Absorption Coefficient Chart

Coefficients by octave band, because a single figure hides the most important fact in room acoustics: almost nothing absorbs low frequencies. Read the 125 Hz column and the 2 kHz column together to see why churches boom.

Coefficients become useful when multiplied by area. Sabins are what the reverberation calculation actually consumes, and this table shows where a typical sanctuary’s absorption already comes from before anything is bought.

Thickness sets the lowest frequency a panel can absorb, not the material. A porous absorber works by turning air movement into heat, and air movement is greatest a quarter of a wavelength from a hard surface. At 125 Hz a quarter wavelength is about 2.3 feet, so a 2 inch panel flat on a wall sits in almost still air at that frequency and does nearly nothing. This is not a defect in the product, it is physics, and no amount of marketing changes it.

Which is why NRC flatters thin absorbers. NRC averages only 250 Hz to 2 kHz, so it ignores the octave where churches have the most trouble. Two inch open-cell foam and a 2 inch fiberglass panel look comparable at NRC 0.75 and 0.85. At 125 Hz they read 0.12 and 0.22, and a 4 inch panel reads 0.65. If the complaint about your room is boom rather than smear, NRC is the wrong number to shop by.

Egg boxes and thin foam tiles do nothing useful. This is worth saying plainly because churches do it every year. Thin foam scatters high frequencies, which removes some sparkle and flutter, and leaves the reverberation time essentially unchanged. A room treated entirely with 1 inch foam tiles measures much as it did before, sounds duller, and has cost real money.

Spend on thickness and on placement before quantity. Sixty 2 inch panels on the rear wall and the first reflection points will outperform two hundred 1 inch panels scattered evenly. Corners are where low frequency energy accumulates, which is what makes bass traps the one thin-looking product that genuinely earns its place. See acoustic treatment for churches.

These are published planning values, not measurements of your room. Absorption varies with mounting method, with the air gap behind a panel, and with how a material has aged. A panel mounted with a 2 inch air gap behind it performs substantially better at low frequencies than the same panel flat on the wall, and no published table captures that.

Coefficients above 1.00 are real and not an error. Some tested materials report values slightly over 1.00 because of diffraction at the edges of the test sample, which makes the panel behave as though it were larger than its measured area. Treat anything at or above 1.00 as total absorption for planning.

Absorption is not isolation. Panels reduce reverberation inside a room and do almost nothing to stop sound passing through a wall to the nursery next door. Sound transmission is a mass and decoupling problem, and it needs construction rather than treatment. Churches conflate these two constantly.

It says nothing about diffusion. Some rooms need scattering rather than absorption, particularly where a church wants to keep its congregational singing alive while removing a specific slap echo. Diffusers preserve energy while breaking up reflections, and a room treated entirely with absorption can end up lifeless.

Occupancy makes the room a moving target. Because the congregation is the dominant absorber, a sanctuary treated to sound right when full will sound bright and reverberant at a midweek meeting of twelve people. Design for the service that matters most and accept the variation.