[time-nuts] OT: Audiophoolery

Magnus Danielson cfmd at bredband.net
Fri Apr 20 19:12:41 EDT 2007


From: Brooke Clarke <brooke at pacific.net>
Subject: Re: [time-nuts] OT: Audiophoolery
Date: Fri, 20 Apr 2007 15:49:14 -0700
Message-ID: <4629436A.1090705 at pacific.net>

> Hi John:
> 
> Many years ago after installing a new component Hi-Fi system into my house you 
> could hear noise coming out of the speakers that was objectionable.  I took the 
> Scott power amp back to the high end audio store where I purchased it and 
> complained.  It was meeting specs.  The problem was the speakers.  Rather than 
> degrade the speakers the Scott was returned and replaced with McIntosh 
> equipment and that solved the problem.
> 
> The speakers were Voice of the Theater.  A 15' woofer set back so it's voice 
> coil is in the same vertical plane as the voice coil in the horn.  The gap 
> between the woofer and the front of the box was filled in with an exponential 
> horn.  This construction kept the phase of the sound at the crossover frequency 
> (500 Hz) in phase.
> 
> With 1 mw drive you could hear a 1 kHz tone from across the room and the noise 
> level on the Scott amp was some big number of dB down from max output.  After 
> calculating the noise level it turned out to be maybe 10 dB above 1 mw.
> 
> So I'd say that excellent speakers may or may not make everything else unimportant.

Horns have so much higher effiency that normal speakers looks like a waste of
energy in comparision. Horns are however also harder to handle in many ways.
The best horns I've heard and used are based on the orthonormal oblate
elliptical form (don't recall the exact term now). They assume that the source
is a flat oblate at the throat and then the horn-wall is orthogonal to the
wavefront as it expands, and thus acts as the acoustical transformer that we
want them to be. I made extensive measurements of these horns, especially in
pseudo-anecoic measurement setups. We peaked at 139 dB @ 1 m in the mid and
136 dB @ 1 m in the top. We never used less than 4 a side. We had amps so we
could burn the elements up before clipping (important!) and we had limiters
that ensured we never burnt any elements (we never did). We hunted noise and
hum until (kids, don't try this at home!!!) you had to lay your head against
the speaker front to hear a very very faint noise while the amps and everything
was at their full level that would play the house clean. You *DO* want good
dynamics.

PS. I *still* have perfectly good hearing! I was never hit by even a temporary
run of rings in the ears when doing PA. I've only had a tone popping up
breifly at other times. Just enought to make me notice them but then gone not
too much later. This can be due to stress.

Cheers,
Magnus




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