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Re: Horn stuff and a post back for Tom

Posted by Wayne Parham (M) on July 10, 2002 at 01:30:58

In Reply to: Re: Horn stuff and a post back for Tom posted by tomservo on July 9, 2002 at 19:07:54:

Hi Tom!

You wrote:

>> What your saying is correct then that the speaker, especially a
>> point source direct radiator "moves around in time", it in effect
>> moves front to back depending on frequency.

and then I replied:

And, it is non-linear around system resonance too.

to which you said:

>> Not non linear but its acoustic phase is at its maximum rate of
>> change at resonance.

It's Non-linear. This is where it's shifting the most. Not a slow movement, but a rapid transition. Only after this point does it become relatively linear.

>> For a woofer, at resonance it's acoustic phase is zero degrees and
>> it goes to near -90 above. At the Rmin point in the impedance,
>> the acoustic phase has risen to zero again and then goes positive
>> above Rmin. By choosing ones crossover, one can intercept the
>> woofer's response with little change from -90 or at zero. In any
>> case one is stuck with what the direct radiating point source does.

One is stuck with it, yes.

Not only the woofers, but the midrange too. If the midrange diaphragm is in resonance in the woofer-midrange overlap region, it suffers the same fate. And again, this is the case for the compression driver.

You wrote:

>> A highly loaded horn is resistance dominated and so one finds that
>> by the time one has a 50% efficient horn, that the acoustic phase
>> is around zero degrees mid band. Over the region the acoustic
>> phase is zero degrees, the driver does not move in time with a
>> change in frequency.

to which I replied:

But the horn in the Unity is nowhere near as efficient as this,
wouldn't you agree? The midrange drivers aren't even loaded at
the apex; The horn is, as you say, a "lossy" horn or what I have
called a "mal-formed" horn. It does not meet the conditions of
the Webster equation.

and then you said:

>> No, I wouldn't agree, I have measured the acoustic power and
>> impedance, the mid system is about +2DB more efficient than the
>> compression driver. Also my horn simulator program predicts
>> results very close to what is measured with an efficiency around
>> 40%.

But midrange is the most efficient band of the system. For one thing, the horn is tuned for the midrange band and for another, you have more midrange drivers than any other. If you aren't hitting 50% here, you certainly aren't hitting it anywhere else. One of the sticking points I've had was that this device was claimed to provide loading from 200Hz to 20Khz, but that's not possible. It hits a corner just above the midrange band, and must be equalized to compensate just like other comparibly sized horns.

By the way, what are you using to model this system? What is the "horn simulator" program you are referring to?

>> Loading at the apex is un necessary, one needs to be acoustically
>> close yes, but that is a function of frequency.

That brings us back to the question - How far are the midrange orifices from the compression tweeter diaphragm, at the apex?

You wrote:

>> Flat amplitude AND zero degrees phase are the conditions needed to
>> preserve the waveshape of the input signal, no wonder so many like
>> horns.

and I replied:

This is true, and it is simultaneously impossible for any filter to accomplish.

then your response was:

>> That is right, only a constant power into resistive radiation load
>> without any significant reactance will do this.

So we are in agreement on this.

You wrote:

>> In the Unity, I can't do anything about the phase response of the
>> woofer, one is stuck with that and in that case the closest one
>> can come is to match the time /phase at and around crossover.

I replied:

I see, yes. You make no attempt to time align the woofer subsystem, finding it impossible.

And then you said:

>> Below crossover it does what all woofers do, at crossover it IS
>> aligned with the mid section of the horn. It is the drivers delay
>> and phase at crossover that one matches.

So what you are saying is that the woofer's time response is skewed, but up near the crossover point, you bring it into alignment with the mids. Then you further match the mids to the tweeter as it nears the crossover point.

Putting aside, for a moment, the alignment problems in the overlap region, and putting aside the rapidly changing phase at diaphragm resonance of the woofers, midranges and tweeters, are you proposing a system that has a slowly changing apparent location? Or does it rapidly change in the crossover regions? And in either case, how do you propose to keep from having problems arraying speakers having this kind of "doppler effect?"

>> At 20 kHz, that "impedance matching" is done well before the sound
>> has reached the exit of the driver. 1WL in circumference is .21 inches!!.

No. Impedance matching requires that the flare is of wavelength scale, and in 0.2 inches, you have not even begun a flare. It's still inside the compression driver, which means you are at the mercy of the motor builder. Whatever flare rate they chose at 20Khz is what you've got.

Your horn cannot do anything of scale smaller than one inch if you're using a 1" exit driver device. It looks like free space at that frequency - Its walls are merely a reflector.

>> Even at 5 kHz, the passage way in the compression driver is large
>> enough to fully load the driver. From that view point, one could
>> say that ALL compression drivers do not load any horn above 5K
>> (1 exit.inch driver)

That's right.

>> On the other hand, what the air see's is in the compression driver
>> is a continuation of a conical horn down to a rather small throat
>> area. From that stand point, if you think of the horn as the
>> total air path, it does indeed fully load the driver and does so
>> all the way down to the low cutoff of the horn.

Again, you're at the mercy of the compression driver maker here. What happens in the top octave is not up to you. So you cannot claim to horn load the device from 200Hz to 20Khz. But that's a dead horse - You've already admitted to using compensation equalization to address this.

I wrote:

Conical horns become peaky at their low end, this is true. They act
like a series of resonators combined with a high pass filter.

and you replied:

>> No, not peaky, it is the relationship of the low cutoff of the
>> horn and power roll off of the driver which produce that response
>> shape.

The low cutoff of a conical horn system of this scale is relatively peaky, as is shown by the plane wavefront model. It can be smoothed with a larger mouth or by constrained space, such as using multiple horns in close proximity or installing against a rigid surface or corner. But the lower cutoff point has a peak, and response after that has a negative slope.

>> If one could say "crank the magnet up to 40,000gauss", one would
>> see the compression drivers high frequency response flat octaves
>> higher.

Not unless the throat were made appropriately small. One cannot have horn loading from a single horn from 20Khz all the way down to bass frequencies. At this scale, we wouldn't even be talking about throat distortion, because we would have to solve issues caused by supersonic speeds of the air mass which would be forced to reciprocate in the hyper-constrained throat.

I know that your response is to remove the lower frequency diaphragms from the throat, and to install them along the walls. But the more we do this, the more we decrease the efficiency of the horn, because the drivers near the mouth are less horn-loaded. Of course, we can forego the horn altogether but we are talking about horns here, after all.

>> The mid drivers response is flat.

I know that you are basing this statement on your measurements. But before you measured the system, how had you "run the numbers" on it? What kind of "proof of concept" modeling was done for the midrange and woofer in this configuration?

There are no suitable horn models for the Unity configuration, so what was the basis?

>> All horns are a resonator(s), look at the impedance curve of an
>> exponential horn as you make it smaller and smaller.

That's right. That's why there's a peak at cutoff.

>> Flat response happens when the right amount of damping is at each
>> end, electromechanical at the throat and acoustic at the mouth
>> (where its 1WL incircumfrence)

That's true. If the mouth is large or the horn is constrained, then this peak is reduced. I like to build horns having a peak at cutoff, tuned for just under the motor cabinet's cutoff. In this sense, it is used for extension rather than efficiency.

>> Do you use a square horn around this size you could experiment with?

Yes.

I've worked with horns that are very similar to this for over 20 years. The ten π horn is such a device, and I've built them in various sizes from just a few cubic feet to 60 cubic feet. So I have quite a bit of experience with large format conical horns.

Take care!

Wayne Parham

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