Guide · For the trade
Soundproof carpet pad and carpet acoustics: what the floor can do, and the IIC and NRC numbers to specify
A soundproof carpet pad does not exist: no pad blocks voices or a television coming through a floor, because airborne sound is stopped by the mass and separation of the floor assembly, not by what lies on top of it. What carpet and pad do well is cut impact noise, the footsteps heard in the room below, measured as IIC, and absorb sound inside the room, measured as NRC. A cut-pile carpet over a 7/16 inch 8 lb rebond or a 40 oz felt pad raises a bare slab from an IIC in the high 20s to the 60s, and lifts the room's absorption from an NRC of about 0.05 to about 0.55.
This guide is for the designer specifying a floor for a condominium, a bedroom over a living room, a home office or a media room, and the junior reading an acoustic requirement on a spec sheet: the two ratings, what carpet does and what the pad does, the assemblies and their approximate numbers, the code minimum, and what to write.
The two numbers on an acoustic spec: IIC and NRC
IIC, impact insulation class, rates a floor and ceiling assembly's resistance to impact sound, tested under ASTM E492 with a tapping machine on the floor and a microphone below; higher is quieter, and the International Building Code asks for IIC 50 between dwelling units, 45 when field tested. NRC, noise reduction coefficient, rates a surface's absorption of airborne sound within a room, from 0 for a surface that reflects everything to 1.0 for one that absorbs everything; it controls echo and reverberation. STC, sound transmission class, rates airborne sound through the assembly, and carpet and pad add only 1 or 2 points to it, which is why a pad is not soundproofing.
What the carpet does and what the pad does
Carpet absorbs. Pile is a porous surface that traps air, so a taller, denser cut pile absorbs more than a short loop; a wool or nylon plush at 1/2 inch and 50 oz does measurably more than a 1/4 inch level loop at 28 oz. The pad adds a resilient layer that decouples the carpet from the slab, which is where most of the IIC improvement comes from, and because it is porous it raises the NRC of the same carpet by 0.15 to 0.25. Thicker and heavier pads do more of both up to a point; a 7/16 inch 8 lb rebond or a 40 oz felt is near the top of the curve, and 1/2 inch adds little and voids most carpet warranties.
Carpet sound absorption and impact ratings by assembly
| Floor assembly | Approximate NRC | Approximate IIC on a 6 in concrete slab | Approximate IIC on a wood joist floor with gypsum ceiling | Note |
|---|---|---|---|---|
| Bare concrete, tile or stone | 0.02 to 0.05 | 25 to 30 | 30 to 35 | Fails code between units |
| Level loop carpet, no pad (glue-down) | 0.15 to 0.25 | 40 to 50 | 40 to 45 | Commercial direct glue |
| Cut pile over 7/16 in 8 lb rebond | 0.45 to 0.55 | 65 to 75 | 50 to 60 | The residential standard |
| Cut pile over 40 oz felt or felt-and-rubber | 0.50 to 0.60 | 65 to 75 | 55 to 65 | Best under wool and for IIC |
The numbers are ranges from published laboratory tests, what to expect rather than what to promise; a field test on the finished floor is the only number a code official or an HOA will accept. Carpet and pad are the biggest IIC improvement available on a floor, and the pad does most of the work.
The best carpet pad for sound
For impact noise, a dense, resilient pad at the full 7/16 inch: 8 lb rebond is the standard and tests well; a 32 to 40 oz felt or felt-and-rubber at 3/8 to 7/16 inch tests as well or better and is the choice under wool; sponge rubber at 3/8 inch is the quietest per inch and the most expensive. The term acoustic pad usually means one of these with a test report; ask for the report and read the IIC delta, the improvement over the bare floor, rather than a single number. Under a Berber, a Wilton, a loop or a stair runner, use the dense 1/4 to 3/8 inch felt or rubber the construction needs and accept a few points less. On a condominium slab, an acoustic underlayment, a 2 to 5 mm recycled rubber or cork mat under the pad, adds 5 to 10 points and is what HOA sound rules often require. The carpet pad types guide covers the pad by construction.
Carpet acoustics inside the room: echo and reverberation
In a room with a hard floor and glass, speech bounces for half a second or more and the room sounds loud; carpet over pad is the largest absorbing surface a designer controls. Carpet the floor and treat one more surface in any room where sound matters: a media room, a home office on video calls, a dining room that seats twelve. Cut pile absorbs more than loop, dense more than sparse, and an area rug absorbs in proportion to the floor it covers, so a 9x12 in a 16 by 20 room does about a third of the job of wall-to-wall. The media room carpet guide covers the theater case.
What to write on the spec
- Carpet: a cut pile or dense cut-and-loop at 1/2 inch or less, 40 oz or heavier, wool or nylon, with pile height and face weight stated.
- Pad: 7/16 inch 8 lb rebond, or 40 oz felt at 3/8 to 7/16 inch under wool; 1/4 to 3/8 inch dense felt or rubber under loop, Wilton and stair runners.
- Underlayment where an HOA or code requires it: a tested acoustic mat under the pad, with its IIC report attached to the submittal.
- Installation: stretch-in over tack strip, pad seams taped; tell the installer the pad is specified for acoustics and is not to be substituted.
- Rating required: IIC 50 or the HOA figure, by field test if the building asks; do not write a number the assembly cannot deliver.
Questions designers ask
Does a carpet pad soundproof a floor?
No. A pad cuts impact noise and absorbs sound in the room, which is usually what the client means by a loud floor, but it adds almost nothing to airborne transmission. Voices and music through a floor are a job for the ceiling assembly, not the carpet.
What IIC rating should a carpeted floor have?
The code minimum between dwelling units is IIC 50 in the laboratory and 45 in the field, and many HOAs ask for 55 to 65. Carpet over a 7/16 inch pad on a slab tests in the 60s and 70s; on a wood joist floor it tests in the 50s, and an acoustic underlayment under the pad closes the gap.
Does a thicker pad reduce noise more?
Up to 7/16 inch, yes; past that the gain is small and the carpet warranty is at risk. Density matters as much as thickness: an 8 lb rebond or a 40 oz felt at 7/16 inch outperforms a 6 lb rebond at 1/2 inch.
Does carpet reduce echo in a room?
More than any other finish of the same area. A hard floor absorbs about 5 percent of the sound that hits it; a cut pile over pad absorbs about half, which cuts the reverberation time of a typical room by a third or more.
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