Re: Seeking advice for buying home audio equipment


<dpierce@xxxxxxxxxxxxx> wrote in message
news:8ef243d1-699e-41f9-bd61-f19b2ca5f435@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Nov 20, 1:53 pm, "Dave" <dspear9...@xxxxxxxxx> wrote:
Now, I made my measurements. Go make yours and let's
compare them.

I would, but, in fact, do not own the equipment required
to make such measurements. We'll have to accept the
validity of your results. Feel free to publish them in this
forum.

As measured:

Infinite baffle - 86.5 dB
50 liters - 86.4
20 liters - 86.7
10 liters - 86.5

These are the averages of several measurements,
each has an error of about +-0.2 dB

Thinking of subwoofers, I would guess that they may
be in a bit of a separate class from smaller loudspeakers
given the large area of the cone vs. the enclosure volume.

Nope, same physics apply: above cutoff, the system is
mass-controlled, not stiffness-controlled, and the efficiency
is independent of enclosure volume.

Couple this with the "long throw" high excursion
of the subwoofer driver and air pressure may become
a factor in the power requirements of the sealed
enclosure units.

Where it DOES have factor is like this: suppose you want
to design a subwoofer with a small volume. Indeed, the
acoustic compliance of such a volume is small. You find
that when you put a high-efficiency woofer with a litght-
weight cone in it, the resulting system resonance is too
high to be a "sub"woofer. So, you add mass to the cone.
This does three things:

1. Lowers the resonant frequency of the system,

2. raises the Qt of the system,

3. lowers the system efficiency.

So, yes, the small, closed box system has a lower efficiency
for a given bandpass, but NOT because the air is stiff, it's
because the moving mass of the cone has to be high to
get the resonant frequency down there. The efficiency suffers
not because the cone is "working" against the air, it's
because the cone is heavy.

If you took the same driver and put it into a tiny sealed box,
a big sealed box, an infinite baffle and a reflex enclosure,
and measured the passband efficiency, you'd get the
same number for each. You'd also get different
responses in the low end as well.

Think of the suspension of a car. If you walk up to the bumper
and try to push it down, it's hard to move because you're working
against the stiffness of the spring. But now try to shake the car
up and down 3-4 times per second. It's REAL hard NOT because
the springs are stiff, but because you're trying to start stop and
reverse a ton or two of mass. In fact, at 3-4 times per second,
it'd be just as hard to shake it if it was floating in outer space,
where the stiffness of the spring has no relevance: you're exciting
it at a frequency where it's the mass that dominates, not the
stiffness. And the point where you transition from the stiffness
dominating to the mass dominating is the resonant frequency
of the car/suspension system.

Same for loudspeakers.

Thanks. That analogy makes sense.

Added resonant mass lowers the frequency which can be reproduced.

So, theoretically one could design a subwoofer of an infinitely small size
with an infinitely low frequency response by using an infinitely heavy
driver driven by an infinitely powerful amplifier.

On a more practical level, that's how Velodyne's DD-10 10" sealed enclosure
subwoofer uses a 310 oz. magnet and a 1250W amplifier to get down to 20Hz in
a relatively tiny box.

Dave S.


.

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