Internal aperiodic devices (eg. Briggs' 'acoustic filter')

[Ian J]

In Briggs' (of Wharfedale fame) classic 'Loudspeakers' he mentions a device called an acoustic filter - essentially a solid shelf brace with slits cut into it- to be used between drive unit and vent in a reflex enclosure. Apparently this reduces excessive cone excursion at low freq. and makes the box 'act as though it were bigger' which would square with it being an internal aperiodic damping device. The only other example I've seen of this is in a current UK kit speaker from World Designs which uses an internal brace with the holes covered by open cell foam to damp the main resonance, though in this case the cabinet is sealed. Has anyone any experience with these internal aperiodic techniques and are they worth it over putting the filter at the cabinet boundary?
Ian

[kelticwizard]

I haven't used one, but they are fairly common. The consensus seems to be that they take a sealed box and change the response curve to that of a lower Qtc. Once you go this route, there no longer seems to be much useful bass output coming out of the port. These seem to be used by people interested in tailoring sealed box response.

For instance, if your sealed box/driver combo has a Qtc of 1.0, putting in one of these will lower it down to 0.8 or so. You are free to use more than one.

Here is a ScanSpeak model of flow resistance vent-many builders simply make their own vent and stuff it to taste.

http://www.partsexpress.com/pe/showd...number=296-546

[sreten]

Hi,

Well KC has not answered the question at all so I may as well chirp in.

For a vented box the general idea is for the vent to work which
requires the main airspace to be relatively undamped, usually a
vented box is foam lined, to damp upper resonances but not
affect the main airspace too much.

A resistive partition inside the box is not going to help much or do
anything particularly useful as I see it. It may allow some wierd
vented arrangement but proper design appeals far more.

Reduction of exccessive cone excursion is another moot point for
vented boxes. This occurs with drivers that have too low Fs and
very large Vas and low Qts such that they are used in boxes
very much smaller than the Vas, whilst a higher Fs and lower Vas
would barely change the in box alignment, it would reduce the
excursion below the vent frequency.

GB and Wharfedale IMO should not be taken seriously as they
where responsible for some truly awful poorly designed products.

For example my Fathers 8" RS/DD Gold driver, twin cone fullrange,
DD = double diaphragm, RS = rolled surround, I discovered later
came with an acoustically transparent surround = no bass at all.


/sreten.

[lovechild]

hi

I never heared of an aperiodic vent inside a bass reflex system.

But internal ap-vents have been and still are used in cb systems.
George E. Short of North Creek Music published this article,
which pretty much explains the idea behind that typ of enclosure.

best
LC

[crazyhub]

Originally posted by sreten

acoustically transparent surround = no bass at all.


/sreten.

Hi sreten,

What do you mean? I would understand "air sealed surround" instead, no?

[sreten]

Originally posted by crazyhub
Hi sreten,
What do you mean? I would understand "air sealed surround" instead, no?

Hi, i mean the roll surround was not fully "air sealed", /sreten.

[Ian J]

Thanks for the replies. I was thinking of rebuilding my floorstanders, which are reflex loaded, as sealed. Internal aperiodic filters look like a way of damping the box at resonance and maybe disrupting the main cabinet height mode - but I guess the effectiveness (and whether it was needed at all), would depend on the Qts of the driver and driver-box system.
Thanks
Ian

[sreten]

Hi,

Rebuilding vented boxes as sealed usually involves a reduction in box
size (and bass extension). Aperiodic would only affect very small boxes
with high Q and at this point for a driver with low Qts so it can be
vented, high Qbox goes with high Fbox, i.e. relatively no bass.

Whats wrong with the floorstanders ? Why rebuild ? what driver ?

/sreten.

[Ian J]

Hi Sreten,
My speakers are actually from a kit - IPL A2mk4 which use Audax HM170Z18 (Qts=0.32, Vas=28l according to IPL) in a 27l floor standing reflex-ported column. The problems are 1) Bass notes can really set the room off (~12ft sq with large semi-circular bay - poss the worst shape?) I was under the impression sealed boxes may do this less, if only by having less bass; and 2)the unfinished MDF cabs look grim having in fact started to go a bit mouldy (though this is prob due to the fact that the place wasn't heated much last Winter). I'd like to try another material, I have some veneered chipboard about from dismantled furniture I thought I might try...
Ian

[Iain McNeill]

That is a difficult room, and the semi-circular bay will reflect sounds to it's focus point creating a big hot spot.

The problem being with the room means that any speaker putting out those frequencies will also sound boomy. Try moving your floorstanders around and see if you can find a null point to drive.

Otherwise, it's time to buy some furnishing.