Initial box lining results!
Monacor MDM3 wool batting 2 layers
Looks like this isn't available anymore. About 1/2" thick batting of wool and IIRC poly. Given the performance of 2 layers (1") I would like to find a new source of wool batting, since my big complaint with the denim insulation is the amount of fuzz and fiber it gives off. Makes me worry it will get into the voicecoil of a driver.
And 1 layer:
1" Sonic Barrier
From Parts Express
1.8" Ultratouch Denim insulation
Available at HD (and other places), reasonably priced, performs well. No wonder the popularity. Now if would just stop shedding so much stuff!
A comparo:
Monacor MDM3 wool batting 2 layers
Looks like this isn't available anymore. About 1/2" thick batting of wool and IIRC poly. Given the performance of 2 layers (1") I would like to find a new source of wool batting, since my big complaint with the denim insulation is the amount of fuzz and fiber it gives off. Makes me worry it will get into the voicecoil of a driver.
And 1 layer:
1" Sonic Barrier
From Parts Express
1.8" Ultratouch Denim insulation
Available at HD (and other places), reasonably priced, performs well. No wonder the popularity. Now if would just stop shedding so much stuff!
A comparo:
Gah, I meant to have the unlined response in those plots, not the response of the source driver from earlier.
EDIT: I guess the source driver response is good to have to. I'll post another set of plots with the unlined box response.
FWIW, for my next speaker build I'm considering using torn-up rockwool batt inside a pillowcase. The pillowcase is to keep the fibers contained and out of a coil gap.
What I'm trying to measure is the amount of radiation that leaves the box back through the woofer that created it. Obviously if I drove the actual woofer with a signal the outside radiation would completely swamp the interior radiation. Thus the internal source driver creating the signal and a dummy woofer to measure what leaves the box. Maybe I should have been clearer on what I was trying to test.
Great work, Augerpro.
The results at 400 Hz are a surprise to me. I would not have guessed that there could be a ~ 15 - 20 dB reduction at 400 Hz for all 3 of the stuffing materials. I would have guessed that a big 400 Hz peak for the empty box would have been a cabinet panel resonance, and stuffing would make little difference to a cabinet panel resonance.... but that is not what the data says. I don't believe this is a panel resonance, it is an acoustic resonance. This is why the stuffing materials have such a large effect.
Notice between 650 and 800 Hz the bare box has a resonance, and this looks like panel resonance. The 3 stuffing materials have much less effect here... The MDM3 2 layer and the Sonic barrier foam are about 8 dB lower than the bare box, and the denim is about 10 dB lower. While this is more of a reduction than I would have expected for a panel resonance, it is far less than the 20 dB reduction seen at 400 Hz or at the upper frequencies.
Another surprise is at 2 kHz. Here the surprise is the bare box... we have large resonance, it is the largest SPL resonance of the experiment. The magnitude of this resonance is as high or higher than the 650 - 800 Hz peak, which is shocking. I believe this is an acoustic resonance, because all 3 stuffing materials have a big impact. 2 kHz is in the region where most insulation materials have their strongest absorption. The foam and the MDM3 have a 20 dB effect, and the denim has a 25 dB effect.
The fact that the data revealed 3 unexpected results shows that value of empirical data.
The results at 400 Hz are a surprise to me. I would not have guessed that there could be a ~ 15 - 20 dB reduction at 400 Hz for all 3 of the stuffing materials. I would have guessed that a big 400 Hz peak for the empty box would have been a cabinet panel resonance, and stuffing would make little difference to a cabinet panel resonance.... but that is not what the data says. I don't believe this is a panel resonance, it is an acoustic resonance. This is why the stuffing materials have such a large effect.
Notice between 650 and 800 Hz the bare box has a resonance, and this looks like panel resonance. The 3 stuffing materials have much less effect here... The MDM3 2 layer and the Sonic barrier foam are about 8 dB lower than the bare box, and the denim is about 10 dB lower. While this is more of a reduction than I would have expected for a panel resonance, it is far less than the 20 dB reduction seen at 400 Hz or at the upper frequencies.
Another surprise is at 2 kHz. Here the surprise is the bare box... we have large resonance, it is the largest SPL resonance of the experiment. The magnitude of this resonance is as high or higher than the 650 - 800 Hz peak, which is shocking. I believe this is an acoustic resonance, because all 3 stuffing materials have a big impact. 2 kHz is in the region where most insulation materials have their strongest absorption. The foam and the MDM3 have a 20 dB effect, and the denim has a 25 dB effect.
The fact that the data revealed 3 unexpected results shows that value of empirical data.
I'll be doing the dacron batting from Meniscus and a pillow stuffing tomorrow. That's all I have on hand. The MDM3 got me looking for wool batting of some sort, and there is a widely available supply for quilts. I found some that is .8" thick, so I'd like to try 2 layers and see how that compares to the denim. If it is good, I would say any 100% wool batting for quilts should perform similarly as long as you can layer to the right thickness.
Great analysis! I'm only right now going over the plots. Here is something to chew on, rear panel response (empty box) and dummy woofer response (empty box):
I also modeled these in boxnotes. I now realize I'm not quite sure what the assumptions are for this program. Is the standing waves generated by the driver at the specified location in a given box, and the response measured (if you could) at that same spot? So looking at the boxnote of the source driver, would you necessarily expect to measure those resonances at the dummy woofer location? And the dummy woofer boxnote wouldn't be relevant at all because it is not creating sound?
