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Acoustic Carpets for High End Audio Applications

Art Noxon

All “acoustics” are not the same. We have sound proofing and then we have sound conditioning. Sound proofing means keeping the sound in the room and sound conditioning usually means getting the sound that’s in the room to calm down. Some carpet acoustic products are about soundproofing, reducing the sound transmitted to your neighbors, downstairs. Other carpet acoustics are about sound conditioning. In high end audio applications, there certainly will be times when we need sound proofing but we always need sound conditioning.

A) Sound proofing vs sound conditioning

Sound proofing is a complicated subject and it’s best left to an expert in soundproofing to choose what you need. Mostly what you do about soundproofing depends on the frequency range you are trying to soundproof. Soundproofing bass is always more difficult and expensive than soundproofing treble.

In the carpet world, soundproofing usually comes in the underlayment. It is a sheet of heavy material, typically MLV or Mass Loaded Vinyl with foam above and below it. It blocks the treble but not the bass.

Here’s a basic idea about soundproofing to always remember. Soundproofing means doing something to the wall, floor or ceiling surfaces of your room so that sound cannot go through them. If the sound can’t go through the surfaces of your room, guess what? The sound remains in the room. Sound proofing usually means that sound is reflected back into the room, where it came from. If you are in a soundproof room, and make a sound, the sound you made will stay in the room, for a long time.

It’s the “for a long time” part of sound proofing that does not work for high end audio. In high performance audio environments, we may or may not care about soundproofing. But what we always care about sound conditioning. We always care about how the “sound of the sound” sounds in our room.

1) Sound proofing stops the sound from leaving the room.

2) Sound conditioning manages the sound that’s in the room.

Whenever you add soundproofing to a high end listening room you have to add, at the same time, extra sound conditioning in order to deal with the extra energy in the room, the energy you kept from leaving the room. One of the ways to introduce sound conditioning to rooms is to add an acoustic carpet.

Carpet acoustics is usually reserved for the higher frequency range, typically the treble range, above middle C, or 250 Hz, and contributes nothing to the bass range.

B) Why acoustic carpets

Nearly all loudspeakers are set up on some type of acoustic carpet. If they are set up on a hard floor or on a non acoustic carpet, a flutter echo will occur between the floor and ceiling, a lingering, ringing type of sound that is almost metallic in quality. In addition, the floor reflection will lower the image of the sound stage. It will also create some comb filtering sensation.

The listener does not seem to mind the ceiling reflection as much as the hard floor reflection. An acoustic carpet also quiets down the general noise level in the listening space, which is good for listening. Some people like a bright room and if so, then the carpet should not be wall to wall but should be an area carpet.

C) What is an acoustic carpet

Most carpets have low thread counts and a sealed back. They are nice looking low cost and liquids can’t penetrate through the carpet and wet the underlayment and the flooring below. For normal residential life this is a good thing. For high end audio, it’s a bad thing. A low thread count, sealed carpet reflects sound, and it’s just about as bad as having no carpet at all.

Acoustic carpets have a high thread count, deep pile and it is sheared. Tight Burber weave carpets wear well but they are not acoustic at all because the threads are tightly packed together. Low density long loop piles are not acoustic. Low pile, even if it has a high thread count and is sheared, is not acoustic. A carpet with an open back and with an expensive felt underlayment can be considered an acoustic carpet.

Acoustical carpets have an NRC rating, a Noise Reduction Coefficient rating. Typically they might be NRC 30, which means 30% of the speech range of sound that impacts the carpet is absorbed by the carpet. Non acoustical carpets might have an NRC 5, meaning 5% of the sound impacting the carpet is absorbed. Notice that NRC is a speech range sound measurement and does not include any consideration given to subwoofer bass range, which is 80 Hz and below.

The classic acoustic carpet is the deep pile Persian carpet. Everyone knows this. The problem is that these carpets are hand made and expensive. Most audio types want a lower cost, machine made version of a Persian carpet. Maybe a pattern or maybe not, it’s the threads that count in acoustics and it’s the threads that cost the money.

With carpet acoustics, we begin with a basic understanding. Long pile tight knap expensive Persian carpets are the reference standard in carpet acoustics. Acoustic carpets are one of two types. a) Long pile, tight knap carpets or b) open back carpets with dense felt carpet pad.

a) If the carpet has a loose knap and a strong, tightly woven sealed back, and most good carpets are made that way, then it will reflect sound. Putting an expensive hotel quality felt pad underneath will make the carpet feel better to walk on but makes little difference to the sound.

b) If the carpet has a loose knap and an open weave, unsealed back, then you are in luck. You can add a dense felt carpet pad, thicker the better for sound, and you will have a quiet carpet.

