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Old May 21st, 2012, 08:20 PM   #1
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Quick and Inexpensivew way to block outside noise

We have three large windows we need to cover to muffle (for lack of completely blocking) traffic noise coming from outside. We're not attempting to block vibrations.

Ideally would be a temporary solution that could be implemented as permanent. For instance, large pink foam panels we could later put into frames on tracks with fabric or wallpaper covers. Or we've thought of insulation fiberglass, but we'd like to keep the panels relatively thin if possible, no more than 4-6 inches.

What are the materials available to us? The space is rented, so we don't want to undertake major renovations.

Thanks.
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Old May 22nd, 2012, 12:53 AM   #2
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Re: Quick and Inexpensivew way to block outside noise

vibrations = sound

Rigid foam and flexible fiberglass insulation is great if you are going for thermal insulation. But they won't do much for sound. Nothing blocks sound except mass or distance (distance = mass of air) You can buy very heavy vinyl sheet which is marketed as a substitute for lead sheeting.

Another way of dealing with traffic noise is to shift recording times to avoid the traffic.
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Old May 22nd, 2012, 06:11 AM   #3
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Re: Quick and Inexpensivew way to block outside noise

Over here they are advertising acrylic magnetic panels that simply clip into your windows and reduce sound dramatically ...sort of "take it with you when you vacate" double glazing....the sites in the USA too have audio tests with and with the panels

Might be worth a try????

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Old May 22nd, 2012, 07:58 AM   #4
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Re: Quick and Inexpensivew way to block outside noise

Jacques,

If the sound is entering ONLY from these windows, I would try multiple layers of sound blankets entirely covering each window. Get the thickest blankets you can, maybe 3 or 4 deep. Use gaffers tape to seal the blankets to the edges of the windows.

Plan B: Similar to the above but add a 2 x 4 frame around each window and install 1/4" plate glass (for its density per cubic inch). Then hang a sound blanket over the glass to eliminate reflection from the glass.

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Old May 22nd, 2012, 10:07 AM   #5
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Re: Quick and Inexpensivew way to block outside noise

Quote:
Originally Posted by Richard Crowley View Post
Nothing blocks sound except mass or distance (distance = mass of air).
Richard, I've got nothing but respect for you. You have probably forgotten more about microphones and sound recording than I'll ever learn. I would never doubt your opinion on mics and associated electronics, and I recognize your years of hard won experience in the field.

But on this, you are completely wrong. And my mechanical engineering degrees just won't let me leave this uncorrected. My old profs would be spinning in their graves. Sigh...

That mass blocks sound is a myth that just won't die. Some materials attenuate sound better than others (lead does a pretty good job, but not because of its specific mass). But if just large masses blocked sound, sonar wouldn't work. Steel I-beams in buildings wouldn't ring like a bell and transmit sound all over the structure. Seismology wouldn't work. And if nothing blocks sound except mass, how does one explain how vacuum completely blocks sound? A vacuum has zero mass, by definition. Remember that tag line from Alien? "In space no one can hear you scream." True enough.

I'm just sayin' that mass isn't the key to blocking sound. Good design is.

To the OP's question, first thing to do is block the direct path of outside sound to the inside. That is, make the windows air tight; seal all the cracks. Caulk the edges of the glazing. Caulk around the window frame. Finally, caulk the windows shut. Literally, so they can't open. If you see a crack, fill it with caulk.

You'll be amazed at how much this cuts down on sound transmission at windows, but it only works if you are caulking the windows completely shut so that you can't open them again without a lot of scraping and mess. I did this on the outside of my wife's home office windows while waiting for the right time to replace the crappy old windows with new ones that actually worked. The technique worked way better than I expected, and netted me several chocolate cakes -- and zero nagging to replace the windows. The crappy old windows are still in place (been five or six years now), with the added benefit of lowered heating bills. ;-) But what made her happy was the major drop in sound levels from the grass-obsessed across the street neighbor's constant use of power tools.

After you've stopped the sound transmitting air leaks, cover the glazing with a sheet of plexiglass cut to completely fill the window frame to the walls, and caulk that in place on the inside. If you can, make the air gap between the window glazing and this new plexiglass sheet about 1.25 cm. This layer of plexiglass may attenuate the sound enough for your purposes and you'll still be able to use the window for light as a bonus.

