Geddes on Waveguides

[schuschu]

Re: Re: Scientific

This was shown in a series of AES paper - some three of them - in the early 90's.

For sure I have had read them. Thank You.

A guy in Achan Germany, Gottfried Behler I believe, did show that HOM exist and were measurable, although he concluded that because they were small they were not an issue. However, he never did any subjective work to prove that these small effects were not audible. We discussed and disagreed on this point. His work is avaialble from various sources.

Of course they do exist, but in wich extent and with what perceptability? Thanks again.

It is difficult to impossible to manipulate the HOM directly. However, it has been proven that minimizing them makes the waveguides subjectively better. ... and there is no evidence to the contrary.

I'm not that sure on that. Especially as no comprehensive examination on subjective perceptibility neither has been published nor discussed yet, has it?

... the data is not something that I should share as now ... If I am wrong then you can relish in the vast time and money that wqas spent on a failure. Only time will tell on this one.

A predicament between commercial and scientific issues, I know.

From what I could tell the paper is not applicable here.

I disagree. The thesis mentioned shows, their is no perception of eben harsh phase or amplitude errors if they are narrow in the upper octaves. Any HOM will show as no other than that at the eardrum. The conclusion from that is, HOM is not perceptable.

We did several audiblity tests on compression drivers and waveguides.

I've read the paper that accidentially pops up as a PowPoinT show on Your site. I wonder how You could distiguish between THD and IM - but that's besides HOM i think.

2) HOM like effects are audible and increase in audibility with SPL level (proven) and

Again I disagree with the perceptability of HOM as beeing proven let be proven its characteristics.

... the available data strongly supports my contention that HOM are what makes horns sound bad.

I've still got no data on that.

Do you refer to your Profs as Mr.? I suspect not, especially in Germany!!

I'm confused. Did I step to a social pitfall? Anecdote: One of my post grad profs (cosmology) was named "Juergen" what is my forename too. One day some fashinable dressed woman stepped before the class. All the student still mumbling, prereading the script and so on. The lady lifted her voice to say: "From now on my name is Karin (german female forename)!" It was the prof. All that nerds raised their heads to see: "Hmh, strong evidence, seems very plausible, o/k ... next please ..."

I'll dismiss the salutation then just to be sure - and hope You didn't read me wrong again ;-)

Ahoy!

 

[phase_accurate]

I had a short glimpse at the German thesis and it looks as if they made the listening tests with a monaural setup (though transfering it to the listener via dummy head and headphones).

I think that perception tests should be performned on stereo setups.

Regards

Charles

 

[schuschu]

I think that perception tests should be performned on stereo setups.

Why not 5.1 then? That would mirror state of the art in audio. To investigate the claimed effect one has to isolate its signature. As is claimed HOM just alters insofar a clean wavefront whatsoever with small but highly annoying alterations of phase and amplitude.

Such alterations can be simulated easily. This is so as the effect may be described within a linear model that is well understood since some 100 years. But I don't have any hint on the quantity of the formentioned alterations needed to infect some testsignal whatsoever with HOM-alike phase and amplitude distorsions. To what quantity would the groupdelay or amplitude ondulation measure if caused by serious HOM? What is the expected dependency on angular direction if it comes to binaural hearing?

I think we're caught in a trap. Why should we as being not the inventor prove the concept? If we don't believe the claim the motivation is low. If we do believe positivly that is true also.

The invention does promise a lot. But me personally don't feel to miss anything with horns - yet.

Wake me for Meal ;-)

 

[Mr. Wizard]

I can't be sure, it stands up to a reasonable look. Could you send me the derivation and I'll check it.

I should mention that I looked for a closed form solution, but gave up after a short while. My next attempt was iterative and this worked. Yours may well be correct and you found what I could not.

Is the last term 2 "times" Tan(a) (2 * Tan(a)) or 2 times "x" times Tan(a) (2*x*Tan(a)). The former can't be correct since the slope would then be zero at x=0. But the second COULD be correct, I'd have to check it.

The last term is (2*x*Tan(a)).

I used a computer algebra system to find the solution, so I do not have a formal derivation. I set:

f(x) = Sqrt(rho^2 + x^2 * Tan(theta)^2)

Then solved the simultaneous equations:

a = arctan(f'(z))
r = f(z)

for rho and z, using the CAS. I then substituted those values into

f(x + z)

and simplified the result, again using the CAS.

I believe that if I understand the target curve, the function is correct.

