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 Post subject: Bass Cabinet Stuffing
PostPosted: November 15th, 2017, 3:54 pm 
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For those of us who are not familiar with my system, it is Altec 2-Way monitor crossed over at 500Hz. HF is handled by the horn and LF is a sealed enclosure of approximately 8cu.ft. with 15" Altec 411 Driver.

The cabinet came with minimum damping, about 2" of fiberglass on 3 sides and bottom. Over the next couple of months, I plan to redo the internal damping of the cabinets.

Tom's Eggs got me thinking about eliminating the parallel surfaces in the cabinet to reduce resonances. But that would be impossible to do without significantly reducing the volume. Came across this website which talks about using Anechoic Chamber type rubber wedges. Want to try out wedges and light acoustimass type of stuffing. Could not find the rubber wedges online. Anechoic wedges are too big and too expensive.
http://www.tnt-audio.com/clinica/speaker_design_pt3_getstuffed_e.html

Attachment:
euterpe_bass-anechoic_bloks2.jpg
euterpe_bass-anechoic_bloks2.jpg [ 2.08 KiB | Viewed 24995 times ]

Attachment:
euterpe_brace1.jpg
euterpe_brace1.jpg [ 16.29 KiB | Viewed 24995 times ]


Looking for ideas of making the wedges easily without breaking the bank. Thinking of 1"x1" base and 2" tall wedges. One idea is to use sculpting foam. Any other ideas?


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PostPosted: November 15th, 2017, 5:04 pm 
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Cogito wrote:
For those of us who are not familiar with my system, it is Altec 2-Way monitor crossed over at 500Hz. HF is handled by the horn and LF is a sealed enclosure of approximately 8cu.ft. with 15" Altec 411 Driver.

The cabinet came with minimum damping, about 2" of fiberglass on 3 sides and bottom. Over the next couple of months, I plan to redo the internal damping of the cabinets.

Tom's Eggs got me thinking about eliminating the parallel surfaces in the cabinet to reduce resonances. But that would be impossible to do without significantly reducing the volume. Came across this website which talks about using Anechoic Chamber type rubber wedges. Want to try out wedges and light acoustimass type of stuffing. Could not find the rubber wedges online. Anechoic wedges are too big and too expensive.
http://www.tnt-audio.com/clinica/speaker_design_pt3_getstuffed_e.html

Attachment:
euterpe_bass-anechoic_bloks2.jpg

Attachment:
euterpe_brace1.jpg


Looking for ideas of making the wedges easily without breaking the bank. Thinking of 1"x1" base and 2" tall wedges. One idea is to use sculpting foam. Any other ideas?



Don't even think about it, Shashi. Besides eating up volume, foam rubber is very reactive. You will not get the end result you desire. Also, polydacron is reactive as well. Wool and/or fiberglass is what you want.

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PostPosted: November 15th, 2017, 5:29 pm 
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Walt,

The wedges help in two was as far as I understand. By diffusing the sound waves they a) delay the arrival of the reflected waves back to the cone effectively making it seem like a larger enclosure, b) the waves will be reflected within the cabinet several times and only a small fraction of them at much lower intensity arrive back at the cone, again making it seem like the cone is in a larger cabinet.

I want to glue the wedges to the all internal walls starting at the back wall and then add the wool stuffing over them. What ever waves pass thru the wool stuffing, are scattered instead of bouncing straight back to the cone.

By significantly reducing the resonances withing the cabinet I am expecting a cleaner bass and by increasing the effective volume reduce the resonance frequency. According the measurements Tom made on my woofer, the resonance frequency in the cabinet is around 39Hz. The woofers are capable of going down to 20Hz in a large enough cabinet. I would like to bring it down to 30Hz.

I am making sense?


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PostPosted: November 15th, 2017, 5:43 pm 
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Shashi:

Looking at the numbers I got for your driver, you would need a box volume of approximately 16 cu ft to get an fB of 30 Hz. Typically with an optimally stuffed box you can expect an increase in the "apparent" volume of the box of about 25%. Therefore, your box under ideal conditions would look like 10 cu ft. That would give an fB of just under 35 Hz. I don't think any amount or type of stuffing will get you to 30 Hz. When stuffed, your current box would give you a Q around .5 which would result in good transient response.

