"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..??!!