INFO NEEDED ON BEVERIDGE-TYPE "ACOUSTICAL LENS" FOR ESLs

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I am seeking information on the design of Beveridge-type "acoustical
lens/waveguide" for electrostatic loudspeakers. It has the property
of taking a line source and bending it 180 degrees to creat a
cylindrical wavefront. Of interest are dimensions of waveguide,
curves and'or plots for shaping the various channels to bend the
soundwave(and even the purchase of a"salvaged" or even ruined lens
from an old B. speaker). Also, printed information and papers on this
item available in the public domain.

Has anyone applied this inventor's cylindrical wavefront concept to
other commercial electrostatic loudspeakers or has fabricated such a
"lens" for personal use?

All leads, ideas, suggestions and material greatly welcomed, with my
thanks:
Bernard H. Merems

 

I'm a little confused, because a line source naturally produces a
cylindrical wavefront.

The Quad ESL-63, http://www.euronet.nl/users/temagm/audio/esl63.htm, and
http://www.quadesl.com/quad_main.shtml, has a similar goal of wavefront
modification. It simulates a point source.

 

I'm a little confused, because a line source naturally produces a
cylindrical wavefront. Guess I shoulda finished college instead of spending
12 years yanking my pud.

 

I am seeking information on the design of Beveridge-type "acoustical

Just what I was about to post.

The drivers in the Beveridge were large ES panels of an unusual design using
ceramic materials (IIRC). They weren't very reliable.


No.

There's little point to it, as you can buy electrostatic speakers with curved
panels (M-L). Or electrostatic speakers that produce a (more or less) spherical
wavefront (QUAD).

 

This is the same kind of scientific bulldoody that caused Drexel to eject my
son from their university. It took them a while, but they finally learned
that pretty much everything that comes out of Bob's mouth is nonsense.


--
Sylvan Morein, DDS

Failed 50 year old loser student.
Failed Temple University
Ejected from Grad program after seven years
Ejected from Drexel University after dissertation judged "bullshit nonsense"
Sued Drexel and Lost
Even took it to the Supreme Court, but they laughed at me!
But I get even with studentsandthelaw.org my harassment site.
My poor jew mother Jane Morein died with a broken heart, watching this
poor twisted loser fail at everything I've ever done.
Daddy Sylvan Morein, who studied hard and became a fair to middlin' dentist,
is now stuck at home with his loser son; unwanted by life or any of the
relatives.
But I've discovered at last my calling: INTERNET WACKO!



Man, am I a Loser!

 

If this is the case, one does not hear any part of the direct-field
signal coming from a long but still finite-length line except that
which is at the same height as one's ears. Virtually all of the
remaining direct signals can only be heard after they are reflected
from room boundaries. The direct-field signal would have to be very
weak in this case. This assumes, of course, that what you say is true
and that there are no direct-field sounds reaching the listener from
anywhere on the line but the part at the same height as his ears.

Worse, if the ear height is not precisely in the vertical center of
the line the frequency response goes to hell even more emphatically.
Still more worse, the situation is amplified if the listener is well
inside of the critical distance, and with fairly long or even
infinitely long, line sources it is nearly (or in the case if
infinitely long lines, absolutely) impossible to get to the
reverberant-dominated side of the critical distance in any normal
listening room. The line-source listener is stuck in the direct field
from the bass range on up, and that direct-field sound is dominated by
comb-filtering effects that are the result of the direct signals from
the line not getting to the ears in a coherent manner.

When Lipshitz did his work, he mathematically calculated just what a
line-source radiator would do, and his calculated curves virtually
always (depending upon the length of the line and the distance to the
measurement microphone) were flat to a point, with a roll off and
sever comb filtering above a certain midrange point. The line both
generated those filtering artifacts and also lost direct-field
radiation efficiency.

If the delayed-signal artifacts from a large and vertically long
radiating source that is not the same distance from the ears over its
surface area are not what is causing those comb-filtering artifacts,
just what is causing them? Lipshitz concluded that long line-source
radiators could not generate a flat first-arrival signal in most
listening rooms. They might be able to produce a flat reverberant
signal in relatively reverberant rooms, but their direct-field
behavior would be anything but exemplary, and since the direct-field
dominates what is heard with such systems the smoother
reverberant-field performance would not count for much. Ironically, it
is their direct-field performance that seems to captivate those who
admire such speakers.

So I hear. However, this would mandate a smoothly reflective ceiling
and floor. While a ceiling of this kind is not uncommon, a carpeted
floor would not allow for a simulated infinite line to form. Hence,
with a carpeted room we are back to a finite-length line and the
problems Lipshitz outlined again make themselves known.

