Headphone Reviews

Sidebar 3: Measurements

Measuring headphones is fraught with practical problems, mainly due to the fact that the target response for a drive-unit that fires straight into the ear canal is anything but flat, given the frequency-response–modifying natures of the pinnae and inner ear canal which are unique for each person. Fig.1, extracted from an excellent 1980 AES paper (footnote 1) from Audio Contributing Editor Jon Sank, shows the envelope of responses he found desirable for conventional headphones to sound flat. For this review, I measured the responses of the headphones both in free space, with a B&K/DPA 4006 ½" microphone approximately 1" away, and pushed against a flat wooden baffle by a G-clamp with the same mike set flush. Neither situation duplicates the environment faced by a headphone forced against the head and firing into the ear canal. In addition, the ½" microphone capsule is not that much smaller than the headphone drive-units, and would thus be expected to influence the local acoustic. Nevertheless, as I regard review measurements as primarily providing support for the listening comments, I felt it sufficient to use a setup that would produce relative, rather than absolute, data. The following curves should be viewed in this light.

The Grado headphones were driven by a Linn Intek integrated amplifier for the measurements, the test signal provided by the DRA Labs MLSSA system. All response curves shown are composites, the data below 1kHz being computed by the MLSSA set to a 5kHz bandwidth, above 1kHz with it set to a 20kHz bandwidth.

Fig.2 shows how the Grado HP 1's free-space response (blue trace) is modified by the baffle (red trace). The exact degree of upper-bass boost and LF extension will be affected by the pressure with which the headphone is held against the baffle. Nevertheless, the HP 1 offers a generous bass response, with usable extension to a low 30Hz, with a bright treble presentation. Note also the generally exaggerated lower midrange, which correlates with the HP 1's warm tonal balance and which will also contribute to the subjectively rather depressed top octaves.

Looking at the HP 1's impulse response in free space (fig.3), the initial pulse is well-presented, with a somewhat ragged tail overlaid with a degree of ultrasonic ringing. The step response (fig.4) has an excellent right-triangle shape but indicates that the Grado headphones invert absolute polarity, the opposite of that suggested by the switch position. The associated cumulative spectral-decay or "waterfall" plot (fig.5) shows three residual resonances at 3kHz, 10kHz, and 24kHz, though these are mild in degree. No wonder the Grados sounded smooth—this is one well-behaved diaphragm!

Fig.6 show the Grado's impedance in free space; it agrees with GAG's in his review: ca 40 ohms across the band, with only a very slight peak at the well-damped free-air LF resonance frequency of 138Hz, and a rise above 10kHz due to the voice-coil's inductance.—John Atkinson

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Footnote 1: "Improved Real-Ear Tests for Stereophones," Jon R. Sank, JAES, April 1980. See also "On the Standardization of the Frequency Response of High Quality Studio Headphones," Günther Theile, JAES, December 1986; and "Headphones: As Close As You Can Get," Edward M. Long, Audio, April 1991.