference between a horn with the foam plug and the same horn without it. Then try this process with other horn shapes, ellipticals, radials even Mantarays and Biradials.

My guess is there will be a place where you can tell, probably around the time you get to the Mantarays. Not sure how far back up the line you have to go to be able to reliably tell the difference. That's what I'd like to know.

I propose a DBT of the audibility of HOM.

I do not doubt the existance of HOM, so that shouldn't be inferred by my post. I have always thought horns with sharp edges sounded harsh, long before this HOM subject was ever discussed. To me, there's definitely something to it.

However, what I am not at all sure about is the amount of psychoacoustics involved. These guys putting foam in their horns all think they hear a difference. I can see how the internal reflections would be absorbed by the foam more than the axial wave. But I can also see how the guy putting the foam in the horn would expect it so sound better, especially after getting excited by the prospect here.

My question is, which is the most responsible for a perception of improved sound quality here: Actual detection or preconceived notion?

So I propose a DBT. Identical speakers with and without foam. Might also be useful to include other CD horn shapes with varying degrees of diffraction.

I think that's really the only way to determine the audibility of varying amounts of HOM. Without that, any comments on the effectiveness of specific flare profiles or foam treatments are subjective opinions, likely biased by preconceived notions.

 

[mige0]

Re: DBT - Audibility of HOM

My question is, what is the threshold of audibility. I'm not talking about subjective values, what is better or worse. I'm trying to learn what level people can hear. That's why I'd suggest a DBT would be useful.

You really don't need any blind test to verify that there is foam plugged in.
Buy some open cell acoustic foam stuff and simply give it a try .

But to relate that change in sonic perception to HOM - ui ui ui - thats the main question ever since.

Michael

 

[Wayne Parham]

Re: Re: DBT - Audibility of HOM

You really don't need any blind test to verify that there is foam plugged in.
Buy some open cell acoustic foam stuff and simply give it a try .

I've heard Summas with the foam. In fact, I'm the person that took the photo below.

• Geddes and AudioKinesis at GPAF 2005

 

[wxa666]

How about this: Do the X/Y mesh thing at some nominal distance in front of the waveguide, say 1M. If Michael's plots are correct, we don't care about the SPL at this point, it's the arrival times that define the shape - plane, warped plane, sphere, whatever, on the assumption that a wavefront actually has a 3D "edge," and that no HOMs are able to get there first. Good so far?

Add a Z axis, forward distance, and map the SPL spectrum of the basic modal plane. Windowing will then reveal the anomalies, perhaps.

I don't get exactly how to identify the basic mode from that. Without a clear definition of a (so far arbitrary) basic mode no "Higher Order Mode", obviously.

Would You say that the basic mode is the mode with defined constant group delay over every useful angle? Or is the basic mode that, from which every deviation in group delay is minimum, may be linear/square weighted by its distance to axial direction? With a horn of 1,7 x 1,2 "" mouth size the average distance will be of strong influence up to some feet distance - even with an OS type.

The question that started the tread lacks rigor in its wording. HOM is well defined within theory. But when it comes to practical issues, for instance measurability the proponents become VERY quiet as far as the topic itself is concerend. We don't know how much of HOM has to be expected from an OS - when realized physically!

Whether or not and how HOM is measurable (our topic here) won't be a question of that funny foam. Whether HOM is perceptable (as bad, sure) or not can not be derived from the foam plug either. That would assume that the foam plug works against HOM specifically. The latter is not yet proven. Due to the proverbial shyness of HOM facing virile measurement gear.

cheers

 

[catapult]

I too think it should be measurable.
As said - quite contrary to Earls opinion – I think its useful to get the lumped HOM measured and I also think it shouldn't be so complicated except for bringing up the reflections that are several decades below in an impulse response graph.
With linear scale, signals down at around 30-40 dB got lost (not possible to identify them) – and i'm pretty sure we will have to look another 20-40dB below that at least.

If you're looking for small differences in the impulse response, you could simply subtract one from the other. There are problems with that of course but I think they could mostly be overcome by using a high sampling rate and averaging a large number of measurements for each impulse. ARTA could do it easily.

 

[wxa666]

Re: Re: DBT - Audibility of HOM

You really don't need any blind test to verify that there is foam plugged in.
Buy some open cell acoustic foam stuff and simply give it a try .

But to relate that change in sonic perception to HOM - ui ui ui - thats the main question ever since.

Michael

Exactly! To look at the foam is the more direct way to identify it's there

 

[bear]

Dr. Geddes,

Just checking, are you saying that the typical fine weave metal mesh in a typical compression driver causes HF rolloff similar to that of what looks like about 10" of open cell foam??

Are we saying then, that absent the issue of UV degeneration of foams, it would be a far better idea to put a short section of open cell foam in the short exit area of a compression driver??

_-_-bear

 

[mige0]

Re: Re: Re: DBT - Audibility of HOM

Exactly! To look at the foam is the more direct way to identify it's there

I've heard Summas with the foam. In fact, I'm the person that took the photo below.

Oh!

Whether or not and how HOM is measurable (our topic here) won't be a question of that funny foam. Whether HOM is perceptable (as bad, sure) or not can not be derived from the foam plug either. That would assume that the foam plug works against HOM specifically. The latter is not yet proven. Due to the proverbial shyness of HOM facing virile measurement gear.

cheers

Right, the foam does not necessarily enter the picture – we *could* use it though – think of that ancient 30m stuffed pipe someone has posted (at Western Electric?).
It was used to act as a infinite pipe to measure the “pure” impulse response of the compression driver itself IIRC .
30m may be a little bit too much for us DIYer but 1m stuffed pipe will show severe changes for sure.

