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Re: Agreements and disagreements

Posted by tomservo (M) on January 25, 2006 at 08:18:01

In Reply to: Agreements and disagreements posted by EGeddes on January 24, 2006 at 12:38:17:

Hi Earl

I saw your comments, thanks for the reality check, sometimes when writing, a second read is in order.

If one uses antenna analogy, one finds a useful concept in the “front to back” ratio, that is the energy radiated within the pattern compared to that outside the intended pattern.
What I have found both with antennas and audio horns is that this ratio is related to acoustic / wavelength size. That is, two horns of identical wall angles, producing 20KHz, one being say 5 wavelengths across at the mouth compared to say 50 wavelengths across, one finds the front to back ratio is higher on the larger horn, less energy is radiated outside the pattern angle or to sides, the rear etc.

On HOM’s yes I should have said more about the angles too, all of it is in play.
I would describe HOM’s as sort of a “edge radiation” effect within the horn, which is caused when there is a transition in angle where the acoustic size is already beginning to exhibit directivity. When the acoustic dimension is small such a change in angle should be unable to produce HOM’s. A first guess would suggest that a 2 inch exit could not produce HOM’s below about 2200Hz regardless of horn wall angle. Is that about right?

On the audibility of the two different horns, I did not include HOM’s but lumped all the horn acoustics into an “etc” dealing mainly with the room interaction instead. Haste leads to an incomplete picture.
Also, I should have explained what I meant by off axis listening.
In my arrangement, sitting at the computer, I am about 90 degrees off axis from the close speaker and the far speaker is obscured by the printer and kids computer.
I can’t get a direct path from either speaker, yet, the spectrum is very similar to on axis and music, voices are quite listen-able here.
Sitting on the couch, on axis and equal distances, one of course gets a stereo image not present at the computer position.

The problem of mating horns at suitable acoustic dimensions has been something I gave a lot of thought to. This lead to the Unity horn patent and now to the Synergy horn patent application. It is possible to have ranges combine at acoustic dimensions which do not support HOM’s or produce lobes.
For our market, one must supply an EASE model for designers to deal with and specify a speaker for commercial sound. To generate that model, spherical measurements must be taken to quantify the speakers radiation in every direction.
Lobes and such are quite visible.
Pat Brown has just finished these measurements on a coax speaker we sell, an SH100, which behaves up to about 16KHz, where the inner horn is too far from ideal.
These will be the normal CLF file at the website shortly.
Pat is doing the SH50 this week, that one has 7 discrete drivers and except for the unavoidable effects of having a square horn (needed as they are used in N number arrays) also should not show lobes from the different ranges interacting.
On the 50, at 1000Hz, there is output from both the compression driver and 4 mids.
The dimension where those signals combine in the horn is about 3.1 inches, a quarter wavelength at 1KHz is about 3.4 inches. In a 50 degree horn, those signals combine coherently as the combined source in the horn is less than 1 /4 wl across.
Same for the woofer to mid transition.
Well, actually I should wait for Pat’s measurements to see to what degree that is the case in the real world.

I really like your solution of putting the lobes to your advantage, clever!.
Sort of a “making lemonade with the lemons” solution, in the words of C,M Burns, “Excellent”.

So far as showing HOM’s, I would think doing these same measurements would possibly show what your looking for.
Many commercial horns (in a high resolution polar plots) show a lot of “thorns” pointing in different directions, my guess is that the polar on the one you have would be “simple” in shape.
That behavior can be visible in the spherical balloon measurements also and might be a good way to visually show the difference between “yours and normal”.
Same for the lobe thing, when you have an actual technical advantage, you have to figure out how to explain it VS the “others”. The graphical display is appealing.

Anyway, Here is a link to the free CLF viewer and also for Pat Brown’s (the Synaudcon fellow) measurement company.

http://www.clfgroup.org/viewer.htm

http://www.etcinc.us/

I didn’t get a chance to reply the other day on the Mathcad question.
I have 2001I, I poked around and saw you could read wave files like you mentioned, pretty cool. When I get some time to spare, I want to talk to you about that.
Anyway, its 10am and I need to get to work.
Best,

Tom

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