Virtual volume increase using wadding

[keladrin]

Hi,

I am designing an 11l reflex enclosure and wish to have some idea of the effect of using wadding on the tuning frequency - I am planning on using 4cm thick cotton woll on all internal walls except the front. What sort of percentage virtual volume increase can I expect (just a rough guide).

Thanks

[richie00boy]

None if you are just lining the walls. A reflex box should not be stuffed either, just lined, or you restrict the reflex action.

The lining will just damp the mid and treble down.

[keladrin]

Thanks Richie,

I am a bit confused now though as I have read some websites that say differently - for instance:

http://home.new.rr.com/trumpetb/audio/fboxjs.html

[AndrewT]

Hi,
the volume increase effect you refer to is used for sealed box cabinets.

The air inside a reflex (ported) box needs to be able to resonate for the reflex action to work as predicted.

The lining is there to control the reflections as stated by Richie. Keep it as thin as possible. Why avoid the front?
Use it on all flat surfaces including any bracing.

[keladrin]

Thanks Andrew,

But what is the website I mentioned on about when it indicates the change in virtual volume for he bass reflex enclosure?

Thanks,

[keladrin]

Hi,

I include some information form another website that clearly indicates that damping materialk effects port tuning so please clarify:


Damping material:
Vb = 29.5 litre
Scan-Speak 8" bass driver.
vent: 72 (ID) x 200 mm


Fig. 9. Impedance of bass driver with no damping (blue) and heavy damping (red).
Heavy damping here means 10 mm polyester foam on all internal panels and 2 sheets of Monacor MDM-3 added - and not blocking the passage to the vent!
The MDM-3 is a great material consisting of 2/3 sheep's wool and 1/3 polyester fibre.

As can be seen, the addition of damping material has a significant impact on the impedance profile and the vent tuning is lowered from 37 Hz (no damping) to 33 Hz (heavy damping) and no surprise here. Again damping material increases the virtual volume of the cabinet and has to be taken into account when deciding Fb and vent length.

Thanks,

[sreten]

Hi,

The volume increase depends on the amount of stuffing.

For reflex boxes the amount of stuffing can vary but the stuffing
must be kept away from the port mouth, e.g. for a tall floor stander
with a low port stuffing the top half of the box is quite normal.

For a lively reflex you generally only line the box, which should not
be kept as thin as possible as stated, it should be as thick as is
reasonable, I'd say a minimum of 1/2" but 1" is much better quality
foam. In a small box 1" will be a very significant proportion of the internal volume.

Lining the box will increase Vb ~ 0.2 x the above propotion, i.e.
for 50% = ~ 10% and if you use BAF or similar behind the driver
but away form the port perhaps another 5%.

My boxes are moreorless completely filled with acoustic foam
with the area near the port filled with BAF and the port still
operates but at a reduced level (Musical Fidely MC2s). I
believe it is like this because Q is marginal for reflexing.

/sreten.

[AndrewT]

Hi,
well what can I add? Your research clearly shows that damping can be used to tune a reflex.

The air inside a reflex (ported) box needs to be able to resonate for the reflex action to work as predicted.

note the use of "predicted". It appears you may be going down the route of extensive tesing and measurement. Will this manufacturer be willing to give you the design information to allow you to replicate or generate your own design?

[keladrin]

Thanks Sreten,

You have been a great help.

[Svante]

I agree (as usual) with sreten , and would like to add a couple of more theoretical thingeemees.

Stuffing a box has two effects.

1. The box appears bigger due to isothermal compression/decompression of the air. If the box is completely filled with an ideal isothermalizing material, the volume increase is 40%. In reality it is less.

2. The stuffing adds resistive losses, which typically lowers the Q values of the various resonances in the system.

Both the above are true for both closed boxes and bass-reflex boxes.

For the bass-reflex (1) is great help, it means that the box can be made smaller.

(2) on the other hand is both positive and negative. First it can reduce the amplitude of the standing waves inside the box. That is good. Second, it adds resistance also near the port tuning frequency. This is bad. The result of this resistance is that a larger box volume is required for a given frequency response. In other words, this counteracts the positive effects of (1).

From the above one can understand that a bass-reflex box should have some amount of resistive damping, in order to kill the standing waves, and this can be acheived with fiberglass or similar. A large portion of the remaining volume can be filled with a material that has a low resistance, but a strong isothermalizing effect, such as polyfill (I think that is the english name for it).

And the space near the port should be left completely free of stuffing, due to the high air velocities there.