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In Reply to: overlooked topic posted by toxicport.e on January 27, 2007 at 22:23:42:
Would that paper be "Optimal Loudspeaker Directional Patterns", by James M. Kates (AES, 1980)?If it was some other paper let me know, so I can try to find it (looked after reading your post but nothing leaped out at me). I don't recall Kates specifying 90 degrees as a generally optimum angle, but will have to re-read the paper.
I think 90 degrees is a darn good angle, but it would be very nice to have an AES paper backing up that opinion.
Thanks!
Follow Ups:
Hello,Here is an excerpt from the conclusion of the paper:
Optimum Loudspeaker Directional Patterns
JAES Volume 28 Number 11 pp. 787-794; November 1980
Author: Kates, James M."The result of our analysis is that loudspeakers should be more directional than conventional design practice.
For a listener located at a distance that is 0,5 to 2 times the loudspeaker separation, a cosine pattern (front lobe) will give good localization performance. For a listener closer to the loudspeakers, the loudspeaker beam-widths must be narrower to give accurate localization over the range of listener positions. Combined with the increase in directionality is a requirement that the loudspeakers be angled in toward the listening area so that the respective maxima of the directional patterns are aimed at the diametrically opposite ends of the listener locus".Best regards from Paris,
Jean-Michel Le Cléac'h
Merci beaucoup, Jean-Michel!I remember reading that part, and frankly it was over my head. I don't know what a "cosine pattern" would look like. Can you translate that into English (or even French)?
What sort of radiation pattern characteristics does a LeCleac'h horn have?
Best regards from somewhere in Idaho,
Hello Duke,
Here is a measurement I did of the directivity of the horn "Jerzual 321" built by my friend Marco Henry (the horn is equipped with the TAD TD-2001 compression driver )
http://www.musique-concrete.com/mesures/321direct.gif
You can find a simuation, performed by John Sheerin using a finite elements method, of a 275Hz horn having a so-called "Le Cléac'h profile" :
http://ldsg.snippets.org/HORNS/images/polarmaps/LeCleach_275_flat_wave.jpg
Please notice that John Sheerin explains that
"For example, on this Le Cleac'h horn the on-axis response falls off at high frequencies but there are lobes just off center that stay at the same 0dB level. In reality, you might choose to eq the on-axis response to be flat. However, there would still be lobes at those frequencies and angles, and they would be the greater in amplitude compared to the on-axis response by the amount shown in these graphs. "
In fact, I never could find those lobes on my own measurements and I am convinced that they are probably FEM calculation artefacts...
Also on axis the frequency response doesn't fall that much as you can see on my mesurements:
on a 320Hz horn
http://www.musique-concrete.com/mesures/321gain.gif
on a 500Hz horn
http://www.musique-concrete.com/mesures/501gain.gif
You can see in the conclusion of the previously mentionned paper from JAES that: " For a listener closer to the loudspeakers, the loudspeaker beam-widths must be narrower to give accurate localization over the range of listener positions. Combined with the increase in directionality is a requirement..."
I agree with that conclusion an increase of directivity is benefitial to a quasi 3D image.
I do myself a difference between "3D image" and "soundstaging".
What I am looking for is a 3D image with a precise localisation and shape of the instruments. (This requires phase stereophony, a rare beast with commercial recording)
Soundstaging is generally never fully recorded in one record due to the limitations of the sterophonic process (unability to record a complete sound field). Specialised music lovers may try to retrieve a part of the missing information using some acoustic treatment of their listening room, playing with constant directivity loudspeakers, more or less reflecting zones in the listening room, etc... Some of them obtain convincing results in having the atmosphere of a jazz cavern by example, but don't play any symphonic music in the same listening room or a choir...
I prefer for myself to accept the limitations of sterophony and privilegiate a better focusing and a better "3D image" at the detriment of the soudstage...
(PS: convincing soundstages and atmospheres of different styles may be reproduced in the same listening room with very good -but rare- multichannels SACD records...)
Best regards from Paris
Jean-Michel Le Cléac'h
Bonjour Jean-Michel,Thanks for the additional information and links.
The high frequency on-axis hole in John Sheerin's simulation is something that I have seen before, but depicted in a different format.
Earl Geddes told me that a round horn or waveguide will always have an on-axis hole in the frequency response. He said it has to do with an axi-symmetric reflection, and is not there in rectangular or elliptical devices.
