Egg cartons, thin foam and other cheap fixes

Room treatment attracts cheap tricks. Some work a little, many do nothing, and a few do harm. Here is how to judge them with a few principles.

Egg cartons

The best-known myth. Egg cartons are thin, light and shallow. Absorption needs material that air can penetrate, with enough thickness. The depth of an egg carton is a few centimetres of cardboard, which does very little for sound, and nothing for bass. They scatter a little at high frequencies, but they are not a replacement for absorbers or diffusers, and cardboard is flammable.

Thin foam on the walls

Acoustic foam of 2 to 5 cm absorbs the highs and does almost nothing below about 500 Hz. A room covered with thin foam sounds dull but still boomy, because the bass is unchanged. The thickness matters: porous absorption works well down to a frequency where the thickness is about a quarter of the wavelength, see Reflection, absorption and diffraction. A 5 cm layer corresponds to a wavelength of 20 cm, which is 1.7 kHz in theory, and in practice absorption falls off below a few hundred hertz.

Blankets, mattresses, carpet

Heavy blankets hung loosely absorb some mid and high frequencies, and carpet on the floor helps with the high end. They do not treat the bass.

"A reflection filter makes my room sound good"

A reflection filter is a curved absorbing screen placed behind a microphone. It reduces the reflections from the wall behind the performer reaching the microphone and the room sound entering from the back. It helps a little for close vocals, but it does not change the room itself, and in a very reverberant room it can only reduce so much. See the idea behind Initial time delay gap.

One bass trap

A single trap in one corner does little. Bass problems come from the whole room, so treating all corners, ceiling-wall edges, and the first reflection points gives much more than one big absorber.

Soundproofing with foam

Absorption makes a room less reverberant. It does not stop sound from getting into or out of the room. Foam does not isolate, see Absorption, diffusion and isolation.

Over-damping

Covering every surface with absorbers makes a room very dead. Voices and instruments sound unnatural, and musicians have difficulty playing in time. Aim for a controlled, even decay, with some diffusion at the back and sides.

A better approach

1. Measure or estimate the room, see the RT60 tool and room mode calculator.
2. Treat the bass first with thick porous absorbers, 10 cm or more, in corners and across the wall, ideally with an air gap.
3. Control the first reflections at the listening position with absorbers or diffusers.
4. Add diffusion at the back to keep liveliness, see the QRD diffuser designer.
5. Listen and measure after each step.

Draft: written by an AI assistant, awaiting technical review.