Extending a high frequency horn to achieve a lower cutoff - diyAudio
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Old 6th September 2007, 11:40 AM   #1
MaVo is offline MaVo  Germany
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Default Extending a high frequency horn to achieve a lower cutoff

Hello,

i have an unsused pair of the horn tweeter Visaton TL16H, which i would like to use as a tweeter above 2000hz.

http://www.visaton.de/de/high_end/ht_horn/410.html

The manufacturer designed it to be used above 5000hz with a 12db crossover. The frequency/amplitude graph indicates a possible lower cutoff. I tested its ability to play with a 24db 2000hz cutoff and moderate listening levels - it sounds nice and stays alive, but there is a drop in amplitude below 5000hz.

http://www.visaton.de/bilder/frequen...s/tl16h_fs.gif

My Question is: Can i extend the horn to load those frequencies? I would rather build an extension for the horn, instead of using EQ. The Horn is 7.5cm long with a throat of 3cm² and a mouth of 28cm² with what looks like an exponential shape. How can i calculate the right horn shape and size to extend the loading to 2000hz?

Thanks in advance
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Old 6th September 2007, 12:48 PM   #2
OzMikeH is offline OzMikeH  Australia
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I can't help you with any calculations.
I have a suggestion for testing by estimation: Use play doh to extend the horn to test if it will work. You can mould it by hand to a shape that looks right.
You should have a similar putty or clay in Germany for children to play with?
If the putty works then you can get the calculator out.
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Old 6th September 2007, 01:29 PM   #3
Geoff H is offline Geoff H  Australia
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Hi MaVo,

I'm not a horn expert, however I do know that the cut off frequency of the horn is determined by the mouth dimensions and the flare rate.

Extending the horn will bring up the mouth dimensions, but will do nothing to the flare rate of the existing horn.

Also, the driver may not be capable of going down more than an octave you desire.

It may be worth the try though, as you already have the horns.

Geoff.
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Old 6th September 2007, 02:02 PM   #4
GM is offline GM  United States
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Default Re: Extending a high frequency horn to achieve a lower cutoff

Quote:
Originally posted by MaVo
My Question is: Can i extend the horn to load those frequencies?
Greets!

Yes, but.......

The mouth already supports ~1836 Hz, the length a ~817 Hz flare frequency, so if you don't mind re-doing the XO, then reducing the compression chamber's (CC) volume (Vb) 29-30% using Mike's Play Doh or similar should shift the gain BW up enough to allow a 2 kHz XO.

Otherwise, since the CC is already tuned for a 2 - 20 kHz BW, just continue the expo flare another ~2.14" (5.44 cm) until the mouth area = ~21.92"^2 (141.4 cm^2). The downside is that now the mouth will be large enough to require a too wide a driver spacing for good summation at a 2 kHz XO point with a driver larger than ~5.25" (13.3 cm).

GM
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Old 6th September 2007, 07:57 PM   #5
MaVo is offline MaVo  Germany
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Thank you all three for your answers, pointing some light on aspects i havent thought of.

To GM: I will try building the extension. The summation will be a problem i have to live with even without the extension, since my low/midrange is 12inch in size.

A planar mid/tweeter like the SLS design of Igor Levitsky would be a good thing to have - and a challenge to build
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Old 6th September 2007, 11:36 PM   #6
Geoff H is offline Geoff H  Australia
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Thanks for the explanation Greg. I would have thought for a lower cutoff freq, a larger CC would be required. I live and learn.

Geoff.
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Old 7th September 2007, 03:55 PM   #7
GM is offline GM  United States
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Greets!

You're both welcome!

MaVo:

Please let us know how it works out.

Geoff:

A compression horn is a 4th order band-pass with the compression chamber's (CC) air cushion acoustically counteracting the horn's, so ideally the CC is tuned to the mean of the desired BW: SQRT(Fl*Fh), ergo to get more LF gain for a given horn the CC must be tuned higher since we have to raise Fl to either narrow up or shift the BW, with the trade-off being a steeper roll-off, while making it larger will raise the horn's F3 with a slower roll-off just like any sealed alignment and what Visaton chose for whatever reason.

Indeed, if MaVo had gone this route, then for a hifi app the horn may need only a resistor to damp the attendant Fc 'ringing' of a ~reactance annulled alignment instead of the proper hi-pass required of the stock unit.

GM
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