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Sennheiser Microphones

The new range of condenser mics from this respected German firm utilise frequency modulation techniques to achieve a wide, flat response. Gareth Stuart checks them out.


Engineer Gareth Stuart gives his appraisal of Sennheiser's omnidirectional MKH20 and cardioid MKH40 condenser microphones.


Before reporting on how these two mics responded to my live and studio tests. I'd like to take a few moments to talk about the basic workings of condenser (capacitor) microphones. I don't really want to get involved with the technicalities of how such microphones work but to point out a few differences caused by Sennheiser's approach, which renders these microphones generally more flexible than most.

PRINCIPLES



Capacitor (condenser) mics differ from 'dynamic' and 'ribbon' mics in that they have to be fed a DC voltage in order to work. The most commonly used voltage is 48V - often referred to as phantom power. One possible explanation of this peculiar name/label is because the voltage travels up the mic cable in the opposite direction to the signal being sent from the mic to an input, but is 'invisible' to that signal. Due to the way they work, standard capacitor microphones incur higher voltages, and higher impedances, than microphones like the MKH20/40 which work according to the RF (radio frequency) principle. They also need transformers to match the very high internal operating impedance to conventional low impedance (600 ohms) line levels. Limitations imposed by using transformers manifest themselves most clearly in this type of microphone's sometimes erratic frequency response and bulky mechanical design.

So, how is it that Sennheiser have managed to overcome the typical problems and produce such elegantly slim mics?

Okay now, sit tight, here comes the crunch... The big difference between these RF principle mics from Sennheiser and conventional capacitor mics is that when sound arrives at the capsule, changes corresponding to the audio frequency produce an FM (frequency modulated) circuit. The rest of the mic's circuit then decodes this information. The key to its working procedure thus lies in modulation and de-modulation of the signal. All the electric circuitry works at very low impedance, unlike normal capacitor circuits where very high impedances of hundreds of mega-ohms are created, and due to this design, it is able to have a transformerless, low impedance output.

And the main benefits, apart from an aesthetically pleasing design, are that these mics are (quote) "specially suited for unfavourable climatic conditions" (which takes care of Britain for starters) and "thanks to the RF principle, impedances...are so low, even the highest humidity will not affect performance." In other words, maximum sweat for minimum fuss, not vice versa!

There has got to be a cynical hardcore out there saying 'Yeah, yeah... heard it all before. High humidity - baahh; my mic'll take 85% humidity at 60 degrees C.' Well, these two mics, so I'm reliably informed by staff at Hayden Laboratories - Sennheiser's UK base, will work even when being sprayed with water!

Unconventional designs call for unconventional tests I suppose, but I have this vision of white-coated technicians, spray guns at the ready, taking it in turn to plug in and squirt each new mic, glancing at each other, eyebrows raised in satisfaction, before stalking off to report on another success!

And so to my mic tests...

I first used the MKH20, which has an omnidirectional pick-up pattern, to record a snare drum. It's not something I'd normally do (or ideally recommend), but having just received the mics, I whipped one out of its case and got cracking with some recording.

The result was very pleasing - a clear, crisp 'snap', full of energy and a faithful reproduction of the snare's true sound. Mind you, I got a faithful reproduction of the rest of the drum kit as well, understandably since any omnidirectional mic will pick up sound equally from all directions - front, back, side etc... But, before you start knocking it, once the snare sound was gated using a dbx 463X over-easy noise gate/expander, I was left with a lovely full, somehow round, snare sound... very good, I liked it.

Another benefit of the mic having such a slim design was the ease with which I was able to position it over the snare drum without impeding the drummer's playing in any way. Also, by switching in a 10dB attenuator pad on the side of the mic, it's possible to obtain distortion-free recordings of sound sources that create pressures as high as 142dB, which makes it ideal for this particular application.

Whilst 'playing' with the mics during the early stages of this review, I thought I'd better give Sennheiser a ring to clear up a few points and at the same time record the conversation so as not to miss any technicalities. No, I don't tap phones, or record conversations on a day-to-day basis, but I thought this could be quite important. So I used the MKH20 with the telephone earpiece as close as I could physically get it to the mic, and it worked quite well - except the person I needed to talk with wasn't in the office. Typical. Still, I've got very clear and relatively noiseless proof of how helpful the secretaries are! I admit this is an unusual application, but it could make a useful radio drama production technique.

