Effect of wadding in opening of dipole - diyAudio
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Old 16th March 2009, 07:30 PM   #1
Dr_EM is offline Dr_EM  United Kingdom
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Default Effect of wadding in opening of dipole

Hello all

Don't think this topic has been covered at all yet, it may well be pointless though so this could be why!

Anyhow, the idea is adding layers of BAF wadding accross the openings of a W-profile (or RiPole, or N profile) sub. I originally tried it as a means to form an acoustical low pass filter to attenuate motor vent noises. This is still the main idea of it, but when trying it it got thinking a little more about it.

I noticed that that firstly (and perhaps obviously) the wadding didn't want to stay in place at higher levels when just placed in the openings. If implemented, the wadding would be sandwiched between drilled/perforated MDF/metal/etc. So, this restiction of air movement, would it be correct to assume this will raise the resonant frequency of a driver installed like this?

Would one also experience a loss of immediacy in transient response due to the restriction of air movement? Or since all the frequencies a low will this make no difference? I couldn't detect much difference listening but the adding wasn't held firmly in place.

If carefully designed could this acoustical filter be combined with the electrical/electronic one to reduce the order of filter required, perhaps at the benefit of reduced group delay? Most likely the thickness of wadding required to form a filter at 80hz or so would be far too great though.

Any input welcome!
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Old 17th March 2009, 08:06 AM   #2
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Ok, I'm no expert on this kind of sub, but I know that wadding is there to cushion the air moving. In sealed boxes, it can make the driver act as though the box is up to 40% bigger.

It manages this because it absorbs some of the air movement in the box, thus reducing the amount of damping the walls of the box induce, because they see less SPL altogether.

Putting it into something like a port would reduce the amount of sound comming out, and (presumably) make the driver act like it's in a bigger box.

Most of us know that different box sizes = different port sizes...

So it may go out of tune...

Like I said - I'm no expert of this kinda thing.
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Old 17th March 2009, 12:49 PM   #3
AMV8 is offline AMV8  United Kingdom
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Dr EM

By adding wadding in the tuning vent you effectively reduce the area of the vent. This will reduce the tuning frequency and also lower the volume just above the tuning frequency. If you assume that the vent was the optimum for the speaker/box size then you will possibly also reduce the vent size below the minimum for the speaker/box size which will lead to some distortion.

Having said all of that I would guess that the room accoustics may play a bigger part in the sound you finally hear.

Don
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Old 17th March 2009, 03:20 PM   #4
jbell is offline jbell  United States
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sounds like an aperiodic vent to me.
search came up with this:
Aperiodic enclosures
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Old 18th March 2009, 10:07 AM   #5
Dr_EM is offline Dr_EM  United Kingdom
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Thanks for your responses!

It seems that the conscensus is that adding stuffing will change the open baffle into a collection of aperiodic boxes? That does make sense but the "venting" will still be very large compared to a typical aperiodic.

These "boxes" are not tuned as such. They are merely a convenient way to fold a a large open baffle into a small size. Both front a rear waves are emitted into the room. Naturally, there is a TL resonance dependant on the size of these openings, but it is not the functional principle of the design and is usually notched out if it is low enough to interfere.
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Old 18th March 2009, 03:17 PM   #6
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Apart from being somewhat aperiodic (not fully unless you stiffen it up to prevent motion)
It will (or should I say should?) Create added mass loading to the driver and to some small extent also work as a passive radiator.
The same way damping material to close to the speaker will in a sealed box.
Although I have no idea how large the effect will be.

I made a 45*45*45cm U-baffle for a 12" a while ago. (not optimal I know now)
At the back I put a 5cm thick acoustic wall panel to cover the opening.
It seems do emit more bass from the back than the front but there could be other reasons for that.
I have not evaluated the design since it was built and it has not been used that much.
It did sound ugly at higher frequencies but that's due to the way too long u-baffle and probably too thin walls.
Now that I have a DCX2496 I might test is some more. Although the driver will be ripped out and put together with three other to form a little bigger sub.
But not until I have found out if damping is good or not. Might lower the fs even more to get deeper bass. But at 18Hz fs I'm not sure it's needed.
Not sure if it will increase or decrease Q ether. Might be frequency dependent also.
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Old 21st March 2009, 04:56 PM   #7
Dr_EM is offline Dr_EM  United Kingdom
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Sounds interesting, keep me updated if you get any more results!

I probably won't be building mine for a long time unfortunately
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Old 22nd March 2009, 02:45 PM   #8
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Quote:
Originally posted by David_Web

I made a 45*45*45cm U-baffle for a 12" a while ago. (not optimal I know now)
(...)
But not until I have found out if damping is good or not. Might lower the fs even more to get deeper bass. But at 18Hz fs I'm not sure it's needed.
Not sure if it will increase or decrease Q ether. Might be frequency dependent also.
Which driver did you use? 18fs for a 12 inch is damn good.

How high did you cut it?
Damped u frame is good roughly for up to 200hz and depends also on the lenght. 45cm is quite much.
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Old 22nd March 2009, 03:04 PM   #9
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Quote:
Originally posted by chris661
Ok, I'm no expert on this kind of sub, but I know that wadding is there to cushion the air moving. In sealed boxes, it can make the driver act as though the box is up to 40% bigger.

It manages this because it absorbs some of the air movement in the box, thus reducing the amount of damping the walls of the box induce, because they see less SPL altogether.
snip
A figure like 40% enlargement was uttered 50 years ago by Edward Vilchur (a founder of Acoustic Research and early developer of the "acoustic suspension" concept). He said the stuffing changed the behaviour inside the box from isothermal to adibiatic. I may have that wrong since I don't know what either means really.

Is there any empirical truth to the assertion that stuffing boxes is like enlarging them? If so, is the adibiatic theory correct or chris661's notion?

BTW, for midranges, among my past favourites was a Bozak (rubber coated aluminum midrange drivers) with well-stuffed largish boxes and no back. Sort of solves all the problems of enclosures, eh. Not feasible for woofers.
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Old 25th March 2009, 11:18 PM   #10
Ron E is online now Ron E  United States
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A speaker with resistive damping acts like a speaker with lower Qms. If the damping moves, it will also affect Fs. It's that simple.
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