If you've been to a small rehearsal studio or a friend's rehearsal basement, you've probably noticed a portion of the room (most probably the ceiling) is covered with egg cartons. Unsure whether or not you've entered a practice space or a shady egg-packing operation, you ask the owner what that's all about, wherein he says that the egg cartons serve as soundproofing by absorbing the sound because of its unique shape. Somehow, you find yourself having a hard time believing him as you walk out of the studio after practice with a really bad case of tinnitus.
Egg cartons for soundproofing is perhaps one of the most popular myths about acoustics. Whether or not this belief was founded by experts or DIY enthusiasts let me break this myth by telling you that egg cartons do absolutely nothing for soundproofing. This should be obvious to you when you have a friend scream directly at your ear with only an egg tray in between the two of you.
In the realm of acoustics, there are different frequencies of sound that interact in a variety of ways with the environment around them. High frequency sound waves, like the sound of a high pitched shriek or your irritating Epson LX-300 dot matrix printer, are easily absorbable by layers of porous material such as cloth, fiberglass and carpets.
Low frequency sound, such as those heard at hip-hop / dance clubs, bass guitars and kick drums are not easily as absorbed (an explanation why you hear only the bass when you're in the vicinity of a club or a car with subwoofs passes by with their windows up).
These types of frequencies can be absorbed by structures that have a higher density and mass, such as concrete walls and increasingly thick layers of fiberglass. While an egg carton may absorb some high frequencies, it will not stop any low frequencies from passing through it, and is therefore a poor choice for sound proofing. Egg cartons can be used for sound treatment, though, as a poor-man's diffuser.
By its very definition, sound proofing is the inhibitory of sound from escaping a given space in which it is allowed to propagate. If even the slightest measure of sound is heard out of a "sound proofed" area, that area is no longer considered sound proof.
Sound treatment, on the other hand, is the manipulation of a room's response to frequencies to create a sonically balanced and favorite listening / performance area. Sound treatment can be achieved through absorption, reflection and diffusion (egg cartons may fall into this category).
If you're helping someone setup a rehearsal studio or are making one at your own home, skip the egg carton myth and look for more effective means of treating your space. Creating a sound proof room may be beyond the budgetary capacity of hobbyists (it's essentially creating a room inside a room), but acoustic sound treatment can be had at a reasonable cost.
You have the option to buy ready made absorbers and diffusers online or at acoustic expert shops, or you can simply make your own by following the many plans available on the internet: all it takes is a printer, some handyman work and a trip to the near Home Depot.