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Re: Horn stuff and a post back for Tom

Posted by tomservo (M) on July 7, 2002 at 21:07:05

In Reply to: Horn stuff and a post back for Tom posted by Wayne Parham on July 7, 2002 at 19:07:49:

Hello again Tom!

You wrote:

> > Similarly When I have asked you at what point the explanation does
> > not make sense, you do not reply.

Again, my points of contention were as follows:

1. I do not believe that the Unity device acts as an acoustic transformer, i.e. provide horn loading over the two-decade
bandwidth claimed.

And by horn loading, how do you say that would be in evidence?
Is it not a comparison of acoustic power out vs electrical power in?

2. I do not believe the Unity device manages to correct the movement in time over the span of frequencies delivered by its
subsystems in this two-decade bandwidth, such that it acts as a point source. You have said, "generally a first or second
order crossover gives the best results" in this design. This very statement means that you had not figured out how to make this
thing work when you wrote it. Not only that, but the midrange drivers have wavelength-scale distances between them and
across their diaphragms, and they enter a common chamber.

The upper and lower drivers DO NOT operate simultaneously except in the crossover region.
By first measuring the inherent fixed delay of the upper and lower drivers, a physical off set can be found which compensates the FIXED delay
time of the driver.
That fixed delay is EXACTLY the same as if the driver were moved back in time the same amount.
Since the delays of all the drivers can be measured, the delays can be compensated by an inverse physical distance.
At the same time one must have that physical distance correspond to the frequency range within the horn that the drivers are intended to drive so
getting all of it tied in took some doing
Also, that physical distance can be used to compensate for the fact that high pass signals emerge from the crossover ahead of the low pass signal.
Since one knows the compression driver has the lowest delay and is ahead of the lf by so many degrees, it is logical to put it to the rear of the
lower section. At crossover, when the spacing and phase are correct, the compression driver sound arrives in synchrony with the lower system
and as one goes to either side of crossover, there is little or no change in phase or amplitude
Remember it is the response slopes of the horn sections that are what add, who's phase and time matters, not what ever electrical filter slope it
took to make it work
Put it this way, if you had 3 different lf point sources and you could put each one at exactly the same distance and then measured them and found
that they each had a separate delay, wouldn't this mean that the sound from an impulse would arrive at different times from each? YES, that is
what you just measured.
Knowing how much each delayed the signal, now can one account for that fixed delay by an adjustment of the physical position. YES
Distance = time

Put the fastest speaker at the end of a closed pipe and then had the second fastest in the middle and then the slowest one at the open end.
Now a signal fed to the system is radiated first by the fastest speaker and then as the wave travels down the pipe, it is reinforced by the second
speaker and then finally by the slowest speaker.
Get the idea.
In the Unity, there is never a situation where more than two ranges produce any single frequency and even then it is only in a narrow crossover
region where the time and phase right, hence the crossover does not show up.

I made those comments to you probably four times over the course of this dialog between us.

> > Measured results are the acoustic reality here, not
> > a "mathematical defense".

This assumes that the measurements are valid. I'd prefer to see the model, thank you very much.

Like I said look into AKABAK, you can model it your self or anything else you can think of.
Again, many of our users have test equipment and measure speakers, if anything people say our
stuff "does what it says", ask around.

> > You asked for the horn size where the sound "adds", I told you,
> > now you want to know the hole size, I'll tell you its 3/4 inch.

Actually, where time response is concerned, let me say this again:

I do not believe the Unity device manages to correct the movement in time over the span of frequencies delivered by
its subsystems in this two-decade bandwidth, such that it acts as a point source. You have said, "generally a first or
second order crossover gives the best results" in this design. This very statement means that you had not figured out
how to make this thing work when you wrote it. Not only that, but the midrange drivers have wavelength-scale
distances between them and across their diaphragms, and they enter a common chamber.

Fortunately the horn does not care what you believe and it does what it does.
What your not getting is that except for acoustic phase which is only + - 90 degrees if minimum phase, that the positions of the drivers do not
change with frequency, hence the static part can be removed by physical displacement.
Also that the electrical filter IS NOT the response and phase which matters, it is the acoustic response and phase, where your focussing is a step
removed from where the combining takes place.

There are a lot of problems that must be solved simultaneously to make this system act as a point source. The parallax
between the array of drivers in overlapping regions and the listening (or measurement) position must be small, in relation to the
wavelengths of the frequencies in these regions. Any orifice or exposed diaphragms must have cross-sections that are small in
relation to the wavelengths of the frequencies in these regions.

