How to Measure Noise-Canceling in Headphones

Testing noise-cancellation with science

Noise canceling headphones

David Becker / Getty Images

There are a lot of noise-canceling headphones on the market. Unfortunately for the consumer, though, the efficacy of noise-canceling circuitry varies radically from headphone to headphone. A few are so effective you might think something's wrong with your ears, while others only cancel a few decibels worth of noise. Even worse, some add audible hiss, so while they may reduce noise at low frequencies they increase it at high frequencies.

Fortunately, measuring noise-canceling functions in a pair of headphones is relatively simple. The process involves generating pink noise through a set of speakers, then measuring how much sound gets through the headphone to your ears.

Set up the Gear

Ear-cheek simulator

Measuring noise-cancelling ability requires basic audio spectrum analyzer software, such as True RTA; a USB microphone interface, such as the Blue Microphones Icicle; and an ear/cheek simulator such as the G.R.A.S 43AG we use, or a headphone measurement mannequin such as the G.R.A.S. KEMAR.

You can see the basic setup in the above photo. That's the 43AG at lower left, fitted with a rubber earpiece that represents an earlobe typical of some people. Earpieces are available in a variety of sizes and different durometers. 

Make Some Noise

Sound check room diagram

Generating the test signals is actually a little tougher if you go by the book. The IEC 60268-7 headphone measurement standard dictates that the sound source for this test should be eight speakers positioned in the corners of the room, each playing an uncorrelated noise source. Uncorrelated means that each speaker gets its own random noise signal, so none of the signals is the same.

For this example, the setup involved two Genelec HT205 powered speakers in opposite corners of our test space, each one firing into the corner to better disperse its sound. The two speakers receive uncorrelated noise signals. A Sunfire TS-SJ8 subwoofer in one corner adds some bass.

You can see the setup in the diagram above. The small squares firing into the corners are the Genelecs, the large rectangle at lower right is the Sunfire sub, and the brown rectangle is the test bench where we do the measurements.

Run the Measurement

Measuring the noise cancelation

To begin the measurement, get the noise playing, then set the noise level so it measures 75 dB near the entrance to the 43AG's fake rubber ear canal, measured using a standard sound pressure level (SPL) meter. To get a baseline of what the sound is outside the fake ear so you can use that as a reference, click the REF key in TrueRTA. This gives you the flat line on the graph at 75 dB. (You can see this in the below image.)

Next, place the headphone on the ear/cheek simulator. The bottom of our test bench is fitted with wood blocks so that the distance from the top plate of the 43AG to the bottom of the wood blocks is exactly the same as the dimensions of a head at the ears. (It's about 7 inches.) This maintains ​the appropriate pressure of the headphone against the ear/cheek simulator.

Per IEC 60268-7, we set TrueRTA for 1/3-octave smoothing and set it to average 12 different samples. Still, though, like any measurement involving noise, it's impossible to get it 100% precise because noise is random.

Confirm the Result

noise cancelling graph result

This chart shows the result of a measurement of the Phiaton Chord MC 530 noise-canceling headphone. The cyan line is the baseline—what the ear/cheek simulator "hears" when there's no headphone there. The green line is the result with noise-canceling switched off. The purple line is the result with noise-canceling switched on.

Note that the noise-canceling circuitry has its strongest effect between 70 and 500 Hz. This is typical, and it's a good thing because that's the band in which the droning engine noise inside an airliner cabin resides. Note also that the noise-canceling circuitry can actually increase the noise level at high frequencies, as we see in this chart where the noise is higher between 1 and 2.5 kHz with noise-canceling on.

But the test isn't finished until it's confirmed by ear. To do this, we use our stereo system to play a recording we made of sound inside an airline cabin. We made our recording in one of the rear seats of an MD-80 jet, one of the oldest and noisiest types currently in commercial service in the U.S.

Note that like every audio measurement, this one's not perfect. Although the subwoofer is placed as far away as possible from the test bench, the test bench is placed on felt feet, and the ear/cheek simulator has compliant rubber feet, at least some bass vibration sneaks directly into the microphone through physical conduction.