How to get optimal enclosure damping?

[cirrus18]

I have just built sealed 25 L enclosures for Peerless 830667 drivers as part of a three-way system crossed over at approximately 350 Hz. The other drivers are in a separate spherical enclosures.

The manufacturer calls for the enclosure to be damped/filled 65% but 65% doesn't mean too much because you can get it to this level by fluffing up the filling a lot and hardly using any material or packing it in tight and using a lot.

My question is how does one know, by listening to the speakers, when you have got the filling correct?

What are the audio differences to listen for between a overfilled or under filled enclosure?

I have got a SP meter, if that is of any use.

[bear]

Generally speaking the role of "fill" in a cabinet is to try to increase the effective volume by a few %... it does that by turning sound energy into heat.

The usual technique is to keep the fill fluffy not compressed.

So 65% would likely refer to the volume of the fill...

Having said that what I would probably do is to measure the impedance curve of the woofer and look at the Q of the peak without any stuffing. Then put stuffing in selectively and retest. The optimum stuffing may be seen by finding the point at which adding stuffing no longer has any appreciable beneficial effect.

That point would also likely be where the stuffing does not increase the volume of the box (lower the F3 or the impedance peak) and it starts to raise the F3.

_-_-bear

[454Casull]

Who knows how they calculated that figure?

Overfilled enclosures don't exhibit much of the virtual increase in volume seen by the driver. Underfilled enclosures simply haven't been filled to the point where the effective volume has gone up as much as it can.

http://www.moodym.com/audio/fiber.html

Filling can also attenuate the back wave which can come through the diaphragm.

[cirrus18]

Thanks guys for your replies, it seems that will harmI just have to experiment.

[sreten]

Hi,

Checking the driver specs and box volume shows that one should
maximise effective box volume (not always the case). So stuff and
monitor the freuency of the impedance peak.
Optimum stuffing is when this is at the lowest point.

Alternatively you can use a rule of thumb, e.g. 100% stuffing
is 2 pounds of fibreglass per cubic foot, 65% would be 1.3.

For the rule of thumb search for numbers you can trust.

/sreten.

[Dr. Ram]

The filling itself is not the objective and neither is the effective volume. The real end-objective is the so-called "alignment" you want the sealed enclosure to achieve (or approximate, if you will). A Bessel alignment has certain properties, a Butterworth alignment has different properties, a Chebychev has different ones. Each property is a tradeoff between transient response, flatness of response, sharpness of rolloff, etc etc (you cannot have the best of everything).

So while there are formulas to help you, you do need to experiment as you rightly concluded, and when you experiment with filling, the scientific way to do it is to decide what alignment you want, and look for response curves and characteristics which tell you what alignment you are approaching during testing.

-Ram