Mutual coupling and tapped horns?

[davygrvy]

I've been looking at the math, and it doesn't look like multiple THs in a pack make use of each other's mouth area like for an FLH. Is this true?

[Don Hills]

Yes, it's true. I've even tried coupled horns, where they have a common mouth back to just before the "mouth tap", and they do not couple significantly until you severely restrict the mouth size. (I was trying to find a way to use mutual coupling to reduce excursion on the individual drivers.) It's probably a good thing, otherwise pipe organs wouldn't be able to line their pipes up next to each other in such magnificent rows. Remember, a "tapped horn" is not a horn. It is a "tapped pipe" or quarter-wave resonator. Horn-like behaviour (output at frequencies above the mouth cutoff) is undesirable and is usually suppressed by a low pass filter.

[davygrvy]

Thanks for the confirm and the pipe organ analogy.

[jbell]

Originally Posted by davygrvy

I've been looking at the math, and it doesn't look like multiple THs in a pack make use of each other's mouth area like for an FLH. Is this true?

From what I've seen/measured. That's 100% correct.

I know that seems to be a big negative for TH's... but here's something to think about. TH have directivity. Really, they do.

FLH Mutual coupling vs TH directivity seems like a good trade off. Outdoor at distance -- I'd take TH over FLH.

[Tom Danley]

No mutual coupling??

Here are 1,2 and 4 TH-115’s, 1Watt total drive in each case.
Keep in mind 50% efficiency would be around 109dB sensitivity.
How large would a conventional horn or band pass speaker be with that response and efficiency?

AVS Forum

Best,
Tom

[jbell]

Tom:
I think my initial thought on davygrvy's 'mutual coupling' question was in relation to a front loaded horn, and how the lower corner is extended as cabs are 'coupled' and mouth area is increased. Your excellent graph shows the lower corner exactly the same for 1,2,4 TH cabinets, and is what I believe was being asked about.

Maybe different terminology should have been used?

[Don Hills]

No significant in band mutual coupling, as your graph proves.
(Where "in band" is below the first main resonance, and "mutual coupling" is defined as multiple mouths combining to function as one mouth of size equal to the combined mouth area.)

You need to be careful, Tom. Speakers like those have the potential to violate the Strategic Arms Limitation Treaty. You run the risk of being nuked from afar.

Actually, looking at that graph - I suspect the 4 box curve would have looked different if you placed the boxes in a line instead of a quad. How far off the ground was the upper pair of TH-115s?

[Tom Danley]

Well as you can see the mutual coupling is alive and well exhibited in the increasing over all efficiency as the number increases.
I think what you mean is the low corner going down as a result of more units, that is a length effect.

For a port or horn, the acoustic length includes some air in front of the outlet, sort of a mouth bubble.
If I remember for a round duct, it is some portion like 2/3 of the duct radius.
In any case, with a horn with a large mouth, when you step from one to four, one has made the mouth bubble significantly larger and so, made the horn significantly longer.
With a Tapped horn, the mouths are small and while the mouth bubble effect is present, going from one to four units has a small effect on the length.
A further issue is that if you “add” on to the length of the front part of the horn, you are altering the drivers location in the system which may or may not mistune the system.

As for placement of the 115’s , this was one upright, two upright and four done two on two, (pairs laying down with the mouths at the center}.

Tom

[Don Hills]

Originally Posted by Tom Danley

Well as you can see the mutual coupling is alive and well exhibited in the increasing over all efficiency as the number increases.

What overall efficiency increase?
You say "1 watt total drive", does that mean one watt total split across 1, 2 and 4 cabinets respectively? Did you allow for the differing impedances?
I suspect we're talking about two different legs of the same elephant...

Originally Posted by Tom Danley

As for placement of the 115’s , this was one upright, two upright and four done two on two, (pairs laying down with the mouths at the center}.

That makes sense, it looked from the graph like there was some "floor reflection" taking place.

[David_Web]

"TH have directivity. Really, they do. "

It's cold outside here now so I can't test it, so how much directivity would one expect?
And how does it behave?