Formula? Lining interior of sealed/ported cabinet.

[rMidi]

I am making a cabinet for a Celestion F12-X200 speaker (this is an FrFr speaker).
I started following their suggested cabinet design (sealed, ported) but found
the dimensions to be not quite right for me.

So, being a programmer I writing a program that allows me to design a cabinet
for the F12-X200 (imperial / metric measurements, Yea from us state-side wood workers for Imperial!)
that allows me to design different sized cabinets, different configurations (Dual F12-X200's, Slant cabinets).

I am about 80% through the mathematics (TS, Tuning, Port Length,...) but in the back of my mind is the
fact that the Celestion Design for the F12-X200 calls for lining the inside of the cabinet with 'Polyfill'.
There is no property data given on the weight / density / thickness of the polyfill itself.

I somewhat understand what lining the interior of the cabinet with poly fill will do but it seems that the effect of it can only be arrived at via empirical data. meaning their is no real science or formulas behind it.
I constantly see sentences (on this four and others) such as:
- 'Most estimates say between 15-25% theoretical volume gain'.
- The use of 'estimates' and 'theoretical' in the same sentence somewhat bothers me.


I took a brief glance at WinISD (which seems to be a really old share-ware type application) and though I do
not have all of the required parameters from Celestion I see that they are applying some sort of formula:
- They use 'Qa' as 'absorption losses'.
It is my understanding that the value of Qa is reflected thusfully::
- 100. Bare Box
- 50. Lined Box
- 10. Stuffed but not over compressed.
- 5. Stuffed and compressed.

And that the lowering of Qa is seen at:
- SPL
- Impedence Charts
- Qtc

They of course are using a formula.

So, is anyone aware of any sort of formula that can be used to model the effects of lining the interior of a ported sealed speaker cabinet?

Thanks

 

[duncan2]

I take it you are talking about internal cabinet resonances but what about cabinet vibration needing bracing for rigidity ?


Even if a loudspeaker cabinet is computer designed it still has to be tested in an

anechoic chamber with expensive calibrated microphones .


DampFact.PDF - Damping_factor.pdf

 

[rMidi]

Thanks for the information Duncan.

The F12-X200 frequency response is 50Hz - 20kHz
It is an FRFR (full range - flat response) guitar speaker.
The lowest frequency a guitar will produce (normal tuning) is ~80Hz.
The cabinet is pretty small (15" x 15" x 20" interior).
The cabinet is made from Baltic Birch.
Joinery is dado+dowels+glue.

With all that said I believe the cabinet itself is rigid enough and would not require internal bracing.

It is my understanding that lining a ported cabinet with poly fill will smooth out some SPL spikes at lower frequencies. I have seen this from empirical evidence based on a few YouTube videos (looks like they are using TrueRTA).

I do understand that designing a cabinet via computer design and the math behind it is really pretty much theoretical.

I am just looking for some sort of formula that 'may' predict what will happen if I where to line the cabinet with some sort of acoustic absorption material.

Thanks.

 

[Galu]

'Most estimates say between 15-25% theoretical volume gain'.

Of course, this theoretical volume gain (5-10% might be more realistic) applies only to fully stuffed sealed cabinets, not to ported ones which are simply lined with absorbent to minimise internal reflections.

You will be aware that there should be no stuffing in the main body of a ported cabinet otherwise the interaction between the bass driver cone and the mass of air in the port would be impeded.

I am not aware of any mathematical formula regarding the lining of a ported enclosure. If nuisance high frequencies are audible through the port, then a thicker absorbent lining would be in order.

 

[Galu]

Looks like there's a mathematical treatment of the effect absorbent material has in a ported enclosure here: AES E-Library >> Absorption Materials in Reflex Loudspeakers

It may be worthwhile looking at the contents of the paper. It examines the effect of absorbent materials on the vortices which form in an unlined ported enclosure.