 

[Sheldon]

Re: Re: Scientific

If I am right, this line will sweep through the industry as the sound quality is markedly better than anything else out there. If I am wrong then you can relish in the vast time and money that wqas spent on a failure.

Unfortunately, based on what we see in the market, I would question whether sound quality is highly correlated with market success.

Sheldon

 

[Patrick Bateman]

Re: Re: Re: Scientific

Unfortunately, based on what we see in the market, I would question whether sound quality is highly correlated with market success.

Sheldon

Here's a little anecdotal evidence that it might:

A few months back, there was a presentation at work. Since the audience numbered nearly one hundred people, they had rented a PA system. Furthermore, the presentation was outside.

So far so good?

I was outside, and noticed something unusual -
It didn't sound awful, unlike most PA systems.

So I checked out the PA speakers, and sure enough, they were a new set from JBL. I'm convinced that some folks at JBL are following this research, as a number of their new speakers use improved waveguides that are somewhat similar to what we are discussing here.

Needless to say, the PA speaker in question used a improved waveguide, one which resembled an oblate spheroidal waveguide.

Compared to the typical PA squawk-boxes, the quality of the JBLs was a night-and-day improvement.

 

[Sheldon]

That would be a good trend indeed. I can see it being easier in the sound reinforcement market, though. Quality in that context is easier to define and demonstrate - can the audience hear the presentation clearly? Of course many of the attributes which enhance clarity reflect a lack of extra stuff which obscures details. I guess that means clear undistorted sound, at least lacking certain types of distortion. Of course those issues need to be addressed in home systems too, but a few more elements are probably required for the highest level of music enjoyment in home context, as we don't have to visuals or venue effects to fill in some of the details.

If I may paraphrase shamelessly, Dr. Geddes was saying that if he is correct on the merits of his speakers, that they would sell like hotcakes , if not they wouldn't. Given the difficulty of selling into the home sound marketplace, success would tend to support his design assertions more than lack of success would negate them.

My own experience on the technical end certainly confirms the utility of foam in cleaning up the vestiges of horn sound in my system.

Sheldon

 

[schuschu]

My own experience on the technical end certainly confirms the utility of foam in cleaning up the vestiges of horn sound in my system. [/B]

Hello, would You mind to give me a link to a description of that fabulous foam tweak*? My English is yust to bad to discuss that topic here. I have to have at least some measurements made by my own.

Thank You

*I really did som ebackground search. Alas, with no success gven.

 

[gedlee]

The last term is (2*x*Tan(a)).

I used a computer algebra system to find the solution, so I do not have a formal derivation. I set:

f(x) = Sqrt(rho^2 + x^2 * Tan(theta)^2)

Then solved the simultaneous equations:

a = arctan(f'(z))
r = f(z)

for rho and z, using the CAS. I then substituted those values into

f(x + z)

and simplified the result, again using the CAS.

I believe that if I understand the target curve, the function is correct.

From what you describe it should be correct.

Thanks

 

[gedlee]

Re: Re: Re: Scientific

Unfortunately, based on what we see in the market, I would question whether sound quality is highly correlated with market success.

Sheldon

This is absolutely the case. But sound quality is not the only place where we are gaining in acceptance. Customers seem to like the looks as they look more modern than square black boxes. For a comparable set of specs, we are the lowest cost solution on the market, and if by some strange coincidence sound quality is important to you then we win hands down.

We believe that we have all the bases covered.

 

[gedlee]

Re: Re: Re: Re: Scientific

I'm convinced that some folks at JBL are following this research, as a number of their new speakers use improved waveguides that are somewhat similar to what we are discussing here.

Needless to say, the PA speaker in question used a improved waveguide, one which resembled an oblate spheroidal waveguide.

Compared to the typical PA squawk-boxes, the quality of the JBLs was a night-and-day improvement.

I know for a fact that JBL are using my technology. Harman has bought more of my books than anyplace else on the planet and I've talked with them about it. They just don't like to give out any credit to anyone outside of the corp.

Its the JBL pricing that is out of sight compared to what we will be offering. So if the Logo is all you care about then but JBL, but if you want the best value buy Ai.

I can see it being easier in the sound reinforcement market, though. Of course many of the attributes which enhance clarity reflect a lack of extra stuff which obscures details. ... Of course those issues need to be addressed in home systems too, but a few more elements are probably required for the highest level of music enjoyment in home context ...