You couldn't be more right about difficult but I have partly by trial and error, partly by model found a set of "rules" which work.
You are right about dimension issues and so on but again that is what the design involves.
Go look at the polar plots our web site, they are 5 degree measurements, you do not see the radiation pattern of a complex source do you.

The three separate subsystems must be made to acoustically
move together towards a common center, and since phase is a function of frequency, fixed position offsets are difficult to
correct with electronic phase. All of these things must be done, simultaneously. That's essentially why there are filters that are
optimized for performance in the time domain and other different filters that are optimized for the frequency domain. You are
claiming that you system meets both conditions at once, and not only that but it provides acoustic loading of all three
subsystem's diaphragms over a two decade range.

Remember it could have any number of divisions and the arrangement allows that when any two ranges are interacting, that they are the correct
time and phase apart and that the region where they interact be small enough to still be a point source so far as loading the horn.

> > A 4 inch radiator has about a 2 inch maximum path to the holes
> > btw, not 4 (think pressure here).

This is the first time you have alluded that the orifice for the midrange drivers is smaller than the drivers. You have not said the
size of the midrange drivers either; My mentiuon of a four inch diaphragm was a guess. I assumed you would try to keep them
small for making an attempt at time alignment. But for all I knew, they may have been 8 or 10 inch units.

No, your zooming in on dimensions is dead on, all that has to be right or it won't work.
Of course at the 300 Hz crossover on the td-1, that switches to two 12" drivers with appropriately larger holes of course.

But since you've made this statement, I'd like to point out that it isn't accurate either. For a 4 inch radiator to have a "2 inch
maximum path" to holes makes the assumption that the hole is infinitelty small. There will actually be a window of distances
from the shortest path to the longets path. This is significant to a person attempting to make a time aligned speaker.

Yes that would be true and is lower in frequency (where it isn't an issue) but at the high end the holes and the trapped compression volume are in
play which makes the air mass in the hole driven by net pressure, higher in frequency the air mass in the hole chokes off the mobility and the hole
gradually becomes more acoustically "solid"

> > Yes, it is difficult to get everything packed in close enough,
> > difficult but possible.

Perhaps. But I'd enjoy further explaination. If the holes are small enough that wavelength scale becomes insignificant, then we
have only the distances between those holes to consider. What are the distances there? What did you decide to do about a
crossover? First or second order? This is important; Not only is phase set by the filter but the overlap region as well.

Yes they are small enough.
Well I'm not going to take all the fun out of this but I'll give you an important clue.
The response of the compression driver (a B&C DE-25) on this horn raw shows a broad peak at about 2.5 KHz, about a 6 dB/oct roll off
above that (the drivers REAL power response) to about 10 kHz where the series inductance adds a second bend.
Below about 2 KHz there is a rolloff which reaches about 4DB/oct to about 1 kHz where it reaches about 10 dB/oct roll off.
The 1 kHz slope reflects the low cutoff of the horn driven at the apex. the 2.5KHz slope is the mass rolloff dictated power response.
A TAD2001 on the same horn has this rolloff at about 4 - 4.2KHz fwiw.
The crossovers job is to flatten the response and trade off the upper/lower frequencies.
Since one can only attenuate with passive circuits most of the time, it is the sensitivity of the driver at the high cutoff that matters.
The design starts with 0 attenuation at say 18 kHz and then "rolls off" with decreasing frequency.
What one is left with when the final acoustic response is measured is different than the electrical filter because the drivers acoustic response is
added to the filter's.

> > You insist there is an anomaly yet when I offer to send you
> > measurements of the unit in the next room your silent.

Send 'em! But I'm more interested in the math. Measurements can be decieving, so I'd like to have some pretty solid models
first.

You will have to wait for the math, the design currently requires 3 different computer programs.
I'm going to bed now buy I'll do some Monday,

> > If there were a problem such as the one you insist, wouldn't that
> > show up in polar or other measurements as a big flaw?

Well, it did in Lambda's Unity. Would you care to tell us exactly why?

