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Old 22nd August 2011, 05:13 AM   #2171
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Quote:
Originally Posted by mwmkravchenko View Post
When measuring below about 300hz the wavelength of the frequency and the output from the horn seem to require a greater measurement distance in which to get an accurate representation of what horn does under normal useful conditions. Trying to measure 20hz at one meter generally looks ugly ragged and terrible. Take the same measurement out a distance and correct for the losses as David showed and you get a more accurate representation of what your box is doing.
Just to clarify - the "losses" Mark refers to are due to the sound energy being radiated into a larger surface area, resulting in the intensity and hence the pressure being reduced at the measuring point. None of the propagated sound power is actually lost.

With bass horn loudspeakers it is desirable to measure the SPL several metres from the horn mouth, ideally under anechoic conditions, and then normalise the results back to the standard reference distance of 1 metre.

For example, if the SPL measured at 7 metres is 90 dB, then the value normalised back to 1 metre would be:

SPL(1) = SPL(7) + 20 * Log10(7)
SPL(1) = 90 + 20 * 0.845
SPL(1) = 90 + 16.90
SPL(1) = 106.90 dB

Kind Regards,

David
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Old 22nd August 2011, 02:30 PM   #2172
Mark Kravchenko --- www.kravchenko-audio.com
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"Losses" is indeed a poor choice in words.


Thanks for the clarification David.

Testing at least one half wavelength back is preferred when possible.

Those dimensions can get pretty large when you are testing subwoofers!
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Old 23rd August 2011, 02:34 AM   #2173
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So, have you an example of two frequency measurement, near and far the cab? I think i t will be interesting to see the difference for all.
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Old 23rd August 2011, 07:40 PM   #2174
Mark Kravchenko --- www.kravchenko-audio.com
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Well I'm not at my computer today.

But I will post a couple of measurements.

If you want to check the veracity of Hornresp versus measurements search my name in this thread. I have posted quite a few times the results of built projects versus the simulations.
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Old 24th August 2011, 03:25 PM   #2175
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Quote:
Originally Posted by David McBean View Post
Hi dirkwright,

Sorry, it's not going to happen... :-).

Kind regards,

David
Oh, that's fine! It would be stupidly complex to add something that took folding into account.
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Old 26th August 2011, 03:53 PM   #2176
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Does anyone know why Hornresp gives different results than this spreadsheet:
Download Section
for tractrix horns? I was inspired by the spreadsheet because a 60Hz horn was so nice and short, but Hornresp made it much bigger. One shows a length of 67.1cm and the other shows a length of 161cm! The expansion curve of the spreadsheet has the 90 degree flare at the mouth whereas the generated by Hornresp does not.

I don't know which one is correct. A lot of people use the spreadsheet, from what I have seen. I gave up and anticipate building a 100 Hz hyp-ex instead. It's 105cm long.
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File Type: jpg 12PE32 tractrix spreadsheet.JPG (72.4 KB, 130 views)
File Type: jpg 12PE32 60Hz tractrix input 2 screen.JPG (56.2 KB, 134 views)
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Old 26th August 2011, 04:33 PM   #2177
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You must run placement 1 in the German spreadsheet to get realistic figures.
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Old 26th August 2011, 04:43 PM   #2178
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Quote:
Originally Posted by revintage View Post
You must run placement 1 in the German spreadsheet to get realistic figures.
Is that because he made a mistake in the spreadsheet? The mouth areas are the same, the throat areas are close, but only the length is different. I used the system designer tool in Hornresp to build the horn there.

Last edited by dirkwright; 26th August 2011 at 04:47 PM.
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Old 26th August 2011, 07:43 PM   #2179
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Quote:
Originally Posted by revintage View Post
You must run placement 1 in the German spreadsheet to get realistic figures.
Even if I set it "1", I get a horn length of 1.96 meters, which is different than Hornresp.
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Old 27th August 2011, 07:19 AM   #2180
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Quote:
Originally Posted by dirkwright View Post
Even if I set it "1", I get a horn length of 1.96 meters, which is different than Hornresp.
Hi dirkwright,

Have a look at the formula in the spreadsheet used to calculate Rm (mouth radius). The velocity of sound value used here is 340 m/sec. Change this to 344 m/sec and you will now get effectively the same results as those calculated by Hornresp - allowing for the very slight differences in mouth area and length due to rounding.

Spreadsheet:

Fc = 60 Hz
Placement = 1
At = 178.8 sq cm
Am = 26158.0 sq cm
Length = 199.63 cm

Hornresp:

S1 = 178.80 sq cm
S2 = 26157.99 sq cm
L12 = 199.62 cm
F12 = 60 hertz

Kind regards,

David
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