Tom


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PostPosted: November 15th, 2017, 5:50 pm 
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Hey Sashi,

What specifically are you trying to change about the existing response of your bass cabinet?

From my experience, wool will improve midrange performance, but doesn't necessarily outperform any other stuff in it's effects on bass performance. I don't know about fiberglass because I don't like working with it. Acousta-stuff however, in my speakers, did improve the weight of the bass sounds a little bit. Ultimately I have chosen something different than these for midrange clarity reasons. I also tend to find that the amount of stuffing is more critical than "what" it is. The proper amount is not arbitrary either, and isn't always predicted by a common formula. You usually have to figure it out with trail and error and a systematic way of increasing and decreasing the stuffing in meaningful amounts. 1/2 lb. per cubic foot is a good place to start, but the exact optimum amount for every different driver/box/stuffing material combination will always be slightly different.

If you are just trying to get more "umph" out of your bass cabinets, or cleaner richer sound, my favorite method is to add a couple shelf braces, or make the cabinet walls thicker, or both!

If you cannot add the extra thickness on the outside of the cabinets, taking a 3/4" loss in volume per wall from adding a layer of material on the inside of all six walls, in my opinion, is a compromise worth making! You want to add hard, resonating material though, not soft absorptive stuff.

Chris


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PostPosted: November 15th, 2017, 5:53 pm 
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Independent of your woofers measurements, and cabinet volume, the easiest way to make your box "ring" at a lower frequency is to add mass to the cabinet walls!


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PostPosted: November 15th, 2017, 7:25 pm 
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Hopefully your box is not "ringing" at any frequency. If it does you have real problems as the output it produces will have nothing to do with the music. Note that I am not talking about tuned port resonance but actual vibration of the enclosure walls.


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PostPosted: November 15th, 2017, 8:38 pm 
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tomp wrote:
Shashi:

Looking at the numbers I got for your driver, you would need a box volume of approximately 16 cu ft to get an fB of 30 Hz. Typically with an optimally stuffed box you can expect an increase in the "apparent" volume of the box of about 25%. Therefore, your box under ideal conditions would look like 10 cu ft. That would give an fB of just under 35 Hz. I don't think any amount or type of stuffing will get you to 30 Hz. When stuffed, your current box would give you a Q around .5 which would result in good transient response.

Tom


Here is where things get confusing for the novice. As Tom pointed out you can only go so far with a sealed box. For Edger Villchur to get down to 42-Hz. before it started to roll off in the bookshelf size box (AR-1) he used a woofer that had a free-air resonance of 5-Hz. My system has a vented box that I changed to what I call a "half-ass" transmission line. I mass loaded my woofers to get a free air of 15-Hz. I get output flat to 31-Hz. by virtue of the vent reinforcement and the longer path from the back of the woofer to the vent. The roll-off beyond 31-Hz. is so sharp it kind of "rains on the parade" of deep-bass performance. Or put another way -- Mother Nature (physics) does it her way or no way.

The acoustic fill is a "game." You cheat Mother Nature into "thinking" it's a bigger box to get the box resonance down. That process is where the sound off of the back of the woofer is converted to heat by absorption. To optimize that absorption you not only need to perform trials with different densities to dial it in but also use material that is not reactive. Meaning that it won't push back (create undesirable resonances).

Then there is the enclosure. The stiffer the better. You can brace it like Tom has demonstrated or use constrained-layer damping on the outside to save interior volume.

There is no quick fix. Fitting the box with anechoic wedges will not change the resonant behavior of the box other than pushing the box resonance up rather than down and causing unforeseen problems with the overall playback quality.

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PostPosted: November 15th, 2017, 9:09 pm 
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The devil is always in the details.


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PostPosted: November 15th, 2017, 9:36 pm 
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Hey Sashi,

This might give you some ideas:
https://www.bing.com/images/search?q=So ... ORM=IRIBIP

They are solid, not foam!

I have heard of people putting this type of diffuser on the back panel inside the speaker, although I don't really think these things work with low frequencies as well as they do with midrange frequencies.

Chris


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