Headphone DSP Calibration – our philosophy on measuring and correcting headphones
Intro
Headphones are the “Wild West” of audio sound reproduction. We have not measured a single headphone that has any resemblance of a flat or neutral frequency response. One only needs to browse this site catalog to see.
No dummy heads
We do not use “dummy measurement heads.” A headphone is meant to be positioned on a human head and adjusted for both comfort and sound. There is a sweet spot. Anyone who has positioned a headphone on one’s head knows this.
Repeatable measurement and analysis techniques
We take 30 high resolution measurements for each headphone. For each measurement, the headphone is taken off, the calibrated binaural mics are adjusted in ear, the headphone put back on, and repositioned to the sweet spot.
Here is an example 30 full resolution frequency response measurements of the Abyss 1266 Phi TC.
There is a clear frequency response pattern up to about 3 kHz where one can see the response deviates from a pattern, other than one of randomness. A 3 kHz wavelength is ~4.5 inches and a 6 kHz wavelength is about 2.25 inches.
The size of headphone ear cup, distance from the driver to the pinnae, canal, drum all contribute to the large peaks and dips in frequency response. Technically referred to as one’s own Head Related Transfer Function (HRTF).
Above the calculated and measured cutoff frequency, the measurement data no longer reflects the actual frequency response of the headphone. Therefore, our FIR filter design philosophy is not to equalize the frequency response beyond that range. We are only interested in calibrating the headphone flat so anyone wearing is getting a flat frequency response being delivered to the ear, independent of one’s HRTF.
Anyone can equalize a headphone flat, but does it sound any good?
Psychoacoustic filtering, frequency dependent windowing and pattern recognition of multiple measurement data is the key to designing a convolution correction filter that maps to how we hear sound. Our data analysis technique extracts only the headphone frequency and phase response while the HRTF is “windowed out.”
In addition, we use high resolution FIR convolution filters that correct both the frequency and phase response of the headphone. 65,536 tap filters at 48 kHz sample rate have a frequency resolution of 0.7 Hz. Thinking in terms of a graphic equalizer, it would be the equivalent to 32,768 frequency eq sliders that are being adjusted. The ultimate in equalization technology.
High resolution FIR convolution filters combined with psychoacoustic filtering and frequency dependent windowing, is the reason our convolution filters provide the “correct” flat frequency response and typically with +- 0.5 dB tolerance from 20 Hz up to the headphone’s cutoff frequency.
Here is an example of the HiFiMan Susvara headphone, measured and corrected frequency response using the techniques and DSP software described above:
For some headphones in the catalog, one will see a corresponding phase response chart. That is because that headphone is exhibiting a non-minimum phase behavior that is also corrected:
Accurate sound is defined as sound reproduction with no frequency or phase distortion of the signal arriving at one’s ears. The Susvara exhibits phase distortion that is then corrected to the ideal minimum phase response.
What about the high frequency response above the cutoff frequency?
Our headphone calibration process is 50% measurement and 50% listening. The listening portion of this process is to determine how much overall high frequency energy is coming at one’s ears. Does it sound balanced and neutral with the overall frequency response. This fine-tuning process is to determine if any overall gain adjustments to the high frequency response is required. Of course, we use Hang Loose Convolver to instantly switch between filter and no filter to compare.
Conclusion
Our high-resolution FIR convolution filters using our advanced DSP analysis and design software produces the most accurate, transparent, and neutral equalization filters possible.
Download a 14-day trial of Hang Loose Convolver with the headphone’s filters loaded. You can compare with filter versus no filter, all in real time and level matched. Hear firsthand what neutral sounds like.
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