If I may paraphrase shamelessly, Dr. Geddes was saying that if he is correct on the merits of his speakers, that they would sell like hotcakes , if not they wouldn't. Given the difficulty of selling into the home sound marketplace, success would tend to support his design assertions more than lack of success would negate them.

My own experience on the technical end certainly confirms the utility of foam in cleaning up the vestiges of horn sound in my system.

Sheldon

I gave up on the home market a long time ago. Nothing rational is a factor their - its all about marketing, ala Bose success. Our focus is pro and if there is a leakage to Hi-Fi or Home Theater then great. If not I won't be surprised. Tom Danley does this same thing, presumably for the same reason.

I would disgaree that good sound qualuity for Pro use is different than for home use. This has not been my experience - at least not with my designs. When one uses horns because they had to to get the SPL levels required then yes compromises where required. But when the waveguide sounds as good as a hi-end tweeter but performs like a Pro sound compression driver then there need not be a compromise.

 

[Sheldon]

Re: Re: Re: Re: Re: Scientific

I would disgaree that good sound qualuity for Pro use is different than for home use. This has not been my experience - at least not with my designs. When one uses horns because they had to to get the SPL levels required then yes compromises where required. But when the waveguide sounds as good as a hi-end tweeter but performs like a Pro sound compression driver then there need not be a compromise.

You caught me speculating there. Admitting up front, lack of extensive experience, training and intellectual rigor, I still wonder about one difference. In the home experience we are trying to create an illusion of a live experience - not necessarily THE live experience, but one that allows us to suspend belief a bit. I do think it makes sense intuitively that a speaker which produces a faithful, coherent, replica of the input is a base requirement. Included in that should be controlled directivity. But, I can imagine that how it's directed can be different depending on the effect that is desired. I wonder if how the sound interacts with the room and walls, sometimes in subtle ways, can be important to improving the illusion. Hence, perhaps somewhat different requirements for sound reinforcement applications vs. a home system? I can imagine that things like rear directed tweeters, etc., might be useful in a home system for enhancement, but not in sound reinforcement.

Sheldon

 

[gedlee]

Re: Re: Re: Re: Re: Re: Scientific

You caught me speculating there. Admitting up front, lack of extensive experience, training and intellectual rigor, I still wonder about one difference. In the home experience we are trying to create an illusion of a live experience - not necessarily THE live experience, but one that allows us to suspend belief a bit. I do think it makes sense intuitively that a speaker which produces a faithful, coherent, replica of the input is a base requirement. Included in that should be controlled directivity. But, I can imagine that how it's directed can be different depending on the effect that is desired. I wonder if how the sound interacts with the room and walls, sometimes in subtle ways, can be important to improving the illusion. Hence, perhaps somewhat different requirements for sound reinforcement applications vs. a home system? I can imagine that things like rear directed tweeters, etc., might be useful in a home system for enhancement, but not in sound reinforcement.

Sheldon

Sheldon

Very perceptive comments. Something that I have considered for a long time.

You are absolutely right that there is no fundamental reason why loudspeakers designed for Hi-Fi in a small room should have the same requirements as a set of speakers designed for a large venue at high levels.

I could ellaborate on the whys, but I am very busy here and I don't have the time for that. But the bottom line is that it is actually a shear coincidence that what one wants for hi-power pro in a large venue is exactly the same as what one wants for Hi-Fi in a small room, but for different reasons. Narrow directivity (for different reasons) Constant directivity (for different reasons), flat frequency response (same reason here), low power compression (same reason), etc. I know that this opinion is controvesial, but I stand by my 40+ years of doing this to claim that it is correct.

 

[FrankWW]

Earl, I bought two used B&C DE500 drivers for a nice price so now I have to bite the bullet and build the speakers. Because I got a good price on the drivers I don't have to compromise too much on the woofers.

Which B&C woofers did you use in the Summas? Has B&C come up with something which is a better value than you used in the Summas?

 

[gedlee]

I don't use the DE500. I use the 15TBX100's as the woofer.

 

[FrankWW]

Thanks, Earl.

Yeah, I was going to use DE 250 but the price was too good on the others to pass them by.

 

[Patrick Bateman]

Earl, I bought two used B&C DE500 drivers for a nice price so now I have to bite the bullet and build the speakers. Because I got a good price on the drivers I don't have to compromise too much on the woofers.

Which B&C woofers did you use in the Summas? Has B&C come up with something which is a better value than you used in the Summas?

