For many, summer means it’s time for a vacation, and travelers looking to beat the time, traffic, and heat of the road take to the sky. Let’s face it, though, airplane travel has its downsides. Once you finally get through security and onto the plane, you have to contend with cabin noise for the entire journey. Read on to learn about how some physics magic and quick calculations can keep things quiet on your flight.
Active noise cancellation began as an idea in the 1930s, and by the 1980s when processing technology caught up to the ideas, devices using the technology had begun to find their way into the hands of consumers. These devices rely on physical concepts called superposition and interference. As most readers of this newsletter are aware, sound travels as waves of compression and rarefaction through air; if you have two sounds, the waves add together…this is superposition. When the peaks of two sound waves of equal level and frequency line up, the waves are “in phase” and the sound energy becomes twice as high. If these same waves are shifted so that the peaks of one match the troughs of the other, these waves interfere destructively and cancel each other out…no waves, no sound! Add an amplified speaker, a microphone or two, and a speedy processor, and you’re most of the way towards a noise cancellation system.
…but there are challenges…
For starters, noise cancellation works best on periodic sounds – that is, steady state noises. Because of the repetition, the processor only needs to run the calculations once to establish an out-of-phase signal and repeat it over and over. These types of sounds include the rumble of an HVAC unit or the drone of an aircraft’s engine. Much more difficult to cancel are transient or aperiodic sounds, such as speech or a beeping car. Due to the ever-changing, unpredictable nature of these sounds, a noise cancellation system must work quickly and accurately to push out an anti-signal in time to cancel with the source.
Another limiting factor is the wavelength of the sound to be cancelled. Low frequency sounds have long wavelengths and therefore present easier targets for the cancellation signal. As a noise increases in frequency, it becomes increasingly trickier to compute the cancellation signal and line it up in time to effectively cancel the noise; 800 Hz is roughly the threshold above which active noise cancellation becomes increasingly less effective. Manufacturers of noise cancelling headphones revert back to passive isolation to reduce intruding high-frequency noise which, conversely, is much easier to block by passive methods than low-frequency noise.
So to enhance the quality of your audio on your next flight, be sure to bring a pair of noise-cancelling headphones. Without the drone of the engines masking the speech of the in-flight movie, your audio environment is that much closer to perfection. Just don’t expect them to cancel out that crying baby!