The aim of this study is to evaluate different damping material of a box of 15 L with a KEF B200 driver.
Materials tested
1. Fine BAF salvaged from loudspeakers over the years. Tested at 0, 100, 200, 300 & 400 g giving a packing density from 6.7 to 27g/l =(kg/m3). Firmly packed at highest density. Evenly distributed at all tests.
Fr lowered from 79 to 70 Hz and Qms from 9.9 to 3.4
2. Pharmacy type cotton.Tested at 0, 100, 200, 300 & 400 g giving a packing density from 6.7 to 27g/l =(kg/m3), some space left at highest density. Evenly distributed at all tests.
Fr down to 70 Hz and Qms 3.5
3. Coarser BAF sheets for bass reflex boxes 83 gram each (Biltema) tested at 0, 83, 166, 249 and 332g (the 5th mat in the pack did not fit) Uneven distribution with the sheets against the back wall and then piling forward.
Down to 73 Hz and Qms 4.7 with less damping material than the previous two.
4. Open foam one sheet 50 mm thick covering the back of the box was 100 g and only one of them fitted.
75 Hz and 7.9 with even less damping material.
To to conclude different types of BAF and Cotton and at least the foam I tested does seem to give similar results in lowering the Fr and reducing the Qms that is increase resistive losses.
Next time around, if there will be one!
Having the driver the other way around so that the magnet structure does not interfere with packing the box. Perhaps also test the african sheep wool that I have.
There is also the question of performance higher up in the bass and midrange where sheeps wool is supposedly superior to everything else. To find any difference in measurements at all, let alone get it to correlate with subjective evaluations, will be more demanding then measure Fr an Qms. Perhaps time domain plots measured inside the box?
Materials tested
1. Fine BAF salvaged from loudspeakers over the years. Tested at 0, 100, 200, 300 & 400 g giving a packing density from 6.7 to 27g/l =(kg/m3). Firmly packed at highest density. Evenly distributed at all tests.
Fr lowered from 79 to 70 Hz and Qms from 9.9 to 3.4
2. Pharmacy type cotton.Tested at 0, 100, 200, 300 & 400 g giving a packing density from 6.7 to 27g/l =(kg/m3), some space left at highest density. Evenly distributed at all tests.
Fr down to 70 Hz and Qms 3.5
3. Coarser BAF sheets for bass reflex boxes 83 gram each (Biltema) tested at 0, 83, 166, 249 and 332g (the 5th mat in the pack did not fit) Uneven distribution with the sheets against the back wall and then piling forward.
Down to 73 Hz and Qms 4.7 with less damping material than the previous two.
4. Open foam one sheet 50 mm thick covering the back of the box was 100 g and only one of them fitted.
75 Hz and 7.9 with even less damping material.
To to conclude different types of BAF and Cotton and at least the foam I tested does seem to give similar results in lowering the Fr and reducing the Qms that is increase resistive losses.
Next time around, if there will be one!
Having the driver the other way around so that the magnet structure does not interfere with packing the box. Perhaps also test the african sheep wool that I have.
There is also the question of performance higher up in the bass and midrange where sheeps wool is supposedly superior to everything else. To find any difference in measurements at all, let alone get it to correlate with subjective evaluations, will be more demanding then measure Fr an Qms. Perhaps time domain plots measured inside the box?
If you do a search on 'stuffing' you'll probably find a number of similar studies done by hobbiests in the past. They all show the same result - pretty much. With good quality stuffing material like fiber glass, for example, the impedance curve will move downward (i.e. flatten) and to the left slightly with increasing packing density - up to a point. Dickason's Loudspeaker cook Book also includes this.
There has been claims that Sheeps wool is much better than fiber other fibrous material and also claims that plastic open foam is useless or "unmusical". My interest in the matter is that ,as a scientist, to find a way to measure these differences if any. So I started picking the lowest branch of a closes box and T/S values. Perhaps there are more esoteric matials that can show things like an increased apparent box volume without affecting Qms or the other way around. But at least in my crude measurements no such animal showed up.
By having this as a precursor to work in the midrange region I hoped to gather some to show how sheeps wool is better than BAF and how plastic foam is inferior.
A theory would be that a very homogenous block of foam could be "monolitic" that is that the whole block becomes, transparent, reflective or resonant at some frequency ranges. Organic material like cotton and wool will be be by the very nature heterogenous and thus not prone to having all parts doing the same thing at any given frequency. If this theory is true is then pillow BAF (one kind of fiber) inferior to Acoustic BAF (several kinds of fibers)?
How do I measure this?
My own idea is currently to measure room accoustics with no and different amounts of damping. Modes and decay times and so on. The only difference is that the "room" is the inside of a loudspeaker.
By having this as a precursor to work in the midrange region I hoped to gather some to show how sheeps wool is better than BAF and how plastic foam is inferior.
A theory would be that a very homogenous block of foam could be "monolitic" that is that the whole block becomes, transparent, reflective or resonant at some frequency ranges. Organic material like cotton and wool will be be by the very nature heterogenous and thus not prone to having all parts doing the same thing at any given frequency. If this theory is true is then pillow BAF (one kind of fiber) inferior to Acoustic BAF (several kinds of fibers)?
