Jim Lesurf in uk.rec.audio:

In article , Arny Krueger
wrote:


My point with the fishing line example is that the box doesn't move
that much, especially compared to the cone. If the box were
suspended, the rule of equal and opposite reactions would hold. Then
motion of the box and the motion of the cone would be proportional to
their respective weights or masses.

If I *could* suspend the sub off the floor I'm sure the people downstairs
would be happy!

I agree with most of the above, but have a slight uncertainty about a
few points... :-)

I'd agree that the action and reaction forces will tend to be equal
and opposite (Newton). However although the movements of the box/floor
will be small in terms of amplitude compared with the speaker cone I
wonder about the radiation efficiency once the floor area is coupled
to the box. Although the displacements are small, the area moving may
be much larger than the area of the speaker cone. Hence I am not yet
sure that the sound levels radiated are simply in proportion with the
masses.

Another point is that the cone/motor is not the entire mass against
which the motor applies force as this must include the moving air mass
loading. That said, I'd expect this to become a tiny contribution at
LF for a typical boxed speaker. However I also wonder how heavy the
cones and motors are for subwoofers as they may need to be very
stiff/strong if the speaker is an infinite baffle.

I'm not sure exactly what the configuration of this sub is called. Quick
bit of ascii art should speak 1k (fixed width font). I think this may be
a 'bandpass' enclosure?
_______
f |__ | | behind the driver is enclosed, with ported enclosure
r | | to the front of it. The port goes very near the middle
o |¯¯ | | wall.
n | \ |
t | D | D = driver
| / |
¯¯¯¯¯¯¯
I'd imagine that the force of compressing the air behind the driver is
considerable, as is the friction of moving air on a non-straight path.
Then again, I've never studied liquid dynamics, so this is really just
guessing.

Not seen any figures / calculations / models of this, but I do wonder
about it. When I fitted spikes to the stands of my ESL63's I did form
the view that the bass was altered. That said, an ESL is a very
different case to a boxed subwoofer, and the ESL63 is pretty light
compared with a typical large boxed speaker, so this might not be a
reliable guide to the general situations.

It'd be hard to imagine that changing the speaker to the size of the
floor wouldn't change the sound at all.

I think the problem is that nobody seems to be able to explain exactly
what is happening, other than the sound is changing. I've read in hifi
mags that spikes DEcouple the speaker by reducing contact area with the
floor. lol!

--
Jim H