On Thu, 09 Apr 2009 10:13:43 +0100, Jim Lesurf
wrote:

In article 49f8d005.182930703@localhost, Don Pearce
wrote:
On Wed, 08 Apr 2009 17:11:00 +0100, Jim Lesurf
wrote:

In article 49f6beb7.178501281@localhost, Don Pearce
wrote:
On Wed, 8 Apr 2009 15:56:08 +0100, "Keith G"
wrote:


Thus if you fit a baffle you can prevent air movement being 'short
circuited' around the speaker unit and get larger pressure variations.
Hence - potentially - higher overall efficiency. Unless the pressure
rise simply reduces the movement to compensate exactly. But is that the
case if the speaker movements are mass controlled?


I think we can assume mass control - which is true for the majority of
the operating range. But compliance control is what you use to calculate
the dimensions of the box. at Fs.

I'm not thinking of any specific 'box'. Just of the more general question
of how the efficience will alter when you place any kind of box, baffle,
etc, around the cone.

Consider two assumptions:

1) That the scale size of the speaker is not significantly larger than the
radiated wavelength.

2) That the movement is mass dominated.

Under those conditions anything you place around the cone can be expected
to change the efficiency as it changes the amount of pressure variation
that a given cone movement produces.

As I said, I've never designed a speaker. But I have put speaker units into
surrounds or baffles. The increase in sound level at mid-low frequencies
has been quite noticable. However I have no idea if that takes you to the
kind of values Keith asked about originally.




But none of these things change the fundamental efficiency (sound
power out / electrical power in) of the speaker.

I am less sure of that. However I've never been though the details. nor
designed any speakers, so you may be correct for all I know. Interested
to see what responses you make to the above.


There is a standard equation that derives sensitivity (dB at 1 metre for
1 watt)

112 + 10 * LOG(9.64 * 10^(-10) * Fs^3 * Vas/Qes)

I don't have the derivation for it, but if you check pretty much any
speaker manufacturer's data, the published sensitivity will match.

Again, I have the feeling that simply isn't the whole story. The coupling
efficiency between cone movement and sound pressure is surely going to be
frequency dependent and also be affected by items which alter the air flow
near the cone.

It occurs to me that horn loading will change this considerably, but it
really isn't equivalent because it sort of alters the assumptions
inherent in the T/S parameters by severely increasing the air mass
(equivalent density, if you like).

Maybe in your terms, using a box or baffle *also* alters the parameters.
:-)

There is also the question of the effect of a surround, box, etc on the
speaker input impedance, thus altering the 'efficiency' when regarded in
terms of input voltage - output sound pressure. (As distinct from input
electrical power - output pressure.)

Why do people put cones into boxes if that has no effect on the sound level
radiated at low frequencies?... ;-

Slainte,

Jim

OK, now you have me thinking, but I know my brain is going to start
hurting if I go down that route - I won't be able to put it down.

But speaker sensitivity is generally given at 1kHz, well away from all
the cabinet "stuff" occurring around the speaker resonance. All the
box is really doing is stopping the out-of-phase stuff round the back
getting involved. Boxes certainly do change things to a certain extent
by diffraction if nothing else, but I think the equation above was
derived empirically as the least-worst estimate.

Having said all that, I would be suspicious of a claim of 6dB
improvement just from any mounting method other than a front loaded
horn.

d