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Old 7th January 2003, 08:05 AM   #1
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Default Purpose of lining an enclosure?

Assuming a ported enslosure - what are the real reasons to line an enclosure?
a) To further reduce individual panel resonances
b) To absorb / reduce standing waves
c) To help absorb unwanted noise (speaker motor, particular frequencies (eg. 200Hz+ for a subwoofer etc...)
d) To keep the speaker warm

Have I got one of the above wrong? Any other reasons?

For a subwoofer - assuming lining "helps" in one or other of the above - does it matter what the material is? (thick felt, some sort of wool, dacron / polyester, egg crate foam???). I've read each of these has different absorption characteristics in different frequency bands but am a bit confused.

If I am going to line the box - I just want to know its worthwhile doing so (or why bother)... (like all those flash interconnect cables )

Thanks,
Dave.
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Old 7th January 2003, 09:52 AM   #2
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Dave,

a, b, and c, and in some cases (though not as much in ported boxes as in closed boxes), to give the present box the characteristics of a slightly larger box.

You should definitely line the box for all the reasons you gave above. See this thread for further discussion on linings and their relative effectiveness.
A couple of questions from a newbie

Hope this helps
Rodd Yamashita
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Old 7th January 2003, 03:35 PM   #3
jmiyake is offline jmiyake  United States
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Well standing waves should not be much of an issue for a subwoofer. The box is many times smaller than a single wave. Also traditional lining would not have much effect on the low frequency produced by a subwoofer anyway.

There is the benefit of enlarging the apparent volume by stuffing, especially in a closed box.

For a subwoofer the main issue is cabinet wall vibration. Standard lining does not have much effect on this. Some amount of mass is required along with damping to absorb this.

You should isolate the woofer from the cabinet. The main cause of cabinet wall vibration is from the physical contact of the woofer basket.

Thickening the walls also reduce the vibration and will eliminate it if the walls are thick enough. Which is why some go to extreme measures in building boxes, ie ... concrete, lead, bricks... Using a sonotube is a real neat way to bypass a lot of these issues since a tube will not expand under pressure and does not have a lot of the disadvantages of flat walls.

You should brace the box extremely well. I think it is best if the braces are single pieces with cutouts and unbroken sections connecting the center of opposing walls.

Bracing reduces cabinet wall vibration but it will not eliminate it. To do this you need to damp the vibration. To do this you need to add some damping and some mass. Some coat the inner surface with rubber or tar, or sand mixed in with some sticky goo.

My favorite method is to provide a constrained damping layer. This is a rigid mass, that is attached by a flexible adhesive. Vibration of the outerwall is transfered toward the rigid mass, and absorbed by the friction between the layers. I sometimes attach plates of hardboard, with about 1/16" flexible chaulk.

For my next subwoofer, I plan on trying a box within a box approach. This is a braced box, with the subwoofer driver attached, contained within another box, with a thin flexible layer between them.

James
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Old 7th January 2003, 04:29 PM   #4
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James,

I agree with most of what youíve said. I also like your arrays on your web site, very nice. They remind me of the old Infinitys.

Although I do have a different opinion on a couple of the point you made. A woofer (particularly a ported sub-woofer) can generate quite a lot harmonic distortion in the lowest octave. The third or forth harmonic can create standing waves within the box that will be transmitted through the port if they are not damped. This is one of the primary reasons for lining a ported enclosure.

Any direct radiating speaker (woofer or not) should be rigidly mounted to the baffle. Decoupling the speaker from the baffle sets up another resonance (most likely in the audible range) that you now have no control over. The baffles on my subs are nearly 2Ē thick with layers of plywood and MDF. The woofer is bolted directly to the baffle with only a thin gasket to form a seal. In this way the speaker frame is fixed to the 90lb. enclosure so the only thing vibrating is the cone.

Rodd Yamashita
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Old 8th January 2003, 01:03 AM   #5
jmiyake is offline jmiyake  United States
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Rodd said:
The woofer is bolted directly to the baffle with only a thin gasket to form a seal.
-----------------------------------------
I am not sure we totally disagree here. The "thin gasket" would be considered decoupling to me. Perhaps not completely, and perhaps not by your intention. I certainly don't advocate loosely mounting a driver.

