Engine air filter for aperiodic membrane?

[Datoyminaytah]

Has anybody used, or heard of using, a rectangular engine air filter (car or lawn mower) for an aperiodic membrane, or at least for a layer of it? I would think they would be easy to design an enclosure around, because they are self-supporting and have a fixed size, but I don't know if they would provide enough resistance.

[Variac]

Just a bump to this thread, because I think it's a brilliant idea!!

How simple and clean to have a round ring auto air filter between a baffle with the driver and the box baffle. They come in so many sizes from lawnmower to diesel ship engine that I would think we could find one that works... Or ring filter could be on the back of the box , covering a large hole, with a circular cover clamping it in place...

Of course flat filters would work as well as ring , probably

Also they would be very consistant I would think, as long as one used the same manufactiurer

Now we need someone who knows something to try it .....

Any suggestions of existing aperiodic designs?

[454Casull]

http://www.linkwitzlab.com/frontiers.htm#A

It may not be widely known that filter media for the filtration of liquids and gases in the chemical and other industries can have applications in acoustics. Such filters may be thin sheets (<1 mm thick) of a non-woven, sintered, stainless steel fibre matrix for filtration levels from 5 to 50 micron. Airflow at a constant velocity v through the filter material causes a pressure drop Dp between input and output sides corresponding to a flow resistance Rf = Dp/v [Ns/m3].

It is common in this industry to specify an inverse quantity which is Permeability P [l/dm3/min] at 200 Pa pressure drop. Flow resistance and permeability are related by Rf = 1200/P in this case. Resistance values between 150 and 3500 Ns/m3, or 15 to 350 rayl in the older cgs system of units (1 rayl = 10 Ns/m3), are obtainable from a single filter sheet. For comparison the free-space acoustic field impedance p/v = rc is resistive and has a value of 414 Ns/m3 = 41.4 rayl.

Materials are available with greater structural rigidity such as
Feltmetal with thickness up to 6 mm and resistance between 6 and 50 rayl. The impedance is resistive and constant over the 20 Hz to 2 kHz range that I tested. Linearity should also be quite good, but I have not measured it.

The challenge remains to build an acoustic termination for the inside of a box. Feltmetal and filters should be readily useable for a cardioid speaker, but for a woofer application their linearity at high volume velocities needs investigation.

[Brushpilot]

Parts Express sells ready made aperiodic vents for less than $8 each, manufactured by Scan-Speak. Why go through all the trouble of designing and building your own out of automotive air filters?
I'm unfamiliar with aperiodic designs, so maybe the store bought vents are not a good choice for your particular system???

[Hybrid fourdoor]

It might go well with the Aluminum flared port in this thread

[Variac]

I think I need one of those flared ports on top of my amp!!
Should make it sound "faster"
Might work well with a cooling fan!
For big boxes you need 3 or 4 of those SS aperiodic vents. For So the auto air filter might be cheaper and easier and look cool!! Also might be more consistant- I think the SS allows you to add more material to change it's response, but the filters would be consistant once a size was established.

[GM]

Have not used a rectangular one, but on the Fullranger forum awhile back I described my speaker system I use for listening to AM stereo talk shows/background music while on-line/doing paperwork. This is a pair of no longer available 8" ~fullrange drivers (RS 40-1271) with a very high 3.3 Qts and the typical treble 'shout', though with good HF BW. They are mounted magnet to magnet (to act as mutual bucking magnets so they don't affect the computer) with a round automotive air filter, all clamped together with some threaded rods/hardware. This adds a bit of mutual mass loading also, which 'tightens up' the LF, increasing mids clarity. The filter is covered with a thin foam filter that's normally added for off road/dusty apps.

Since the aperiodic loading lowers the effective Q considerably, rolling off what little LF there is with a 75Hz Fs driver, I used an old, discarded receiver's bass/treble tone controls to apply a 'happy face' EQ, then put a 25 ohm pot in each driver's line to dial in a somewhat more extended/flatter response, at the expense of efficiency of course.

Sitting on top of the receiver with the drivers facing L/R, I use their cardboard shipping boxes stuffed with fiberglass insulation and some plywood scraps as crude adjustable baffle boards to reflect the mids/HF back at me. What little mids/HF coming out of the filter/foam 'baffle' adds a little center/surround channel-like ambiance. Crude, cheap, yet surprisingly natural sounding overall.

BTW, this concept is not very innovative on my part, but an adaptation of several stereo consoles made in the early '50s, which in turn used design elements from much earlier mono designs.

Anyway, WRT using rectangular ones, there are designs with the vents that run down the sides of the BR cab, so making an aperiodic version using automotive air filters should work well, and if additional attenuation is required you can fine tune by using foam, as I did.

GM

[Datoyminaytah]

> Parts Express sells ready made aperiodic vents for less than $8 each, manufactured by Scan-Speak.

Thanks, I did not know that. I just discovered aperiodic membrane enclosures a few days ago and didn't know such premade parts were available. I'll take a look at them.

[el`Ol]

I was thinking about mounting an air filter behind open baffle speakers (back side open). I asked the market leader in Germany, they told me MTU airplane filters have the size I need, price 590 Euro per piece plus VAT. Is there a material from which I can build such a cylinder myself?

In case you are still interested, VW Rabbit filters are rectangular.

[kan3]

You don't want a premade vent - how exactly do you plan on adjusting the amount/area of the material used to give the desired reponse with one of those?