Proper Use of Furni Pads/Sound Blankets

Z

Zander Burstein

Member
I haven't been able to get much feedback on this subject but I feel this information can come in handy in not so great locations for sound.

Does anyone have tips or tricks for sound blankets? I know they're used to minimize sound reflection, but I'd love some examples. I've seen a few mixers clip a furni-pad to a C-Stand and walk it in towards the talent when in an open space. Things like this could really help me when I'm recording in a not so ideal setting.

Thanks
 
I'd like to hear more on this as well, so I'll chime in first. The two "tricks" that I know of are the following:

1) Move them away from the walls - generally, the closer you can get the more space they cover, and it helps to dampen lower frequencies.
2) Leave folds - don't have them straight unless you really need the coverage, as this also significantly improves the diminishing of lower frequencies.
 
I was under the impression that absorption of lower freqs (all freqs really) was based on density...so moving away from walls, not sure how that affects (by all means please explain...im curious)? Also, sure, i think folding them over will cut lower freqs, but it wont help much. One former recording engineer told me even the expensive 2"+ thick acoustic foam doesnt cut much below 2k unless attached to something substantially denser.
 
Jema said:
I'd like to hear more on this as well, so I'll chime in first. The two "tricks" that I know of are the following:

1) Move them away from the walls - generally, the closer you can get the more space they cover, and it helps to dampen lower frequencies.
2) Leave folds - don't have them straight unless you really need the coverage, as this also significantly improves the diminishing of lower frequencies.
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A mentor of mine places furni's at the corners of a room. To be honest though I can't remember the reason why. I'm sure someone here could explain the science of it.
 
Direct sound travels in a straight line and in an average size room in a house it reverberates around all the walls. Enough of it can make recorded speech sound off mic and unintelligable.

Good damping at the rooms right angled corners up to about 5-6' helps stop voice frequencies reflecting at these corners.

If you're booming actors, depending on other factors like glass surfaces, wooden floors, lighting, mic placement etc. corners of a room are good places to work on first. Of course a good pair of headphones and enough set-up time are both essential to get more experience with this.

Cheers.
 
I forgot about the corners - that's another good tip. It's somewhat of a "hot spot" for lower frequencies, and placing something really dense and thick might do a difference (I haven't done any real test on this myself).

Regarding the distance from the wall, it has to do with the wavelengths. An absorbing material is the most effective when placed at a distance from a reflective surface that corresponds to 1/4 of the wavelength. So to get a little space helps.

I was under the impression that absorption of lower freqs (all freqs really) was based on density...so moving away from walls, not sure how that affects (by all means please explain...im curious)? Also, sure, i think folding them over will cut lower freqs, but it wont help much. One former recording engineer told me even the expensive 2"+ thick acoustic foam doesnt cut much below 2k unless attached to something substantially denser.
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It's true that they don't cut "much", but it's better than nothing. Do a search on some absorption koefficiants to find lists on them for different materials. The best for low frequencies are large furnitures, but not what I would call practical.
 
Sound does not only travel left to right and front to back, it also travels up and down. So some carpeting on the floor will also be a large help. It also help reduce crew noises.

The idea is to create lots of odd angles as well; when the sound keeps reflecting off of parallel walls it creates "standing waves" where certain frequencies can start reinforcing each other.
 
Sound travels like a pool ball on a pool table, but a three dimensional pool table.

The number of blankets and placement of them depends a lot on the practicality of the situation.

Floors, doors, walls, ceilings, big tables, windows, any hard, flat surface is your enemy. Parallel hard flat surfaces are the worst. Treat one of them so the sound doesn't flutter.

Regards,

Ty Ford
 
From a musical recording background I can say that short of a custom built studio, furnishings are about the best way to break up sound. A bookcase full of irregular sized books is about as good as it gets (and makes for a good background too). It is heavy (mass absorbs sound), irregularly shaped (diffuses the sound, prevents standing waves) and is ideal to break up a parrallel wall. A bookcase with matched sets of books is okay but less effective. Simply pulling some forward and pushing others back gives the same effect. Soft furnishings are good, especially for high frequencies and echo. Carpets, the thicker the better work well. You can put down underlay on a hardwood floor in scenes where it isn't in shot in the same way as you put an off camera mattress down for a stunt-fall. This will limit any echo and standing waves between the parrallel floor/ceiling. I've got a roll of thick rubber underlay I picked up off the side of the road for free after a neighbour had their carpets redone. I've used it in much the same way as sound blankets while recording music.

