by Ron Kurtus (revised 18 March 2012)
Noise cancellation is a method to reduce or completely cancel out undesirable sound, such that you can't hear it. It is often called Active Noise Cancellation because the electronics involved actively cause the noise cancellation in real time.
In the case of a single sound frequency, you can add the same frequency 180° out of phase to cancel the sound. It is much more difficult with complex sounds, such as the spoken word. The most common electronic noise cancellation device consists of special earphones.
Other types of devices that do not require earphones are being developed. Note that this is an emerging technology and is often not available for common noise problems.
Questions you may have about such a device are:
- How can you cancel out simple waveforms?
- How is complex sound cancelled out?
- What are some noise cancellation applications?
This lesson will answer those questions. Useful tool: Units Conversion
Simple wave cancellation
Although sound is a compression wave, each tone or frequency can be represented by a sine wave of a given wavelength.
Simple sine wave for single sound frequency
Slightly out of phase
Suppose another pure sound of the same frequency was emitted, but just a fraction of a second later. That means it is slightly out of phase from the first sound or sine wave. The visual representation of the two waves would be as in the next illustration.
One pure sound a fraction of a second after the next
Graphing the result
You can graphically see what the resulting wave would look like by adding values above and below the centerline (zero). This is illustrated by the green dots in the following drawing. The green line is approximately a sine wave of the same frequency.
Sum of two waves slightly out of phase
You would hear the same frequency, except that the sound would be twice as loud as the original. Note that since the frequencies are the same, you would not hear beat sounds.
(See Beat Frequencies in Sound for more information on that subject.)
Finally, if the sound waves were 180° or one-half a wavelength out of phase, the sum of the waveforms would be zero. They would cancel out each other and there would be no sound.
Sum of waves equals zero sound
How is it possible in air?
Although it is difficult to accept that adding two sounds can result in zero sound, you need to remember that sound is a compression wave in air or other media. That means the wave first compresses to an amount greater than normal air pressure. This is the positive part of the sine wave graphic. Then the air expands to a pressure less than normal air pressure. This is the negative part of the sine wave—the part below the zero centerline. Adding the positive pressure and negative pressure will give you the normal air pressure.
Canceling complex waves
Electronically, it would be simple to detect a simple tone, adjust it 180° out of phase and add it to the original, thus canceling out the sound. Unfortunately, most sound is much more complex than a simple sine wave.
Complex waveform of a spoken word
Spectrum of frequencies
A spoken word consists of a spectrum of frequencies of different amplitudes. This means that to cancel out each waveform, the electronics would need to filter each frequency separately, determine its frequency, create the same frequency and amplitude at 180° out of phase, and then add it to the original.
Since there are so many frequencies and fractions of frequencies in some sounds, it is impossible to cancel them all out with this method. Instead, the electronics selects a narrow band of frequencies and averages out the result. This is a fairly good job at noise cancellation, but it is not 100%.
There are a number of great applications for active noise cancellation devices.
Noise cancellation almost requires the sound to be cancelled at a source, such as from a loud speaker. That is why the effect works well with headsets, since you can contain the original sound and the canceling sound in an area near your ear. In applications where the sound comes from many areas, such as in a room, it is difficult to cancel the sound from each area. But, scientists and engineers are working on solutions.
One obvious application is that people working near aircraft or in noisy factories can now wear these electronic noise cancellation headsets to protect their hearing.
Students can wear these headsets to cancel out unwanted noise, while being able to listen to their own music while they study. These headsets are now fairly inexpensive, costing around $50.
Honda is now using noise-cancellation technology in their Japan-only Accord station wagon.
The way it works is that a microphone connected to the car stereo system picks up all the sound inside the car, including music or such from the stereo. Then the noise-cancellation system subtracts the sound of the music coming from the stereo and produces noise-canceling sound waves that match the frequency of unwanted sound.
The noise-canceling sound waves are also sent through the stereo speakers, along with the music. This technique greatly reduces the low frequency vibration noises in the car, without dampening the car's audio system.
Unfortunately, Honda is not shipping the noise-cancellation system outside Japan.
Space satellite antennas
Some space satellites have long antennas. If such an antenna would start to vibrate wildly, it could throw the satellite out of orbit and out of control. By detecting the waveform or any vibration in the antenna, it can be suppressed in the same way that noise is suppressed.
A piezoelectric device creates an electrical signal when it detects a vibration. It also will vibrate according to an electrical signal it receives. Putting piezoelectric devices on the antenna can result in vibrating the antenna in an opposite phase, thus eliminating the dangerous motion.
Use in apartments
One cool idea is to suppress unwanted noise from another apartment by vibrating your wall out of phase with the sound from next door.
For example, suppose the person in the next apartment has his stereo on real loud. In some cases, you can even feel the wall vibrate. By placing devices on the wall, you can detect the wall vibrations and generate vibrations in an opposite phase. Thus would completely cancel out the noise coming from the other apartment.
Although there are some new devices available that use your walls as a speaker by creating vibrations, it has not yet been combined with detectors and noise reduction electronics. This is an invention that is begging to be developed.
Active noise cancellation is a method to cancel out undesirable sound in real time. It is relatively easy to do with a single sound frequency, but it is much more difficult with complex sounds, such as the spoken word. Noise cancellation earphones are the most common devices. Other devices that do not require earphones are being developed.
Look at achieving the impossible
Resources and references
There are different areas for information on active noise reduction.
DARPA DSO: Defense Sciences Office sponsors research and development in the application of smart materials to active noise control problems. Defense Advanced Research Projects Agency
Noise Control and Noise Cancellation- From DARPA MTO, which sponsors research and development in the application of active noise control to air-coupled sensors and related applications
Siemens develops noise cancellation system for cars - Article from EETimes.com.
Products and companies
NVH Technologies - A supplier of active noise cancellation and active vibration cancellation solutions
Pro-Tech Communications - Noise reduction headsets
Top-rated Noise Reduction Headphones - These headsets available through Amazon.com range from $25 to $200
Silentium - Company provides products and solutions for the reduction of acoustic noise generated in electrical products
Noise Reduction in Speech Applications by Gillian M. Davis, CRC Press (2002) $101.30 - Comprehensive introduction to modern techniques for removing or reducing background noise from a range of speech-related applications.
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