2" 223Hz tractrix horns

[djk]

"Would "any inside corner" refer to the square cross section of the
Edgar type wood horns that everyone (including me) is making lately?"

Yes, it does. For frequencies above 300hz (Fc) Dr Edgar make mainly round horns. The only rectangular models in his 'standard' lineup are 80hz (Fc) and lower.

"I
wouldn't think that wouldn't be a problem considering that microwave RF
transmission lines, that is waveguides, are usually rectangular in cross
section. They propagate energy nicely without reflections. The acoustic
wavelengths at high audio frequency are a fraction of an inch and so
are the electromagnetic wavelengths at microwave RF. Microwave
waveguides and horn throats are also around the same size. From what I
have seen, similarities like this usually illustrate similar
characteristics. I ask because I have a fair grasp of TEM mode stuff in
coaxial lines, but I never really understood waveguides. They are used
at frequencies that are well above the world of L-C filters so I never
bother to look into them. If there is a direct correlation between horns
and microwave waveguides, I need to do it."

Although there may be similarities, I'm not sure how they apply. All the microwave RF stuff I worked on was digital with error correction. The waveform could get badly mangled and still work. Audio seems to be touchy, if not harmonically related, things 50dB (or more) down can be audible.

Geddes goes so far as to stuff his round horns with foam to reduce reflections, even though they start out much better than rectangular horns. Many people have been using foam in HF horns for years, in bends, corners, and at the mouth.

I like the elliptical horns better than rectangular ones, they do sound different. Geddes did some for Adamson Acoustics (a while back before they went belly up) . I would have liked to have heard the 8" Kevlar compression driver on elliptical horn that ran down to 200hz, and the mating 2KhzHF elliptical with the TAD2001.

http://www2.link.langhofer.at/adamson/mh225.pdf

Bill Woods has a 500hz elliptical horn I would also like to hear.

http://www.acoustichorn.com/products/500/

(unfortunately I see no easy way to make these as a DIY project)

[Al Klappenberger]

"Would "any inside corner" refer to the square cross section of the
Edgar type wood horns that everyone (including me) is making lately?"

"Yes,
it does."

I thought you would say that. The slot used in constant directive horns is obviously and extreme case of a square cross-section. I think it's pretty well known what they can sound like.

I have noticed that Dr. Edgar also makes the salad-bowl round horns. The horn I copied from his article doesn't use a compression driver but what looks like typical direct radiator drivers instead. That makes for a shorter horn because of a very large throat. Maybe that's for good reason! A 2-inch horn with the same mouth size as a 1-inch will be a lot shorter. That should be and advantage even without considering the lower pressure of a 2 inch diameter throat for the same SPL.

The only way I could see to make a horn with an elliptical shape would be to make it out of a pile of many layers each with a slightly larger size and pile them up. That wold not be easy even in an industrial setting! I suppose it's lucky for guys like me that the K400 is such and old design and very easy to improve on!

I was thinking more about the analogy of microwave guides to acoustic horns. I am not up on microwave wave guides at all, but the issue of "modes" comes in play. Also there is TE versus TM which I also don't really understand. All I have dealt with is the effects of reflections themselves (Smith chart) many times though. It's obvious to me what that could do inside a horn. In the acoustic situation, we are dealing with pressure waves in a medium that the wave guide is already "primed" with (air). In a microwave guide all the energy is electrostatic and electromagnetic even though the air is still there. The air is just in the way! In a lot of cases it's replaced with dry nitrogen to reduce losses. This is even done in coaxial lines, like the feed-lines to the antenna of a TV station. My gut hunch is that there has to be some parallels though. Microwave horns fed by waveguides definitely exist!

AL K.

[Bacek]

>I would have liked to have heard the 8" Kevlar compression driver on
elliptical horn that ran down to 200hz, and the mating 2KhzHF elliptical
with the TAD2001.

My friend is preparing for me constant coverage, elliptical horn for APT-50. I will see/measure if they are better than usual Crites like horn in this place. We are also planing some surgery on phase plug to see if we can squeeze some more from apt.

[DrWho]

"It is concluded that the reflections from the mouth
termination of long horns is responsible for the characteristic sound
and that for studio monitor applications, a midrange horn should have a
length not exceeding 340 mm and should be free of flare
discontinuities."