No, I got that all right. But instead of shorting the terminals you could connect them to the preamp. Since the SB will act as a piston till quite high frequencies, it will serve you as a quite good dynamic microphone. Be it rather poorly damped at fs, but you are not looking that low anyway.
Advantages are you could enlarge the time window and you measure the actual pressure on the woofer, not the derivative you use now.
I see. I was led to believe shorting the terminals would mimic being hooked to an amp. I *could* hook it to an amp, but that would also require running wire in a whole bunch of boxes every time I build one. Would it be worth it?
I can widen the window on my saved impulse responses. Can't remember why I settled on 50ms, but I can just as easily set it at 500ms and re-plot everything.
I can widen the window on my saved impulse responses. Can't remember why I settled on 50ms, but I can just as easily set it at 500ms and re-plot everything.
hifijim> at some point I need to build another sealed box, but with the preferred lining, and measure side and rear panel response again. Presumably the levels will be much lower with lining, but that almost makes me think of these boxes (the actual wood structure) in the 400-1000hz range as...almost transparent. Am I thinking about this right?
Here is what Markbakk is getting at... Currently you are using a microphone to see how much sound is transmitted through the SB woofer... But, if you connected the SB woofer to a preamp, you could use the woofer as a kind of microphone. You could directly record the acoustic signal impinging upon the woofer cone, instead of letting the signal migrate through the cone and then recording it with a mic outside the box.
However... I am not sure this is the same thing. It may be that the signal which impinges upon the cone is not the same as the signal that gets transmitted through and radiated from the cone.
I have read that when an enclosure panel is in resonance, it is almost acoustically transparent. This makes sense to me.
There are two mechanisms for sound to penetrate an enclosure wall.
(1) The first is normal sound transmission. A pressure wave strikes the surface of a wall and compresses the surface of the wall. The sound moves through the thickness of the wall as a wave of elastic compressive - tension stains until it reaches the opposite surface. it then is radiated into the air. For 3/4 inch plywood, we would expect there to be a 15 - 25 dB loss, depending on frequency... about 15 dB at 250 Hz, about 20 dB at 1 kHz, and about 25 dB at 5 kHz.
(2) The second mechanism is structural resonance. The mass and stiffness of the panel together form a dynamic system with a natural frequency. If the wall is driven with a signal at its natural frequency, the wall flexes in and out of plane. It acts as a diaphragm, which is very different than mechanism (1) above. The driving signal can be either an acoustic wave or a direct force from the speaker driver, but the direct force from the driver is usually more significant.
When in resonance (case 2), the normal 15 dB to 25 dB sound loss through the wall does not apply, because the wall itself is moving back and forth.
Most enclosure designs, including very high end enclosures, basically ignore mechanism (1). With a good stuffing or lining, sound transmission through the cabinet walls is reduced by at least 20 dB, and possibly much more than that. The effect on the frequency response of the speaker system is very small. The speed of sound through plywood or MDF is much faster than air, so there is no energy storage when mechanism (1) is dominant.
Most enclosure design is focused on minimizing the effects of mechanism (2)... i.e. resonance. It is complicated because there are six walls in a box, and they mutually couple. Cabinet stiffening can transform the six cabinet walls into many more virtual surfaces. Anyone who claims that this is simple engineering or basic physics is blowing smoke. It is very difficult to model and simulate. The fixity of the edges are difficult to predict, and the effectiveness of the bracing is hard to know. Are the cabinet walls under pre-load (clamped during glue-up)? What is the effectiveness of the adhesive bond line (i.e. how stiff is the adhesive)? How much internal damping is there in the cabinet walls? All in all, a tough modelling problem.
The work you are doing is totally relevant, and very needed.
augerpro - Thanks for doing all this extra work to find the right path!
hifijim - I am a newb and appreciate the writeup!
Techlite Insulation 0079-2448SS100-SH-0000-00 $61.18 24" x 48" Melamine Foam Insulation Sheet 1" Wall | Zoro.com
I don't know where other folks get thier melamine, but I have used this in the past to quiet a vent. If you wet it and squeeze out the excess water it will dry and hold a shape. I'll gladly chip in some bucks if you want to try it.
The other foam you used has lots of air bubbles. Melamine looks more solid to the naked eye. There was a mention on this site with a link to NASA:
NASA Technical Reports Server (NTRS)
Sound Damping and absorption material
As a final plus, melamine wont burn.
@augerpro you should put a paypal link on your webpage.
hifijim - I am a newb and appreciate the writeup!
Techlite Insulation 0079-2448SS100-SH-0000-00 $61.18 24" x 48" Melamine Foam Insulation Sheet 1" Wall | Zoro.com
I don't know where other folks get thier melamine, but I have used this in the past to quiet a vent. If you wet it and squeeze out the excess water it will dry and hold a shape. I'll gladly chip in some bucks if you want to try it.
The other foam you used has lots of air bubbles. Melamine looks more solid to the naked eye. There was a mention on this site with a link to NASA:
NASA Technical Reports Server (NTRS)
Sound Damping and absorption material
As a final plus, melamine wont burn.
@augerpro you should put a paypal link on your webpage.