Sound proofing, acoustical foam and pop filters

My friend Ed Helvey has probably forgotten more about voiceover and audio work than I will ever learn. He recently demonstrated some of what I mean in a post about these subjects on the Yahoo! Voiceover Group. Ed has kindly given me permission to reprint his remarks here. They are included here with one or two edits to correct typos, etc.
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Sound Proofing! I see the term misused regularly. There is only ONE way you can produce a Sound Proof room and that is density and isolation – now that is two things, however, they really combine to create one thing – Sound Proofing. Density is NOT a function of Auralex or any other kind of acoustical foam – never has been, never will be. Density is created by building walls that are made of dense materials and transfer minimal sound, which is a function of physics, Sound waves are vibrations of the air that travel like the waves that radiate out from where a stone falls into a body of water. Those waves will continue out from the source and dissipate the further they go depending on the initial energy that created them – ie. smaller stone (quiet voice) less energy, the waves will dissipate sooner – big rock (yelling loudly, loud amplified guitar) lots of energy, the waves travel farther. Now, to stop those waves – you can build a wood box around the area where you’ll drop the stone – and the waves will strike the walls of the box and basically stay within the box. But, drop a big rock in that same area and you’ll create enough energy to cause the waves to hit against the wall so hard they will vibrate the wooden wall and the water outside the box will pick up some of that energy and the waves will continue farther. Now, if you make that a cement wall instead of a wooden wall, it will allow less energy to transfer.

This same law of physics applies to sound. Wood has certain sound transfer characteristics, drywall has other characteristics, lead sheeting, vinyl sheeting, plaster, each have different characteristics. Also – AIR is another sound insulator. So, when you are talking about building a sound proof room or booth, get your checkbook out an prepare to spend a lot of buckaroos to create this sound proof box. It should be isolated (there’s the other word) from the surrounding structure by using certain kinds of materials, rubber, springs, and such (all specially designed for minimal sound energy transfer) that “float” the room and create the least possible direct contact between the outer building structure and the isolated room. And then, you want to construct a room within a room – again, physically isolated from one another. You want to use layers of wall construction materials that could include plywood of varying thicknesses and drywall, again, varying thicknesses (the thickness of the materials relates to the frequencies that the materials will resonate at and transfer sound energy). The two sets of walls should be covered on both sides of each wall with two to three layers of the various materials in various thicknesses and all seems concealed – using wood and or metal framing – maybe different framing in each set of walls and it couldn’t hurt if one set of walls is 4″ interior thickness and the other wall 6″ interior thickness. And, while you could pack those walls with compressed fiberglass insulation, better yet would be to fill each wall with dead sand through holes in the top of the walls that you’ll later seal. Also, the wall materials should be applied so that one set of materials is mounted vertically – 8 to 12 feet high by 4 feet wide and the next layer should be horizontally applied – 4 feet high and 8 to 12 feet wide. There should be at least a couple inches of air space between the two walls and it would be a good consideration to hang a sheet of lead or specially designed vinyl between the two walls. I’m not going into the complexity of designing the window, the door system, the ventilation system and the cable ducts to carry the signal outside the room if so desired. When you’re done with this “sound proof” room, you’ll probably equal the cost of the construction of most of your house, maybe more.

So, when you are talking about sound proofing THIS is what you are talking about – stopping the transmission of sound from either direction – into or out of the controlled environment. Most of us do not need this kind of sound proofing because we can reasonable control the amount of noise generated in our surrounding area – unless maybe you live next to an ordinance test site, are directly under the landing or take off pattern of a major airport or Air Force base, or next to a huge construction site and similar. There was a recent article in one of the trades about the land next to the Capitol Records Tower in Hollywood. They are building a huge condo complex directly adjacent to the Tower and there were all kinds of lawsuits, special zoning provisions, etc., etc., etc. to guarantee the integrity of the sound environment for the Capitol Tower, some of their echo chambers and such are below ground and the construction equipment, blasting, and even the auto traffic in the eventual parking garage below the condo will have impact on a historic site where some of the great music of our time has been produced. Physics is an amazing study and sound is one part of the science of physics since it is energy.