Howard Ferstler

 

Most of the explanations posted here about the errors or deficiencies of line
sources are incorrect or misleading. This is especially true when considering
the interaction of the speaker with the room. Many designers of cone-type
speakers (especially those from New England) would like you believe that the way
a speaker interacts with the room is the single most-important factor in sound
quality, and, therefore, minimizing room interaction is the only way to get good
sound.

This is quite untrue. I've owned enough different speakers and lived in enough
different places to know that, if basic speaker design -- good drivers, correct
crossover points with appropriate slopes, proper consideration of driver
alignment and diffraction, dead cabinets, etc -- are not taken into account, no
amount of attention to the room interaction with the is going to turn a poor
speaker design into a good one.

With respect to line sources...

If the ceiling and floor are reflective, a line radiator extending from floor to
ceiling appears to be three times as long as its physical length (not infinitely
longer). This reduces the frequency at which the line source starts to
vertically "beam" by 2/3.

At wavelengths shorter than the line source's effective length, the vertical
component of the wavefront is planar. As the wavelengths become comparable in
length, the radiator starts "beaming" until at very long wavelengths, the
wavefront is spherical.

The same rule applies to the driver's lateral dispersion, except that its width
is usually smaller than the wavelengths being reproduced. At "low" frequencies
a circular wavefront is projected. As the wavelength approaches the width of the
driver, the wavefront becomes increasingly "lobed" and "beamy." At wavelengths
significantly shorter than the width, the lateral wavefront becomes planar.

The inability to build a line source with ideal (ie, vertically planar,
laterally spherical) radiation at all frequencies is often used as an excuse to
dismiss planar speakers. "Unfortunately," properly designed planar speakers will
_always_ be superior to cone-type speakers, for exactly the same reasons that
condenser and ribbon mics will _always_ outperform dynamic mics. "I kinna change
the laws of physics!"

To wit... In order for a driver to be small enough for broad dispersion in all
directions (and thus be relatively free of beaming at some frequencies), the
driver can no longer be sufficiently low in unit mass to be highly damped by the
air load. QED.

I owned Acoustat Sixes and currently own Apogee Divas. I invite Stanley
Lipshitz, Howard Ferstler, et al., to visit, with their own recordings, and
decide whether a good planar speaker more-accurately reproduces what's on the
recording than a cone speaker. Because that's what high fidelity is all about.

Not so. The planar wavefront reaching his ears is the result of destructive
interference, considering the line as a huge number of Huygens spherical
radiators.

I'm very curious as to where this "certain midrange point" occurs. If one places
8' Acoustats in an uncarpeted room, they are "acoustically" 24' tall.
Wavelength-wise, that's about 50 Hz. By the time you get to 200 Hz or so, you
should be well within the region where the drivers project a planar wavefront.

There are no delayed-signal artifacts. Read your Huygens.

One of the advantages of a line-source radiator is that it converts a liability
into an asset. It "uses" the ceiling and floor reflections to cancel out the
spherical radiation components that _would_ produce combing effects.

You have hit the nail on the head.

Why do you think people who own electrostatic and orthodynamic (often
incorrectly called ribbon) speakers are so fond of them? It isn't because they
have some characteristic sound that appeals to the listener's taste, but because
they have signficantly LESS "sound" of their own. THEY COLOR THE SOUND LESS THAN
CONE SPEAKERS, for well-understood reasons. This is plainly audible to anyone
with normal hearing, who knows what live sound "sounds like." And no amount of
mathematical posturing is going to change that.

I take a certain snide pleasure in realizing that the people who believe the
mathematics -- rather than what they hear -- are denying themselves a great deal
of musical pleasure. Well... "I told you so."

 

Line sources will by their very nature, and depending upon the

Please define what _you_ mean by comb filtering.

That has nothing whatever to do with what I said. Read it again.

An interesting, theory, as the larger a driver is with respect to the wavelength
it reproduces, the greater its efficiency (in the sense of making a good
impendance match with the air).

Then it's not a line source, so how does it demonstrate your argument.

The point of a line source extending from floor to ceiling is that it uses the
ceiling and floor reflections to eliminate the comb-filtering effects that (I
think) you're complaining about. At least at those frequencies where the
wavelength is shorter than the effective length of the line.

Uh -- yes, they do, if by accuracy you mean very low coloration. There is a
qualitative difference that does not show up in a simple frequency-response
curve.

This terrible fallacy needs to be put to death once and for all.

A line source "longer" than the wavelengths being reproduced projects a
vertically planar wavefront. The flatness of wavefront is due to destructive
interference among the spherical components of the source (considering the
sources as made of gazillions of spherical point radiators).

