Re: [Phys-l] Advertising bad physics!

["Rick Tarara" <rtarara@saintmarys.edu>]

----- Original Message ----- 
From: "John Clement" <clement@hal-pc.org>
> An example of this is putting isolators under speakers to prevent 
> coloration
> due to interactions.  But most of the interactions are mediated by the air
> which carries far more energy, so it is a 3rd order effect.  And 
> interaction
> through the floor is impossible on a concrete slab floor.  Also imaging is
> conveyed by the first signal to arrive at the ear and the interaction
> through the floor will be a delayed signal which contributes nothing to
> imaging.  So isolators are worthless under speakers!  But raising speakers
> slightly can dramatically change the sound and reduce some room resonances
> so using cheap wooden shims would probably produce the same effect but 
> would
> not look as nice.  Technobabble is used to sell a cheap fix at an 
> expensive
> price.

Imaging can, I think, be influenced by delayed signals--if such signals end 
up out of phase with the original.  Bob Carver used this in a remarkable 
$300 add-on that he called Sonic Holography.  Here is the basic idea.  When 
you listen to a sound source directly in front of you, you get the same 
signal in each ear, at the same time, and at the same loudness.  For sources 
to your right or left, the signal arrives at your ears at slightly different 
times and louder in one ear than the other.  This latter works fine for 
stereo speakers, but the 'centered' signal has a problem.  If you put the 
same signal in each speaker, the sound does arrive at each ear at the same 
time and with the same intensity--just like a live source--BUT a few 
microseconds later, the left speaker signal gets to your right ear and the 
right speaker signal gets to your left ear--each somewhat diminished because 
your head is in the way.  This is enough for our brain to say that something 
is not quite right.  The audible effect is that stereo speakers tend to 
produce a very two dimensional sound stage--right, left, center, but 
basically seemingly all from the same distance.  What Carver did was produce 
a box that took the some of the signal from one speaker, delay it a few 
microsecond, put it out of phase, and then sent it to the other speaker.  If 
one sits midway between the speakers, then this effectively 'erases' the 
cross-over signal--the one from the left speaker to the right ear.  If done 
only with those signals that would be common to both speakers, the result is 
to 'fix' the 'two speaker doesn't produce a proper centered signal' problem. 
Audibly the effect can be spectacular.  Sound sources are much more 
localized in space and seem to come from close, far, right, left, (even 
right of the right speaker and left of the left speaker).  Some recordings 
are quite interesting when heard on this system.  Despite being a 'studio' 
recording, the original Jesus Christ Superstar album sounds like a stage 
play--with voices well positioned and moving about the stage in a realistic 
way.  The point is, this is done with delayed signals--they can effect 
imaging.  Of course having your speakers out of phase can do so as well--but 
now you get a diffuse sound with almost no localization (in early 4 speaker 
setups, one often did this intentionally with the rear channel setup to 
increase the spaciousness of the sound.)

Rick