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Re: tapped horns vs 1/4 wave length

Posted by tomservo (M) on April 7, 2007 at 16:12:54

In Reply to: tapped horns vs 1/4 wave length posted by jnorv on April 6, 2007 at 21:13:27:

Hi

As Bill suggests, bass horns are usually a quarter wavelength long at the low cutoff.
Like wise the Tapped horn is approximately a quarter wave length long at its low cutoff.
Horns are resonant devices like a transmission line but horns have sufficient resistance to damp the resonances into insignificance.
When a horn mouth is too small for example, these resonances are unmasked and when the peak to dip magnitude reaches some level, it is unacceptable.
In the case of the small bass horn, the lowest peak is the quarter wave resonance, the velocity and pressure distribution are as is shown on Martin’s site for a T-line.
The tapped horn has both sides of the driver in the horn path, one face is at the “normal” end for a horn, the other side (opposite acoustic phase) intersects the horn, usually near the mouth.
At the low cutoff, the acoustic path between the two side comprises about 90 degrees of phase, leaving about 90 degrees difference between the front and back radiations where they sum within the horn. At the low cutoff, because of the phase shift, effectively only one side of the radiator at the far end feels the load from the quarter wave resonance.
As the frequency climbs and where the big dip would normally be, now, the acoustic path is about 180 degrees and both sided of the radiator add in phase, driving the horn with a larger surface area. That increase in driving area (ideally) offsets the big wide dip in radiation resistance between the first two peaks.
It is a case where the resistance at both ends of a horn sets the Q of the resonance’s, the Tapped horn allows the driver end source resistance to in effect change with frequency to accommodate the change in the horn resistance.

An unexpected side effect of a variable source in the Tapped horn is the measured group delay for a given corner frequency, is less than a Vented box and even a conventional horn with the same LF corners.
The Tapped horn is only useful when a conventional horn is enough smaller than ideal to have large peaks and dips. When size is no object, a regular horn can be made to be more efficient.
There is some additional info on them and some examples at our web site
http://www.danleysoundlabs.com/

Best,
Tom Danley
.

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