Measuring noise-canceling functions in a pair of headphones 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
Measuring noise-canceling ability requires:
Basic audio spectrum analyzer software, such as True RTA.A USB microphone interface, such as the Blue Icicle microphone.An ear/cheek simulator such as the G.R.A.S 43AG, 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 in the lower-left corner, fitted with a rubber earpiece representing an earlobe typical of some people. Earpieces are available in a variety of sizes and different durometers.
Make Some Noise
Generating the test signals is a little more challenging if you go by the book. The IEC 60268-7 headphone measurement standard dictates that this test’s sound source should be eight speakers positioned in the corners of the room, each playing an uncorrelated noise source. Uncorrelated means that each speaker gets a random noise signal, so none of the signals is the same. For this example, the setup involves two Genelec HT205 powered speakers in opposite corners of the test space, each one firing into the corner to disperse its sound better. 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 in the lower-right corner is the Sunfire sub, and the brown rectangle is the test bench where the measurements are done.
Run the Measurement
To begin the measurement, get the noise playing, then set the noise level to measure 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 artificial ear, so you can use that as a reference, click the REF key in TrueRTA. This key gives 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. Fit the bottom of the test bench with woodblocks, so the distance from the top plate of the 43AG to the bottom of the woodblocks is the same as the head’s dimensions at the ears. (It’s about 7 inches.) This setup maintains the appropriate pressure of the headphone against the ear/cheek simulator. Per IEC 60268-7, set TrueRTA for 1/3-octave smoothing and set it to average 12 different samples. Still, like any measurement involving noise, it’s impossible to get it 100 percent precise because noise is random.
Confirm the Result
The chart below 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. The noise-canceling circuitry has its most potent effect between 70 Hz and 500 Hz, which is typical. It’s a good thing because that’s the band in which the droning engine noise inside an airliner cabin resides. The noise-canceling circuitry can increase the noise level at high frequencies, as seen in this chart where the noise is higher between 1 kHz and 2.5 kHz with noise-canceling on. But the test isn’t finished until it’s confirmed by ear. To do this, we used 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. 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 on felt feet. The ear/cheek simulator has compliant rubber feet; at least some bass vibration sneaks directly into the microphone through physical conduction.
Did You Know?
Inside a plane’s cockpit, noise levels can reach up to 85 Db. Pilots use aviation headsets to reduce the impact on their eardrums. Consumers can now buy aviation headphones, too, so if you are experiencing high noise levels and regular headsets aren’t getting the job done then look into aviation headsets. They just might fit the bill for you.
