Broadband and Mid / High panels can look identical from the front. The difference is the core: Broadband panels have a 4" core and Mid / High panels have a 2" core, both 6 PCF (6 pounds per cubic foot) rigid mineral wool. How thick acoustic panels should be depends on how low you need them to absorb, and the thicker core keeps absorbing further down in frequency. Bass traps, clouds and diffusers each need a different depth, listed in the table below.
Depth by Treatment Type
For porous absorbers, the deeper the treatment, the lower it reaches. Tuned traps also depend on the mass of the membrane or diaphragm, so a heavier membrane can tune lower in a shallower box. Each row links to the post or product page that explains it.
| Treatment | Overall Depth | Where It Works |
|---|---|---|
| Mid / High Wall Panel | 2.75" (2" core) | Upper mids and highs. 0.75 at 250 Hz, 0.18 at 125 Hz. |
| Broadband Wall Panel | 4.75" (4" core) | Down into the low mids. 0.88 at 250 Hz, 0.60 at 125 Hz. |
| Ceiling Cloud | 3.5" or 5.5", plus the air space above it | Same range as its core. The air space above adds bass absorption to a 2" core. |
| Fabric Track Wall | Set surface by surface | 2" infill for flutter and the upper mids. 4" or more for the low mids. |
| Porous Bass Trap | 6" to 12" | A 6" trap down to about 225 Hz, a 12" trap down to about 110 Hz. |
| Diaphragm Bass Trap | 9-1/4" or 12-1/4" | From about 80 Hz up on 2 ft wide traps and 60 Hz up on 3 ft and wider, through the mids and highs. |
| Membrane Bass Trap | 7-1/2" to 13-3/4" | A narrow band at one tuning: 45, 60, 70 or 85 Hz. |
| Well Diffuser | 4" to 8" in a home room | Scatters from about 965 Hz up at 4" deep and 480 Hz up at 8" (7-well design). |
Panel figures are published lab data for the bare board. Porous trap depths follow the tenth wavelength rule. Diaphragm and membrane trap ranges are predicted values. Diffuser ranges come from the published quadratic residue design formula.
How Thick Should Acoustic Panels Be?
A porous absorber, such as mineral wool or fiberglass board, turns sound into a small amount of heat through friction as air moves through its fibers. It only works where the air is moving.
At a hard wall the air can't move, so air particle velocity is zero at the wall surface. It rises to its peak a quarter wavelength out from the wall. A panel hung flat on drywall sits in the slow moving air close to the wall, so the deeper the panel, the more moving air it reaches.
Wavelength is the speed of sound divided by frequency, about 1,125 ft/s at room temperature:
- 125 Hz: 1,125 / 125 = 9 ft. A quarter wavelength is 27".
- 500 Hz: 1,125 / 500 = 2.25 ft. A quarter wavelength is about 6-3/4".
- 2000 Hz: 1,125 / 2000 = 0.56 ft, about 6-3/4". A quarter wavelength is under 2".
Both cores are thick enough for the upper mids and highs. At 125 Hz, neither is anywhere near 27" deep, so the extra 2" of the Broadband core is what moves the number.
What the Published Data Shows
The table below uses published ASTM C423 data sheets for two 6 PCF rigid boards, one stone wool and one fiberglass. For each band it shows the lower of the two published values, with anything above 1.00 shown as 1.00.
The fiberglass sheet states Type A mounting, laid flat against the test surface. That is the closest lab mount to a panel hung on Z-clips or cleats. The stone wool sheet does not state the mounting. These are values for the bare board.
| Core | 125 Hz | 250 Hz | 500 Hz | 1000 Hz | 2000 Hz | 4000 Hz |
|---|---|---|---|---|---|---|
| 4" core (Broadband) | 0.60 | 0.88 | 1.00 | 1.00 | 1.00 | 0.91 |
| 2" core (Mid / High) | 0.18 | 0.75 | 1.00 | 1.00 | 1.00 | 0.97 |
From 500 Hz to 2000 Hz the two cores read the same. The difference is at the bottom: the 4" core absorbs more than three times as much at 125 Hz, 0.60 against 0.18.
Why NRC Doesn't Separate Them
NRC, the noise reduction coefficient, averages the coefficients at 250, 500, 1000 and 2000 Hz and leaves out 125 Hz. On both published data sheets, the NRC rises by 0.05 or less from 2" to 4", while the 125 Hz value more than triples, from 0.18 to 0.60 in the table above.
Compare the 125 Hz column, not the NRC. Our post on what NRC means covers the rest of the data sheet.
When Broadband (4" Core) Is the Right Call
Broadband panels use the 4" core. Choose them for:
- Mix and Control Rooms: Low mid buildup affects every decision made at the desk.
- First Reflection Points: The spots on the side walls between the speakers and the listening position. To find them, sit in the mix chair while someone slides a mirror flat along the wall. Wherever you can see a speaker in the mirror is a reflection point.
- Rear Wall: The wall behind the listener, which sends sound straight back to the mix position.
- Small Rooms: A spare bedroom puts every wall close to the listener, so reflections arrive early and strong across the whole range.
Covering a room in 2" panels alone absorbs the mids and highs and leaves the bass, so the room ends up dull and boomy at the same time.
When Mid / High (2" Core) Makes Sense
Mid / High panels are 2.75" deep, against 4.75" for Broadband. Use them:
- Tight Spots: Where depth is limited, such as next to a door or in a hallway.
- Flutter Echo: Between two parallel hard walls, where the problem is high frequency ringing.
- Bass Already Handled: In rooms where low frequencies are covered by Diaphragm Bass Traps or bass traps.
- Speech Rooms: Offices, podcast rooms and conference rooms, where intelligibility depends mostly on the mid and high bands.
A Common Layout
Many rooms mix the two. A sensible order:
- First: Broadband panels at the first reflection points on the side walls.
- Second: Broadband panels on the rear wall.
- Third: Bass traps in the corners, where they do the low frequency work a flat panel can't.
- Fourth: A ceiling cloud over the listening position. The cloud follows the same choice: 5.5" Broadband with a 4" core or 3.5" Mid / High with a 2" core.
- Last: Mid / High panels where depth is tight or where flutter echo remains.
Check It Yourself
A measurement shows which bands need work before you pick a thickness.
- Use a free room measurement program and a calibrated USB measurement microphone at the listening position.
- Play the program's sweep through your speakers and look at the decay time in each octave band, shown as T20 or T30.
If the 125 Hz and 250 Hz bands ring much longer than 1000 Hz, the room needs depth: 4" cores and bass traps. If the decay is even and the problem is flutter or harsh highs, 2" cores will do the job. Measure again after the panels go up, with the mic in the same spot.
Sources
- Manufacturer published Technical Data Sheet: 6 PCF Stone Wool Multipurpose Board, ASTM C423 Sound Absorption, 2026.
- Manufacturer published Product Data Sheet: Rigid Fiberglass Board, ASTM C423 Type A Sound Absorption Coefficients, Pub. 10021736, 2026.
- ASTM International, ASTM C423-23, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, ASTM International, 2023.
- ASTM International, ASTM E795-05(2012), Standard Practices for Mounting Test Specimens During Sound Absorption Tests (preview with the Type A definition), ASTM International, 2012.
Need a Second Opinion?
Every panel is built to order in North Los Angeles in 50 fabric colors. Not sure which your room needs? Send photos and rough dimensions through our free room review form. Reach out to us for design assistance.