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Sennheiser SL Range Headphones

For accurate monitoring, the correct choice of headphones can be essential. Mike Skeet checks out three of Sennheiser's SL range.

Sennheiser are well known and respected for both their headphones and microphones. In the former field, they pioneered, more years ago than I care to remember, the 'open aire' or 'supra-aural' type of headphone which happened before the current vogue for 'Walkman' lightweight phones. Their first model was the HD414 and since its introduction, several million have been manufactured. Sennheiser have added to the range at different times and have now brought out a new series. SL stands for nothing more than Slim Line, but I've heard that progress has been made with the diaphragm and magnetic materials, and the phones are improved in these areas. Here we look at models HD410SL, and HD414SL and HD420SL.

I must declare an interest here - I have favoured the early HD420 model for years having had around a dozen models for use in binaural stereo demonstrations, when I was involved in that mode of recording. It should, I thought, be obvious to me what design changes had been made.

'Open Aire' Advantages

The backs of diaphragms of 'open aire' phones are as the name implies left open instead of being boxed in. They sit on the ears and do not enclose them, and it seems that supra-aural is the term for this type of headphone. Closed phones have the backs of the diaphragms 'boxed in' and the phone must seal to the head which implies circumaurally covering the ears. Closed phones can thus be hot and uncomfortable and are frequently heavy and audible colouration of the sound reproduction at mid and low frequencies characterises enclosed phones more often than not. One can imagine the pressures involved in the small rear enclosure and the standing waves which could constrain and colour reproduction. Essentially, closed phones are like small sealed loudspeaker enclosures. LF performance is achieved in the main by 'absorbing' rear of diaphragm output and thus preventing cancellation with the front output.

Often 'open-aire' phones have the advantage of light weight and sit comfortably on the ears. The diaphragm is largely free of uneven or one sided acoustic constraint, but won't the LF cancel if it can get 'around' to the front? It's all a matter of the ratio of distances involved. The ear is relatively near one side compared to the other and a small physical baffle construction suffices. Having examined the very soft plastics material used for the diaphragms of Sennheiser headphones, I suspect that LF performance is also a function of a large low frequency resonance in the system.

In the earlier days of open phones there seemed to be positive mistrust of a headphone that didn't enclose. For some people a disadvantage appeared to be the sound leakage to others nearby and even that one could still hear room environment sounds. Of course, if one listens in noisy situations this is so, but I find open phones quite satisfactory even in situations where loudspeaker listening would be slightly obscured. In fact, you have the added advantage of being in contact with the outside world. It is feared by some that the leakage can create a howl round when vocal overdubbing in a recording environment, but in my opinion it occurs only if far too much level is fed to the phones and hence the loop gain, (mic to phones, phones to mic), becomes positive.

Headphone Impedances

At one time we had the ridiculous situation of headphones having 8 impedance transducers. Why ridiculous? To couple satisfactorily to amplifier loudspeaker outlets one needed series resistance in order to reduce the sensitivity. Efficient power transfer is simply not needed. In fact, headphones need around 1/1000th of the power of a typical loudspeaker to get a similar listening level so there is no need to opt for maximum power transfer by optimum impedance matching. Many 8 ohm phones were in fact demolished by being overdriven (leaving aside the dangers of hearing damage). Invariably, you still had too much sensitivity and consequently the apparent signal to noise ratio of your amp seemed poor with hum and hiss intruding.

The original HD414s used 2,000 ohm impedance transducers, with the result that they were virtually indestructible. Indestructible, that is to everbody except one joker who had used a 13 amp plug as his connector... the inevitable happened, someone plugged them into the mains and managed to destroy them!

So the idea of highish impedances for phones was born and is now followed by other manufacturers. The review models are of 600 transducer impedance and can be used on loudspeaker feeds without series resistance. This results in listening levels very similar to those produced by average loudspeakers. Don't worry about the amp having a high impedance load connected or even the chance of an open circuit - any amp so affected is not worth having and should be treated with the contempt it deserves.

