Purpose
Headphones work better in noisy environments if they provide isolation from external sounds. Open-back headphones provide the least isolation, closed-back headphones the most, particularly if they incorporate active noise cancellation (ANC) which markedly improves isolation performance at low frequencies. This measurement determines the test headphone’s isolation across the audible frequency range.
Method
This test uses a loudspeaker and electronic turntable to mimic a diffuse sound field. A total of 18 automated frequency response measurements are performed at 20 degree rotation intervals of the artificial head, which are then averaged. The first series is performed without the headphone in place; subsequent series are performed with the headphone on the artificial ear, with each mode of operation (eg. passive, active, ANC, if relevant) measured separately. To prevent structure-borne vibration influencing the measurement the artificial head is compliantly isolated from its stand at a natural frequency of ~6Hz, while the effect of other external sounds is suppressed by testing at ~85dB SPL at the artificial head and by applying both time and spectral averaging. To ensure representative results at low frequencies, the headphone’s input connector is short-circuited to provide electromagnetic damping of the drive unit. Post-processing of the isolation measurements is used to construct an attenuation table which estimates the headphone's perceived degree of isolation for different types of environmental sound.
Measurement parameters
Test signal: pink-spectrum periodic noise
Sampling frequency: 48kHz
Bit depth: 16-bit
FFT length: 16,384pt
Frequency resolution (measurement): 2.93Hz
Frequency resolution (graph): ISO third-octave steps
Measurement averages: 10 time / 10 FFT per head angle
Test duration: 12.5 minutes per series