You can see this sort of on-axis hole by clicking on the link below and scrolling down to page 11. This is a speaker that uses an oblate spheroid waveguide, which is an axi-symmetric device. There you will see a family of off-axis curves, which gives a somewhat less complete, but easier-for-the-layman-to-understand, presentation of the information contained in a polar map.
The curves are taken at 7.5 degree intervals. Notice that the on-axis curve dips below the first few off-axis curves in the range between about 4.5kHz and 9 kHz. I think this is the same phenomenon shown in John Sheerin's simulation, which doesn't go high enough in frequency to show that the on-axis hole closes back up at higher frequencies.
I also have measurements taken of a much smaller round waveguide than the one in Earl's paper, and it shows a similar on-axis hole in the response but at a higher frequency.
Anyway, I prefer to listen to round horns and waveguides from slightly off-axis, as the response is usually smoother there than directly on-axis. Again John Sheerin's simulation doesn't go up high enough to fully justify that conclusion regarding the LeCleac'h horn, but it looks like the trend is there.
Best regards,
Duke
Hello Duke,The strange thing about what you said about Earl Geddes's Summa is that Earl Geddes developped his waveguide in order to avoid those HOMs that seemingly are at the origin of the hole you mentionned.
For what it seems the waveguide shape succeed only partly to reduce those HOM. I think this is one of the reasons why, more recently, Earl Geddes filled entirely the waveguide with some foam.
Did the mesurement is done with the foam or not? If the answer is yes, most probably the origin of the hole on axis in the directivity response is yet to find...
Best regards from Paris,
Bonjour Jean-Michel,It is my understanding that the on-axis hole is an artifact of a virtually inevitable mouth reflection. It would take an infinite radius "lip" at the mouth to totally eliminate it. A larger radius "lip" does produce a measurably smaller on-axis hole.
The measurements I linked to are with the HOM[higher order mode]-absorbing refractive foam insert in place. The on-axis dip isn't a HOM artifact, so design features that address HOMs have no effect on it.
The Summa is designed to be listened to somewhat off-axis, which has other benefits aside from smoother first-arrival sound - but that's another story.
A la prochaine,
Hello Duke,IMHO: in the case of a waveguide filled by foam, the origin of such a hole of 3 DB cannot be a reflection of waves from the mouth.
Best regards from Paris,
Jean-Michel Le Cléac'h
In my opinion you are incorrect.No mouth can be made such that there is no diffraction and as such this diffraction will meet at the axis in anti-phase at some point. This causes a hole. It is not an HOM effect and is unaffected by the foam. A larger mouth radius makes it smaler and a larger mouth makes it go lower in frequency.
I have tested this all and it is quite correct.
I have also stated numerous times that no waveguide has NO HOM, only some have a lot less. The drivers create HOM even if the waveguide does not.
Duke is dead right in what he says. He has learned his lessons well.
Earl Geddes
Hello Jean-Michel,What do you believe causes the on-axis response hole?
Do you believe it has one cause in the case of your horn (as modelled by John Sheerin), and another cause in the case of Earl's waveguide?
Merci,
Duke,As I told you I don't see myself such a hole on my measurements some of them can be seen at:
http://www.musique-concrete.com/mesures/321direct.gif
due to the precision of the measurement, if this axial hole
exists it is far less than 0.5dB with my horn.I don't want to criticize John Sheerin simulations, there are very enlightning about differences between horn shapes. A limitation to those simulations is that they are only performed in 2D not 3 D (someone will surely say this has a minor influence in the case of an axial horn...)
You know that the profile of horns calculated with my method is curving back at the mouth. Diffraction is therefore reduced. This explains both the very pure wave shape on impulse reponse and the very smooth electric impedance curve.
Most waveguides are cut too sharply at the mouth and for sure diffraction will occur with more evidence.
Best regards from Paris
Jean-Michel Le Cléac'h
I really cant remember much about it,Il probably look it up soon.When I do,il tell you if thats the one!Regards
Mike.e
There are three constants in life: death, taxes, and the inevitability of a (speaker)wire thread being closed -SY
Hello,May be the paper you were talking about is:
J. Rodenas, R.M. Aarts, and A.J.E.M. Janssen.
Derivation of an optimal directivity pattern for sweet spot widening in stereo sound reproduction.
J. Acoust. Soc. Am., 113(1):267-278, 2003.http://www.extra.research.philips.com/hera/people/aarts/papers/aar03c.pdf
That's an interesting pattren but the method requires an array of 2 loudspeakers + FIR.
Best regards from Paris,
Jean-Michel Le Cléac'h
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