Acoustic treatment works by controlling sound reflections inside a room.
In theory, how acoustic treatment works is simple: absorb, diffuse or trap sound energy before it bounces around the room.
You can hear the problem in an untreated room: clap your hands in a bare bedroom and listen for a ringing echo.
That ring is sound bouncing back and forth between hard, parallel surfaces, in Auralex’s words, and it colors what you record or mix in that space.
Absorbers and bass traps convert those reflections to heat and diffusers scatter them, and Auralex’s home-studio guide says using all three can help control sound considerably, even in a room that is less than perfect.
The starting point is understanding which type of treatment targets which problem, because absorption, diffusion, and bass trapping each work through different physical mechanisms.
Below, this guide explains how each type of acoustic treatment works at a physics level, why thickness and placement matter, and the order Auralex’s home-studio guide gives for treating a room.
Absorption panels convert sound energy to heat through friction in porous materials. Diffusers scatter sound in multiple directions. Bass traps use thick material in corners to absorb low frequencies. Auralex’s home-studio guide starts with bass traps in all of the corners, then treatment behind the monitors and at the first reflection points.
How Acoustic Treatment Works on Reflections
Sound is pressure waves moving through air. When those waves hit a surface, three things can happen: the sound reflects (bounces back), absorbs (converts to heat), or transmits (passes through to the other side).
In an untreated room, most surfaces, drywall, glass, concrete and hardwood, are hard and reflective. Auralex’s Acoustics 101 gives ½-inch drywall on studs absorption coefficients of 0.10 or less from 250 to 4000 Hz.
Those reflections combine with the direct sound from your speakers or instruments. Some frequencies get louder when direct and reflected waves arrive in phase, and others cancel out when they arrive out of phase.
The result is a frequency response at your listening position with bumps and dips in it: Auralex’s Acoustics 101 describes a room mode as a bump or dip in a room’s frequency response, set by the room’s dimensions.
Acoustic treatment works by intercepting those reflections before they reach your ears. Absorbers remove energy from the room.
Diffusers redistribute it more evenly. Bass traps target the low frequencies.
For a broader overview of the three treatment types, see what is acoustic treatment.
How Does Sound Absorption Work?
Sound absorption works through a simple physical process: sound waves enter a porous material, and the air molecules vibrating inside the material’s fibers lose energy through friction.
That lost energy converts to a tiny amount of heat — not enough to feel, but enough to significantly reduce the reflected sound.
The process is frequency-dependent. High frequencies have short wavelengths and lose energy quickly in even thin materials.
Low frequencies have long wavelengths and need much more material depth to slow down.
This frequency dependency is why a single type of panel cannot solve every acoustic problem. Effective treatment requires matching material thickness to the frequency range you need to control.
The standard measurement is NRC (Noise Reduction Coefficient), which Auralex’s Acoustics 101 defines as the average of a material’s absorption coefficients at 250, 500, 1000 and 2000 Hz. NRCs are not percentages, Auralex adds, and they can run above 1.0: a material at 1.10 simply absorbs more sound on average than one at 0.50.
Auralex lists drywall on studs at an NRC of 0.05. For a 2-inch fiberglass acoustic panel board, Owens Corning lists unfaced 703 at 1.00.
The three main absorption materials are fiberglass (rigid boards like Owens Corning 703), mineral wool (rockwool), and open-cell foam.
Why Thickness Matters
Thickness sets how low a panel reaches, and Owens Corning’s 700 Series sheet shows it for unfaced 703 board at 125 Hz: 0.05 at 1 inch, 0.15 at 2 inches and 0.56 at 4 inches.
At 500 Hz, the same sheet gives 0.63 for 1-inch board and 1.14 for 2-inch.
At 250 Hz, 4-inch board scores 1.26 against 0.71 at 2 inches.
Foam follows the same rule: Auralex says thicker acoustic foam is better at absorbing low frequency sounds, so 1- and 2-inch foam leaves more of the bass untreated than thicker material.
Auralex’s home-studio guide calls bass trapping very important in smaller spaces. The guide to egg cartons and other foam stand-ins covers why those fall short.
For broadband absorption that reaches into the bass range, those figures point to the thicker boards.
Mounting With an Air Gap
Panels can also be mounted with an air gap between the panel and the wall, and mounting changes the numbers: Auralex’s Acoustics 101 warns that a material tested flat on the floor and one spaced off it by several inches can’t be compared.
Owens Corning’s 703 figures are for boards placed against a solid backing, so they don’t show what a gap adds.
For small room treatment, where wall space is limited, try a gap and listen: Auralex’s home-studio guide suggests trying different placements yourself and listening to the results.
Standoff brackets or spacers hold a panel off the wall.
Auralex’s Acoustics 101 describes a version of this: spanning a peak with a piece of Studiofoam, forming a small, triangular-shaped airspace behind it, which it calls viable and perhaps even more effective than two separate pieces. For board, ROCKWOOL gives its Rockboard 60 mineral wool a density of 6 lbs per cubic foot and, at 2 inches, 0.28 at 125 Hz.
How Does Sound Diffusion Work?
