Myth: "Carpet is good acoustic treatment"

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Soundman2020
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Myth: "Carpet is good acoustic treatment"

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Postby Soundman2020 » Tue, 2020-Jan-21, 15:46

Another common myth among first-time studio builders, is that carpet is a good thing, acoustically, and helps to make your studio sound great! Carpet on the floor, carpet on the walls, and even carpet on the ceiling....

Well, guess what? It's a myth!

In fact, carpet is a pretty good way of trashing your room acoustics, since it does the exact opposite of what small rooms need.

Small rooms need a lot of bass trapping (the smaller the room is, the more it needs), they need some controlled absorption in the mid range on a descending curve (more for low-mids, less for high-mids), and little to no absorption in the high end. Carpet does the exact opposite: It sucks out all of the high end wonderfully, absorbs some of the mid range randomly on an ascending curve, and does nothing at all to the low end. So not only is it useless, it actually makes things worse. Carpet will make your room sound dull, boomy, muddy, and dry. It will suck out your crash and ride, wreak havoc on the hi-hat and some of the snare, trash the toms, and make the kick sound like a wet cardboard box inside a concrete pipe.... It will mute the life out of female vocals, turn an electric guitar to mush, and make a good acosutic guitar sound like a ten dollar toy guitar with rubber strings on it.

Control rooms need to have neutral acoustics. Take a look at ITU BS.1116-3 (available here) to get an idea of how tight a control room needs to be kept, acoustically. It is flat-out impossible to achieve that with carpet alone. And live rooms, rehearsal rooms, vocal booths, etc. need to have lively acoustics, with some sparkle, air, life, a little brilliance, etc. Carpet turns all that to mud.

Carpet absorbs high frequencies very well, mids to a certain extent but randomly and rising with respect to frequency, then does absolutely nothing at all to low frequencies. That's the opposite of what a small room needs. All small rooms need huge amounts of low frequency absorption, some in the mid-range but less and less as frequency rises, with little to none in the high end. Here's what carpet actually does, according to a test that was done by Riverbank laboratories, one of the most respected acoustic test labs in the world:
carpet-absorption-spectrum-RVBK-S01.jpg

All of the absorption is in the high end! Nothing at all in the lows, and only some in the mids.

Compare that graph with this one that shows roughly what a typical small room ACTUALLY needs:
Needed-room-cruve-opposite-of-carpet-ENH.jpg
Needed-room-cruve-opposite-of-carpet-ENH.jpg (32.1 KiB) Viewed 512 times
Needed-room-cruve-opposite-of-carpet-ENH.jpg
Needed-room-cruve-opposite-of-carpet-ENH.jpg (32.1 KiB) Viewed 512 times

You can see that this is pretty much the opposite of what carpet gives you.

Now, if you use a really thick pile carpet, then the curve improves slightly...
thick-pile-carpet-absorption-chart.png
thick-pile-carpet-absorption-chart.png (28.99 KiB) Viewed 512 times
thick-pile-carpet-absorption-chart.png
thick-pile-carpet-absorption-chart.png (28.99 KiB) Viewed 512 times

... but it is still skewed completely in the wrong direction, and way short of what is needed: There's still nothing in the low end.

But what about if you put some type of underlay under the carpet?
Effect of underlay with carpet.png

Yes, that does, indeed, get you more absorption... but still in the wrong place on the spectrum! There's still nothing useful below about 500 Hz... and considering that the majority of acoustic problems in a small room occur below about 250 Hz, carpet stops absorbing about one entire octave above where it is needed!

Carpet makes your room sound dull, boomy, thuddy, muddy, lifeless, unpleasant, etc.

Secondly, if it is on the floor (as it usually is), that means it destroys the reflections from the floor that your brain relies on to build an "acoustic picture" of the room. All your life, wherever you go, your ears are exactly the same height above the floor, and your brain is very, very accustomed to figuring out the acoustic signature of the room based on the reflections it hears from the floor. If you sit down, your brain recognizes that, and adjusts it's "image" of the room accordingly. It does not use the ceiling or the walls for that, because the distance from your ears to the walls and ceiling changes all the time, many times per second as you walk around, so the "signature" is not constant or consistent. Ceilings are different heights, and when you walk outdoors, there is no ceiling at all! But there is still a floor, and it is still the same distance from your ears as every other floor.

If you have carpet on the floor, your brain no longer has any reflections to use for this.

So forget the carpet. You'll find it really hard to have a good acoustic setup in a room with thick carpet on the floor. Take a look at some photos of high-end pro studios: how many of those have thick carpet on the floor? :) There's a reason for that...


- Stuart -




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