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Re: acoustics near a surface (was: sound bug)



James Braunsdorf wrote:
...
Sound Bug that is a small device that can
be attached to hard surfaces to use the wall or tabletop as a speaker.

This sounds like snake oil to me.
Think about the design of an ordinary loudspeaker.
The main components are
-- the electromechanical actuator ("voice coil")
-- speaker cone
-- enclosure

The objective is to achieve a decent impedance match
between the actuator and the air. Large, lightweight
cones and/or horn-shaped enclosures achieve this.

Let's see if the advertised item is a good idea.
Make a checklist of all the physical properties you
want a speaker cone or enclosure to have, (size,
strength-to-mass ratio, etc.) and then check off
whether a tabletop has each such property.
The answer should become rather obvious.

====================

As a tangentially-related issue: Suppose you take
a small but basically-decent speaker and put it within
a foot or so of a flat surface, such as a floor or
tabletop. The wave going directly to your ear will
interfere with the wave that bounces off the surface.
This will cause a near-zero in the transfer function
somewhere in the middle of the audio range. It just
cracks me up when a self-proclaimed "audiophile" spends
lots of money on equipment that is flat to +- 1dB, and
then puts it in a room that has all sorts of zeros and
high-Q poles.

The reciprocity principle suggests that similar remarks
apply to microphones. It cracks me up when somebody
buys a $1000.00 omnidirectional microphone and puts
it on a little stand 6 inches above a tabletop.

Actually, the situation is better for microphones.
The impedance-matching issues are different, so
mikes tend to be smaller than speakers. You can
get so-called "boundary microphones" which are
designed to be placed on a surface; they use
the surface essentially as an "enclosure". You
can stick them on a tabletop, a wall, or (!) the
lid of a piano.

To close the circle: You could design a speaker that
was optimized for resting on a table in the middle
of the room, and using the tabletop as part of its
"enclosure", but it would be very unlike the "sound
bug" described above.