Eggs are often packaged in flat cardboard crates, and these can be the basic building blocks of your new recording room. The flat crates are useful because they do not have lids and are larger and square. They're basically ready to use.
Use these crates to line the walls of the room you want to improve the acoustics in by tacking them to the walls with small nails. They should fit up against one another and completely cover the walls and ceiling if possible.
Some professionals do not recommend this method. According to Soundproofing.org, egg crates often are criticized for being ineffective at absorbing certain frequencies. In fact, according to the website, the sound transmission often seems enhanced when egg crates are used.
Foam rubber mattress pads with the "egg crate" design can also be cut into pieces to line the walls of a studio for acoustic purposes. Tack these pieces of foam up instead of buying the slightly more expensive pieces designed for soundproofing for some of the same effects, but remember there is a trade-off in quality.
According to Soundproofing.org, these pieces of foam designed for mattresses will deteriorate faster and are extremely flammable in many cases.
Carpet-covered walls will certainly reduce echo and stifle sound, making for an improved sound quality over bare walls, but the type of carpet will determine the results. A thicker shag carpet will absorb more sound than a simple, flat indoor-outdoor carpeting.
It's advantageous if you have access to spare, unused carpet pieces that will cover the walls, but perhaps this homemade wall covering would be too expensive to buy new. The results will be inferior to actual soundproofing products and may cost just as much for a thorough job. It will help with acoustic absorbency but will not do a lot for actual soundproofing, according to Soundproofing.org.