If I am "missing" something here, please tell me what it its.

You're missing the point completely. In a planar wavefront, the parts of the
wavefront "above" and "below" the listener never reach his ears at all (until
they're reflected off the side and rear walls)! That's precisely the point.

But -- DUH! DUH, DUH, DUH! That's true for ANY driver with a broad radiation
pattern. The "direct" output of a 2" dome midrange driver that doesn't reach the
listener strikes the ceiling, walls, and floor and combs with the direct sound
(if the sound lasts long enough).

One of the biggest arguments in favor of a floor-to-ceiling line source (whether
made of planar or cone/dome drivers) is that it _minimizes_ these sorts of
effects. I think you have this exactly backwards.

NO, YOU DON'T.

WRONG, WRONG, WRONG. You don't understand what a planar source it. See above.

Now I understand the fallacy in Mr. Lipshitz's calculations.

Not in the region where the source is longer than the wavelengths being
reproduced.

He believes that because his reasoning -- and consequent mathematical model --
is wrong.

Yes -- if you use invalid reasoning to create the mathematical model.

 

A choppy response curve that is the result of suckout cancellations.
It may have nothing to do with any inherent problems with a
line-source driver itself, but instead simply is a result of the fact
that signals from the large, flat surface area of the line cannot all
get to the ears simultaneously. Because of this, at certain (but not
all) frequencies you get cancellation nulls.

I am at a loss. Does the driver have to be damped by the air load? I
was under the impression that damping is mostly accomplished by
electrical characteristics and the suspension materials in the driver.
As long as a driver can deliver flat output over its
crossover-controlled bandwidth it will be sufficiently damped.

Actually, its efficiency is reduced, due to the increased mass and the
beaming and comb-filtering artifacts.

Actually, it was two lines, with a short tweeter line (about a foot as
I recall) and a midrange line somewhat longer (about two feet). The
tweeter line was quite narrow (about half an inch) and the midrange
line was considerably wider (several inches). It certainly qualifies
as a line-source system. Where it surpasses systems that have very
long lines (or one long line) involves the shorter distances from each
segment of the line to the ears. At longer wavelengths this is no big
deal, but a system with a very long line that is trying to reproduce
higher frequencies there are going to be powerful comb-filtering
effects. This has nothing to do with driver design and everything to
do with the size of the driver in relation to the wavelengths handled.

I do not know how this can happen. The line cannot be the same
distance from the ears over its full length. Because of this, when
some signals from certain parts of the line are hitting the ears
signals from other parts of the line are going to be hitting it
anywhere from slightly out of phase to 180 out. As the frequency
changes the null points will change, since although the speed of sound
remains constant the out-of-phase points will be different at
different frequencies.

This is the opposite from the way it is. Where the wavelengths are
longer than the line the time between negative and positive sweeps is
so long that for all intents and purposes the line is in phase over
its entire length. However, at shorter wavelengths the the phase
characteristics gradually shift as you move away from any point of the
line that is not the closest to the ears. The gradual delay causes the
incoming signals to gradually go out of phase with those that are
coming from the closest point on the line. As you move further up or
down the line they go back into phase (but be 360 degrees out,
actually) and then further up or down they will start going back out
of phase.

Look at it this way. Let's say that you are hearing a 5 kHz signal
coming from the center of the line. As you move up and down the line
several inches the source is further from your ears and so that part
of the signal is delayed slightly in time and the signal is not fully
in phase with the signal coming from the closest part of the line. At
some point up and down the line that signal will be 180 out with the
part coming from the closest section, and so you would get a
substantial cancellation dip. At higher or lower frequencies the dip
would be elsewhere, and the series of cancellation dips makes the
at-ear response curve resemble a comb filter.

What causes this? Certainly, standard HD and other distortion products
are sufficiently low with dynamic speakers. And many dynamic speakers
can go a LOT lower in frequency and with considerably less distortion
down really low. A subwoofer will correct this limitation with
flat-panel jobs, of course, and many have dynamic woofers, anyway.

Actually, my theory is that some listeners rather like the choppy
frequency response (which makes some frequencies stand out) and treble
rolloff. They also like the reduced reverberant-field strength and the
fact that most of what reverberant-field strength there is is
reflected from the front wall and therefore somewhat delayed. This is
a pleasant artifact that has nothing to do with the reproduction
issue.

OK, are you saying that you do not hear all portions of the line as a
direct-field signal? If that is the case, the only part of the line
you hear is that directly opposite the ears, with all the other sound
from the line passing over your head or under it. I do not think you
will find this to be possible. Rather, you hear the whole line, but
because the signals coming from the various parts of it are delayed in
time in relation to the part of the line that is closest to the ears,
those phase-generated comb-filtering effects are very real.