There's nothing magical that happens at 340mm...exact dimensions should always be referenced to the bandwidth, and even then...there's a billion other factors that could make that magical number shift around too.

When
it comes to reflections in a horn, the phase plug plays a huge role.
All of the horn expansion equations make various assumptions about the
manner in which the wave propogates through the horn, which means they
also start with the shape of the wavefront that enters the throat.
Ideally, one would know exactly the wavefront shape at the throat of
the horn and then choose a custom flare rate that provided the best
impedance match. There's also the issue that the wavefront shape
naturally changes with frequency. Phase plugs are there to minimize the
frequency shifting effects and control the wavefront shape over a much
wider bandwidth. All that to say, randomly swapping out horns without
regard to the driver and phase plug isn't demonstrating good system
level design, and the conclusions from such an approach are a bit
misleading in my mind.

Along those lines, throwing away energy (by adding foam inside a
horn) seems like more of a band-aid than a direct solution to an actual
problem.

And just a side comment...I don't want my speakers to
sound like direct radiators. I would much rather them be true to the
original source. I don't disagree that horns can add their own imprint
to the sound, but so do direct radiators (and to a much larger extent
in my opinion)...

[djk]

The 340mm figure was arrived at by listening to about 360 horn and driver combinations.

"I don't want my speakers to
sound like direct radiators"

Is the glass half empty, or half full? The JBL 2370 horn sounded almost identical to the Audax PR17 direct radiator. This statement could have been rephrased to the Audax PR17 direct radiator sounded almost identical to the JBL 2370 horn. Would this make you happier?

I suggest you read the entire thesis (or at least the excerpt in Speaker
Builder) to understand Holland's remarks in context.

"My friend is preparing for me constant coverage, elliptical horn for
APT-50."

$27 http://www.loudspeakersplus.com/images/XT120.pdf

$69 elliptical tractrix http://www.usspeaker.com/homepage.htm

$7 http://www.parts-express.com/pe/showdetl.cfm?Partnumber=270-314

[Guest David H]

Is the glass half empty, or half full?

From an engineering standpoint, the glass is twice the size it need to be.

[DrWho]

The 340mm figure was arrived at by listening to about 360 horn and driver combinations.

Sounds like shooting in the dark to me...shotgun engineering doesn't usually yield meaningful results.

Is the glass half empty, or half full? The JBL 2370 horn
sounded almost identical to the Audax PR17 direct radiator. This
statement could have been rephrased to the Audax PR17 direct radiator
sounded almost identical to the JBL 2370 horn. Would this make you
happier?

Well I've not heard the PR17, but if what you say is true, then the PR17 must not sound very good...

The
JBL 2370 is a pretty nasty horn, with some not so great specs either.
In fact, JBL's tech note on the horn starts off with an introduction
explaining that performance was sacrificed for space considerations:
http://www.jblpro.com/pub/technote/tn_v1n07.pdf

I believe horns are capable of sounding better than direct
radiators which is why I wouldn't want a horn to sound like one. I've heard horns that sound much better than the 2370, so I
think it's fair to claim that I've heard horns that sound better than
the PR17 as well. Why would I want to mess with those horns and settle for less?

$27 http://www.loudspeakersplus.com/images/XT120.pdf

I've got a pair of the XT120 in my listening room right now. I
think they sound very good, but I also think they beam way too much and
the off-axis
polars are extremely difficult to deal with when voicing the system. I've spent
several days voicing these horns and still find myself wanting to tweak
it. It's actually the longest I've spent voicing a horn. I've had no
where near the difficulty with the K402 (30 minutes), K510 (1 hour), JBL2380 (3 hours) or
the stock tractrix
in my Chorus II's (1 day).

I think the XT120 is an incredible buy
for the price, but it's not very constant coverage and I don't think it
demontrates that elliptical is always better than rectangular (whatever the
reasons may be). Of course it's also a $27 horn, so it's probably to be expected that it can't compete with $800+ horns.

[Bacek]

I've got a pair of the XT120 in my listening room right now. I
think they sound very good, but I also think they beam way too much and
the off-axis
polars are extremely difficult to deal with when voicing the system.

Thats why our goal is to create horn which wont beam too much, and has as much as possible constant coverage of listening area.

[Al Klappenberger]

Dennis,

I am courious if you have any idea how one could measure reflections in a horn. In an RF transmission line, reflections show themselves as impedance variations at the source. Dr. Edgar has a plot of throat impedance in his article on the midrange horn. Maybe this is how to measure it. Do you know how to measure throat impedance and what it should look like?