1) How to tell if your carpet is acoustic

The easiest way to tell if a piece of carpet is acoustic is to hiss at it and if it’s acoustic, it won’t hiss back. What’s good about learning how to do this test is that you don’t need any test equipment and you are always ready to make one of these tests.

Let’s first learn how to do the test. In an open room, away from reflecting surfaces, say “Tsssss”. Listen with your ears to the sound as it leaves your mouth and wraps back around your face and into your ears. Raise your hand as if you were going to cover a yawn and Tisss again, this time you hear a different sound. It has a different tone and it is louder. This is the sound of a treble reflecting surface.

Next, practice this again with a piece of paper. Hold a piece of paper with the thumb and forefinger of each hand, resting on your lap. Go Tiss and while hissing steadily raise the paper to position in front of your face, about 2” away and again you hear the sound of treble reflections. It’s amazing to think of how much sound just a thin sheet of paper reflects. Now you are trained.

Get a carpet sample, or a remnant or just peal the carpet off the floor at a corner and fold it back a few feet, enough that you can comfortably hiss at it. Put your lips close, 2” from the knap side of the carpet and go Tssss. If the sound bounces back, it’s not acoustic, if the sound seems to diminish, it’s acoustic.

2) How to tell if a carpet back is open or closed

The reason carpets are closed back is so you can spill and wash them without getting the underlayment material soaking wet and even worse, get the floor underneath wet. But these waterproof carpets are very bad for sound. Get a remnant or sample or pull the carpet up at the corner. Put your lips against the back side of the carpet and try to blow through the carpet.1) The carpet has a closed back if you have a hard time or can’t blow through it.2) The carpet has an open back if you can easily blow through it.

If the carpet has a closed back, and most carpets are closed back, then nothing you add behind the carpet will contribute to sound absorption. All the absorption must come from the carpet pile.

If you discover you have a sealed carpet in your listening room, your choices will be to 1) change the carpet and buy an open back carpet and felt pad2) Rip the carpet out, buy and install an "acoustic rated" carpet. 3) Leave the bad carpet in place and buy an acoustic rated carpet and put it on top the bad carpet.

If the carpet is open backed, then you can add top quality carpet felt, not foam but the very best, hotel grade felt underlayment. You will get sound absorption from this felt underlayment. If the carpet is open backed, the type of pile and knap does not matter much, as long as the sound can get into the felt underlayment.

The best situation would be to find a sound absorbing knap with an open back. Then you can use foam or other type of underlayment and have an even better acoustic carpet.

Caution! With open back carpet, you cannot use underlayment that is sealed on the top. Sealed underlayment often has a blue skin on the top. When this is pressed down by the open back carpet it turns the open back carpet into a closed back carpet. Use an underlayment that does not have a seal. You do not want to put the sealed side of the underlayment against the floor. This makes a humidity trap.

3) Finding the right carpet

Acoustic rated carpets are few and far between. The classic acoustic carpet is, as we all know, is the deep pile, dense knap Persian carpet. It is beautiful and expensive, because the fibers are long and densely packed. There are a few machine made carpets that are acoustic rated. They do look the same and are expensive. The pile is 5/8ths to 3/4 inches deep and the knap is very dense, lots of threads, tightly packed together. They are still not as good or tight as a Persian carpet. An acoustic rated carpet says “acoustic rated” on the sales tag or brochure.

Regarding acceptable acoustic carpet choices for an audio room:General guidelines: -longer threads, more dense pile-Use 40 ounce (7/16" thick) expensive felt pad. -Carpet backing is not sealed with plastic. -Thicker carpet, thicker felt pad the better. -Deep pile Persian carpet is ideal.

Three approved choices for roll carpet:

  1. Stainmaster carpet "Dazzling Light" (Acoustic rated)
  2. Stainmaster carpet "Calypso Royal" (not rated, but good)
  3. Gulistan carpet "Touch of Love" (not rated, but good)

ASC does not supply acoustic carpet or carpet accessories at this time.

Art Noxon is a fully accredited Acoustical Engineer with Master of Science degrees in Mechanical Engineering/Acoustics and Physics. A Professional Engineer since 1982, he is licensed in Oregon to practice engineering in the public domain with the specialty area of acoustics. A prolific inventor, he developed and patented the iconic TubeTrap, the original corner-loaded bass trap/treble diffuser, 150 other acoustic devices, and counting. Lecturer, writer, and teacher of acoustics, he has presented 7 AES papers, numerous magazine articles, white papers and blogs. He is president of Acoustic Sciences Corporation, the company he founded in 1984.