Bass is the hardest one to deal with by using mass alone because bass frequencies require a LOT of mass to absorb them (we're talking lead). Fortunately a lot of bass energy will pass right through an object (including walls). The down side is that bass (such as low rumbling from traffic) can come back the other way too. A high pass filter while taking or in post can help there, the wall already acting as a low pass filter you should be able to cancel most external sources. In a mix you can usually be quite aggressive with the high-pass filter settings for dialogue without it sounding wrong once blended back into your ambient/music soundtrack. A touch of reverb can help it blend if its still not quite right. The other big issue with bass (as already noted) is the fact that it tends to gather in corners. You can deal with this using bass traps (available from music suppliers) but again diffusion can go a long way. Simply pile up set dressing into every corner you can. Without corners to grow in, most unwanted bass will dissipate and leave.

I guess my point is the most important thing is to get the room right. Sound blankets are more a bandaid to minimise the issues you can't fix practically. Being black (unlike my carpet underlay) they also double as light blankets, good for flagging lights and blocking windows.
 
For bass control, bass traps that are either wall-installed slot absorbers or systems with pliable diaphragms to absorb those frequencies seem to work best, but they cost a lot to design and install.

Regards,

Ty Ford
 
Egg Born Son said:
It is heavy (mass absorbs sound)
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The myth that won't die. Sigh...

If simple mass absorbed sound, SONAR would be useless. If mass absorbed sound, steel I-beams in buildings wouldn't ring like a frellin' bell and transmit sound all over the structure. If mass absorbed sound, stone cathedrals would be silent.

Certain types of mass are efficient at converting vibration into heat so that it can dissipate. Many are better at transmitting the sound than they are at dissipating it.

The reason that the wedge and pyramid shaped sound deadening surfaces inside an anechoic chamber works isn't because it's massive (it's not at all massive), it's because of its design.
 
The reason it won't die is because it is a good rule of thumb to give someone without dropping a 1000 page book about acoustics on them. It is true that it must be the right mass to be effective but a bookcase isn't a steel I-beam nor a stone cathedral (in which there's a lot more at play than mass). I don't think anyone would use those items for this purpose and an apartment is not an anechoic chamber. The point of the 'myth' is to illustrate that a blanket is going to be better than a sheet. By pointing out that a lot of mass is needed to block bass I was merely illustrating that you can't (without a foot or more of concrete).

Maybe I wasn't clear enough (I do tend to ramble) in pointing out I was advocating diffusion as the primary and best method of cleaning up a sound space, making the space sound nice rather than isolated. For music my second port of call is roof insulation (I believe its called glass wool in the US). You can get it in rolls with a foil backing which works great because the fibre glass strands are effective as diffusion and the foil takes the energy out (it takes more energy to vibrate the foil than the vibrations give back). I didn't mention it because unless its a permanent space its probably more effort than you want to go to.

In the real world you rarely have what you want and knowing a few tricks to make do with what you have will get you a long way. I've never recorded in a high end studio or even a low end one for that matter. I'm usually in someone's lounge, garage or home studio with mattresses and my rubber underlay used stuffed into corners or erected into baffles, cleaning up the acoustics as best I can.
 
Let's go on a sunday drive...

Let's be sure were differentiating internal acoustics from external noise abatement.

Internal acoustics is treating the space. External noise abatement is keeping the neighbor's dog from being heard while you record or keeping your guitar amp sound out of their space.

For most noise abatement issues, although the best block does involve mass, simple mass is not where near effective as D-I-D; Density-Isolation-Density. Two masses that are isolated by a space so they don't connect. That's usually used to keep the noises from one space from getting into the other. Putting sound blankets on your wall won't do much for the dog and guitar amp scenario because there's only one mass; the wall.

For Internal acoustic issues, you need a balance of diffusion and absorption. The bookcase idea mentioned earlier is a very good example of diffusion. Blankets and foam panels are absorption. Fiberglass is not used a lot anymore because small particles of it will work their way free and can be inhaled. Blankets and foam panels are not as effective at low frequencies. Bass traps need to be designed and built. If you're on location and low frequencies really are a problem, you need to find a better location because there isn't enough money to do the job.

Think of a space like a coke bottle. When you blow over the top, you provide enough energy to excite the bottle to its resonant frequency and it whistles. Some spaces are very resonant; some aren't. Spaces can be resonant at one or any number of frequencies. And there's also the issue of where the sound is generated from within the space. If your talent is loud enough, he/she can excite the space with his/her voice and all sorts of things will be heard.

As mentioned earlier, spanning sound blankets across the ceiling works as does covering the floor and building a sound block of blankets just out of frame. That's a lot of blankets, but usually far less than it would take to blanket the entire room. The idea is to stop reflections from the big, hard, flat spaces, and also to reduce the energy of the voice to keep it from hitting the walls, ceiling and floor. Building a sound block of blankets just out of frame achieves this, by blocking and absorbing some of the energy BEFORE it makes the room ring.

Regards,

Ty Ford
 
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