Lowther questions....
 

As is being repeatedly pointed out in this group, I am something of an
ignoramus when it comes to certain (most) things 'audio' -some because I
couldn't care less about them (CD) and others because I haven't devoted a
lifetime to studying/working in audio and I'm too damn old to start now!

But I do have a couple of questions - surprisingly perhaps, I like Lowther
speakers (as any number of others have done for the last 70 or 80 years it
appears, but there ya go) and I was skimming through this article (yes, I
know....)

http://www.hi-fiworld.co.uk/hfw/olde...hornspkrs.html

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...)

And this phrase also: "This relied on the same basic twin-cone driver allied
to the new PM4 magnet unit, which has the most unbelievably powerful
gap-flux of 24,000Gauss - stronger than anything else I have ever seen."

....is interesting because one of my pairs of Lowthers has a 'Flux density'
figure of 2.1 Tesla (where 1 Tesla=10,000 Gauss) which is pretty close and
implies *plenty beeg cojones* in some way, but what does it mean? What does
it do? In my ignorance, I imagine it is the 'strength of the return spring'
of the speaker's 'motor' - ie how quickly it can be snapped back from an
excursion??

How silly or wide of the mark is that?

TIA

(It'll be Don, I suspect - if anyone at all....)

Lowther questions....
 

On Wed, 8 Apr 2009 15:56:08 +0100, "Keith G"
wrote:

As is being repeatedly pointed out in this group, I am something of an
ignoramus when it comes to certain (most) things 'audio' -some because I
couldn't care less about them (CD) and others because I haven't devoted a
lifetime to studying/working in audio and I'm too damn old to start now!

But I do have a couple of questions - surprisingly perhaps, I like Lowther
speakers (as any number of others have done for the last 70 or 80 years it
appears, but there ya go) and I was skimming through this article (yes, I
know....)

http://www.hi-fiworld.co.uk/hfw/olde...hornspkrs.html

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...)

And this phrase also: "This relied on the same basic twin-cone driver allied
to the new PM4 magnet unit, which has the most unbelievably powerful
gap-flux of 24,000Gauss - stronger than anything else I have ever seen."

...is interesting because one of my pairs of Lowthers has a 'Flux density'
figure of 2.1 Tesla (where 1 Tesla=10,000 Gauss) which is pretty close and
implies *plenty beeg cojones* in some way, but what does it mean? What does
it do? In my ignorance, I imagine it is the 'strength of the return spring'
of the speaker's 'motor' - ie how quickly it can be snapped back from an
excursion??

How silly or wide of the mark is that?

TIA

(It'll be Don, I suspect - if anyone at all....)



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).

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. The physical
spring of the suspension is described in terms of how big a volume of
air would give the same restoring force. It is called Vas (I'm
guessing that is short for volume, air, spring or somesuch). That's
why small cabinets mean less bass - the air spring is stiffer and the
resonance moves up further.

When you put a port in the cabinet something interesting happens. The
air in the port now becomes a second mass bouncing against the springy
air inside the box. This also resonates, so if you make the frequency
of this the same as that of the speaker you get what is called a
coupled system. When that happens the resonance splits into two, one
moving higher in frequency and the other moving lower. It needs
careful tuning for this to happen accurately, but if you look at the
impedance plot of a properly designed ported speaker you will see the
two peaks at low frequency.

Another way to design a speaker is to attempt an infinite baffle. What
this means in practice is that you put enough air into the box that
its spring effect is so floppy that it leaves the mechanical spring of
the driver essentially unchanged. There are two ways to do this. One
is a huge box, and the other is the transmission line; this simply
absorbs the energy from the back of the speaker over a long enough
distance that it is like it has simply drifted into space. There are
versions that don't absorb all the energy, but allow the line to
resonate, producing a similar effect to the simple port. You've made
several of these.

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

d

Lowther questions....
 

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

Lowther questions....
 

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

As is being repeatedly pointed out in this group, I am something of an
ignoramus when it comes to certain (most) things 'audio' -some because I
couldn't care less about them (CD) and others because I haven't devoted a
lifetime to studying/working in audio and I'm too damn old to start now!

But I do have a couple of questions - surprisingly perhaps, I like Lowther
speakers (as any number of others have done for the last 70 or 80 years it
appears, but there ya go) and I was skimming through this article (yes, I
know....)

http://www.hi-fiworld.co.uk/hfw/olde...hornspkrs.html

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...)

And this phrase also: "This relied on the same basic twin-cone driver
allied
to the new PM4 magnet unit, which has the most unbelievably powerful
gap-flux of 24,000Gauss - stronger than anything else I have ever seen."

...is interesting because one of my pairs of Lowthers has a 'Flux density'
figure of 2.1 Tesla (where 1 Tesla=10,000 Gauss) which is pretty close
and
implies *plenty beeg cojones* in some way, but what does it mean? What
does
it do? In my ignorance, I imagine it is the 'strength of the return
spring'
of the speaker's 'motor' - ie how quickly it can be snapped back from an
excursion??

How silly or wide of the mark is that?

TIA

(It'll be Don, I suspect - if anyone at all....)



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).

