Nov 02, 2007

Seems like yesterday when I was writing about getting acoustical experts involved in matters of residential sound insulation - oh wait, it was.

Now comes an article from the Staten Island Advance that asserts “[t]here are easy ways to contain sound at home and keep the neighbors from complaining:”

If noise is absorbed into, say, soft surfaces with plenty of surface area, such as a medium to dense fabric like draperies, the sounds die, because they cannot bounce off them. So soft surfaces contain sound and work to minimize transmission of noise to other rooms.

Umm, no.

The author is making one of the common mistakes that many laypeople make: confusing sound insulation (i.e. sound proofing) with sound treatment (i.e. room treatment).

Room treatment (bass traps, absorption, diffusion) is all about the effect of the acoustics within the room. Room treatment changes room acoustics properties like reverberation time (echoes/reflections), standing wave amplitudes, and speech intelligibility. Room treatment can reduce interior noise levels, but substantial absorption over a large area is required to significantly reduce noise levels. However a relatively small amount of absorption can cause a perceived reduction in noise levels by reducing reverberation - humans will perceive two sounds (such as a direct sound and a reflected sound) arriving at nearly the same time as one louder sound rather than two separate quieter sounds.

Sound-proofing/sound-insulation (double-walls, double-studs, resilient channel, “QuietRock”, mass-loaded vinyl, etc) is all about reducing sound outside the room. If you want to listen to your favorite action/sci-fi/sports program at full volume without waking up the kids in the next room, or you want your neighbors to stop complaining about your barking dog, you need sound insulation. Doing things like adding egg-crate foam to the walls or heavy curtains to the windows will change the acoustics within the room, but not outside the room.

Think about an open window or door: it makes an ideal sound “absorber” since it reflects 0% of sound waves, but it makes a lousy sound insulator since it allows 100% of sound to be transmitted from one room to the outside. If you have have a totally enclosed room with a loud noise source (say a rock band or table saw) and you remove one of the exterior walls, the sound level in the room will be reduced but the sound level outside the room will increase substaintially.

I suspect that the curtains the author installed hasn’t reduced the noise level at the neighbors’ apartments. Yet the article implies that the treatment was a success because the neighbors have not complained. So, why the disconnect?

My guess: the landlord/friend/author likely told the neighbors that “soundproofing” was installed. The neighbors still hear the barking, but they know that the dog’s owner has installed noise mitigation. As a result, either the neighbors decided to cut the owner a break since they know the owner has tried to solve the problem, or their knowledge of the mitigation efforts (which they assumed was installed by someone with expertise) leads them to genuinely believe that the noise they’re hearing has been reduced. Plus, there is the fact that rambunctious puppy tend to mellow with age, and it may be that the puppy is barking less and less as it grows older.

There’s another lesson in here about non-experts trying to apply acoustic knowledge acquired for a regulatory purposes, usually through specialized training courses (like the one I’ll be assisting in later this month) or internet research. These types of courses and learning methods are not designed to turn laypeople into experts. We give these types of training courses to help non-experts intelligently read and interpret our guidance. When it gets time to implement your project, hire someone that knows what they’re doing.

I’ve been doing acoustical engineering on a professional basis for 13 years now and I’m still learning something new everyday. A non-expert isn’t going to crack the profession after a couple of days of reading Wikipedia or taking a 3-day course.

permanent link