Yep, this is exactly why.
Nick made his horns more slick looking than ours, in doing so filled in the corners more, nice and smooth, very nice looking.
Ours had very little smoothing in the corners, only where it was needed near the compression driver.
This difference changed the distribution of energy buy making the directivity increase a little high up, so when he tried the crossover we came up
with, it had a plateau in the response. Nick went ahead and designed a crossover and also sent me one of his deluxe horns to play with.
When I had time (I do have a job too) I used his horn and came up with another crossover which I sent him.
Nicks horn shape also made the directivity perturbation somewhat more of an issue as well, some time later I found that the reflection could be
largely or entirely eliminated by placing a pad of acoustic foam at the right point of the outer mouth.
Nicks measurements did not include that foam and I don't recall which crossover (his or my first one) it was.
I do remember telling him that it looked like periodic reflections and to try it outside.

> > I have the TEF and a speaker in the next room, want to see what
> > the mid horn section looks like?

Send it. Send the response curve too, unequalized if you please.

> > Want to see a time measurement of the mid /high drivers while we
> > are at it?

And the bass subsystem too. I'd like some reference measurements too, to set a baseline.

But again - I'm more interested in the math. If the model isn't solid, then the measurements may be a fluke.

The bass system on the one in the other room is a direct radiator (2 10 inch woofers on the sides on either side of the horn)
The woofers are about 2 ms behind where they should be but I can't put them where they need to be.
No Fluke, HP, TEF machine mostly :-)

> > It is about the acoustic loading of all horns of that shape not
> > just ours, yours too if it were similar in shape.

I don't claim acoustic impedance matching over two decades.

Only driving the horn from the apex automatically prevents it from going down very low.
At the apex, it has the expansion rate of a 1 kHz flare or so.

> > Yes that is correct but it will only operate when driven at a
> > point suitable for the frequency involved.

You mean, by placing the diaphragms along the walls, making a large open space for a "throat?"

Yeah more or less, the horn has to be mostly solid everywhere or it isn't a horn (or is a lossy horn).
The sound for each range is from drivers mounted to the outside and communicate the pressure through holes.
One cannot interfere with the interior of the horn as even a harmless looking addition can change what it does (as in Nicks corners).
Each range of drivers has to communicate to the horn where the dimensions make it a point source.
For any given frequency, there is an active region of the horn (acoustic transformation).
This starts where the driver couples in and extends to approximately where the mouth would be 1 wl in diameter.
The horn beyond that point does continue to determine the radiation angle up to some larger size (manta ray paper)

> > We sell servodrives because they are the most powerful subwoofer
> > there is (by weight, size, cutoff and power), possible
because
> > the servomotor approach allows one a range of T&S parameters that
> > cannot be made with normal VC motors.

Do you plan to put this type of motor in a Unity horn?

No not at the moment, our business is pro sound and permanent installations.

> > I figured that it would be interesting to turn loose a "modern"
> > bass horn design based on the most suitable drivers that
could
> > be made for a reasonable price. Predictions for it place it right
> > up with a BT-7, a group will be greater than 50% efficient and
> > flat to 30 Hz.

Is it a Unity device? Is it claimed to have a two-decade response curve? And if not, why? if you can do it in the Unity, why
not do it in this "LAB horn?"

With that design, as it is so small acoustically even a large pile has little directivity.
Not caring about that, I used a hyperbolic flare.
This kind of woofer is used for concerts, special effects Raves etc, with our BT-7's only 16 were needed for the largest sound systems used for
band like Garth Brooks, U-2, michael Jackson and so on.
That is to say that those 16 horns (two groups of 8) produced enough bass to keep up with 140-180 speaker box per side sound system.
The lab sub should be very similar.
Should one not want the 200+ acoustic watts at 30 HZ per box, I think then one could indeed build a full rage horn this way, figure about 2-3
octaves range per.
While over kill for the home, there is an against the wall /corner position which looks good for a pair of them.

> > To you, all that must seem mind boggling from a business point of
> > view but again "it is not about the money"

Not at all. In this respect, we are very much alike.

> > I suppose by selling the Servodrive we are doing a disservice to
> > the other manufacturers of woofers but we are not putting them out
> > of business.

So you think the Servodrive is the "end all" motor type for woofers, yes? Would you say that a rotating motor with a belt and
pully arrangement is much better than say, a large linear motor of similar stregth and driving a similar mass?

No it isn't the ultimate but its usefulness is in that
It allows T&S parameters that cannot even now be built with VC drivers.

Those parameters allow one to do things you can't with a normal driver.
> > So far as hurting other Pro sound mfr's, the LAB sub has a number
> > of companies worried.