Now, acoustical foam – is for sound control and sound conditioning – NOT, NEVER, CAN’T BE for sound proofing. There are all kinds of acoustical conditioning materials and devices – foam is one of them, and the configuration of the foam is also one of them – wedges, pyramids, egg crate, as is the kind of foam – chemical formulation, density, etc. you also have bass traps to control those low frequencies – and yes bass is one of the hardest things to control because of the very long wavelength and the amount of energy – bass is omni-directional whereas higher frequencies are directional in nature. Different thicknesses of foam will control different frequencies. Diffusers are usually a harder material applied to the walls to break up sound so that instead of reflecting directly back toward the source, it will be reflected in many directions thus breaking up standing waves. Another useful sound conditioning technique is to make sure you don’t have any parallel surfaces. That means your room is odd shaped. So, this is why there are specialists in acoustical design. They measure the room, the characteristics, the construction of the “host” structure and consider the use(s) for the room and then design the acoustical environment utilizing a combination of these various kinds of materials and devices – and many of them can do it with a flair and make the room look really, really cool. Larger studios will have a dead area and a live area – a trap area for drums and piano, many will have moveable panels built in the walls to change the acoustical characteristics depending on the needs of a specific recording project. Once again, get your checkbook out and prepare to start vying with the U.S. gov’t in creating a national debt.

I’ve found for most of us and the work we do from day to day – a small, simply constructed booth in an isolated part of the house furthest from family traffic (if you have a family) and outdoor sound (maybe you’re lucky nough, like I’ve been, to live in a nice rural environment in a masonry & brick structure far from any outside traffic). I then used a combination of 4″ and 2″ Auralex foam and some carpeting to create a dead environment. This allowed me to work the mic anywhere I wanted to, even a foot off mic and sound like I was right on the mic. I placed the mic a little above my reading stand and avoided pop filters, wind screens and the “Popping P.” Was it a perfect design? Nope, but it worked like a champ and my cost with all materials and paying someone to construct it for me (I got a bit lazy) – probably about $1200 to $1500.

Now, wind screens and pop filters. After all I’ve said about various kinds of materials to this point – I think it should be obvious that ANYTHING between your mouth and the diaphragm of the microphone is going to have some impact on the sound transfer characteristics (back to physics 101, again). So, YES, there will be some difference if you use a foam wind screen or a pop filter – that I characterize as a needle point ring or whatever they call it in the craft field and a piece of nylon hosiery stretched on the ring. attach it to a little gooseneck and – viola – a $30 pop filter you could build yourself for about $6.00 in readily obtainable materials. Be that as it may, I’d rather pay someone to do it for me.

Here’s the thing – EVERY mic has different characteristics regarding frequency response, susceptibility to plosive sounds, etc. And each speaker has different vocal characteristics and techniques. So, the trick is to match up the speakers characteristics and techniques with the mic that is most complimentary – and that is why we audition different mics until we find the one that we are most comfortable with and happy with the resulting product. Now, here is another little secret from physics 101, an omni-directional mic is less susceptible to plosive problems then any direction mic (even with a wind screen or pop filter). And, here is another secret from the same book – an omni-directional mic typically has a more natural and transparent sound then a directional mic. And, one final secret; An omni-directional mic becomes a directional mic the closer you get to it and/or the deader the space is that you use it in. Oh yeah, and you can be on mic no matter where you are with an omni-directional mic. So, what am I saying? If you have a dead booth and either an omni mic or a mic with switchable patterns, try it in the omni mode and you may discover a sound you may love and give yourself a lot more flexibility. Oops, one more lesson from the book, omni-mics don’t have a proximity effect – you know, the build up of bass frequencies the closer you get to the mic. Some of us may like to use that effect from time to time, but sometimes it can be a problem (and by the way, that’s also one of the reasons directional mics are more susceptible to plosive sounds).

So, I hope this little primer in sound proofing, sound conditioning, physics and mic technology is useful with regard to this subject and the voice-over artist. And, BTW, if you use a pop filter or a wind screen and you feel you’ve lost a little tiny bit of high frequency (actually mostly mid range), a little tweak of your parametric equalizer will compensate for that, which is precisely one of the major uses of an equalizer.

Have a terrific day, everyone.

Enthusiastically,
Ed Helvey
The Virginia Sound Man
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Thank you for the very interesting tutorial, Ed.