How do I measure this?
My own idea is currently to measure room accoustics with no and different amounts of damping. Modes and decay times and so on. The only difference is that the "room" is the inside of a loudspeaker.
There are exotic materials being marketed as such. An example would be Twaron's Angel Hair: TWARON Angel Hair, the new and innovative damping material for loudspeaker systems am room acoustic adjustments
Being thinner haired than sheep wool it is said to have even better properties used as damping material. Claiming that less material is needed for the same amount of damping.
A test on a dutch forum: forum.zelfbouwaudio.nl • Toon onderwerp - Twaron dempings materiaal.
(10 gr. Twaron had comparable results to 46 gr. long hair wool)
Being thinner haired than sheep wool it is said to have even better properties used as damping material. Claiming that less material is needed for the same amount of damping.
A test on a dutch forum: forum.zelfbouwaudio.nl • Toon onderwerp - Twaron dempings materiaal.
(10 gr. Twaron had comparable results to 46 gr. long hair wool)
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Your idea of taking inside-the-box measurements would seem on the face of it a nice academic exercise. We, as listeners, only hears what's going on outside the box. So, then how do you propose to translate or correlate what's going on inside the box to what we already know what's going on outside the box?
Since you are a scientist, you might be interested in reading Dr. Hanlon's derivation of why FG outperforms PET in closed box stuffing applications.
Content warning
Don't worry about the 'warning' notice. It's harmeless.
There was a scare some decades back from a mine where perlite was contaminated with asbestos. They both form under similar conditions with in the earth.
Today this is not an issue. I do alot of flower gardens, potted plants etc and don't like perlite as it is gritty and brittle.
2-Comments
1- Have you considered Anechoic Enclosure Construction? Enclosure walls are of laminar construction as shown here:
w - wood
f - felt pad
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2 - The attached article was published on Rod Elliott's website and may be of interest here.
Regards,
WHG
1- Have you considered Anechoic Enclosure Construction? Enclosure walls are of laminar construction as shown here:
w - wood
f - felt pad
wwwwww
wwwffffffff
wwwwww
wwwffffffff
wwwwww
2 - The attached article was published on Rod Elliott's website and may be of interest here.
Regards,
WHG
isn't rock wool just a name of a manufacture's fiberglass insulation?
In the HVAC world it is the the stuff that really makes you itch. It is brown instead of yellow or pink.
I'm familiar with the rock wool of the 60's and 70's used in Acoustic Research closed box speakers. It does just as well as fluffed fiberglass used in wall insulation. The difference being rock wool has much glass shot in it and its density is higher than fluffed Fg. Therefore less weight of fluffed FG is used to replace the rock wool to get the same box Q and Fc.
Box stuffing discussions always get confusing because there are basically two different stuffing scenarios that eventually become mixed as posts accumulate during a thread's life.
IIRC, the OP was experimenting with CLOSED BOX stuffing densities and different materials. In that case, stuffing serves two purposes. One is to dampen standing waves and the other, make the box perform as though it is actually bigger than it is. An optimized stuffing job can produce performance comparable to boxes as much as 25-30% larger in volume than they actually are.
Vented box stuffing is done more along the lines of lining the walls to dampen reflections only.
IIRC, the OP was experimenting with CLOSED BOX stuffing densities and different materials. In that case, stuffing serves two purposes. One is to dampen standing waves and the other, make the box perform as though it is actually bigger than it is. An optimized stuffing job can produce performance comparable to boxes as much as 25-30% larger in volume than they actually are.
Vented box stuffing is done more along the lines of lining the walls to dampen reflections only.
Hi Speedie, I did some experiments about 8 years ago with different materials. I tried some carpet underlay (which was not too bad) but it was the dimpled rubber type not foam. I suspect that the foam stuff would be closed cell which may not work too well.
I did try for a while to get hold of some silent step underlay but couldn't get any. In the end I went with some industrial sound damping material which works very well for getting rid of reflections. Details here --> http://www.diyaudio.com/forums/multi-way/167178-speaker-dampening-materials-what-do-people-use.html Post 11 has a good comparison of with and without nearfield measurements, which is why I suggested them earlier as a good test.
edit: good point speakerdoctor
The stuff I have just posted is more along the lines of damping reflections, it does very little in the way of increasing box volume. In fact the tuning of the box stayed almost exactly the same with or without the foam (although it took up a significant fraction of the volume of the box)
Tony.
I did try for a while to get hold of some silent step underlay but couldn't get any. In the end I went with some industrial sound damping material which works very well for getting rid of reflections. Details here --> http://www.diyaudio.com/forums/multi-way/167178-speaker-dampening-materials-what-do-people-use.html Post 11 has a good comparison of with and without nearfield measurements, which is why I suggested them earlier as a good test.
edit: good point speakerdoctor
The stuff I have just posted is more along the lines of damping reflections, it does very little in the way of increasing box volume. In fact the tuning of the box stayed almost exactly the same with or without the foam (although it took up a significant fraction of the volume of the box)Tony.
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