Acoustic center of horns

[mrkramer]

Just out of curiosity.

As far as I know, for a standard cone speaker the acoustic center is roughly at the voice coil. What defines the acoustic center for a horn?

MrKramer

[Saturnus]

The same.

[John Sheerin]

Only at certain frequencies. Due to phase shift of the enclosure and/or crossover, the acoustic center can be at very different places than the voice coil. Ultimately it is not that simple and the best way to figure it out is to take a measurement.

[ente]

.. John is right a "fixed acoustical center" is a fairytale .
Maybe this article from John Vanderkooy helps a little (even if it deals not with horns):

http://www.aes.org/sections/uk/meetings/a0604.html

Regards
Heinrich

[mrkramer]

That was an interesting read.
It seems to say that up to midbass at least, the acoustic center is actually in front of the cone! It suggests that the center moves back as driver size increases. Moves forward as baffle size increase.

Really not what I expected/thought I knew. Throws a bit of a monkey wrench into time alignment...

MrKramer

[Jmmlc]

Hello,

I guess Vanderkoy's study has for goal to define an acoustic center from the point of view of the pressure field at quite long distance.

It is useful when, as an example, we want to simulate the pressure field at any place inside a given auditorium. In the simulation software we have to position the point sources equivalent to the loudspeakers.

What Vanderkoy said is that we don't have to postion the point source at the coil of the loudspeakers (as often done) but at a place in front of the loudspeaker and he gave some innformations of the methode to define this center.

Now , IMHO, the notion of acoustic center depends on the purpose. By example if we want to optimize an alignement/crossover the acoustic center as defined by Vanderkoy is nearly useless, we have better to consider arrival times at different frequencies at the listening point. The most useful tool in that case is the group delay curve or more simply a spectrogram or a waterfall.

http://www.diyaudio.com/forums/attac...amp=1211535153

http://www.diyaudio.com/forums/attac...amp=1211535985

For a horn, at frequency much higher than the acoustical cut-off, the center is nearly at the coil of the driver and as John said it will be moved back progressively as the frequency decreases to the acoutical cut-off. (= the distance between the listening position and the acoustical center increases). This is related to the group delay curve which one is derived from the phase curve which is itself related to the evolution of the acoustical reactance and the acoustical resistance of the horn with frequency).

http://www.diyaudio.com/forums/attac...amp=1212045399


At the cut-off, group delay equivalent to a distance of 0,3 to 1 meter ( 1 to 3 feet) can be found depending on the cutoff frequency of the horn.

http://www.diyaudio.com/forums/attac...amp=1212684090


Best regards from Paris, France

Jean-Michel Le Cléac'h

Originally posted by ente
.. John is right a "fixed acoustical center" is a fairytale .
Maybe this article from John Vanderkooy helps a little (even if it deals not with horns):

http://www.aes.org/sections/uk/meetings/a0604.html

Regards
Heinrich