When I was testing my Trachorn prototype early on, I inserted my calibrated mike right into the throat through a hole in the side as close to the driver as I couldget it. It showed me where the horn rolled off and the SPL in the throat, but that's all I could get from it.

Al K.

[pocket.change]

I'm another that would like to ask "how does it sound" ???

The measurement pathology has its place but I've never seen a dyno

make a pass at a drag strip yet and bet no one else has either.

IMO the proof is "listening exclusively" with how it sounds [to me].

How a transducer renders audio is a pointless exercise (but it is a nice way to enjoy an afternoon)

especially if [i'm] being plyed with [your] Glenfiddich and (good) Cigar.

[DrWho]

The measurement pathology has its place but I've never seen a dyno make a pass at a drag strip yet and bet no one else has either.

But the best teams ARE using a dyno to help them get down the drag strip faster. The engine analogy is perfect for audio because there are so many variables that fight against each other when trying to extract more performance. Rule of thumbs only get you so far. The art of engineering is having such a strong handle on the physics, that one can intentionally fabricate that which produces the desired results...whether it be a fun to drive motor or a speaker playing good music.

I think knowing how a transducer renders audio is only pointless when you don't use it to the advantage of the music...

[djk]

"I'm another that would like to ask "how does it sound" ???"

Dangerous words on this forum.

"I am courious if you have any idea how one could measure reflections in a
horn."

There was an article in Speaker Builder
Magazine (Reflecting on Echoes and the Cepstrum, 1996) and Liberty
Instruments offered some dedicated test equipment.

http://www.libinst.com/cepst.htm

[Al Klappenberger]

Dennis,

"how does it sound" ?

This is a question I usually never answer. I'm just one person whit no better ear than anyone else. How something sounds to me is totally unimportant to anyone but me!

Thanks for the link. I have bookmarked it for later.

Al K.

[Al Klappenberger]

I took a quick look at the article. It looks like it confirms something I noticed when I was playing with the time-capture impulse stuff in my new FFT analyzer (HP 3563a) on a Crites CT125 tweeter sitting on the bench. With wide-band noise, there were a lot more fine ripples in the response curve than were there when I did the same curve using an impulse. I haven't had the new analyzer long enough to try any serious acoustic measurements with it. That is going to happen when I start playing with the 2-inch horn Bill Martinelli will be making for me shortly. Anyhow, the article seems to be looking at a reflection from outside the horn. How can you look at reflections within the horn? It seems you need to load the horn mouth with a plane wave tube that's 2 feet in diameter! What? A 50 gallon drum stuffed full of fiberglass insulation maybe!

Al K.

[ZombieWoof]

if u do the 55 gallon thing be sure & take some photo,it's a THAT I'd like to see moment !

[Al Klappenberger]

That would be a thing to behold, wouldn't it?

AL K.

[Guest David H]

Interesting idea AL, sonotube might be a better choice than the 55 gallon drum. Available in all sizes.

[Al Klappenberger]

I was kidding about the drum full of fiberglass, but it actually might be possible. Fiberglass insulation comes in rolls. The right length to make a roll that would fit snugly inside a plastic trash can might absorb enough sound to emulate an anechoic chamber. The lid could be cut to fit adapters for various horn mouths. It might be a way to measure the frequency response of a horn, but I don't know how that would help determine internal reflections though. My gut hunch is that it would still have to be computed from throat impedance somehow. Maybe from the difference between the driver response measured on a plane wave tube and the response measured somewhere with the horn terminated in the "trash-wave" tube! ?? ( [+o(] OH BROTHER!)

Al K.

[djk]

A plane wave tube has a conical absorbtive wedge with the narrow end at the entrance, getting full width at the far end.

[Al Klappenberger]

I think building a "proper" plane wave tube into a trash can is a bit of an oxymoron to begin with! Klipsch had several of them on the wall in the Hope factory when I was there for the first Tour-of-Hope. They were very long and made into a transparent tube. You could see the wedge shaped absorber inside. Microwave dummy loads for waveguides use the same wedge shaped absorber. Making one properly that could be used with a mid-range horn, would be a bit of a chore! The best you could hope for would be a jury rig!

Al K.

palnewt.pdf

palnewt.pdf

palnewt.pdf