Horn Mouth Size, Dinsdale vs. Klipsch

[boom3]

I am throwing this out for those who've built and measured bass horns. I think Dinsdale was writing about the Klipschorn when he wrote this. Appreciate any opinions and observations from experience. 

 

 

Dinsdale, in "Horn Loudspeaker Design, Part Two (Wireless World, May 1974)

wrote:

"A plan view of a corner horn shows that the room itself provides a natural extension of the horn mouth. Many listeners have observed that comer horns can provide bass notes from fore-shortened horns, well below the limit dictated by the mouth area (footnote 25). It is tempting to reduce the mouth area still further below the 3dB limit established earlier and rely instead on the corner placement itself to supply the additional mouth area and horn length. In the author's experience, this technique cannot be justified because although the bass response is undoubtedly there, careful listening reveals an uneven response over the first two octaves above the cut-off frequency which will often detract from the realism offered by the horn. It is therefore recommended that in cases where overall enclosure size is a limitation, a correctly-designed horn with a cut-off frequency of (say) 80Hz will give a more satisfying and linear response than a foreshortened horn whose expansion constant has been set to 40Hz but whose length has been limited to give a mouth area corresponding to 80 Hz." 

 

Footnote 25 refers to  Klipsch, P. W., "A Note on Acoustic Horns", Proc. /.R.E., July 1945.

 

​Later in the article, Dinsdale says that a 40 Hz corner horn must have a mouth of 1133 square inches, and a length of 15.3 feet for an exponential horn with a flare constant of 5. These tables are in Part 2 of the article.

 

I have read elsewhere that "corner horns can "fudge" on the mouth size but not the length".

[Chris A]

So what's the question?  Is Dinsdale talking about a Klipschorn bass bin?  Clearly, yes, at least the UK equivalent--Vitavox Cn191--which are horribly expensive loudspeakers, but the company is apparently still in business. 

 

 

I have read elsewhere that "corner horns can "fudge" on the mouth size but not the length".

 

Yes, with the irregular FR in the 100-250 Hz region that is noted, but easily EQed out.  You can't EQ out modulation distortion from direct radiating loudspeakers, but can easily EQ out FR irregularities--just like the loads of EQ applied the RIAA inverse curve for vinyl where upwards of 40 dB of relative gain is being used there. 

 

No one that I know wants to put a 15.4 foot path length, 1133 in² bass bin mouth area horn into their living room.  Most of the designs that I've seen of late are tapped horn subs.  Mine have 21' path length, but a very, very undersized mouth (170 in²) but has a cutoff frequency of about 14 Hz in a package that is 45" x 48" x 11".

Edited January 11, 2015 by Chris A

[mustang guy]

15.3' is 18Hz. The low extension is there, but the high end wouldn't be. 200Hz maybe?

[boom3]

I let this topic slide a few days to see if more responses would roll in, but I appreciated Chris & Mustang Guy's thoughts. The essential question is, how do we account for  the performance (well documented) of a corner horn that has a shorter path than the "theory" would suggest? Both Dinsdale and a popular online calculator (http://www.mh-audio.nl/Horn/Basshorn.asp) say that the length has to be quite a bit longer than the Klipschorn or other documented designs.

 

In his first paper, Paul said that a horn operated close to a wall and floor junction would have a length of about 80 inches, and that operation in a corner allows the path to be half that much. The logic of the corner horn mouth size is well understood. "Mirror" images "effectively multiply" the size by 8. This assumes that all the boundaries are perfectly rigid, which in practice most domestic walls are not. A non-rigid boundary is a "cloudy mirror" and maybe we can get a 1/6 reduction in effective size, but in most circumstances, I feel that the "gospel" of "8 times" is wishful thinking. 

 

But I've never run across the reasoning that says the horn path can be 1/2 that of a "wall-floor horn". Even our maestro himself said that nobody can miniaturize a 32 foot wave. 

[Chris A]

Using the corner of a room to compete the horn's length is exactly the concept of a corner horn, but note that Klipsch and others have used exponential horn profiles, which have a definite "cutoff frequency" (unlike conical horns) and a horn expansion profile that matches that cutoff frequency.  Below that cutoff frequency, the horn doesn't work very well in terms of output SPL and phase relative to mid-band.  That's why you get such a sharp 30-40 Hz cutoff using Klipschorns in corners.