I next recorded an alto sax (track) - with all the mic settings in their normal positions. Again, the quality was very good.

As well as the 10dB pad, the MKH20 may be operated in a mode whereby off-axis sounds can be given a frequency boost above 2kHz. 'Wait a minute,' you say, 'if this is an omnidirectional mic then surely on-axis signals are boosted as well in this mode.' This is true and, if you so wish to, you can use this switch purely as an HF (high frequency) lift. But that's not quite what Sennheiser had in mind.

The thing to remember is that omnidirectional mics tend to become, to some extent, directional at high frequencies.

Before I explain why, it's necessary that you should imagine frequencies in terms of their wavelengths. For example, a 20,000Hz signal has a wavelength of 1.7 centimetres - this figure is arrived at by dividing the speed of sound (340 metres per second) by the frequency. Please also accept that this type of microphone ceases to be omnidirectional at 2 to 3 octaves below the frequency for which sound wavelengths equal the microphone diameter.

Now here's the reason why Sennheiser include such a switch, and the circumstances in which it should be 'properly' used.

Consider the HF sounds approaching the mic from a 90 degree angle: if one complete wavelength should equal the diameter of the diaphragm then it will produce roughly equal inward and outward forces. Consequently the diaphragm won't be inclined to move too much, and a reduced output level for that frequency will occur. This decrease in output is due to phase cancellation. Naturally, as the microphone is incapable of reporting these HF advances from the side, its 'field of vision' (pick-up pattern) is narrowed and it becomes more directional at high frequencies. However, if you flick this nifty switch on the MKH20 and boost those high frequencies, the microphone will be able to sustain its omnidirectional characteristic at much higher frequencies than would be otherwise possible. Clever stuff, eh?

The 'proper' time to use this switch is when making a traditional stereo recording using a pair of spaced omnidirectional mics, with the individual mics held vertically and their diaphragm pointing straight up or down. Even though I didn't get a chance to try out the MKH20 in this capacity and gauge my own results, it would appear that these mics have been used on location by the much-respected classical engineer, Tony Faulkner, to record the Berlin Philharmonic, and he reckons they compare favourably with Schoeps microphones. That definitely puts them high on the value for money chart!

MKH40



In keeping with the MKH20, Sennheiser's MKH40 mic also has two switches: a similar functioning 10dB pad, and a bass roll-off filter in place of the MKH20's HF lift switch. The bass roll-off filter effectively reduces the microphone's response to frequencies below 120Hz. So, for example, if the mic is positioned on a stand near a gangway, or likely to be subjected to periodic stage/ground vibrations, then this roll-off filter will help to curb the rumbling sounds associated with those vibrations.

At the front of the mic, protected by a type of wire-mesh screen, is the diaphragm assembly or 'capsule' which can be seen through the slats which run round the top of the mic's body. These aren't just a pretty design feature - they allow the microphone to produce its directional characteristic.

The MKH40 is a cardioid capacitor microphone (cardioid, meaning heart-shaped, is the favoured term for a directional mic, as it gives a graphic representation of the field 'seen' by the microphone). It is designed to pick up signals on-axis (those from the front), and cancel out those approaching from the rear.

The slats, or apertures if you prefer, present soundwaves with two possible paths for reaching the mic's diaphragm. Sounds occurring in front of the microphone travel directly to the front of the diaphragm, and also enter via these apertures, are further hindered by an acoustic labyrinth which creates a phase-shift in sound waves passing through it, and eventually they arrive slightly delayed at the rear of the diaphragm. Because those sounds arrive at the rear of the diaphragm later than at the front, there exists a phase difference and consequently the diaphragm is allowed to move to and fro.

However, in approaching the mic from the rear, soundwaves which travel to the diaphragm via the acoustic labyrinth, arrive at the same time as those which travel around the mic's exterior to the front. This gives rise to equal amounts of (sound) pressure being applied simultaneously to either side of the diaphragm, leaving it motionless. Thus those sounds are rejected. Well, at least that's the theory.