Rodd said:
The third or forth harmonic can create standing waves within the box that will be transmitted through the port if they are not damped.
-----------------------------------------

I find this an interesting point one that I am not familiar with. If so it is an issue frequently ignored in subwoofer design (which doesn't mean it isn't true).

I would like to understand more about this.
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Old 8th January 2003, 01:55 AM   #6
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I'd like to add something to what Rodd said.
Don't forget that--unless you're using some sort of brick wall filter--you're still going to get some information above the cutoff frequency for the crossover. If the cabinet has a resonance, it won't take much energy to get it going.
Furthermore, regardless of the crossover point and driver energy, you're going to be taking in frequencies from the main speakers, even if they're all the way across the room. This is easy enough to demonstrate. Unhook the sub, put a mic in the cabinet (easiest if it's a ported design), and play the main speakers. If you listen to the output from the mic via headphones, you'll hear some pretty interesting things going on inside the cabinet. If your headphones are open style and the music from the mains is too much, then record the mic signal on a cassette deck and play it back afterwards.
Incidentally, this is a mild argument against using reflex cabinets. Sealed cabinets and transmission lines are less prone to this kind of thing.
Another source of higher frequency information is cone breakup. This will vary quite a bit with driver design.
All in all, there are more high frequency things going on in subs than meet the eye.

Grey
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Old 8th January 2003, 02:41 AM   #7
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Thanks Gray, I totally overlooked the idea of the low-pass rolloff rate. Furthermore, with sub-woofers you want a relatively steep high-pass rolloff rate if you donít want to contaminate the mid-bass output with the sub-woofer output particularly if your xover point is approaching 100Hz or higher. I learned first hand that this can just destroy your imaging. 3dr or 4th order slopes are called for here and 12db/oct cutoff may be too shallow.

While Gray was replying to this thread, I did find a great reference site with a number of AES presentations on the large signal performance of direct radiating loudspeakers. There is a lot of discussion about excursion related distortion products.

http://www.klippel.de/pubs/default.asp

WRT my speaker gasket, I use a thin, soft, closed-cell foam that compresses to virtually nothing. So for all practical purposes, the speaker is rigidly mounted to the baffle.

Rodd Yamashita
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Old 8th January 2003, 06:35 AM   #8
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Default 6db/slope crossover ok?

What you said about crossover slope worries me. I just realised the plate amp I intent to by only has a 6dB slope! - which doesn't seem steep enough to me?

I have not measured my mains - so can't say what a good integration slope would be (I can't control the main's xover since my receiver is an old 2 channel and I'm not about to pry into my mains - the drivers are not screwed into the cabinet in the conventional way)

Should I be looking at an 18dB slope (or greater?) Can I achieve this by putting an inductor in series to the parallel DVC wiring to the Adire Shiva driver from the amp? (or would the size of the inductor be prohibitively large at 200Hz or less).

I might need to read up on active xovers....

Thanks for your replies,
David.
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Old 8th January 2003, 06:41 AM   #9
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Dave,

What hardware do you have, amp, speakers, brand, model, specs, etc?

Rodd Yamashita
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Old 8th January 2003, 07:46 AM   #10
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Old (well, 10 year old) stuff - but still works as good as the day it was bought

- Denon 40 watt receiver - DRA-335R
- Wharfedale 505.2 bookshelf speakers - 1" aluminium dome, 6.5" polyprop woofer. 8 Ohm impedance, rated 100watts.

the wharfedales claim to use a "bayonet fixing" on the woofer, sealed enclosure - approx 10Litres. the woofer has 3 slots just past the butyl / rubber surround (which is an odd shape - its concave - probably to avoid suck back at excursion). Probably needs some proprietary tool to remove. A friend had his reconed / coiled when he burned his out 7 years ago (funnily enough the woofer crapped out before the tweeter - he only had a 60 watt receiver).

Bright sounding setup - woofer sounds a bit nasal as well (tweeter probably crossed over too high and woofer beign asked to do a bit too much - maybe needs a contour or notch filter network). I'd like to tone them down a bit - should do some measuring. Probably could do with xover component upgrades

Anyway I'm lacking in the bass department - thought I'd start with a reasonable sub and work my way up through the system

Dave.
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