 

The comment about miniaturizing 32-foot sound waves is a favorite of mine.  I think of that every time I read people writing that Klipsch horn-loaded loudspeakers are "huge".  In fact, they're not, at least not in terms of the lowest frequencies that they reproduce.  The corner horn is actually a bit of marvelous invention when you think about it.  Direct radiators are 10x-15x less efficient in terms of constant driver area, but they're much, much easier for design engineers to produce.  But IMHO, they don't sound nearly as good.  YMMV.

Edited January 18, 2015 by Chris A

[boom3]

Thanks Chris, agree on all points...but...if you re-read the quote from Dinsdale, he's taking issue with Klipsch (and it seems a lot of other designers over the decades). I re-read the reference (the Klipsch paper from 1945) and it doesn't directly address the issue of corner horn length. So I am still wondering what the mathematical basis is for a corner horn 1/2 the length of a wall-floor horn.

 

I tried running this thru the calculator as a fraction of the 47 Hz Fc Paul assigned to this generation of the Klipschorn, and the nominal 40 inches gets me nowhere near 1/4 lambda. At this point I am almost tempted to say that Paul assigned an envelope size and built what he could within it, and, due to his genius, it came out a serendipitous mixture of appropriate compromises. If it hadn't been successful we wouldn't be discussing it now, eh?

[thebes]

. This assumes that all the boundaries are perfectly rigid, which in practice most domestic walls are not.

 

I obviously have no chops at all as far as the more technical aspects of your post but this comment intrigued me greatly. Could there in fact be a material that we could cover our walls with, or replace our walls with, that would engender this rigidity?

 

In other words, if I change out the sheet rock, for say, two inches of plaster, one inch thick mahogany, whatever, would it make the highs higher and the lows, lower.

 

Not trying to shift the conversation but was just curious.

[cradeldorf]

 

. This assumes that all the boundaries are perfectly rigid, which in practice most domestic walls are not.

 

I obviously have no chops at all as far as the more technical aspects of your post but this comment intrigued me greatly. Could there in fact be a material that we could cover our walls with, or replace our walls with, that would engender this rigidity?

 

In other words, if I change out the sheet rock, for say, two inches of plaster, one inch thick mahogany, whatever, would it make the highs higher and the lows, lower.

 

Not trying to shift the conversation but was just curious.

 

Doubtful, maybe but most likely only measurable in micro amounts. now if you went with say 2" of marble LOL, If you went softer... then yes except you'd lose highs and lows would probably be smoother. don't know about anybody else but I hate when I lose my high.

[boom3]

 Thebes, more rigid walls won't extend the bandwidth. They will slightly increase efficiency and reduce resonances, which is why many dedicated HT rooms are going to double sheetrock. Paul's "false corners", IIRC, are two 3/4 ply boards with 2 x 4 studs on 8 inch centers, that extend 4 feet from the corner and just as tall as the Klipschorns.

[thebes]

I hate when I lose my high

 

Takes the quality of your stereo system down a notch or two, doesn't it?

 

Thanks for the responses guys. Kinda figured that the situation would be, and I am familiar with PWK's design for the false corner. Now all I need is some corner horns to experiment with. Anybody want to trade Cornwalls for Khorns even up?

[Marvel]

In his first paper, Paul said that a horn operated close to a wall and floor junction would have a length of about 80 inches, and that operation in a corner allows the path to be half that much. The logic of the corner horn mouth size is well understood. "Mirror" images "effectively multiply" the size by 8. This assumes that all the boundaries are perfectly rigid, which in practice most domestic walls are not. A non-rigid boundary is a "cloudy mirror" and maybe we can get a 1/6 reduction in effective size, but in most circumstances, I feel that the "gospel" of "8 times" is wishful thinking. 

Okay, I haven't studied papers enough but I can't see the"multiply by 8" at all. The horn length isn't longer and the mouth is only doubled. Unless you do something weird with the wide spread oh the two mouth exits.

Bruce