You think so, do you? Trembling, are they?

I know that a pair of the contributors cannot reveal there names because they fear being fired if mgmt found out they were involved.

If you were one of the companies who sold big bass horns and saw what this one looks like it was going to be, wouldn't you be scared?

Sorry that came across as being cynical. I'm sure that the horn is very good. But I'm going to remind you of some word you
said to me, earlier in this discussion:

You must imagine the world revolves around you I guess and that no one has ever given any of this thought before
you.

Sorry, couldn't resist.

Yes, I resorted to snide comments too sorry .

> > AS to the product you mentioned, I can easily produce 10 Hz in a
> > vastly smaller package than the right horn...

On this point, I agree with you 100%. Bass horns - and particularly ones that are required to do very low frequency - are not
small. They suffer from what one person on this forum has called poor "bulk efficiency." They use up a lot of "real-estate" and
more total output can be generated in less space if power is not a limiting factor. So I'm with you all the way in that line of
reasoning.

What my ears tell me I want is to have everything above about 80 - 100 Hz come from one spot, I have a large set of flares here but as it is not an "official project"

it is kind of on the back burner.

> > ...and at this point I also have to build things that people want,
> > not only that since at the moment we do not make HF drivers, the
> > 100 kHz part would be pretty tough too.

Probably not terribly important for sound production either. Maybe if you were generating sound using a beat frequency from
the interferiometry of two ultrasonic sources. Now that's a cool idea!

I worked on a project doing something like that for Gary Kendall at Northwestern U. and then helped Gene Pitts write

an article about for Audio a few years ago.

I used a focussing electrostatic transducer, was able to get ~155dB at the focal point at 75 KHz.

The thing worked but gave people headaches and used a lot of power

> > Why would I go on a public forum like the LAB, where there are
> > some 50,000 people looking on, in the heart of one of our
> > companies business, then say "I can make a bass horn that I say
> > will stomp any similar sized VC driven horn out there",
> > then turn over a design for one and get a driver manufacturer
> > (Eminence) to make a "perfect" driver to order for it to boot.

Are you saying that, here and now? Do you think this horn of yours will "stomp" every other VC driven bass horn out there?

I would say that give that the driver(s) is near the edge of what can be built and exactly suited to the job that it will be hard to beat

so far as its acoustic power vs size and input power. Even at 30 Hz, the excursion limit is a huge acoustic output (14mm linear Xmax)

You ask me "why." So I'll tell you what I think. If you have said that then I suppose you are either very confident or very
arrogant. Honestly - and no offense because I use Eminence too and like 'em - But if you think that an Eminence based
product will "stomp" similarly sized offerings using more expensive drivers then it isn't confidence, Tom.

It's arrogance.

We will see.

You asked.

> > Why would I be "saying all that with a straight face" as you put
> > it here?

I think it's probably going to be a great horn, but I think you're probably a bit overly optimistic if you think this particular
combination of components will be "a bass horn that will stomp any similar sized VC driven horn out there."

again time will tell and the drivers should start arriving at the distributors in a couple weeks.

> > Why would I go as far out on a technical limb as you feel I and
> > discuss the design with hundreds of people at AES and other trade
> > shows if it is bogus?

I dunno. Attention? Arrogance? You tell me.

maybe you're just excited about it. That would be a much nicer thing to believe. Yes, that's it. You're just excited about it.

(But Tom, "stomp any similar sized VC driven horn out there?" With a hundred and fifty dollar woofer? Come on.)

Again, time will tell and soon

> > Why would I do all this when so many of our customers do have test
> > equipment, measure speakers themselves and would easily find out
> > if it was not real?

OK. I give up. What the hell are you doing?!! Are you crazy, or what?!! [grin]

Or what....

> > Either I had to be pretty sure about what I was saying or not care
> > at all what happens if I am wrong, I can tell you it isn't
> > the latter.

So, yeah [snicker], that Eminence-powered LAB horn is going to just bust everything else right off the market. Why even
think about any other bass horn? Forget Altec, JBL and TAD. Here comes the LAB!!!

Well the BT-7 has been the most powerful sub there was in our markets anyway,

why don't you think I could push the envelope with an Eminence driver?

> > Do you know anyone more knowledgeable in acoustics than you?

If anyone, it must be you.

No Wayne I mean somebody you trust not to be tricking you.

Cheers,

Tom

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