If you need still more attenuation, cover the window and frame and 10cm or so of wall all the way around with a sound blanket. Gaffer tape the blanket to the wall all the way around, leaving no gaps. None. Very important.

Beyond this and you're deep into diminishing returns. That is, you've probably attenuated the sound through the windows below the level of sound transmitting through the walls. So doing more window treatments won't accomplish anything. If you need more after this, then you're into a whole new and much more complicated ball game, which is why there are people who specialize in the design of sound studios.
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Old May 22nd, 2012, 12:47 PM   #6
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Re: Quick and Inexpensivew way to block outside noise

Thanks Buce. Excellent advice. However, we can't permanently caulk the windows shut as 85% of the time we're not shooting (just editing) and we like to open the windows (besides, that solution would require installing central air).

I was thinking specifically of another studio I saw on a main boulevard that uses custom-built panels on wheels measuring about 10' X 6' X 1', presumably filled with fiberglass insulation. When needed, they wheel them in front of the windows, clip them to each other and it works well enough to shoot video without ruining the sound.

I can't use something this large, as storing them would be another separate headache, but I was thinking of something on a smaller, more portable scale - hence the panels filled with foam or fiberglass insulator.
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Old May 22nd, 2012, 01:51 PM   #7
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Re: Quick and Inexpensivew way to block outside noise

Actually, I'm going to agree with both Richard and Bruce.

The perception of sound is caused by vibration of our eardrums. The original sound source was also vibrational, e.g. the head of a bass drum, the oscillation of air in an organ pipe, etc. The vibrations from the source need to be transmitted to the eardrum (or microphone element) by some means. Since our eardrums (or microphones) exist in an environment surrounded by air, the sound is almost always transmitted, for the last part of its path, through airborne means. And, indeed, sound does not travel through a vacuum (unless there is some sort of mechanical coupling that traverses the vacuum... and the physical reality of that is pretty slim).

Someone standing in an open field, listening to a carillon a mile away, is hearing the music entirely through airborne transmission... that's pretty easy to accept. How, then, would we stop that person from hearing the carillon?

If we put that person inside a perfectly sealed "room" (but a room filled with air, in which the person could survive), would he still hear the carillon? That depends on the construction of the "walls" of the "room."

If the "room" were a latex balloon, twenty feet in diameter, of course the person would still hear the carillon. Informally we'd say the sound "penetrates" the balloon, but what really happens? The sound waves striking the outer surface of the latex causes the latex to vibrate to a minute degree, in sync with the sound waves. Since the latex is vibrating, the inner surface is vibrating, too. That causes minute changes in air pressure inside the balloon, and those cause the person's eardrums to vibrate; therefore the person still hears the carillon.

Now, instead of a latex balloon a few mils thick, let's make the balloon out of solid rubber (similar to that used for automotive tires) and let's make it 1/4" thick. When the sound waves strike the outside of this rubber membrane, it will vibrate a bit, but not nearly as much as the thinner, more flexible latex balloon. The person inside will still hear the carillon, but the sound will be attenuated (and also probably changed in timbre... a related fact that we can ignore for now). If we keep making the rubber membrane thicker and thicker, the sound level will become lower and lower, until eventually it's not discernable.

What If the room were constructed of a 12" thick poured concrete slab, with 12" thick poured concrete walls and ceiling; would the person hear the carillon? If that room were perfectly sealed, the answer is: no. The sound waves from the carillon would strike the outside of the concrete, but the energy of the sound waves would be too small to cause the concrete to vibrate (at least in any measurable way). Since the concrete is not vibrating, the air pressure inside the room remains perfectly static, and the person would not hear the carillon.

But what if there were a hole through one of the walls, exactly one inch square? Most of the sound waves from the carillon would hit still solid concrete as above. But one square inch of the sound waves would enter that hole on the outside of the wall, continue through the hole, and exit from the inside of the wall. Those waves would spread out through the room, causing changes in air pressure within the room. The person in the room would hear the carillon, although at a greatly diminished level compared to outside (unless he put his ear directly over the inner opening of the hole).

If you make the hole bigger, more of the sound waves will find their way into the room, and the perceived level will be louder.

So, as I read in college 40 years ago, and as I've verified many times over the intervening years, there are two distinct forms of sound transmission that need to be addressed: airborne sound and structural vibration. Using caulking will, indeed, reduce the sound leakage from outside. Using mass will, indeed, reduce the sound leakage from outside. Using both, intelligently, will significantly reduce the sound leakage from outside.

