The (Very,) Basics of Human Hearing

Human hearing does not have a flat frequency response, and the frequency response is sensitive to Sound Pressure Level, (SPL,) as demonstrated in the Fletcher Munson/ISO 226:2003, equal loudness curves.

The Implication

Headphone frequency response can not be "flat," and must be compensated to accommodate the non-linear characteristics of human ear drum pressure, as diagramed in "The Artificial Ear Dilemma: The challenges of modern handset testing," Gunnar Rasmussen, Figure 14, (a highly recommended paper that has a wealth of information on the issues of headset measurements.)

It is important to note that the ear drum pressure is NOT linear on the external diffused sound intensity over frequency. This is because the ear canal, (about 1/4" in diameter, and 1" long,) is closed at the ear drum end, and open at the entrance to the canal-i.e., the ear canal forms a Helmholtz resonance, which is centered about 3 kHz., with a Q of about 1/2. (The acoustic guitar uses the same principle of the Helmholtz resonator-that's what the hole in the face is for; an acoustic amplifier, just at lower frequencies.)

It turns out that the non-linearities that headset manufacturer's design into their products to accommodate the frequency response of the human ear, (the ear canal is closed at both ends when wearing headphones-so manufacturers compensate the frequency response of their products to duplicate the ear's natural frequency contours,) is fairly easy to measure using the principle of acoustic reciprocity, (which does not include the effects of the pinna,) i.e., place the cups of a headphone together, and using one cup as a dynamic speaker, and, the other cup as a dynamic microphone, measure the frequency response of the combined transducers; the electrical connection can be made with a used headphone extension cable, bringing out the "hot" line of the cup used as a microphone to a plug board, (and on to the microphone input of a PC-perhaps through a large capacitor to isolate the DC voltage supplied by the microphone input of the PC.)

An Example

A Sennheiser HD280 Pro headphone was connected to the microphone input of a PC running Linux, through a headphone extension cable, with the left channel cut, and brought out through a 10uF polypropylene capacitor to the mcirophone input of the PC. The right channel was driven by the line out of the PC, (e.g., loopback,) which was running the Baudline FFT time-frequency browser designed for scientific visualization of the spectral domain, and the headset transduces swept in frequency, from 20 Hz. to 20,000 Hz., 100 dB SPL:

Figure I is a plot of the Sennheiser HD280 Pro acoustic reciprocity measurement from the right channel cup to the left channel cup, (held together with a rubber band.) The actual non-linearity of the frequency response is multiplied by factor of 2, (since the frequency response of the two identical transducers are being measured.) This graph should be compared with "The Artificial Ear Dilemma: The challenges of modern handset testing," Gunnar Rasmussen, Figure 14.

The Point?

Manufacture's have to design and manufacture headphones for the general population. However, the dimensions of the human ear canal will vary by a factor of about 2 over a population, (and thus the frequency characteristics of the ear canal-compensated by the manufacture for the population "average.") Also, there is difference in the dimensions between the left and right ears of a single person, (see "The Artificial Ear Dilemma: The challenges of modern handset testing, Figure 8, for example.) Although headphones do deliver more accurate sound than loudspeakers, (for example, group delay is indiscernible in speakers until it gets gross, but is readily detectable in headphones,) but headphones are not without their problems.

Testing for Compatibility

One can test a given set of headphones for compatibility with one's particular anatomy by measuring one's minimum threshold of hearing, (the bottom graph in the Fletcher Munson/ISO 226:2003, equal loudness curves,) while wearing the headphones, using a PC and sine wave generator, after calibrating the volume control of the PC sound card.

License

A license is hereby granted to reproduce this software for personal, non-commercial use.

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So there.

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