Sound Advice

Back in 1957, author Arthur C. Clarke (of “2001: A Space Odyssey” fame) published a fictional compilation of shaggy-dog stories told in the White Hart pub by a henpecked college professor. The first of these “Tales from the White Hart” is the recollection of a lab technician named Fenton who succeeded in building a machine (the Fenton Silencer) that can absorb all of the sound in a room. The machine consists of a microphone that picks up sound, the requisite hodgepodge of electronic gadgetry and a loudspeaker that is driven precisely backwards from the soundwave. In this way, the sound energy in the room is completely absorbed and stored as electrical energy in the machine. One of Fenton’s college friends borrows the device for a harmless prank and, well, I bet your library has a copy of the book in the stacks.

The Fenton Silencer was one of the public’s first encounters with the concept of Active Noise Control. The story foretold of the day when electronic silencing would be commonly used to control unwanted noise.

How ANC works

To see how ANC works, consider the two 15-inch subwoofers in your neighbor’s teenage son’s car. Although their stroke isn’t considerable, these two air pistons pump in and out together at, say 20 times every second, pressurizing and evacuating the interior of the car with every beat. (By the way, studies show that your neighbor’s kid will probably need hearing aids by the time he’s 30.) Since the speaker diaphragms move in and out at the same time, they help each other pressurize and evacuate the car. But, if you were to crosswire one of the woofers (red to black, black to red) the diaphragm on that speaker would move in when the other one moved out and vice versa. The speakers would work against each other with a considerable drop-off of subwoofer volume as the result. In ANC terms, the noise from one speaker has been cancelled by anti-noise from the other speaker.

Unlike the Fenton Silencer, the sound in the car hasn’t been absorbed and converted to electricity. It has just been cancelled. Instead of working together to pressurize the air in the car, the woofers oppose each other and play something analogous to Slinky with the small air volume between them.

If your neighbor won’t let you crosswire his son’s subwoofers, you can still demonstrate ANC for yourself. If you have a component stereo, place the speakers about 1 inch apart and facing each other. Tune in an AM radio station and set all of the equalizer bands except one as low as they will go. Set one band as high as it will go. Adjust the volume appropriately. With both speakers on, the volume will be the loudest. Unplug one of the speakers and the volume will obviously drop. But reconnect that speaker crosswired and the volume will drop again!

Experiment to determine which equalizer band gives the best results. If you don’t have a component stereo, but do have a computer with Internet access then you can download a program that demonstrates ANC using your computer’s speakers. The program was written by the Vibration and Acoustics Laboratory of Virginia Polytechnic

Institute and State University and

can be found on its Web site at www.val.me.vt.edu. The program gives an impressive demonstration but its 1 KHz test signal will give you a headache in short order.

In 1957, practical application of active noise control had yet to wait for the invention of high-speed digital processing. After the invention of the digital signal processor, the press was filled with accounts of ANC work being done in laboratories and the fascinating new devices which would soon be here. Well, it has been a full 70 years since the first laboratory experiments with ANC and over 20 years since the invention of the DSP chip, so where are all the wonderful ANC devices? OK, maybe not a Fenton Silencer, which can absorb all the noise in a room, but how about noise-canceling walls, ceilings, windows and doors? Right now, the main stumbling block to widespread use of ANC is the so-called “quarter-wave limitation.” Simply put, ANC only works in spaces that are small compared to the length of the soundwave.

For example, the low-frequency sound from a subwoofer typically has a wavelength of 20 feet or more. Any space that is smaller than 1/4 of 20 feet (5 feet or less) is “small” at these low frequencies. Car interiors, which are usually less than 5 feet across, fit this definition and so ANC will work to control low-frequency noise inside car interiors. The quarter-wave limitation ensures that the pressure from the soundwave is nearly the same throughout the space. The pressure everywhere in the car goes up when the speaker diaphragm pushes out and the pressure everywhere in the car goes down when the speaker diaphragm pulls in.

This fact allows the “anti-noise” speaker to be placed anywhere in the car to counteract these pressure fluctuations and cancel the noise. Soundwaves that are shorter than four times the size of the car interior do not cause the pressure to be evenly distributed throughout the space. In fact, the soundwave pressurizes sections of the car interior while evacuating other sections of the space.

The sound field quickly becomes complex and we no longer know where to put anti-noise speakers to cancel the original noise. Table 1 is a list of how small a space must be for ANC to work for different frequencies of sound (lower frequency = longer wavelength). Because of the quarter-wave limitation, present ANC devices are designed to work in very small spaces or in moderately small spaces but only at low frequencies. Active noise control of rooms and other large spaces will have to wait for someone to solve this quarter-wave limitation.

Present ANC devices

One of the most successful applications of ANC to date is the active headphone. These headphones contain microphones to pick up outside noise, a DSP chip to make quick calculations and speakers inside the earcups to play anti-noise that cancels the outside noise. These are now commonly worn by helicopter and propeller aircraft pilots to cancel propeller-generated noise. Since the space between the pilot’s ears is about 7 inches, you can see from the table that these headphones are good at cancelling only noise from about 500 Hz and below. Airlines will soon provide ANC signals for their passengers through the aircrafts’ entertainment sound system.

Another current application for ANC is in active duct silencers that control low-frequency ductborne noise in building heating, ventilation and air conditioning systems. As the size of the space gets bigger, the maximum frequency that can be controlled drops. From the table, you can see that the frequency of sound that can be cancelled in an HVAC duct depends on the width of that duct. Frequencies less than 250 Hz can be controlled in ducts smaller than 1 foot wide; less than 125 Hz in ducts 1 to 2 feet wide, but only frequencies lower than 62 Hz can be controlled in ducts 2 to 4.5 feet wide. Because of this, commercially available active duct silencers are built no wider than 2 feet to achieve the most sound attenuation over the widest frequency range.

The largest spaces in which active noise control is being used are the cockpits of planes and expensive automobiles. As shown in the table, the dimensions of these spaces allow only the very lowest frequencies to be controlled. Currently, active noise control is an expensive and inefficient way to shield these spaces from outside noise. However, it is justified here because it is lighter than the heavy, limp mass sound insulation normally used to insulate these vehicles and since cost is a secondary consideration.

Although the quarter-wave limitation makes general application of ANC for room acoustics impractical as yet, college laboratories that aren’t discouraged by impracticalities are experimenting with ANC walls. The Vibration and Acoustics Lab at Virginia Tech and the Department of Mechanical and Materials Engineering at the University of Western Australia have each built and tested walls that use active noise control. Virginia Tech’s wall was built to absorb sound in a room. Like the Fenton Silencer, the wall contains a microphone, electronic circuitry and a wall panel that moves like the diaphragm of a speaker to absorb most of the sound that hits it. The result is that it reflects almost no sound from 100 Hz to 2,000 Hz. UWA’s wall was designed to block sound. Using microphones, electronics and shakers inside the structure, they have been able to achieve a 28-decibel increase in transmission loss with its electronic wall.

But don’t worry, we will all be comfortably retired before the soldering iron replaces the finishing knife.