Diffusion takes the opposite approach from absorption. Instead of removing sound energy from the room, diffusers scatter it in multiple directions so that no single strong reflection reaches your ears.
The room stays lively and natural-sounding, but without the problematic echoes and comb filtering that come from flat, hard surfaces. Vicoustic uses diffusion on the rear wall where back wall absorption would render the room too dead, and Auralex says good diffusors can help a small room sound larger.
Diffusers work by presenting surfaces at varying depths to incoming sound waves. When a sound wave hits a diffuser, different parts of the wave reflect off surfaces at different distances, causing the reflected wave to break apart and scatter.
QRD And Skyline Diffusers
Two diffuser designs are QRD (Quadratic Residue Diffusers) and skyline diffusers, both based on mathematical sequences; Vicoustic’s Multifuser DC3 is a skyline diffuser for walls and ceilings.
QRD diffusers use a series of wells at varying depths calculated from a quadratic residue sequence. The depth differences cause the reflected sound to scatter in a pattern set by that sequence.
One-dimensional diffusers scatter sound in one plane: Vicoustic’s 1D diffuser spreads it left and right when the unit is vertical. Two-dimensional designs scatter in all directions.
Skyline diffusers use blocks of varying heights arranged in a grid pattern. The effect is similar to QRD — different block heights create different reflection path lengths, scattering the sound evenly across the room.
The effective frequency range of a diffuser depends on its well depths and overall size. Deeper wells scatter lower frequencies.
The diffuser must also be large enough relative to the wavelength — a small diffuser only scatters high frequencies. This is why rear-wall diffusion needs room: Auralex’s Acoustics 101 gives it a front-to-back dimension of at least 10 to 12 feet.
In Auralex’s stereo mix room, diffusion goes on the rear wall and rear ceiling. For mixing room treatment, it puts large amounts of absorption at the front of the room to kill early reflections.
How Do Bass Traps Work?
Low frequencies are hard to control because their wavelengths are long. At the 1,130 feet per second Auralex gives for sound, a 100 Hz wave is about 11.3 feet long and a 50 Hz wave about 22.6 feet.
Thin panels absorb little of them, as the 1-inch figure above shows.
Bass traps use thick absorptive material, fiberglass or mineral wool, placed in the corners.
The physics is the same as standard absorption, but scaled up. More material depth means more friction over a longer path, which captures more low-frequency energy.
For foam in the corners while you plan a fiberglass build, the JBER 16-Pack Column Corner Bass Traps give you 16 columns, four for each corner.
JBER 16-Pack Column Corner Bass Traps
Why Corners Are Critical
Corners are where low frequencies build up: Auralex’s home-studio guide calls the buildup of low frequencies, especially in corners, one of the most common problems in small rooms.
That is why its first step in any small room is bass traps in all of the corners.
Where a corner can’t be treated vertically, Auralex says bass traps can also be placed horizontally, as along the ceiling-wall junctions.
Standing waves — the resonant frequencies determined by room dimensions — create patterns of peaks and nulls. Auralex’s Acoustics 101 calls a room mode a low frequency standing wave and sorts the modes by the surfaces they run between: two parallel surfaces for an axial mode, four for a tangential mode and six for an oblique one.
Auralex says its absorption and diffusion products are going to alleviate standing waves and room modes, and that rooms with angled walls still need bass traps.
A before-and-after measurement at your listening position shows what the traps change, and the corner guide covers getting more accurate bass there.
Bass traps go in the corners of home theater setups too.
The Bottom Line
Acoustic treatment works by controlling three things: absorption removes excess sound energy, diffusion scatters it evenly, and bass trapping targets the low frequencies.
Auralex’s home-studio guide gives the order: start with bass traps in all of the corners, the first step in any small room.
Then add treatment behind your monitors and at the first reflection points, and absorption panels or clouds above the listening position. Add diffusion to the rear wall last, if the room sounds too dead after absorption.
If you are deciding whether your space needs treatment, see is acoustic treatment necessary. For the complete picture, the home studio treatment plan walks through every step.
To build your own, the DIY acoustic treatment guide shows how to make panels.
Frequently Asked Questions
Does acoustic treatment block sound?
No — acoustic treatment and soundproofing are completely different things. Treatment controls sound reflections within a room to improve what you hear inside it.
Blocking sound between rooms requires mass, isolation, and sealed air gaps — which is soundproofing, not treatment. Auralex says acoustic foam is not sufficient by itself to keep sound in or out of a room.
How does acoustic foam work?
Acoustic foam absorbs sound through the same friction mechanism as fiberglass — sound waves enter the open-cell structure and lose energy as air molecules vibrate against the foam walls.
The limitation is thickness. Auralex says thicker acoustic foam is better at absorbing low frequency sounds.
For bass absorption, you need thicker materials like rockwool or rigid fiberglass. Our acoustic treatment cost guide breaks down what each material costs per square foot.
What is the difference between absorption and diffusion?
Absorption removes sound energy from the room by converting it to heat. The room gets quieter and reflections decrease.
Diffusion scatters sound energy in multiple directions without removing it. The room stays lively and natural, but without the focused reflections and echoes that cause problems. Auralex’s stereo mix room uses both: absorption at the front of the room and diffusion on the rear wall.
