Thinking inside the box
I have trying out different damping material in TLS recently. This lead me to think about ways to how to measure how effective damping material are at attenuating sound waves bouncing around inside a box. I used Omnimic and a Seas metal dome in a closed box of about 150x250x500 mm.
This is how it looks outside with the Seas some facing inwards through the old tweeter hole. The woofer cut out is blocked by a MDF disk.
This blurry picture show the microphone poking through a hole drilled in the back wall and centered on axis of the tweeter.
This is the the time response in the empty box.
This is the box filled with damping material with a small path between the dome and the microphone. With a cutout in front of the dome parts of this ringing can be caused by this. So this is the golden standard to compare with ( felt covered walls and then cotton to fill up the remaining empty space.
Lets start with the sheets of BAF, foam and felt.
Felt covered walls look very similar to the reference.
The Accoustic foam lacks the diagonal ridge 5-15 kHz but has other complex ringings.
The BAFmat looks more like the empty box but with slightly faster decay. Not impressive
Lining the walls with cotton looks really good
Then for the two volume fillers
BAF wool 100 gram looks really bad to my surprise, perhaps higher densities are needed but then it becomes hard to keep the path to the microphone open due to the springy nature of the BAF.
Last out is the sheeps wool that I have tried to tease out 100 gram and the result is depressing. Even tesed out it does not cover much of the surfaces or volume.
If I would do it again I would, thinking inside the box:
Use a small fullrange driver all the way down to 100 Hz or so. Use software that would do time domain response at lower frequencies. If I would build a purpose box I would have the loudspeaker more in the middle and countersunk facing inwards, on a removable baffle.
I have trying out different damping material in TLS recently. This lead me to think about ways to how to measure how effective damping material are at attenuating sound waves bouncing around inside a box. I used Omnimic and a Seas metal dome in a closed box of about 150x250x500 mm.
This is how it looks outside with the Seas some facing inwards through the old tweeter hole. The woofer cut out is blocked by a MDF disk.
This blurry picture show the microphone poking through a hole drilled in the back wall and centered on axis of the tweeter.
This is the the time response in the empty box.
This is the box filled with damping material with a small path between the dome and the microphone. With a cutout in front of the dome parts of this ringing can be caused by this. So this is the golden standard to compare with ( felt covered walls and then cotton to fill up the remaining empty space.
Lets start with the sheets of BAF, foam and felt.
Felt covered walls look very similar to the reference.
The Accoustic foam lacks the diagonal ridge 5-15 kHz but has other complex ringings.
The BAFmat looks more like the empty box but with slightly faster decay. Not impressive
Lining the walls with cotton looks really good
Then for the two volume fillers
BAF wool 100 gram looks really bad to my surprise, perhaps higher densities are needed but then it becomes hard to keep the path to the microphone open due to the springy nature of the BAF.
Last out is the sheeps wool that I have tried to tease out 100 gram and the result is depressing. Even tesed out it does not cover much of the surfaces or volume.
If I would do it again I would, thinking inside the box:
Use a small fullrange driver all the way down to 100 Hz or so. Use software that would do time domain response at lower frequencies. If I would build a purpose box I would have the loudspeaker more in the middle and countersunk facing inwards, on a removable baffle.
What ever happened to good fashioned Fiberglas house insulation?
Also, what about thick Poly batting? Also a popular alternative.
Or a combination of the two, 2" Fiberglas covered by 1" Poly?
I'm curious about the Acoustic Foam you used, was this flat, or some type of convoluted foam? And how thick?
Still very informative information.
There is a company that makes house insulation of the recycled Blue Jeans. So, mostly cotton. The house insulation is about 3.5" to 4" thick and not especially dense, but the same company also makes denser cotton acoustic panels. (All flame retardant). When I used Fiberglas inside speaker cabinet, I usually took 3.5" house insulation and carefully split it into two 1.75" sheets. That seemed about the right thickness.
Although not cheap, most place like Home Depot sell small rolls of 2" Fiberglas insulation, specifically for speakers and acoustic panels.
One company with cotton insulation is Bonded Logic -
Bonded Logic - Acoustical Products - OEM Acoustical Products
https://www.google.com/search?q=blu...KBISdyQHs_IGYCw&ved=0CFUQsAQ&biw=1024&bih=611
Though obviously, cotton has the ability to absorb moisture far more than FiberGlas or Poly. I suspect the same from Felt. What that means in the long run is unclear.
Also, though probably not the cheapest, they make standard ceiling tiles from Ridged Fiberglas. That's not ridged as in Corvette body ridged, but in the same sense that Owen Corning 703.
It is usually in about 3/4" thickness covered with a thin layer of white textured vinyl. I've often wondered how well this would work if you simply pealed the vinyl layer off and used the ridged Fiberglas in acoustic panels or inside speaker cabinets.
Sorry ... just rambling.
