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Old 30th December 2001, 10:18 PM   #1
pkgum is offline pkgum  Australia
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Default unibox

in unibox..whats the difference between 'physical Vb' and 'Vb'?

which one do I use to build a box?
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Old 31st December 2001, 11:14 AM   #2
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Don't know unibox, but normally this is the difference:

Physical Vb : the 'real' geometrically measured volume

Vb : the volume the speaker 'thinks to see'.

The filler material (dacron, glasswool etc.) changes several parameters in your box. One of the most important is that the volume seen by the speaker looks greater.
In simple words, airflow is slowed down when passing trough the filler, hence the time needed by a soundwave to reach the end of the box is bigger, like if the box has a bigger volume.
As rule of thumb, for a typical glasswool panel ov 2" you may consider:
Filler on box walls : vol. increase about 10%
Filler on all box : vol increase about 20%
Filler pressed in the box : about 30 %

In reflex enclosures stay with filler on the walls, filling up the box will add other important effects on the vent behavior.
In sealed box you can use more filler, just remember that the Q factor will decrease more than proportionally.
Anyway if the software takes in account the effect of the filler, it should also calculate for you the right parameters.
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sandro
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Old 1st July 2006, 01:59 AM   #3
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Came across this searching on "Unibox".

"In simple words, airflow is slowed down when passing trough the filler, hence the time needed by a soundwave to reach the end of the box is bigger, like if the box has a bigger volume."

The increased effective volume is entirely unrelated to teh speed of sound.

It's due to ther filler absorbing the heat of compression of the air, reducing the pressure rise t that of a bigger box.
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Old 1st July 2006, 07:58 AM   #4
Svante is offline Svante  Sweden
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Quote:
Originally posted by noah katz
Came across this searching on "Unibox".

"In simple words, airflow is slowed down when passing trough the filler, hence the time needed by a soundwave to reach the end of the box is bigger, like if the box has a bigger volume."

The increased effective volume is entirely unrelated to teh speed of sound.

It's due to ther filler absorbing the heat of compression of the air, reducing the pressure rise t that of a bigger box.
I also prefer your explanation, but it is a fact that if the compression would be isothermal in normal air, the velocity of sound would be lower.

So, it is sort of right, anyway. Actually, I think that if you enter this lower c in the equation for acoustic compliance:

Cav=V/(rho0*c^2)

I think you end up with the right numbers.

But as I said, I also prefer the heat exchange explanation.
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Old 1st July 2006, 08:03 AM   #5
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"So, it is sort of right, anyway. "

Just because the sound speed is affected doesn't increase its relevance.

While it might be true that the box is acoustically bigger in a wavelength sense, it's the softer air spring that lowers the driver/box resonance.

IOW it's a purely mechanical phenomenon, not acoustic.
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Old 1st July 2006, 08:59 AM   #6
Svante is offline Svante  Sweden
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Quote:
Originally posted by noah katz
"So, it is sort of right, anyway. "

Just because the sound speed is affected doesn't increase its relevance.

While it might be true that the box is acoustically bigger in a wavelength sense, it's the softer air spring that lowers the driver/box resonance.

IOW it's a purely mechanical phenomenon, not acoustic.
Now I don't want to get into a semantic argument over this, as I said I also like the isothermal explanation better. And I agree that the time that it takes for the sound to propagate from one wall to the other is largely irrelevant for the topic. But there is a side of it where c can be seen as an "indicator" of what has happened to the medium. And c can be used to calculate the compliance, so in a sense c alters the compliance.

...but it is not because of the longer time it takes for the sound to reach the walls.

... and also, acoustics is essentially mechanics, when you get down to the nuts and bolts of it.
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Old 1st July 2006, 06:29 PM   #7
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"And c can be used to calculate the compliance..."

How so?

c depends on density.

So far nothing but tenuous potential connections between sound velocity and the effective volume increase have been made, when there exists a well established thermodynamic (better than my calling it mechanical) explanation.
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Old 1st July 2006, 08:22 PM   #8
Svante is offline Svante  Sweden
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Quote:
Originally posted by noah katz
"And c can be used to calculate the compliance..."

How so?

c depends on density.

So far nothing but tenuous potential connections between sound velocity and the effective volume increase have been made, when there exists a well established thermodynamic (better than my calling it mechanical) explanation.
Well, as I said, the classic equation for calculating the acoustic compliance is Cav=V/(rho0*c).

In turn c=sqrt(gamma*p0/rho0), which turns the compliance equation into

Cav=V/(gamma*p0)

gamma=1.4 for diatomic gases, p0 is the (atmospheric) pressure.

Now in the conditions for these equations is adiabatic compression. If you instead assume isothermal compression and the gas law (what is that in english?) pV=nRT=constant, the equation turns into:

Cav=V/p0

Neither of these equations depend on c, which is what I guess you are aiming at, and that is partly why I prefer to look at it the same way as you.

Now I think I'll back off this discussion.
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Old 2nd July 2006, 01:20 AM   #9
MJK is offline MJK  United States
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Quote:
How so?

c depends on density.
c is a function of the ratio of specific heats gamma which for an adiabatic process in air has a value of 1.4. For isothermal the value is 1.0, but I am sure you know that already. Using these in the equations provided by Svante one can see that gamma, c, rho, and the rest of the terms in the equations can be used in different combinations to calculate the box compliance Cab. I prefer to keep track of c in a fiber fill enclosure because at some point it is needed to accurately determine the standing wave frequencies. The speed of sound does change a little in fiber filled enclosures.

If you are interested in reading a paper that attempts to mathematically solve for the behavior in a fiber fill enclosure I recommend :

"Thermal Time Constants and Dynamic Compressibility of Air in Fiber Filled Loudspeaker Enclosures" by Gavin Putland published in the JAES Vol 46 No 3, March 1998.
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Old 2nd July 2006, 01:27 AM   #10
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"c is a function of the ratio of specific heats gamma which for an adiabatic process in air has a value of 1.4. "

Are we talking about the same c? I thought it was the speed of sound.

And aplogies to everyone for being grouchy. I have a sore spot from all the uninformed opinions/explanations I used to read and believe before I got an engineering education.
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