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

The phrase "They are very efficient - in a cabinet, the PM7 hits over
103dB/watt." comes up.

Now, this has always mystified me, especially since I asked an 'expert'
at a famous (but fairly recently changed hands) 'speaker company'
about this very thing and he didn't think the sensitivity of a speaker
(Fostex in the Buschhorn cabinet, at the time) could be changed
(increased *or* decreased) by the enclosure it was used in! So, who is
right here? (Makers claim for the PM7A is a sensitivity of 96 dB at
1m/1kHz/1 watt...)


Dead right. The efficiency of a speaker is built into the driver when it
is designed. I guess you have heard of the Thiele Small parameters; they
describe the sizes, masses, springiness, damping - all the things that
the designer will choose when he specifies his speaker. Anyway,
efficiency can be calculated straight from a couple of those parameters
(can't remember which right now).

I have my doubts that is the entire story. For a speaker unit in free space
the acoustic coupling between the cone/piston movement and the air will
vary with frequency. The electromechanical efficiency will tell you how
much cone displacement you get for a given electrical signal. IIRC over a
fair range speakers tend to be mass-limited where the wavelength isn't tiny
compared with the cone scale-size.

But then there is the question of how much air pressure variation you
radiate for a given cone displacement/velocity. This also affects the
efficiency.

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?

In practice also the efficency may improve if judged in terms of *volts* in
if the change in arrangement drops the impedance. So values quoted in terms
of presuming a given drive *voltage* may not be the same as those based on
the power power into the coil.

Anyway lets start with return springs. There are two. The first most
obvious one is the rubber suspension and it's easy to see how that
works. You end up with a spring and mass (the cone) which makes for a
resonance called Fs. As soon as you put the driver in a cabinet you add
a second spring alongside the first - this is the springiness of the air
in the cabinet. The net effect is a stiffer overall spring which moves
the frequency of the resonance upwards.

Does that always shift the compliance limited range up to wavelengths
significantly shorter than the cone scale size? I assumed not.


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.

Slainte,

Jim

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

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:

The phrase "They are very efficient - in a cabinet, the PM7 hits over
103dB/watt." comes up.

Now, this has always mystified me, especially since I asked an 'expert'
at a famous (but fairly recently changed hands) 'speaker company'
about this very thing and he didn't think the sensitivity of a speaker
(Fostex in the Buschhorn cabinet, at the time) could be changed
(increased *or* decreased) by the enclosure it was used in! So, who is
right here? (Makers claim for the PM7A is a sensitivity of 96 dB at
1m/1kHz/1 watt...)


Dead right. The efficiency of a speaker is built into the driver when it
is designed. I guess you have heard of the Thiele Small parameters; they
describe the sizes, masses, springiness, damping - all the things that
the designer will choose when he specifies his speaker. Anyway,
efficiency can be calculated straight from a couple of those parameters
(can't remember which right now).

I have my doubts that is the entire story. For a speaker unit in free space
the acoustic coupling between the cone/piston movement and the air will
vary with frequency. The electromechanical efficiency will tell you how
much cone displacement you get for a given electrical signal. IIRC over a
fair range speakers tend to be mass-limited where the wavelength isn't tiny
compared with the cone scale-size.

Nothing is ever the entire story, but all I have done so far with
speakers suggests that as a first approximation it seems to work out.

But then there is the question of how much air pressure variation you
radiate for a given cone displacement/velocity. This also affects the
efficiency.

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.

In practice also the efficency may improve if judged in terms of *volts* in
if the change in arrangement drops the impedance. So values quoted in terms
of presuming a given drive *voltage* may not be the same as those based on
the power power into the coil.

Anyway lets start with return springs. There are two. The first most
obvious one is the rubber suspension and it's easy to see how that
works. You end up with a spring and mass (the cone) which makes for a
resonance called Fs. As soon as you put the driver in a cabinet you add
a second spring alongside the first - this is the springiness of the air
in the cabinet. The net effect is a stiffer overall spring which moves
the frequency of the resonance upwards.

Does that always shift the compliance limited range up to wavelengths
significantly shorter than the cone scale size? I assumed not.


I wouldn't have thought so.


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.

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).

d

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

--
Change 'noise' to 'jcgl' if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

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

"Don Pearce" wrote


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.


Don't bust a blood vessel over this, Don - I can continue to live in
*ignorance* of this, it doesn't matter that much; I'll throw it onto the
'Feck Nose' heap with all the other little mysteries I have encountered in
life, but not yet conquered! (As opposed to throwing it on the 'Who GAS'
heap where stuff like CDs lie.... ;-)

There's another issue, as of this morning - looks like I've got a
microphonic *valve socket* on an amp I have just re-activated and it's got
me wondering about cobbling together one of these 'Tripath' amps pro
temps....?? Just curious - David Holgate (past UKRA poster) was very
enthusiastic about them offlist a while back!!