Well, if the line were made up of those gazillions of point radiators
and we suddenly and temporarily shut down all but the top and bottom
you would still hear the top and bottom playing. Turn the central
section back on and you would still here them, but this time, because
they are different distances from the ears, much of what they
reproduce would be phase shifted from what is coming from the nearer
parts of the line. The line cannot generate a wavefront that can be
heard simultaneously from its entire surface.

Well, you are basically saying that segments above and below the
nearest part of the line have their signals cancelled out by those
central signals. This is true, but only at certain frequency points.
Hence the comb-filtering effect.

But the direct-field signal does get there as a coherent wavefront.
Sure, reflected sound arrives later, but those reflected signals are
coming from all over and they also are continuously delayed over a
long period of time, relatively. With the line-source radiator, you
are getting a comb-filtered direct-field signal. Except at the
crossover points, you do not get that with conventional systems.

Nope. The line cannot be the same distance from the ears over its
entire length. Sure, some of the output is nulled by cancellation
effects. However, some parts of the line are in phase with the nearest
segment (360 degrees, 720 degrees out, etc.) and those frequencies are
what make up the tops of the comb-filtering curve. The nulls make up
the bottoms of the dips. All points in between are the result of
partial cancellations. Lipshitz illustrated this clearly in his AES
paper.

You do at some frequencies. The line cannot cancel all the sound
coming from the segments above and below the nearest section. Only
some parts points can be cancelled. The result is the comb filter. The
dips and peaks only disappear where the wavelengths are longer than
the length of the line. This generally means the lower midrange. Above
that you get comb filtering. Lipshitz actually outlined this pretty
clearly.

Right, right right. The cancellation effect you note does not happen
continuously and uniformly at all frequencies. It happens in a choppy
manner.

Read his article. I gave a reference. Actually, the source of the
article was the result of a debate he had with Roy Allison in the
Boston Audio Society magazine "Speaker." Allison pointed out the
deficiencies with long, uncontrolled length line sources and Lipshitz
contested that point of view. He then went on to do some research and
his AES paper ended up supporting what Allison had said.

Backwards. The comb-filtering effects disappear at lower frequencies,
but become more emphatic as the frequency climbs.

I find it interesting that you contest the findings of an audio icon
like Dr. Lipshitz, particularly since you have not read his paper.
Contact the AES for a copy of the lecture and demonstration he
presented.

Get a copy of the Lipshitz paper (with John Vanderkooy): "Acoustic
Radiation of Line Sources of Finite Length," presented at the 81st AES
convention, November, 1986. Available from the AES as preprint 2417
(D-4).

Howard Ferstler

 

Cancellations do occur, but they do not occur uniformly. That is the
reason for the comb-filtering effects that Lipshitz noted.

In addition, in order to get even the level of performance that
Lipshitz noted the listener's ears must be at the vertical center of
the line. If you move above or below that point problems really do
arise, and the direct-field balance changes appreciably.

If what you say about the "Huygensian" interference effects were true
the system would sound pretty much the same at all listening heights,
because all you would ever hear as a first-arrival signal would be the
part of the line at that same height. This obviously does not happen.
The sound changes as height changes, because the cancellation nulls
change as the distance to the various parts of the line change.

Howard Ferstler

 

Bernard, et al.

I posed this question to one of the designer's sons (Ross Beveridge),
whom I had gone to school with. He has a short not-highly-technical
writeup on the history and differences among the different models.
See
http://www.cs.colostate.edu/~ross/personal/harold/BevHistory2.html

Ross tells me that his brother, Rick Beveridge is carrying on
development of this type of speaker. A google search turns up:

http://www.beveridge-audio.com/Front_Page.htm

which has at least some details, e.g. in
http://www.beveridge-audio.com/White_Paper.htm

I haven't explored the site beyond a quick look.
I hope the above links were not already known/obvious (I don't see
them mentioned elsewhere in the thread as of 27Aug. Did I miss
something?)
Hope this helps.

-- Larry

 

Howard Ferstler wrote...

A PhD does not prove anything, other than having convinced a group of other PhDs
that your dissertation has value.

Neither Dr. Lipshitz nor Dr. Toole have ever struck me as having had a profound
insight into anything.

 

There are times when a PhD means plenty, and it is easy for someone
who has not gone through the motions to say that the enterprise was
easy and/or meaningless.