Bruce, you have covered the importance of caulking very thoroughly. It's amazing to realize that a 1/16" wide crack, around the 16 foot perimeter of a small residential window, is an opening of twelve square inches! (1/16in * 16ft * 12in/ft = 12 sq. in.) That nearly invisible crack lets in a huge amount of airborne sound!

But once all the openings are sealed, you do have to consider structural vibration. Even if the walls are 12" thick poured concrete, if you have a 3' x 5' single pane window, that's 15 square feet of glass that will easily be set into vibration by outside sounds and that will transmit a lot of sound into the room, too.

Anything fibrous (fibreglas or mineral wool batting, sound blankets, moving blankets, carpeting, mattresses) will attenuate sound transmission (passing through from one side to the other) by virtue of friction of the air trying to move back and forth through the blanket. But they will be much less efficient at lower frequencies, because low frequencies tend to have a lot more energy than high frequencies. To effectively stop low frequencies, you do need mass. So if you try blanketing, and still hear LF rumble from the traffic outside, you aren't done yet!

If, for example, I were faced with a 3' x 5' window set in an 8" thick wall, I would build a frame to fit snugly into the wall opening (which will be, perhaps 3.5' x 5.5'). I'd build it out of something rigid, like 2x4 lumber with its plane perpendicular to the plane of the wall. I would screw and glue one piece of 5/8" sheetrock to one face of the frame. I would then stuff the frame full of high density mineral wool (much more effective than low density fibreglas). Finally, I would cover the remaining side of the frame with two thicknesses of the same sheetrock, using construction adhesive between the two layers. The different thicknesses on the two sides of the frame ensure that the two sides have different resonant frequencies; that way, an outside sound that just accidentally hits the resonant frequency of one face will not cause the other face to resonate (which would transmit the sound into the room). Finally, I would snugly fit that assembly into the wall opening, completely covering the window, and would then carefully seal it in place using caulk. (If visibility is absolutely necessary, you could use two different thicknesses of plexiglass, lexan, etc., and omit the mineral wool, but the result would be much less effective than the construction described above.)

Of course if the building's exterior wall has a plywood outer face, and a thin sheetrock inner face, both rigidly attached to 2x4 framing, the walls themselves are going to vibrate, and they will transmit more sound than the above "window blocking unit" will. So let's hope your building has some reasonably hefty masonry walls, if you want it to be really quiet.

PS: I've had to cut this short due to my own scheduling constraints. I hope it is adequately clear and detailed. If not, please let me know, I will be glad to give expand on this theme, as time permits.

Last edited by Greg Miller; May 22nd, 2012 at 02:43 PM.
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Old May 22nd, 2012, 05:43 PM   #8
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Re: Quick and Inexpensivew way to block outside noise

Jacques I'd look at this problem from the other end ..

1) How much traffic noise is coming in from outside? Light traffic or big trucks with low rumble? Are you close to a freeway?
2) Why do you need to muffle or block it?
3) If it's intruding into your sound recordings, try another approach. Different mic or mics. Different room, not during peak traffic.
4) Are you on the ground floor? Are the walls double brick, stone or weatherboard?
5) As you don't want to damage your rented property, try looking for another recording location?

You could spend days and many bucks and still not get a satisfactory result so Google 'Sound Insulation in Montreal' there's a lot there ..
just an inspection by pros, might save you some time and hassles and they'll probably ask you all the above questions on the phone anyway.

Cheers.
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Old May 22nd, 2012, 06:50 PM   #9
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Re: Quick and Inexpensivew way to block outside noise

At one 48 hour film project screening, a team filmed at a friend's restaurant - right under a freeway underpass! It didn't help that they used poor recording techniques (probably camera mounted mics, from the sound of it.)

Realistically, they should have found a different location or used ADR (which is tough within 48 hours.) Good technique would have helped, but blocking outside noise would have been anything but cheap or inexpensive. It probably would have been cheaper to build a fake restaurant set than to try to soundproof the real one!
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Old May 22nd, 2012, 07:33 PM   #10
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Re: Quick and Inexpensivew way to block outside noise

Thanks Allan.

It's a ground-floor loft on a busy boulevard where we occasionally need to shoot green screen video. We're not hoping to block the vibrations of the occasional truck, which travel through the ground, but would like to muffle engine noise. Closing the windows makes a big difference, but we need something extra that won't cost us more than a) buying our own place (not viable at this time) or b) renting a proper green screen studio (at $700+ a day, it would cut too much into our profit margin).