The MKH40 is most capable of eliminating low frequencies approaching from behind - and can attenuate frequencies up to 1 kHz by 25dB. This figure decreases with frequency increase, but as it's just about impossible to design a delay network (acoustic labyrinth) which creates the correct phase-shift at all frequencies, Sennheiser's figures again compare favourably with the typical response of other high grade cardioid mics.

ON TRIAL



I first tried out the MKH40 as a live vocal mic - on a female (alto) voice. To set the scene, imagine the band rehearsing in a fairly 'cosy' room, with drums, bass, guitars and keyboards for the vocals to contend with. It was necessary to obtain a high enough level from the mic to make the words intelligible, and yet not so loud as to induce feedback. The whole idea of using a cardioid mic rather than the omni MKH20, for this purpose, was to keep feedback to a minimum as the MKH40 attenuates those sounds occurring 180 degrees off-axis ie. behind it.

So what happened then? Well, in order to amplify the mic's signal to a useful level it was necessary to equalise out most of the top end (high frequencies) to avoid feedback. The result of this rather drastic action left such a thin-sounding voice, that I was inclined to use a much cheaper dynamic mic designed to cope with all these problems more effectively.

Now, don't get me wrong, that's not a criticism of the mic - it's just a way of discovering its suitability for a certain role. Used as a live snare drum mic, it worked extremely well.

I decided to carry out a few more studio tests making sample recordings on my digital system, using a Sony PCM501ES, to see how Sennheiser's MKH40 coped with my voice, acoustic guitar, classical guitar, and electric guitar - both distorted and clean types.

Hmmm, I thought. Well it seems to sound fine, perhaps a little lacking in the top end on the acoustic instruments, but that's probably down to my poor mic positioning. I decided I needed a decent mic to compare it with, so I used my AKG 414EB and re-recorded the distorted guitar. The reason for choosing this particular sound was that it's just one of those that has such life when you hear it live, but inevitably loses that spirit upon recording.

But the MKH40 test recording was pretty good... in fact, very good... and I was dying to see what my 414 would make of it. So I swapped mics over and re-recorded the electric guitar. No, I thought, there's something wrong here... must have chosen a directivity pattern other than cardioid on the AKG 414. But no, I had set it the same - cardioid and with a 10dB pad. I couldn't believe the difference in signals coming from the MKH40 and that coming from the 414. For the AKG 414 to give a decent level, almost equal to that of the Sennheiser MKH40, I had to boost it by 10dB. On listening back to the two recordings, well there was no comparison, the MKH40 was just so good. The recording had such fantastic bite - amazing. And what a revelation!

Sennheiser are marketing these microphones with high quality studio and outside broadcast (OB) recording in mind. In this bracket, they are certainly money-savers as they compare so favourably with higher-priced standard capacitor mics. Sennheiser stress that their low noise characteristics and distortion-free transmission of very loud signals make them ideally suited to the digital medium, and my tests with a Sony PCM501 bore that out.

As to their use in a sound reinforcement system - they're not necessarily a luxury for this type of work as they are capable of producing high quality results both on stage and in the studio. I'm sure one flexible MKH would be cheaper than buying a studio mic and a stage mic, so why not kill two birds with one stone?

Somehow the marvellous humidity handling capacity of the two mics seems of little relevance to me and, in a way, comes across just as a potentially useful by-product of the radio frequency principle. For specialised uses, a specialised product is generally always the best bet, but I suppose if you find that a diver's wristwatch, capable of handling pressures at a depth of 200 metres also keeps better time than a normal watch, you'd buy it for its timekeeping accuracy...

For their low self-noise level, high sound pressure handling capacity, extremely clear sound, and relatively low price, I think the Sennheiser MKH20 and MKH40 microphones represent good value for money.

The price of the MKH40 is £323.40 plus VAT. The MKH20 price is yet to be finalised but is expected to be similar.

(Contact Details)



Previous Article in this issue

Frankie Goes To Liverpool

Next article in this issue

Elka EK22/EK44 Synthesizers


Sound On Sound - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.

 

Sound On Sound - Nov 1986

Review by Gareth Stuart

Previous article in this issue:

> Frankie Goes To Liverpool

Next article in this issue:

> Elka EK22/EK44 Synthesizers


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