Incidentally, I never brought Toole up, but one thing that he has done
that you have not done is very serious and carefully documented
research involving the performance of loudspeaker systems. While I do
not agree with all of his findings (I put greater stock in overall
power response than he does, and I am convinced that 1/3-octave
analysis is detailed enough for practical purposes), I sure as hell
would not say that he lacks profound insight. His AES papers and
articles (and his pop-magazine writings, too) are consistantly
insightful and have been very useful to quite a number of people -
including me.

Admittedly, it is tough dealing with what individuals like Lipshitz
and Toole say when one is wrapped around a non-yielding belief system
that demands that certain aspects of audio be mysterious and esoteric.

Enjoy your speakers.

Howard Ferstler

 

Howard Ferstler wrote...

You obviously don't know me well. It is not I who has the "unyielding belief
system." Indeed, I've been griping for years about the lack of real curiosity (a
trait most so-called "scientists" are seriously lacking) and good, hard science
in the audio industry.

Neither Lipshitz nor Toole have done anything that contributes to a better
understanding of how to achieve accurate, realistic sound reproduction.

About 15 years ago I sat in on Dr. Toole's stupid, worthless speaker-evaluation
tests. All these do is tell the researcher what the listener likes. I've read
some of his papers, and consider the results to be "pre-ordained," based on
invalid views of what makes a "good" speaker and how speakers should be
measured.

To put it bluntly -- Drs. Lipshitz and Toole are the people with the
"non-yielding belief system." They think they can use "scientific" methods to
determine "the truth," without bothering to consider subjective reactions --
especially when those reactions go against what they already "know" to be
correct.

Mr. Ferstler, haven't you EVER questioned ANYTHING in your life? Haven't you
ever longed to UNDERSTAND something? Or do you just believe what "science" tells
you?

 

Looks to me as if this is precisely what high-end enthusiasts should
prefer. Are you saying that products should not be built to satisfy
what listeners like?

Actually, I tend to favor the Consumer Reports approach. They do favor
speaker power response, and measure it very well, even if their report
printouts only show a very rough approximation of their actual
measurements. However, Toole does indeed use listener preference in
his research (as does Consumer Reports, interestingly enough), with
most listeners preferring speakers with very good detail and
soundstaging, and precise imaging from the sweet spot. (Consumer
Reports obviously has different listener-preference criteria.) I
cannot see how any serious high ender would prefer anything else.

At least they bother to do their hands-on research by means of the DBT
technique. Possibly, this approach is not "scientific" enough for you.
Perhaps you have another, more esoteric approach to product
evaluation.

As I noted, the Toole speaker tests concentrated primarily on listener
preferences and how they related to measurements. What he was after
was consistent results from listener to listener when it came to the
measured sound of preferred speaker systems. I think that given what
he considers important to good hi-fi sound reproduction, that approach
is about all you can do with speakers. The idea then is to see how
much alike the most-preferred speakers happen to be. That gives
manufacturers guidelines about how to use measurements to build good
speakers. Sounds like a fine idea to me. Objective criteria determined
by subjective preferences.

Sure. I questioned your ideas about line-source radiators. I regularly
question how assorted audio freaks can rhapsodize about the sound of
wires or well-built amps. I question how people can elevate personal
preferences to reference standards.

There is a big difference between wanting to understand something and
wanting to believe in something. I certainly do not believe the
mountain of mumbo jumbo that passes for knowledge inside the realm of
tweako-freako audio.

Well, since you distrust "science," just what do you use as an
epistemological reference when it comes to evaluating audio gear?
Where do you get your standards?

Howard Ferstler

 

....

I'm glad you prefaced this with the 'intelligent and knowledgeable
listeners'. See below.

Nearly every week I'm stuck at a traffic signal and a sound-emitting
device on wheels, with its windows rolled down, sits next to me,
putting out noise of a thumpa-thumpa, expletive laden, fulla
distortion variety. Some folk apparently prefer sounds that ought
be banned by the Geneva Convention under the torture of civilian
section.

....

'flat-response, low-distortion, adequate bandwidth' seemed to be
the criteria most intended to be broken by the example I gave above.


....

Excellent parens to acknowledge that for some 'an audio system
is a musical instrument'. I would add that for some, an audio
system is an instrument of random annoyance to innocent bystanders.

....

Supports my comments above about drive-by blasterds (blastards? -
blasturds?)

....

George Christoph Lichtenberg (1742-1799) suggested such questioning
as essential.


(This thread has too many cross-postings - I don't know who it is
suitable for - I read only the sci.optics, so this could be way off
topic.)

 

A bit of research in the Netherlands about envoronmental noise
complaints. Number 1 was radio - above road, train or air traffic, or
industry. Not 'radio' and not 'hi fi' though.


Thomas