We're already counting on using the right microphone to pick up less noise. Mostly lav mics.


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Old May 23rd, 2012, 03:06 AM   #11
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Re: Quick and Inexpensivew way to block outside noise

Jacques .. green screen may reflect some traffic noise into the mics .. you need to run some tests, with and without.

Then I'd heed the valuable advice above to reduce it as economically and much as possible .. and then experiment by placing a cardioid mic facing the noisy traffic (windows) and recording a 'traffic only' track when you record a voice track with your videos.

With suitable slates to sync it all up, in post put the 'traffic only' track 180 degrees out of phase with the voice tracks and raise its gain in post when intruding traffic noises appear. Your NLE will have that OOPhase option.

That could help and you could refine this process as you go along.

Cheers.
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Old May 23rd, 2012, 06:31 AM   #12
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Re: Quick and Inexpensivew way to block outside noise

Greg Miller, YOU have just BLOWN MY mind.

Your post taught me what years of questions and several books didn't.

The downside is now you will have to take care of even my most basic needs for life because your post turned my mind to mush.
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Old May 23rd, 2012, 08:03 AM   #13
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Re: Quick and Inexpensivew way to block outside noise

Thanks Allan, very good points. We're using a muslin screen, which as a fabric won't bounce sound.

Tell me more about OOPhase. I find it VERY interesting. Is there a tutorial on line that I can read to teach me more about this technique?

Thanks everyone, as always you've given me a lot more detailed info than I had anticipated, which is why I love this forum. :-)
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Old May 23rd, 2012, 11:49 AM   #14
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Re: Quick and Inexpensivew way to block outside noise

In a perfect world, where the extraneous noise is constant, predictable and 'din' like, the reverse phase method could work. (some NR apps use this algorithm) However, close-by traffic is rarely consistent, so IMO and by your description of the environment, it would be an exercise in futility, but go ahead and experiment if you have the time for educational purposes.
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Old May 23rd, 2012, 06:57 PM   #15
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Re: Quick and Inexpensivew way to block outside noise

Even if the extraneous noise were constant, the "reverse-phase" method is only a laboratory theoretical curiosity, not a practical real-world solution. The phase of sound changes with frequency and is different from millimeter to millimeter. The only place you can get truly out-of-phase sound is immediately next to the microphone, and that will pick up the desired sound, also, rendering the technique useless.

Now there ARE "noise-cancelling" microphones used for communication application (like aircraft pilots, etc.). But they are severely limited in frequency response, and they require that the microphone essentially be touching your lips. Neither of these conditions is conducive to conventional media production practice. The Grateful Dead took two laboratory-grade reference microphones (because of precise frequency-response matching) and taped them back-to-back to make "entertainment-grade" noise-cancelling microphones so they could use their legendary "Wall of Sound" where they had 30-foot high stacks of speakers behind them from one end of the stage to the other. Of course you couldn't use conventional microphones in a situation like that because the acoustic feedback would be a show-stopper. The rumor is that they couldn't use Shure SM57 mics because no two are close enough to be able to use the cancellation method. And they still had to hold the mics literally on their lips, and they weren't whispering.

Noise cancelling headphones (Bose, et.al.) work by sampling the sound inside the headphone ear cup and synthesizing an out-of-phase signal to counteract the external noise. But they are only effective in the lower portion of the audio spectrum. They depend on the rigidity of the ear cup to simply block higher-frequency noise by the conventional method. And they only work in a very tiny space (inside the earcup). You can't use that scheme out in a room, for example, or even in an announce booth. Again, because the pressure-level waveform of the "sound" is different at every point in the room.

I have used the dynamic noise filter in Adobe Audition (ne. CoolEdit Pro) to sample a "profile" of constant noise (such as hum) and then do a mathematical transform to partly remove it. But it is fiddly, time-consuming and NEVER completely satisfying. You must always make a trade-off decision between the level of noise removal vs. how much trauma it does to the remaining signal. IME it is NOT a suitable long-term solution to depend on.

I would consider building custom-fitted inserts to block the windows with 2x4 frames and several layers of drywall, etc. There are dozens of tutorials online about making sound-blocking walls from common construction materials. They all depend on the mass of the materials (drywall, solid wood, etc.) to reduce transmission of sound.
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