[rescue] miss me?

[Tim H.]

On Tue, 27 Aug 2002 00:17:12 -0400
Joshua D Boyd <jdboyd at cs.millersville.edu> wrote:

> Basically, should there be fundamental design differences between the
> 18" used as a woofer in a large speaker assembly, versus the 18"
> speaker used as the sole speaker in a large keyboard/PA amp?
> 
> I realize of course that more speakers is better.  At least, when we
> aren't talking about head phones.
> 
> -- 
> Joshua D. Boyd

The design of a speaker (and the box is just as important as the driver)
is based on what frequencies it is supposed to reproduce.  A speaker
designed entirely for low frequency reproduction needs a very stiff
cone, and a voice coil appropriately sized to manhandle the mass of the
cone plus the resistance of the surround and spider.  (the spider is the
stiff, usually fanfolded, usually cloth with stiffeners, thing that
actually keeps the voice coil in the middle of the magnet gap)  A wide
frequency driver (such as a guitar speaker) will usually be a more
flexible cone, which allows the voice coil to reproduce higher frequency
sounds, but also muddies up the lows a bit as well.  They will often
have whizzer cones to help with high frequency reproduction. (otherwise
the primary radiating surface for high frequencies actually is the front
edge of the voice coil, the dust cap itself is usually acoustically
transparent) Shiny plastic or metal domes serve the same purpose as a
whizzer.  

A multi-driver set up allows for more accurate sound reproduction, which
could very well be what you don't want for an instrument, especially
guitar, because the defects in the speaker are an important part of the
"classic" sound.

More speakers is not always better, even from a power handling
perspective, because as soon as you have more than one speaker making
the same noise you wil get acoustical nodes and nulls, where a specific
frequency will be very loud or almost inaudible depending on relative
speaker postition to the listener.  This is especially noticeable with
low frequencies, which have wavelengths long enough to wrap around your
head, so both ears are on the same side of the wave.

On the topic of subwoofers, the bigger the speaker (given similar other
specs) the lower the sound it can reproduce, as long as it is strong
enough to push that much cone and the air coupled to it.  However, bass
is only non-directional if the wavelength of the sound is more than
twice the shortest dimension of the radiating surface, so a big concert
sub is actually directional down into the frequencies that are often
sent to a sub in the living room, so the good sub in a concert situation
is a lousy sub in the living room. Example, based on speed of sound =
331.45 m/s.  A 30 inch sub is going to be in a cabinet with a face at
least a meter accross, so we need 331.45mps/2*1meter=165.73Hz or lower
to be non-directional.  Many home systems cross the sub over as high as
180 or 200Hz

But that's my last word, because this really belongs on geeks now, and I
don't have time to keep up with geeks anymore :-)

Tim