Also, I've read your comments on suspension amd springiness again, but still
don't know exactly what it is that is directly affected (or improved,
presumably) by 'high flux density'...??

(But I guess that can go on the heap also!! :-)

"Keith G" wrote



There's another issue, as of this morning - looks like I've got a
microphonic *valve socket* on an amp I have just re-activated and it's got
me wondering about cobbling together one of these 'Tripath' amps pro
temps....?? Just curious - David Holgate (past UKRA poster) was very
enthusiastic about them offlist a while back!!


Or just grab one of these:

http://cgi.ebay.co.uk/High-Quality-C...QQcmdZViewItem

"Keith G" wrote



There's another issue, as of this morning - looks like I've got a
microphonic *valve socket* on an amp I have just re-activated and it's got
me wondering about cobbling together one of these 'Tripath' amps pro
temps....?? Just curious - David Holgate (past UKRA poster) was very
enthusiastic about them offlist a while back!!


Or just grab one of these:

http://cgi.ebay.co.uk/High-Quality-C...QQcmdZViewItem

??

At that price it ain't worth plugging the iron in, let alone starting to
source the bits!!

On Thu, 9 Apr 2009 14:04:07 +0100, "Keith G"
wrote:


"Don Pearce" wrote


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.


Don't bust a blood vessel over this, Don - I can continue to live in
*ignorance* of this, it doesn't matter that much; I'll throw it onto the
'Feck Nose' heap with all the other little mysteries I have encountered in
life, but not yet conquered! (As opposed to throwing it on the 'Who GAS'
heap where stuff like CDs lie.... ;-)

There's another issue, as of this morning - looks like I've got a
microphonic *valve socket* on an amp I have just re-activated and it's got
me wondering about cobbling together one of these 'Tripath' amps pro
temps....?? Just curious - David Holgate (past UKRA poster) was very
enthusiastic about them offlist a while back!!

Don't know the tripath, but when it comes to microphonic, it is always
the valve at fault. Anti-microphonic sockets just provide some
suspension to keep the valve steady. Unfortunately a common error is
to carefully fit a nice anti-microphonic socket, then completely wreck
it by stiff, short wiring anchoring the thing firmly to the chassis.

Also, I've read your comments on suspension amd springiness again, but still
don't know exactly what it is that is directly affected (or improved,
presumably) by 'high flux density'...??


Flux density? More is better. The force exerted by the wire is
proportional to it (and the current, and the number of turns in the
coil).

d

"Don Pearce" wrote in message
news:4a00f601.258178859@localhost...
On Thu, 9 Apr 2009 14:04:07 +0100, "Keith G"
wrote:

There's another issue, as of this morning - looks like I've got a
microphonic *valve socket* on an amp I have just re-activated and it's got
me wondering about cobbling together one of these 'Tripath' amps pro
temps....?? Just curious - David Holgate (past UKRA poster) was very
enthusiastic about them offlist a while back!!

Don't know the tripath, but when it comes to microphonic, it is always
the valve at fault.


Noop. Simple checking with 3 different valves (out of 4 available)
demonstrates the problem to remain with the socket always. I suspect the
soldering or it just needs proper, tricky cleaning - the problem is that its
PCB mounted, so not trivial to just swap it out and move on!! The nuisance
is that it pops and spits quite hard when its warming up and that ain't good
on a Lowther! (Voice coils wired inside and out and one will fall off with
that sort of behaviour, apparently!)


Anti-microphonic sockets just provide some
suspension to keep the valve steady. Unfortunately a common error is
to carefully fit a nice anti-microphonic socket, then completely wreck
it by stiff, short wiring anchoring the thing firmly to the chassis.


Sure. Apart from the physical coupling, hardwiring anywhere/anything too
tight in a valve amp which is going to get quite hot is not a good idea!



Also, I've read your comments on suspension amd springiness again, but
still
don't know exactly what it is that is directly affected (or improved,
presumably) by 'high flux density'...??


Flux density? More is better.


Yep. That much is obvious - if nothing else, the figure goes up with the
price!! :-)


The force exerted by the wire is
proportional to it (and the current, and the number of turns in the
coil).


OK, I'm getting the idea - my initial thoughts were probably not to far out
then: I suspected it was like shortening a dog's lead - the higher the flux,
the shorter the lead. Which, I guess, is what give Lowthers their 'speed'
and supreme clarity - or, to put it another way, their entire *lack* of
'flubberiness'!

"Don Pearce" wrote

Don't know the tripath,

There's a fairly comprehensive write-up here, if you are interested:

http://www.tnt-audio.com/ampli/tripath_amps_e.html

(I believe the 'Charlize' amplifier is named in honour of an attractive
actress...??)

But this is what David Holgate was enthusing about - a 'Gainclone':

http://cgi.ebay.co.uk/20W-Stereo-DIY...1%7C240%3A1318


(Try eBay auction No. 310125391530 if that link don't work!)