It sounds like you're thinking about cloning the Summa. I've swiped quite a few idea from it for one of my projects. If I am not mistaken, I was the first person to use Dr Gedde's HOM-reducing plug in a DIY project. If you're interested in my take on that endeavor, read on...

In my project, I used the HOM-reducing foam plug along with a DIY oblate spheroidal waveguide. My project used a different compression driver (neo BMS). My project went in my Accord(!), which explains why I didn't just go and BUY a Summa!

Even though I have built DOZENS of waveguides, I spent about THREE months getting my project to sound good. I'd estimate I spent TWO HUNDRED hours getting it "right."

Which begs the question, what is your time worth to you? Are you prepared to invest hundreds of hours in getting this "right?" Of course it depends on your priorities. Personally, I think DIY projects are the right choice most of the time. For instance, you can often clone commercial speakers for a fraction of the price. This is because commercial speakers typically use drivers which cost between ten and fifty dollars. With the Summa this is not the case; as you've discovered, the drivers are about TEN TIMES as much as a typical commercial speaker. In addition, the cabinets are unusually exotic. Another reason to go the DIY route is because it's the ONLY option. This is why I did it - my waveguides went into my Accord! Last time I checked, a Summa wouldn't fit on my dash.

So before you make any sawdust, have you considered just buying a Summa or a SP15?

 

[FrankWW]

Hi Patrick, (it's John isn't it?),

I want new speakers, good ones. I can't afford, (or at least honestly justify to myself), spending the money on buying sp15s.

I've done a lot of research and the the choice is either a Unity or Summa type of speaker. They appear to me to be the best applications of physics to design requirements.

Neither is a particularly cheap build but I think a Summa type speaker is the easier project to get right because it's less complex.

I've followed your speaker building adventures through the various online fora and learned some things for which I'm grateful.

 

[Patrick Bateman]

Hi Patrick, (it's John isn't it?),

Yes my real name is John, I use a pseudonym bcuz God only knows I spend too much time posting about speakers!

I want new speakers, good ones. I can't afford, (or at least honestly justify to myself), spending the money on buying sp15s.

I've done a lot of research and the the choice is either a Unity or Summa type of speaker. They appear to me to be the best applications of physics to design requirements.

Neither is a particularly cheap build but I think a Summa type speaker is the easier project to get right because it's less complex.

It's true, getting the midranges correct on a Unity is a P.I.T.A! But Dr Geddes has spent a loooooong time adjusting the crossover in the Summa / SP15 so that the polar response is consistent. When I look at how reliable the off-axis response curves are, it blows me away.

Originally posted by FrankWW I've followed your speaker building adventures through the various online fora and learned some things for which I'm grateful.

Thanks!

If the cost of the project is a challenge, I think there's a couple viable solutions.

The first solution is the obvious one - make a two way using less expensive drivers. I gave this option a go about two years ago using a less expensive 12inch (P-Audio SN-12MB, about $150 IIRC) and a 1inch neo compression driver (BMS neo 1", about $90 IIRC.) I never finished this project, because it would have required a few hundred hours of time, and I never found it. In addition, using a twelve simplifies the crossover but compromises the in-room response.

The second solution was less obvious, but I stumbled across a JBL pro-sound box that uses a 15inch woofer and a 1inch compression driver paired to waveguide. And it's c-h-e-a-p. Finding the MP215 at my local prosound store was a fluke, but I've been happily using them in my home theater for a year now with few complaints. You can find them on froogle for about $400; I paid $250 for mine on clearance IIRC.

The MP215 has big glaring flaws, but those flaws could be fixed without much work. It's big weakness is a cheap compression driver. But what if you swapped out the compression driver for a BMS or a B&C, then re-designed the crossover? With a decent measurement set-up, I'm confident that the crossover could be re-designed in about 50 hours. By going this route, you've probably saved 75-90% of the time it would take to build a full-on clone. Best of all, it's a task that's accessible to almost any amateur speaker builder. I truly think the results would be spectacular; the MP215s are REALLY amazing for the price.

As for building a true "clone" of a Summa, I think that would take a minimum of a year of p/t work, and that's a HUGE investment. I'm 110% certain it would end up costing FAR more than the real thing.

 

[lovechild]

Patrick,

very interesting post, thanks!

How about one more solution, building an active Summa clone.
I believe using a DCX or simular would make the XO part much more comfortable and maybe not even much more expansive. If I remember correctly, Earl had said that quite some equalizing is needed to get the Summa right - for the less experienced DIYer this is probably the biggest challenge.

best,
LC