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Re: dam my horns image - me too !

Posted by tomservo (M) on January 8, 2005 at 07:17:24

In Reply to: Re: dam my horns image - me too ! posted by KerrB on January 5, 2005 at 09:40:09:

Hi

You have said a couple things which strike me as misconceptions or at least worth comment.
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"CD horns DO sound different than other horns ( esp. Tractrix ) in the Sweet Spot.

CD horns sound dull on-axis.

CD horns INcrease the proportion of high frequency sound off-axis by proportinally DEcreasing HF sound on-axis.

It's a process of SUBtraction not ADDition. A horn is a passive device, it does not amplify ( add/increase energy ) sound, it couples the air pushed and pulled by the driver to the air in a room ( or air outside ).

Which frequencies are coupled well and which are not, depends on the size and geometry of the horn.

And a horn can also guide/ project/ direct the air/sound waves to areas where they are needed.

Diffraction horns, Compund Diffraction horns, Constant Dispersion horns, Controlled Dispersion and Constant Directivity horns are all the same thing basically. The common feature ( usually ) is a narrow mouth ( slot, etc. ) ... because a large horn mouth ( needed for LF loading ) necessarily " beams " high frequencies."
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Because there are so many ways a horn can be "wrong" in one way or another, I will keep my comments to "ideal" examples of these types.

If one measured the acoustic power vs frequency of a compression driver such as on a resistive load (Plane Wave Tube), one see’s that all compression drivers have a hf roll off which begins in the 2-4KHz area due to the moving mass and one see’s a second knee in the roll off where the Rdc and Le act (usually around 8-15KHz depending on the driver / size) and yet another knee is present as one reaches the front volume / throat mass filter corner.

Comparing the PWT response to the "on horn" response, one finds the difference is due to the horns directivity and its ability (or not) to present a proper load to the driver.
Assuming one had the driver mounted on a truly "CD" horn, the measured hf response is the same as in the plane wave tube, that is, the actual acoustic power response of the driver is what you see and it is the same anywhere within its coverage angle.

Placing the same driver on an exponential or Tractrix horn, one finds the response is much different that the CD as a result in the difference between the directivity and loading.
With these horns, one finds (on axis) the response is a lot flatter (in the ideal match, it is flat on axis) because as the drivers acoustic power falls, the beam width narrows proportionately, focusing the decreasing acoustic power into a smaller and smaller area.

AS one can see, if one wants "flat response" from the CD horn, one has to "fix" the hf roll off inherent in the driver connected to it.
This EQ is partially present in the narrowing coverage angle of the curved wall horn and so the two types sound very different (before the response is normalized).
Once the amplitude and phase are corrected, there is no reason that the two types would sound significantly different at least on axis and the arguments about the Tractrix flare having some magical edge diffraction suppression are not born out by polar energy measurements.
A "perfect" CD horn has the same frequency response on axis as anywhere else in the horn pattern.

At the low frequency end of the horns range, the conical horn (the simple way to make a CD horn) one finds the acoustic loading of the conic horn is much less than the expo or Tractrix (curved wall horns), this is seen the any text book plot of the loading vs frequency for the various horn types.
If one examines the rate of area expansion and remembers that the slower the area expands, the lower the horns cutoff is, one finds the apex of the conic horn has a very fast expansion (and so has less loading at low F’s).
The basis of the Unity horn patent (generally Conical horn) is in recognizing that if one moved forward of the apex, that the expansion rate slows progressively and where the expansion is appropriate for the frequency, the conic horn "loads" properly.
This means that as the frequency falls, one must move the driver further from the apex to continue loading to lower frequencies. Fortunately, this also places the drivers "front to back" in a position where the inherent delays can be accommodated by the crossover.

All CD horns are not the same.
Diffraction horns and the more complex shapes are used when a significantly different vertical and horizontal coverage angle is needed or to avoid the "pattern flip" associated with significantly non - equal angle horns exhibit as they loose pattern control at the low end (set by mouth size and angle).
These horns also measure as if they had two acoustic origins, one for the Vertical and a second for the Horizontal angle.
Personally, I feel that for the home or where fidelity is a major concern, that these horns are too problematic to be a good choice.
I mostly use simple conical horns which are closer or are equal angles H and V (minimizing or avoiding pattern flip), at high frequencies, they are driven at the apex with a source which is acoustically "small enough" (given the horn wall angle) to produce proper polar patterns.

Fwiw, the comment about "the large mouth needed for low frequencies producing a narrower hf dispersion" is only true for the curved wall horn like a Tractrix or exponential flare.
This is because if the horn mouth is large, it was also made to go down low and so has a slower expansion rate, defined by a narrower wall angle where the directivity is set.
For the conical horn, the mouth size has no impact on hf coverage angle, that is set by that wall angle and "if" the driver exit and geometry is producing hf directivity by itself.
This last point is where Earl’s latest horn work seems to be, that is recognizing that the horn actually starts within the compression driver and at high frequencies, it is already defining or helping to define the coverage angle. Another way to say it is that the driver exit size and shape are acoustically "large enough" at high frequencies to be defining the radiation pattern and so the horn you mount the driver to must accommodate or account for the part of the horn within the driver .

Hope this helps

Tom Danley

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