The HD600 headphones are Sennheiser's flagship open-backed model, representing the state of the art in conventional moving-coil transducer design. The diaphragm design minimises standing waves, thereby reducing distortion, and is driven by an extremely lightweight aluminium voice coil to provide the best possible transient response. The all-important magnet assembly employs very powerful neodymium ferrous magnets. The entire motor system has been optimised to minimise harmonic and intermodulation distortion, while also providing a high sensitivity and a wide dynamic range capability. The design incorporates Sennheiser's 'loudness diffusion correction' — a form of equalisation intended to make the sound more closely resemble that created by loudspeakers in a good acoustic environment.
The specifications show a sensitivity of 97dB/mW, a frequency response of 16Hz to 30kHz (-3dB points), and distortion below 0.1 percent. The impedance is right in the middle of the normal range at 300Ω, making this an easy load for headphone amplifiers. Mechanically, the headphones are a comfortable weight at 260g, although they exert a fair pressure on the skull around the ears — these are circumaural designs — of 2.5N (Newtons). Sennheiser are one of the few manufacturers who bother to specify this important aspect of headphone comfort, although I don't suppose most readers will be able to imagine what 2.5N of pressure feels like. So, let me say that these headphones initially feel 'tight' but completely stable — there is no tendency at all for the HD600s to slip off if you tilt your head down, which makes them ideal if you want to read, type, mix, edit, dance, or indulge in any other mobile activity while listening!
The earpads are made from a very comfortable and cool velour material over foam, mounted on a stiff plastic backing which clips into the frame of the earpiece. They are therefore easily replaced and cleaned — headphone hygiene is more important than many realise. The drive unit is mounted in the centre of an elliptical frame, with a fine metal grille covering the external side of the cavity and a foam layer and earpads on the inside.
The earpieces are supported from metal strips which can be extended with a stiff ratchet action from the padded headband, to accommodate a wide range of head sizes. The earpieces can also swivel and rotate over a limited but entirely adequate range of angles — certainly, pushing an earpiece behind one ear is easy and comfortable.
In classic Sennheiser style, a 'Y' lead cable connects the headphones, with stiff, detachable plugs at the bottom of each earpiece (keyed to ensure correct polarity). This kevlar-reinforced oxygen-free copper cable is three metres long and terminates in a gold-plated 3.5mm stereo jack plug complete with a push-on adaptor to the larger 6.3mm standard stereo headphone jack plug standard. I'm not a fan of straight cables, as I find myself continually having to untangle the cable, or tripping over it! However, I know many users prefer them over coiled cables — each to their own.
The HD600s sound wide, open, spacious, and very extended at both frequency extremes. Being open-backed, there is a fair amount of high-frequency spill from these headphones, which makes them less than ideal in the studio when recording vocal overdubs, for instance. However, in general high-quality and reference monitoring applications this spill will be of less concern than the influence of external sound passing through to the listener. You really do need to use these headphones in a quiet environment — trying to assess sound quality while the same signal is blaring over loudspeakers is completely pointless!
I found the bass end was tight, tuneful and accurate, the mid-range clear, neutral and detailed, and the high end open and 'airy' — the last being a particular strength of open-backed designs, of course. Dynamic and punchy on rock and pop music, these headphones really excelled on classical music, with immense detail and resolution, and a silky smooth string quality.
The HD600s were comfortable to wear for long periods without being physically or sonically fatiguing, and I actually forgot I was wearing them on several occasions... until I tripped over the cable! These are excellent headphones which easily justify their position as the flagship of Sennheiser's open-backed dynamic range. If you are looking for a headphone capable of reference-quality monitoring and the open-backed characteristics are acceptable to you, then you will struggle to find anything better than the HD600.
- True reference-quality sound.
- Extremely comfortable.
- Straight cable.
Sennheiser's flagship open-backed headphones represent the state of the art in dynamic transducer design.
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