Transducer Design: Dynamic.
Coupling to the Ear: Circumaural.
D.c. Resistance: 290 ohms.
Absolute Polarity: Positive.
Cord: 9 feet long; straight, light, and flexible; runs from both earcups; stereo mini-plug connector with 1/4-inch adaptor.
Adjustments: Headband slides in bail, which has detents.
Weight: 6 oz.
Price: $199.
Company Address: 6 Vista Dr., Old Lyme, Conn. 06371.
Sennheiser originated the idea of 'open-air" earphone design, and following this tradition, the HD-540 Reference II does not have a tight seal at the rear or front of the diaphragm. The first commercially successful earphone de signed to be used without a tight seal between the transducer elements and the ear was, as I recall, the Sennheiser HD-414. It had small foam ear cushions that sat on the outer ear, or pinna.
I wondered how it could produce any bass and still remember my surprise when I tried it on. I had always thought that a tight seal was necessary to obtain good bass, but the HD-414 changed my mind forever. The lack of a tight seal usually reduces bass out-put, but Sennheiser designed the dynamic transducers of the original HD-414 to account for this; now they have done it for the transducers in the HD-540 Reference II. Since these ear phones are not sealed, the attenuation of outside sound is negligible, and it is quite easy to hear outside noises and conversations.
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EARPHONE EVALUATION
GENERAL COMMENTS: Good adjustment to head; very comfortable for long term listening; clear and open sound with slightly recessed upper range.
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The Sennheiser HD-540 II ear phones are very light, weighing in at only 6.0 ounces. The plastic bail, like many other current designs, is made as one continuous molding. The head band, of simulated-leather vinyl with stitching along the outside edges, has seven foam-filled sections on the side that sits against your head; it is attached to a plastic piece on each side of the bail. Near each earcup, the bail has horizontal ridges which act as detents and hold the sliding plastic pieces in place after a comfortable position has been selected. As with other earphones that use this type of system, earcup positioning is most easily adjusted by removing the earphones, sliding the plastic pieces, and then trying them on again. There are no markings on the earcups to show which channel is which, but the connectors that plug into the 'phones are marked "L" and "R," and the plug for the right channel is red.
The plastic earcups are very good looking, with a band that looks like brushed chrome separating the gray outside part from the velvet ear cushions. The outside part has slots that allow the rear of the transducer to be open to the air. The slots are covered by black cloth that keeps dust and dirt from entering the earcups. The ear-cups have a recessed area at the bottom, just below the raised nameplates, to allow the wire to exit; the wires have strain reliefs and join together, after about 6 inches, into one cable.
The ear cushions have a ring of velvet that sits against your head, and a soft cloth covering at the inside and outside edges. The cloth allows an open path between the earphone transducers and the outside air. I liked the feel of these ear cushions, and they were very comfortable. The inside diameter of the ear cushions was large enough for them to fit easily over my ears and sit against my head without pinning my ears. I remember how the HD-414s used to irritate my ears after 15 or 20 minutes, so I really appreciate the comfort provided by the large, around-the-ears cushions of the HD-540 IIs. They were still comfortable after an hour or more of listening. The HD-540 II can swivel enough to fit your head properly, and the tension of the plastic bail, which holds the earcups against your head, is very light. After a few moments, you almost forget you are wearing these earphones.
The subjective sound qualities of the Sennheiser HD-540 II were rated by members of a listening panel against those of the Stax SR-Lambda Pro. The panel members listened to a variety of program material and were asked to write down their comments. Measurements that I had made earlier showed that the Sennheiser's bass output was very extended, with no prominent bump to exaggerate the bass. Below 80 Hz was a gentle roll-off of 6 dB per octave. Comments by panel members-such as "not boomy," "good bass," and "very extended"-correlate well with my measurements. Comments about the middle register being "less recessed than the reference," "very good on voice," and "articulate" correlate with measurements that I made with the B & K HATS manikin.
(See the article "As Close As You Can Get" in the April 1991 issue.) Sennheiser has designed the HD-540 II ear phones to have a diffuse-field response that is similar to the B & K measuring system's. The SR-Lambda Pros, which I use as reference, showed a dip at 2.8 kHz when measured with the B & K system's diffuse-field response (see review in the April 1991 issue), and the listening panel all commented that the HD-540IIs sounded a little brighter than the reference ear phones.
The listening panel's comments of "a little bright" and "zippy" regarding the upper middle register also correlate well with the way the Sennheiser ear phones reproduced a 500-Hz square wave: In the output spectrum, the natural harmonics at 3.5, 4.5, and 5.5 kHz were accentuated. The harmonics at 11.5 and 12.5 kHz were also a little prominent compared to those below and above them, which may have contributed to the "zippy" comment being made. The shape of the 500-Hz square wave was excellent, although a slight tilt showed some low-frequency phase shift; the initial part of the square-wave response indicated that there is some high-frequency phase delay.
Fig. 1--Output vs. time (bottom) for 20-kHz cosine pulse (top).
Figure 1 is the output of the HD-540 II earphones for a 20-kHz cosine input. The output, after the input has stopped, correlates with the listening panel's overall impression that the Sennheisers were very good but not as open and clear as the reference ear phones. There seems to be a little too much output around 5 kHz compared to the upper range, which causes the otherwise excellent response in the up per range to be slightly obscured. The output pulse from the HD-540 IIs also shows that they produce a positive acoustical output for a positive electrical input. Absolute polarity was easy to hear with these earphones, especially on voice.
I measured frequency response with a Fast Fourier Transform (FFT) analyzer by using the same 20-kHz cosine pulse with a B & K 4133 condenser microphone directly in front of the ear phone element. The frequency response showed a rise at around 5 kHz and some sharp dips at around 7 and 14 kHz; the phase response also showed irregularities at 7 and 14 kHz. I couldn't find anything obvious that might have caused this; the ear cushions and other exposed parts are cloth-covered foam, which should pro vide excellent absorption of any reflected sound. The relationship between the amplitude and phase-transfer responses of the output show that the response is minimum phase, so these dips are probably not as bad as they might appear to be.
The HD-540 II produced a very smooth sound when reproducing pink noise. I would describe it as being like a spray of water from a garden hose rather than the sound of a waterfall and its accompanying low-frequency roar.
The impedance of the HD-540 II ear phones is about 300 ohms, so the response shape and output level are affected hardly at all by the source impedance which supplies the signal to them. These earphones are relatively sensitive and produce very high sound levels with very little input power.
The listening panel and I found the HD-540 II earphones very comfortable; it is easy to forget that you are wearing them. The panel gave the Sennheisers a rating of "very good" for overall sound quality and also a "very good" for physical attributes. I think that they share some of the qualities of the Stax electrostatic reference earphones, with a slightly less analytical quality and a very slight veiling of the sound. For long-term listening, the HD-540IIs are excellent. Considering the level of musical enjoyment they provide and how comfortable they are, their price makes them an excellent value.
-Edward M. Long
(Source: Audio magazine, Jun. 1992)
Also see:
Sennheiser Electronic Corporation (ad, Nov. 1984)
Beyerdynamic DT-911 headphones--review, teardown and analysis (Jan. 1993)
Headphones: History and Measurement (May 1978)
Build a Stereo Headphone Amp (May 1983)
Sennheiser MKH 40 Microphone (Jan. 1988)
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