From the Desk of Art Noxon

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.

Persian Rug Acoustics

The timeless image of HiFi is a high flying Persian carpet sporting a listening chair and a pair of loudspeakers. Yes, it’s true. A good sound system set up on a Persian carpet can transport any audiophile into a wonderful adventure in some other place and time.

But, what about this Persian carpet? Does it have to be a real Persian carpet, or just look like one? How about a cheap Chinese Persian carpet? What kind of Persian carpet? The old original short-knap type or the more modern thick and plush type? And what does a Persian carpet have to do with the sound anyway - does it just help with one's imagination or is there something real about the Persian carpet?

They are Expensive

Persian carpets are very expensive. A 1” thick and very plush carpet sized 8x10 is just barely large enough to outfit a listening setup, the chair and two speakers. It costs $4000 [Costs are from 2007]. That figures out to be around $50/square foot. Sure, it looks good and it certainly feels good but does it sound that good? $50/sq ft brackets the upper end of the most expensive acoustic products available. An ordinary sound panel costs in the range of $7/sq ft, while a high end sound panel might cost as much as $20/sq ft. Aside from its beauty and that we can walk on it, the Persian carpet runs 3 to 6 times more expensive than sound panels.

Used in Audio

The Persian carpet shows up in two parts of the audio community. It is often found in recording studios where the sound is being made. It is also found in the HiFi listening room, where sound is being replicated.

In the recording studio, the microphone is doing the listening. It is positioned well off the floor, often overhead of the vocal or instrumental talent. The Persian carpet is found on the floor directly below the mic. Usually the talent and mic stand are both standing on the Persian carpet. The carpet is used to kill or at least diminish the floor bounce which causes “comb filter” coloration in the sound collected by the overhead mic.

In the HiFi listening room, the audiophile is doing the listening. They sit towards the back of the carpet, near the tassels at one end. The loudspeakers are located at the other end of the carpet, again near the tassels of the front end of the carpet. Again, the carpet is used to kill or at least diminish the strength of the floor bounce, which otherwise would cause a “comb filter” effect, coloration of the listened to sound.  

A second important reason to use non-reflective carpet under the speakers is to eliminate the vertical "flutter echo" effect. When high frequency drivers recreate snap or clap type sounds, the wavefront expands in a hemispherical pattern.  Part of that expanding wavefront expands down the front of the speaker, hits the floor and bounces back up, hits the ceiling and bounces back down and so on.  The upward moving portion of the wavefront hits the ceiling first and then expands down towards the floor and so on.  These multiple reflections back and forth between parallel surfaces create a flutter echo that sounds like a lingering low tone.  

Sound travels 1.1 feet/second so with a 9' ceiling we have one reflection every 8 milliseconds which gives a pseudotone of 1000/8ms = 125 Hz.  We need high frequency sound absorbing carpet to eliminate what we hear as a buzzy sounding bass tone that lingers behind any sharp clap kind of sound in the program material.   When you stand behind your speaker and clap your hands over top, in front of the speaker, you should not hear any flutter echo or Zing type of sounds.  

In recording studios the higher ranges are controlled by sound panel absorbers and diffusers.  In home HiFi setups the carpet is a critical component in the room acoustic package. Sound absorbing carpets help to quiet down the listening room. It absorbs the lingering, reverberant sound and quiets down the room.  

Wall to wall carpet creates an overly quiet space.  Hard wood floor with a large sound absorbing throw rug creates a quiet listening zone in an otherwise bright sounding room.  When dealing with early reflections, remember that above the wall reflection point is a ceiling-wall double bounce reflection path and with throw rugs, the floor-wall double bounce reflection path.  Wall to wall carpet does not have the floor-wall reflecting path. 

In both cases, the studio and the playback environment, the Persian carpet primarily acts to minimize the floor reflection from degrading the otherwise clean sound generated by the sound source. The desire to clean up the sound becomes the welcome mat over which the Persian carpet enters to join as a legitimate member of the audio community.

Carpet Facts

Let’s look at some of the rules-of-thumb that fly about the air regarding Persian carpets.

Thick carpets do not need a carpet pad but thin carpets can use a carpet pad.

  1. Thick carpets are so stiff and massive, they do not need anti slip pads.
  2. Thin carpets are light and floppy and do need anti slip pads.
  3. Old style short knap tight pack carpets reflect sound, pad or not.
  4. Newer long knap tight pack carpets absorb sound even without pad.
  5. Thick carpets can be 1” deep and weigh over 1#/sq ft.
  6. Thick carpets provide tippy support, audio spikes should be used.

Sound Absorption

Sound is absorbed on the surface of objects. Fibers have a huge surface area to weight ratio, which is why fiber-made objects make good sound absorbers. The density of glass is 2.5 times that of water. The density of wool is 1.3 times that of water. Wool fiber weighs ½ the weight of glass fiber, all other dimensions being equal.