Steve/bluewizard
Also, what about thick Poly batting? Also a popular alternative.
Or a combination of the two, 2" Fiberglas covered by 1" Poly?
I'm curious about the Acoustic Foam you used, was this flat, or some type of convoluted foam? And how thick?
Still very informative information.
There is a company that makes house insulation of the recycled Blue Jeans. So, mostly cotton. The house insulation is about 3.5" to 4" thick and not especially dense, but the same company also makes denser cotton acoustic panels. (All flame retardant). When I used Fiberglas inside speaker cabinet, I usually took 3.5" house insulation and carefully split it into two 1.75" sheets. That seemed about the right thickness.
Although not cheap, most place like Home Depot sell small rolls of 2" Fiberglas insulation, specifically for speakers and acoustic panels.
One company with cotton insulation is Bonded Logic -
Bonded Logic - Acoustical Products - OEM Acoustical Products
https://www.google.com/search?q=blu...KBISdyQHs_IGYCw&ved=0CFUQsAQ&biw=1024&bih=611
Though obviously, cotton has the ability to absorb moisture far more than FiberGlas or Poly. I suspect the same from Felt. What that means in the long run is unclear.
Also, though probably not the cheapest, they make standard ceiling tiles from Ridged Fiberglas. That's not ridged as in Corvette body ridged, but in the same sense that Owen Corning 703.
It is usually in about 3/4" thickness covered with a thin layer of white textured vinyl. I've often wondered how well this would work if you simply pealed the vinyl layer off and used the ridged Fiberglas in acoustic panels or inside speaker cabinets.
Sorry ... just rambling.
Steve/bluewizard
Below is a picture of used material. 80-100 gram of each ppart from the foam that is 143 g inludining the paper backing (self adhesive). Top left is the BAF wool that is the same or similar to the polly batting.Then there is pharmacy cotton and lastly the african unwashed sheep wool.
The tree sheets at the front is top to bottom
recycled cotton fibers 100g and about 10mm thick
Accoustic grade foam 143g with paper backing the selfadhesive.
BAF sheet for bass reflex speakers 82grams about 15 mm thick and denser and coarser than the BAF roll at the back. Not Scotchbrite but with a shape that keeps.
Here I used enough sheet material to cover back, sides,top & bottom of the box but the 3 volume filling material I used roughly equal mass.
The tree sheets at the front is top to bottom
recycled cotton fibers 100g and about 10mm thick
Accoustic grade foam 143g with paper backing the selfadhesive.
BAF sheet for bass reflex speakers 82grams about 15 mm thick and denser and coarser than the BAF roll at the back. Not Scotchbrite but with a shape that keeps.
Here I used enough sheet material to cover back, sides,top & bottom of the box but the 3 volume filling material I used roughly equal mass.
Pardon me for saying so, but I would have never guessed that the internal 'insulation' material would have that much effect on high frequencies. I've always regarded the amount and type of insulation as a means of fine tuning the bass response.
So, I'm amazed at what I'm seeing. As with everything in audio, everything is always more complicated than it seems.
The more you know, the more you know you don't know.
Steve/bluewizard
So, I'm amazed at what I'm seeing. As with everything in audio, everything is always more complicated than it seems.
The more you know, the more you know you don't know.
Steve/bluewizard
Technically it really doesn't have much effect on very high frequencies unless it's a full-range speaker.
In regards to bass response especially sub bass response, wadding/stuffing is poorly understood by most. It simply have 2 effects: a) it creates friction which dampens internal pressure, this is good for a closed cabinet but bad for a ported cabinet unless the effect is taken into account in the design, and b) it reduces the effective speed of sound which makes the cabinet appear larger to the driver, it does also mean that it affects the cabinet tuning which is good if you want a lower tuning frequency in a closed cabinet but in ported cabinet this has to be weighed against the added loss due to friction.
On a final note in regards to wadding/stuffing, if you design a ported speaker intended for outdoors use, even just occasional, you might want to consider designing the speaker without wadding/stuffing as it traps humidity which will affect the tuning as described above, and when this humidity is released again it will affect all internal parts that are affected by humidity changes (most materials are to some extent).
So, you were playing pink noise or a sine sweep with the tweter and measuring CSD? Results are fine, but I question the validity - the tweeter has a closed back and the box is for the woofer only! You should study that range (with harmonics)
1) the box has vibrations/resonances (the structure and dimensions)
2) the sound inside the box makes modes and resonances, backwave (dimensions, fundamental Fq dependent)
3) stuffing has an effect to air volume that driver "sees"
All these should be recognized and discussed spesifically.
1) the box has vibrations/resonances (the structure and dimensions)
2) the sound inside the box makes modes and resonances, backwave (dimensions, fundamental Fq dependent)
3) stuffing has an effect to air volume that driver "sees"
All these should be recognized and discussed spesifically.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.