The review models are in fact available as SL-2 versions with 50 ohm nominal transducers (or is it 32 ohm, as that figure is also mentioned on the data sheets). These versions would be better suited to 'Walkman' applications as they are also supplied with 3.5mm plugs with adaptors to the 6.3 (quarter inch) standard. Whilst on the subject of versions, the basic SL model has an interesting 4 pin DIN male/female affair which is supplied with a standard 6.3mm jack plug adaptor, but allowing other headphones to be stacked in the DIN part: another advantage of higher impedance phones being usable in parallel on low impedance sources without any effect on the levels of others connected. There is also the SL-13 version that terminates in a standard 6.3mm jack plug only. All leads have steel conductors, are 3M long and have gold plated two pin plugs to fit into the transducers themselves.

Sennheiser HD410SL

These are the lowest priced and the lightest of the review models at 82g less leads, but they still perform to a high standard. The transducers slide up and down the head band with click stops and the slightly dished yellow foam pads are removable for washing with detergent and water.

The sound is undoubtedly smooth, with negligible colouration and a balanced top and bottom extension though with seemingly less extension at both extremes compared to the other SL models.

At this price, these represent good value for money, considering their performance.

Sennheiser HD414SL

It's no surprise that these are a radically different design from the old 414s as they emerged many years afterwards. Personally I didn't like the old models because they were too uncomfortable to wear, had relatively small supra-aural pads and a decided upper middle colouration/lift in the response. The SL model replaces all this with an undeniable smoothness, and the LF is also more extended than on the lower priced HD410SLs.

In construction the two models are in fact very similar with what appears to be the same one piece headband, but the transducers are more substantial, as are the washable foam pads that incorporate a noticeable dish which aids wearing comfort.

They are a good buy across the board, having a decent LF extension and being comfortable to wear.

Sennheiser HD420SL

These have physical similarities to the earlier plain HD420s, but like the rest of the SL range, the transducers slide up and down the headband ends. Also included is a fixed subsidiary inner headband which aids comfort because it is more flexible and moulds itself to ones head shape. The ear pads are made from a textile covered foam and the performance is similar in many ways to the two less expensive models but features rather more than a suggestion of smoother HF. In this respect, the old 420s were much more effective than the old 414s and the HD420SLs seem slightly but noticeably further improved in this area. LF is more extended than on the other new SL models and appears very similar to the old 420s (my favourite headphone, as I believe I've mentioned). However, I must admit that the new version has now taken over in my affections. There is a slightly different sensitivity (though of no consequence) between the old and new models. I merely mention this because different listening levels can easily alter the aural impression made, particularly at the frequency extremes.

Interestingly both the old and new models tend to be more susceptible than most other phones to final ear position for the best HF perception. However once one is aware of this fact,the correct position is repeatedly obtainable very quickly and easily.


My home studio monitoring system currently uses Quad ESL63 electrostatic loudspeakers that are certainly amongst the least coloured loudspeakers available. It is certainly creditable that the HD414SL and HD420SL models can hold their own in such company! By comparison, the old 414s show their colouration and the HD410SLs their curtailed extremes.

If you want a range of headphones from which to choose that are all smooth, uncoloured and comfortable but offer a little more in the way of performance, the more money you wish to spend, then the Sennheiser SL range must be auditioned. Good recorded balance and quality will of course depend on how well you are able to judge the results but quality control starts with the end of your system; monitoring with the aid of these phones will certainly show up any unwanted colourations and improve the balance control of your system.

Prices : HD410SL £17.39, HD414SL £24.34, HD420SL £30.43 - all prices exclusive of VAT.

Previous Article in this issue

Aces BM1082 Mixer

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Jo Partridge

Home & Studio Recording - Copyright: Music Maker Publications (UK), Future Publishing.


Home & Studio Recording - Jun 1985

Donated & scanned by: Mike Gorman

Review by Mike Skeet

Previous article in this issue:

> Aces BM1082 Mixer

Next article in this issue:

> Jo Partridge

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