We have seen that both fiberglass fibers and soft wool fibers are made in about the same diameter, 8 microns. (1 micron equals one millionth of a meter.) The surface weight of a plush 1” thick Persian carpet is 1.1 #/sq ft. The bulk fiber density of a Persian carpet is about 13#/cubic foot. But the fibers of the Persian carpet have half the weight as equivalent fiberglass fibers. A fiberglass sound panel with the same number and size of fibers as a Persian carpet would weight twice as much as a Persian carpet. To compare to a 1” carpet, the sound panel would have a surface weight of 2.2 #/sq ft.

A typical sound panel is made out of bulk fiberglass whose density is between 4 and 5 #/cu ft. The standard 2” sound panel has the surface weight of about 0.75 #/sq ft. To have a surface weight of 2.2 #/sq ft, a sound panel would have to be 6” thick.

A 1” thick plush Persian carpet has the same amount of acoustical fiber as a 6” thick sound panel. Now that’s an impressive comparison.

Efficiency Matters

There’s fiber density and then there’s fiber orientation. All fiberglass sound panels are made with the fibers laying flat, in the plane of the panel. This means that the sound waves hit the fibers broadside. In the Persian carpet, the fibers don’t lay flat, they stand up and the sound waves impact the fibers head on. This may seem to be a minor point but it isn’t a minor point.

In either case, end on or broad side, the sound absorption process takes place on the surface of the fiber. But when sound hits a fiber broadside, part of the back pressure felt by the sound wave is due to the energy needed to accelerate the air around both sides of the fiber. When sound hits the carpet fibers head on, only the tiny head of the fiber parts the sound wave. Almost no back pressure due to air flow acceleration is felt.

There are two kinds or resistance to waves. We have similar example in electrical circuits. The resistance to electrical flow is called impedance. There are two kinds of impedance, real and imaginary. Real resistance is what a resistor provides. Imaginary resistance is what a coil or capacitor provides. In wave propagation, real resistance is measured by the back pressure due to the work being done by wave friction as the air moves back and forth along the surface of a fiber. Imaginary resistance is also a back pressure. But no work is being done. It is only due to the inertial force required to accelerate and decelerate the air flow of the wave as it moves around the fiber.

Sound panels, with their fibers laying sideways to the incoming waves produce a significant amount of non-work, inertial back pressure. The Persian carpet, with all its fibers facing the incoming sound wave, produces a back-pressure due to pure acoustic resistance. The carpet has no acoustic inertance, or back pressure that comes from accelerating air around the broadsides of the fibers. The Persian carpet offers more absorption per fiber than a standard sound panel because the only acoustic back-pressure offered by it is due to acoustic friction.

Green Acoustics

The new wave in acoustics is green acoustics. This is a material that in some way provides a traditional service in a low energy format. It might be a version of acoustic material that lasts longer than similar materials so that the rate of energy consumption to provide this material is overall reduced. Or it might be a material that is simply less energy dependent to produce, transport or market. Persian carpets are acoustically green.

The traditional acoustic material is fiberglass, melted sand. It takes a lot of heat to melt sand and that heat comes from fuel. So far, no one has been able to melt glass in a solar oven. The cost of fiberglass sound panels is directly proportional to the cost of fuel. To make fiberglass, sand is put in an oven and the temperature is raised to 3500 degrees Fahrenheit, and then it melts. Once it is a liquid it is spun into fine fibers in the same way cotton candy is made from liquid sugar

On the other hand, Persian carpets are made from wool, which grows and is harvested from animals. This form of manufacturing fibers consumes little to no fossil fuel and weaving the carpet is by hand. Since the food grown to feed the animals who supply the wool and people who weave the wool comes from sunlight, the manufacture of wool. fibers and fabrication of Persian carpets is in effect; powered by solar energy. The Persian carpet is an architecturally green, environmentally friendly acoustic product.

Rough vs Smooth Fibers

Fiberglass fibers are long and smooth surfaced, essentially, very thin glass wires. Their surface is as smooth as glass, naturally. When sound passes by a fiber, the air in the vicinity of the fiber moves back and forth. The air molecules in direct physical contact with the fiber remain stuck to the fiber. But other molecules, farther away from the molecule are free to move around. Sound causes them to rub against the surface of the fiber. The amount of surface a fiber has is proportional to the amount of sound it can absorb. Glass fibers, like polyester fibers and even silk fibers are smooth, glassy and offer the minimum fiber surface to the sound wave.

Wool fibers have a rough surface, like shingles on the roof of a house. The spacing of the shingles is about the same as the diameter of the fiber. The roughness of the shingles is about 1/10th the diameter of the fiber. Each shingle is called a cuticle, and looks like a micro-fingernail. A micro photograph of a wool fiber looks very similar to what you see in a photograph of the trunk of a palm tree. And the different types of wool fibers, varying with the type of animal that grows them, correspond to the different types of palm tree trunks.

Shingled wool fibers have a rougher surface than glass fibers. The wool fibers have approximately 30% more surface area than an equivalent sized glass fiber. Since acoustic friction is proportional to surface area, wool fibers provide 30% more sound absorbing surface than equivalent sized fiberglass fibers.

Although cotton is directly produced by solar energy, it is not shingled, but looks smooth, like glass, polyester and silk fibers. Cotton diameter is typically larger than wool, in the range between 15 and 25 microns, which is about twice the diameter of fiberglass and wool fibers.