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[email protected] dpierce@cartchunk.org is offline
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Default Seeking advice for buying home audio equipment

On Nov 20, 11:16 pm, Chris Hornbeck
wrote:
Also to be noted is that the 3 dB efficiency gain of ported
systems is really only applicable in the narrow range of the
port's output.


Not correct. Most importantly, the port does not ADD
to the woofer's output over the narrow range, it REPLACES
the driver's output over this narrow raange.

But, more importantly, the 3 dB increase in efficiency is oft cited
and equally often misunderstood. Taking any give driver and
putting in a ported enclosure WILL NOT increase the efficiency
of the system by 3 dB.

The 3 dB results because, for a given enclosure volume
and bandwidth, the driver used for a ported version CAN BE
as much as 3 dB more efficient in the optimum case
(maximum efficiency C2 sealed alignment vs maximum
efficiency lossless B4 alignment results in a 3 dB increase
in system efficuency).

If you have a specific driver, your system efficiency is set,
no matter WHAT enclosure you put it in. The issue is
what type of enclosure, how big and what's the bandwidth
that results.


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GregS[_3_] GregS[_3_] is offline
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Default Seeking advice for buying home audio equipment

In article , Doug McDonald wrote:
wrote:


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.



I've wondered what would happen if you left the mass alone and
simply boosted the bass signal so you get the same speaker movement.
Does this screw up relative phase? If so, what happens if you
use a digital filter before the power amp so as to fix that up?

Would the low bass efficiency be the same as if you had
weighted the cone?

Doug McDonald


I would think efficiency would go down with mass vs watts/equalization,
but just a guess. I give an example of extreme equalization, the Bose
901 with about 18 dB boost at 30 Hz.

greg
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Arny Krueger Arny Krueger is offline
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Default Seeking advice for buying home audio equipment

"Doug McDonald" wrote in
message

I've wondered what would happen if you left the mass
alone and simply boosted the bass signal so you get the
same speaker movement. Does this screw up relative phase?


Relative to what?

If so, what happens if you use a digital filter before the power amp so as
to fix
that up?


In general, bass equalization works, as long as you don't exceed the dynamic
range of the driver.

Would the low bass efficiency be the same as if you had
weighted the cone?


Weighting the cone just cuts the efficiency of the driver in its mass-loaded
range.


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[email protected] dpierce@cartchunk.org is offline
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Default Seeking advice for buying home audio equipment

On Nov 21, 1:28 pm, Doug McDonald
wrote:
I've wondered what would happen if you left the mass alone
and simply boosted the bass signal so you get the same
speaker movement.


You have to stop boosting at some point, so your equalization
has to have a pole or two at some low frequency. That adds
an order or two to the ultimate transfer function.

Does this screw up relative phase?


No. the natural rolloff of the system is minimum phase
so adding a complementary minimum phase equalizer
has no overall phase penalty.

If so, what happens if you
use a digital filter before the power amp so as to fix that up?


There's no need to.

Would the low bass efficiency be the same as if you had
weighted the cone?


"low bass efficiency" is a somewhat curious, misleading
and, overall, meaningless term.

Your goal is to achieve a certain frequency response over
a specific bandpass. The efficiency of the system is
specifically defined to be the passband efficiency. Below
and above the passband, the efficiency drops. That's
why the response rolls off. Changing the amount of power
you apply via an equalizer doesn't change the efficiency
at all.

Now, if what you mean is how does one compare the
efficiency using mass loading vs equalization at some
low frequency, that's a different question.

In essence, if you want to go down an octave, below
the normal cutoff, you either have to add enough mass
(in this case, by quadrupling the mass) or you have to
put 4 times as much power into the system at that low
frequency. The effect is at that low frequency, you have
to put that same power in to get the desired sound
pressure at that very low frequency.



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Chris Hornbeck Chris Hornbeck is offline
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Default Seeking advice for buying home audio equipment

On Wed, 21 Nov 2007 04:41:15 -0800 (PST), wrote:

On Nov 20, 11:16 pm, Chris Hornbeck
wrote:
Also to be noted is that the 3 dB efficiency gain of ported
systems is really only applicable in the narrow range of the
port's output.


Not correct. Most importantly, the port does not ADD
to the woofer's output over the narrow range, it REPLACES
the driver's output over this narrow range.


That's also a useful way of thinking about it, but it's
important for newcomers to the topic to also keep in
mind that *all* of the speaker's output, direct and
through the port, comes from the driver.

At progressively lower frequencies the driver's direct
("front") output begins to fall, then the port's (the
driver's indirect) output rises and falls over a resonant
region. With luck and some work, these add to a more-or-
less flat summed response.


But, more importantly, the 3 dB increase in efficiency is oft cited
and equally often misunderstood. Taking any give driver and
putting in a ported enclosure WILL NOT increase the efficiency
of the system by 3 dB.

The 3 dB results because, for a given enclosure volume
and bandwidth, the driver used for a ported version CAN BE
as much as 3 dB more efficient in the optimum case
(maximum efficiency C2 sealed alignment vs maximum
efficiency lossless B4 alignment results in a 3 dB increase
in system efficuency).


And by defining bandwidth to be the -3dB point. It certainly
has the potential for confusion. 3dB arises again in the
summing of front and (resonantly polarity-inverted) rear
radiation. Many possible interpretations, but yours above is exact.


If you have a specific driver, your system efficiency is set,
no matter WHAT enclosure you put it in. The issue is
what type of enclosure, how big and what's the bandwidth
that results.


Excellent point. In America the term "reference efficiency"
is often used for your concept above.

Much thanks, as always,

Chris Hornbeck
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Chris Hornbeck Chris Hornbeck is offline
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Default Seeking advice for buying home audio equipment

On Wed, 21 Nov 2007 14:57:45 -0800 (PST), wrote:

On Nov 21, 1:28 pm, Doug McDonald
wrote:
I've wondered what would happen if you left the mass alone
and simply boosted the bass signal so you get the same
speaker movement.


You have to stop boosting at some point, so your equalization
has to have a pole or two at some low frequency. That adds
an order or two to the ultimate transfer function.


You might enjoy this:

http://www.linkwitzlab.com/publicati...nd%20Crossover

for an elegant single-stage solution for a two pole resonant
system. Look in the third section, IIRC. Timeless.

Much thanks, as always,

Chris Hornbeck
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