You may have noticed that we are often asked about Tandy's Pressure Zone microphones in the Feedback page but it's a long time since they received any attention within the editorial section of the magazine. This month we set the matter to rights.

Pressure Zone Microphones or PZMs as they are commonly known were developed by Crown. Other similar microphones from different manufacturers working on the same principle are often referred to as boundary effect mics. The Tandy's model (which is actually made under licence from Crown) has attracted a great deal of interest from the home recording enthusiast and the pro market because of its unbelievably low price and its surprisingly good sound quality. But first, what is the advantage of a PZM mic and how does it differ from a conventional mic?

Most mics receive a mixture of direct sound and reflected sound. When this is all added together, the result can be badly coloured and dull due to phase cancellation which is why most conventional studio mics sound better close to the sound source. The PZM on the other hand picks up only reflected sound; the direct sound is rejected by virtue of the mechanical construction of the capsule and its mounting. You will have probably gathered by now that the PZM is rather odd in appearance, and that's because most of what you see is the plate used to reflect sound into the capsule. The capsule itself is positioned just above the plate and is in this case a small electret amplified by a battery powered FET pre-amp located in a box somewhere along the lead. Because low frequency sound has quite a long wavelength, the mic as it stands has a pretty poor bass response because the boundary plate just isn't big enough. To record full-range sounds, the mics need to be mounted on a much larger plate such as a wall, a tabletop or a piece of flat material such as plywood. A piece about 4' square will give good results and you can mount one mic on each side for stereo use if you like.

The advantage in picking up the reflected sound only is that clarity is maintained, even at a distance, and the effect when played back through headphones is much more akin to actually being there than it would be had it been made with conventional dynamic mics. This makes these mics ideal for picking up ambient sounds and they excel when miking up acoustic instruments. In particular they're very effective as inexpensive overhead drum mics. Used in this latter application, there is no need to use a larger boundary plate as you already have plenty of bass from your close up mics. However, the top end detail that you do pick up makes a world of difference to the overall sound and is far more airy than is normally achieved using dynamic mics, even expensive ones.

The pick-up format of these mics is a little unconventional; they pick up sound in a hemispherical pattern, unlike the standard omni which accepts sound from everywhere around it. This wide pattern does mean that you have to be careful to exclude unwanted sound so they are not too practical for live use in a rock situation.

When recording a drum kit at home, I tend to use either two of these mics on stands suspended above the kit, or taped to the walls if I intend to use more of their contribution in the final mix. Taping to the walls gives a much improved bass response but plays havoc with the wallpaper.

If you want a natural sounding drum kit rather than a close miked, produced sound, try using just the wall mounted mics on their own or placed on the floor in front of the kit.

Problems with a Piano

The acoustic piano can be a complete pig to mic up if you can't afford exotic mics, but PZMs give a good sense of stereo width if simply taped under the piano lid in the case of a grand, or taped to nearby walls in the case of an upright. The plates are sensitive to handling noise so it is advisable to mount them where they won't be knocked or subjected to undue vibration. It's also easy to forget that pedal noise is often a problem.

Other acoustic instruments can be recorded simply by placing the mics on top of a low table in front of the performer. I have recorded solo singer/guitarists this way straight onto 2-track with very satisfactory results.

There's a self-adhesive foam shield supplied with the mic and I found the performance to be far better with this fitted. Without the shield, the mic is susceptible to popping on vocals, even at a distance. You can actually get a very good vocal sound using these mics close-up taped to a wall or mirror but additional pop shielding is a distinct advantage — simply use a nylon kitchen strainer positioned a few inches in front of the mic, remembering to remove any pieces of grot first.

Sense and Sensitivity

The problem with any ambience mic used on an acoustic, rather than amplified, sound source is that of sensitivity. As supplied, these mics give roughly the same output as your average SM58 or similar for the same input level with the sound source positioned about a foot in front of the mics. If you've ever tried to record an acoustic guitar with an SM58 or similar at several feet from the instrument, you'll be aware of a problem. Even with the mic gain turned full up on the desk, you're hard pushed to get the meters away from the bottom stop.

There's a partial solution in the form of special high voltage batteries which can be substituted for the 1.5v battery in the pre-amp to increase the gain and hence the sensitivity. Sadly these cost an absolute fortune so it's far more cost-effective to connect a 9v PP3 battery in place of the existing one. This will give about 4dB of extra gain which, though nothing to write home about, is not to be sneezed at. If you're feeling brave, you can then connect two PP3s in series, which would give you a grand total of 18v and approximately another 4dB of gain. When doing my own modification I chickened out after that and didn't go for the 27v jackpot, but if anyone wants to try it, please let us know how you fare. Not only does the extra voltage increase the gain but it also gives more headroom which is useful if you're working close to a loud drum kit.

Of course these two extra batteries won't fit inside the pre-amp housing but you can avoid this by buying a couple of PP3 type clips, again from Tandy, and soldering these to the existing battery terminals inside the case. Then you could couple up the batteries and tape them to the outside of the pre-amp box.

Even with this extra gain, you'll still find yourself struggling a bit at a distance but matters are definitely improved. As to the extra headroom, I've a friend who swears he records bass drums by sitting the mic on a folded blanket inside the bass drum but I can't verify this from experience.

Possibilities and Potentials

It's always interesting to experiment with the new capabilities offered by any mod. Amongst the more successful experiments was the miking of congas, and an attempt to create a loud, live, raunchy electric guitar sound by taping a PZM mic to the wall directly behind a small combo that was being driven to destruction by an exuberant guitarist.

Moreover, if you're in the habit of miking up the reverb of a real room, a pair of these mics on opposite walls will do the trick very nicely. Try to arrange it that the speakers point away from the mics, that way you will get a more reverberant sound.

For live recording, try suspending a piece of plywood by one edge from the ceiling and fix a PZM mic to each side. One edge of the ply should be facing the musicians and, as mentioned earlier, the ply should be three or four feet along each side if you are hoping for a good bass response. Remember too that the directional characteristics of the mics will mean that you pick up as much of the audience as of the band if you are not careful; though this can be an advantage in some situations if you want to capture the live atmosphere.

Summary

It would be wrong to consider PZMs as replacements for conventional mics, but at £25 each, they're an effective augmentation to your existing mics, affordable by virtually anyone involved in recording but the results that they are capable of producing can belie their budget price.

As ambience mics or for miking traditional acoustic instruments, these mics offer distinct advantages over conventional moving coil or even electret mics in terms of their clean, open sound and, used in pairs, the sense of stereo depth is both impressive and convincing. But one area in which these mics fall short of their up-market counterparts is in their gain matching; any two are likely to differ in output level by a few dBs for the same input sound pressure level. However, this does not represent a problem, provided you set up the levels independently using the meters on your desk or recorder.

Those of you with long memories may remember that in March and July 84 issues we ran articles on phantom powering and balancing these mics, so don't be confused by a further article on a related topic. To be quite frank, if you're using these mics in your home studio, the battery life is so long and the propensity for picking up interference so small that a balancing mod is hardly worth the bother.

So, that's our hint for this month: definitely a must for anyone with a PZM or two. And for those of you that haven't, there's absolutely no excuse — buy one this instant!

Information on the Tandy Realistic PZM is obtainable from: Tandy Corporation, (Contact Details).