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. The physical
spring of the suspension is described in terms of how big a volume of
air would give the same restoring force. It is called Vas (I'm
guessing that is short for volume, air, spring or somesuch). That's
why small cabinets mean less bass - the air spring is stiffer and the
resonance moves up further.

When you put a port in the cabinet something interesting happens. The
air in the port now becomes a second mass bouncing against the springy
air inside the box. This also resonates, so if you make the frequency
of this the same as that of the speaker you get what is called a
coupled system. When that happens the resonance splits into two, one
moving higher in frequency and the other moving lower. It needs
careful tuning for this to happen accurately, but if you look at the
impedance plot of a properly designed ported speaker you will see the
two peaks at low frequency.

Another way to design a speaker is to attempt an infinite baffle. What
this means in practice is that you put enough air into the box that
its spring effect is so floppy that it leaves the mechanical spring of
the driver essentially unchanged. There are two ways to do this. One
is a huge box, and the other is the transmission line; this simply
absorbs the energy from the back of the speaker over a long enough
distance that it is like it has simply drifted into space. There are
versions that don't absorb all the energy, but allow the line to
resonate, producing a similar effect to the simple port. You've made
several of these.

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

d


OK Don, many thanks for taking the trouble with that very comprehensive
reply.

I was starting to respond when I saw summat from Jimbo come in, just now -
I'll scrute that before I say more but what I had started to type was:

Just to be crystal clear: The phrase "They are very efficient - in a
cabinet, the PM7 hits over 103dB/watt." as per the article is *blx* then, if
the driver itself is only quoted at 96 dB??

Does that bit hold up OK? - Because what I have is a clear and direct
contradiction between the implication of the phrase in the article I
referenced above and what I was told by the *better half* of WA a little
while back and what you seem to be saying - that the cabinet doesn't come
into it..??!!

Lowther questions....
 

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

Lowther questions....
 

On Wed, 8 Apr 2009 17:34:47 +0100, "Keith G"
wrote:

OK Don, many thanks for taking the trouble with that very comprehensive
reply.

I was starting to respond when I saw summat from Jimbo come in, just now -
I'll scrute that before I say more but what I had started to type was:

Just to be crystal clear: The phrase "They are very efficient - in a
cabinet, the PM7 hits over 103dB/watt." as per the article is *blx* then, if
the driver itself is only quoted at 96 dB??

Does that bit hold up OK? - Because what I have is a clear and direct
contradiction between the implication of the phrase in the article I
referenced above and what I was told by the *better half* of WA a little
while back and what you seem to be saying - that the cabinet doesn't come
into it..??!!



If "in a cabinet" means a conventional box, then yes it is probably
blx. The cabinet is only really "doing stuff" at the speaker's
resonance. Above that, where measurements are made, it is simply
stopping the stuff round the back getting to the front.

See my reply to Jim, where I concede that if the thing is attached to
a horn (front loading, not the type you made) things are different.

d

Lowther questions....
 

"Don Pearce" wrote in message
news:49f9d301.183695000@localhost...
On Wed, 8 Apr 2009 17:34:47 +0100, "Keith G"
wrote:


Just to be crystal clear: The phrase "They are very efficient - in a
cabinet, the PM7 hits over 103dB/watt." as per the article is *blx* then,
if
the driver itself is only quoted at 96 dB??

Does that bit hold up OK? - Because what I have is a clear and direct
contradiction between the implication of the phrase in the article I
referenced above and what I was told by the *better half* of WA a little
while back and what you seem to be saying - that the cabinet doesn't come
into it..??!!



If "in a cabinet" means a conventional box, then yes it is probably
blx. The cabinet is only really "doing stuff" at the speaker's
resonance. Above that, where measurements are made, it is simply
stopping the stuff round the back getting to the front.

See my reply to Jim, where I concede that if the thing is attached to
a horn (front loading, not the type you made) things are different.


Well then, buggrit - is the guy in the HFW article right or wrong then with
the phrase 'in a cabinet...' which is vague, if nothing else. When I asked
about the Fostex drivers and was told categorically the cabinet could and
did not influence the sensitivity of the drive unit, the cabinet was
specifically the type called 'back loaded horns' which you prefer to call
TLs (Buschhorn Mk2).

Actually 'categorically' is a bit strong - IIRC, it was more a case of 'I
don't see how it [the cabinet] could alter the sensitivity' or somesuch!

(I'll have to have a shufti at the other stuff later, my day's suddenly
caught up with me!!)

Lowther questions....
 

"Don Pearce" wrote


If "in a cabinet" means a conventional box, then yes it is probably
blx. The cabinet is only really "doing stuff" at the speaker's
resonance. Above that, where measurements are made, it is simply
stopping the stuff round the back getting to the front.


Which brings us to the subject of Open Baffle speakers....

??

I gotta stop reading this and get my chores done, I'm all behind!!

Lowther questions....
 

"Keith G" wrote in message

"Don Pearce" wrote


If "in a cabinet" means a conventional box, then yes it
is probably blx. The cabinet is only really "doing
stuff" at the speaker's resonance. Above that, where
measurements are made, it is simply stopping the stuff
round the back getting to the front.


Which brings us to the subject of Open Baffle speakers....

??

With a driver that is has as limited of a range and as peaky response as a
Lowther, does it matter that much?

Lowther questions....
 

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