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Old 2nd April 2010, 03:31 PM   #1
johnr66 is offline johnr66  United States
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Default TH for 8" driver

Hi guys,
I'm hooked on tapped horns after the success of my 6 foot monster. I now want to build a horn with an 8" driver and reasonable size. I came up with this design. It's outer size is only 28x19x10.25 inches. Very easy build with all right angle cuts except where the driver board meets the outer panel.

I haven't picked out a driver yet. I don't plan to go beyond 50 watts rms. I hope to get strong output in the 45Hz and above with some extension down to 40Hz. This leads me to some questions:

How does the increasing volume from throat to mouth affect performance? I see horn designs here with very small increasing volume and other with much.

Is there a way to estimate minimum frequency response before roll-off from length of horn path? The path of this horn is about 84".

Thanks, John

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Old 2nd April 2010, 03:47 PM   #2
AndrewT is online now AndrewT  Scotland
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an ideal question to ask of Hornresp
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Old 2nd April 2010, 05:56 PM   #3
godfrey is offline godfrey  South Africa
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The "quarter wave" resonance should be about 50 Hz, but I expect you'll get reasonable output (with higher excursion) quite a bit below that.

The throat area looks like a good match for Eminence Alpha8 or Beta8 drivers.

Like Andrew suggested, try it in HornResp and see what happens.
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Old 2nd April 2010, 07:11 PM   #4
johnr66 is offline johnr66  United States
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Quote:
Originally Posted by godfrey View Post
The "quarter wave" resonance should be about 50 Hz, but I expect you'll get reasonable output (with higher excursion) quite a bit below that.

The throat area looks like a good match for Eminence Alpha8 or Beta8 drivers.

Like Andrew suggested, try it in HornResp and see what happens.
How did you arrive at 50 Hz resonance? Did you use HR? I agree, that HR is important to check design, but just looking for a quick way to estimate box resonance.
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Old 2nd April 2010, 07:21 PM   #5
AndrewT is online now AndrewT  Scotland
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if an estimate is good enough for you then use that.
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Old 2nd April 2010, 07:43 PM   #6
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It might get closer to 40 Hz if it behaves according to theory.

84 inches = 213 cm = 2.13 m

344 m/s / (4*2.13) = 40.3 Hz for the theoretical minimum 1/4 wavelength resonance of this cabinet based on theory alone.

I typically start with the driver, not the cabinet sketch, but your approach is equally valid.
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Old 2nd April 2010, 08:27 PM   #7
godfrey is offline godfrey  South Africa
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Quote:
Originally Posted by johnr66 View Post
How did you arrive at 50 Hz resonance? Did you use HR? I agree, that HR is important to check design, but just looking for a quick way to estimate box resonance.
I used an old home-made spreadsheet that's really for front loaded horns. Minimum cone velocity was at about 50Hz. I assume resonance will be about the same in a tapped horn?

Resonance in a horn is never at exactly a quarter wavelength due to the flare and acoustic impedance at the mouth.

I'm interested to see what you get out of HornResp. So far I've been too lazy to learn to use it. If I was building a horn I would though.

Cheers - Godfrey
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Old 2nd April 2010, 09:16 PM   #8
godfrey is offline godfrey  South Africa
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Some rough calculations for horns...

Ideal throat size for a given driver (for best impedance match, efficiency etc):
St = 415*Re*((Sc/BL)^2)*(Qts/Qes)
where: St is throat area in square meters
and Sc is effective cone area in square meters

Usable frequency range for a given driver on a horn:
Fminimum = Fs*Qts
Fmaximum = 2*Fs/Qts

The above is for high-efficiency horns.
YMMV, especially with "tapped horns".
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Old 3rd April 2010, 01:43 AM   #9
johnr66 is offline johnr66  United States
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I was using sound traveling at around 13,500 inches per sec/(4*84)=40.2Hz (similar to what Litttlemike said), but if quarter wavelength is not true in horns as Godfrey states, I guess I need to figure out how to use HR. At least I can match up a driver then.
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Old 3rd April 2010, 03:48 AM   #10
godfrey is offline godfrey  South Africa
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Hmm...
Tried out HR. Response drops off fast below about 60Hz. Works better as a FLH, with a rear chamber of about 20L - Decent output down to 40Hz.

Then again I'm a total noob at HR and TH so maybe I'm doing something wrong.
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