This would be a good starting point as HOM in pipes is within our reach to predict / calculate / simulate. Also HOM is HOM be it in pipes or horns – at least from a pure measuring point of view. We then could take another step and try our skills with horns.

I had a roughly 12cm / 5” of heavy fine pours foam
(used for acoustic dampening) in the throats of my double horn and got roughly 15-20dB attenuation form that 12cm / 5”.

This should already show up in the impulse response of the driver but haven't measured the driver nor stuck my Earthworks mic into the throat

If you're looking for small differences in the impulse response, you could simply subtract one from the other. There are problems with that of course but I think they could mostly be overcome by using a high sampling rate and averaging a large number of measurements for each impulse. ARTA could do it easily.

Yeah but I always hope someone else would take the burden first..


Michael

 

[ZilchLab]

I remove the screens in all of my speakers and recommend everyone else do likewise. In my measurements the foam attenuates at almost exactly the same amount as the bug screen meaning that swapping the foam for the bug screen is a wash as far as loss is concerned. But because the foam is very widely dispersed throughout the waveguide it is far more effective on some types of waves than others. The flat screen will have its greatest effect on the zero mode, while the foam plug will have its greatest effect on the HOMs. This was all looked into in some depth years ago. It was not left to chance!!

We see the lowpass effect of the foam in your ALMA presentation, where you sent us to look for HOMs.

Do we presume there's no downside to the requisite boosting of the VHF compensation contour an additional 4 -6 dB to compensate?

 

[catapult]

Michael, I think you mentioned that you are having trouble finding the right foam in Austria. Check with stores selling fish tanks where it's used as a filter. The Poret foam some people are buying here in the US is imported from Germany.

Manufacturer: http://www.emw.de/
US dealer: http://www.swisstropicals.com/Poret Filter Foam Pricelist.html

 

[KSTR]

Or check out modulor.de, a german retailer for all sorts of materials etc.

 

[Sheldon]

Broadly spoken - have you ever seen a world class singer stuffing his mouth with foam to get rid of her / his "vioce chord / throat HOM" ?

That's a red herring.

Voice is a musical instrument. So is a trumpet, which would generate HOM, but we're not going to stuff it either, because then it wouldn't sound like a trumpet. A horn or waveguide with audible "horn character" might sound just dandy with a brass ensemble. In fact, for some it might sound better, or more "live". Problem is, we might not like our voices to sound like horns, or our violins to sound like horns. An electric guitar amplifier/speaker combination is a musical instrument. Specific forms of distortion are in fact part of the desired sound. Not quite so nice if everything is run through that filter though. I like my hi fi speakers neutral.

Sheldon

 

[mige0]

Thanks - yes this stuff seems to be used as filters in Koi ponds or for mufflers and so on.

Just wasn't on the right track as a search for acoustic foam brings up the stuff for heavy (room) dampening in the first place.

Found out that there can be specs for quite different airvolume put through for one and the same PPI (poures per inch) value - so the 30PPI Earl has speced my not be enough to get *exactly* the right one - possibly have to try.

Am working on implementing Soongsc promising contour currently - hope to get some resuts soon (measurements).


Michael

 

[mige0]

That's a red herring.

Voice is a musical instrument. So is a trumpet, which would generate HOM, but we're not going to stuff it either, because then it wouldn't sound like a trumpet. A horn or waveguide with audible "horn character" might sound just dandy with a brass ensemble. In fact, for some it might sound better, or more "live". Problem is, we might not like our voices to sound like horns, or our violins to sound like horns. An electric guitar amplifier/speaker combination is a musical instrument. Specific forms of distortion are in fact part of the desired sound. Not quite so nice if everything is run through that filter though. I like my hi fi speakers neutral.

Sheldon

Yes sure! I'm aware that it isn't apples with apples.

But beside I found the picture funny - did you ever (!) think of HOM listening to a voice??? - even now that you *know* there is HOM in our voices ?? - and also do voices (in general) get nasty if you listen to them loud (close)?

Michael

 

[Sheldon]

I found the picture funny

Me too. But it's still a red herring. No, I never worried about HOM in natural voices, or horn instruments, or third order harmonics in saxophones. Some people don't like some voices, or saxophones, or maybe guitar amp distortion. I like some voices better than others, and can listen to some louder that others. I think stuffing foam into the ones I don't like so much, might help some (I have no immediate plans in this regard), whether or not my distaste has to do with HOM.

Sheldon

 

[wxa666]

Yes sure! I'm aware that it isn't apples with apples.

But beside I found the picture funny - did you ever (!) think of HOM listening to a voice??? - even now that you *know* there is HOM in our voices ?? - and also do voices (in general) get nasty if you listen to them loud (close)?

Michael

Hi,

Before You go on in debateing all Your devices infection with HOM, singer included, would anybody please answer the originating question:

How HOM is measured?

I know the cure You are searching for, but You won't. If You are in belief that HOM affects Your pleasure, for instance if You actually use that called unqualified diffraction ** from namely JBL as an example, by chance of course, why not praise foam appliance in one of the other threads dealing with the superiority of OS and their originator? Foam up, but elsewhere.

Thank You very much!