Pressure Zone Microphones
Amcron, Milab and Sennheiser ‘pressure zone’ mics.
Here is a fascinating variety of microphone which has only been around for some five years. The original concept and development work was instigated by the American company, Crown who registered the name and initials PZM as trade marks; but presumably not the basic idea, as there are now four (or more?) other manufacturers producing similar mics, all of whom have different names for their product. So joining the Crown products (known as Amcron in the UK) we have models from Milab, Sennheiser, Beyer and Schoeps. Here we look at those from Amcron, Milab and Sennheiser.
A very small electret transducer is mounted very close to a rigid plate or boundary, either facing down to the boundary (Amcron and Milab) or facing out and flush with the surface (Sennheiser). The transducer responds to the sound pressure at this boundary, with one particular attribute. This is that the physical presence of the microphone does not itself become an influence on the frequency or phase aspects of its performance since the pressure change it is detecting is phase coherent with no cancellations at different frequencies and more particular at different angles of sound wave arrival. The above produces a hemispherical pickup pattern with very uniform frequency response at all angles within this hemisphere.
The size of the actual plate on which the transducer is mounted determines the low frequency response in an interesting way. There is a 'shelf' drop of 6dB below a point where the basic size is a quarter wavelength of the frequency concerned. As shown in Figure 1, mounting a PZM or boundary mic on to a larger, rigid, non-resonant surface lowers the shelf's onset frequency. Also there are no proximity effects as the basic transducer is pressure operated.
This is a 125 x 150mm rectangular plate with the small electret transducer facing down near its centre with an XLR connector on the edge of the plate. The mic is supplied in a sturdy storage case which also houses the PX18 battery or external phantom power connector and a 1.5 metre lead, for the mic to power connection. The PZM 30GP submitted for review came in a gold colour and is listed as being 150 ohms, suitable for 200 ohm mic inputs. A minimum of 1 kilohm load is suggested. (Cost is £259).
This is a black rectangular plate with the very tiny transducer facing upwards at its centre. A neat magnetically held protection plate is provided to cover the capsule and this is replaced by a similarly held mesh cover when in use.
The MKE 212-3 is cleverly designed to be used with the powering module K3U in the existing Sennheiser range. The K3 modules are the ones normally used for the detachable capsule electret range. The K3U allows both internal battery powering or external phantom powering and features a two position low frequency cut switch. The boundary unit is separated from the powering module by around 3 metres of cable. It is stated as being of 1 kilohm source impedance, needing 4.7 kilohm minimum input impedance circuitry. Many 200 ohm mic inputs suit this situation. (£219.36. Powering Module K3U costs £40.95).
This has a round plate and is the smallest of the three, being 70mm in diameter. It is powered by external 48V phantom with the mic being connected via a 2 metre lead to its amplifier and XLR plug for further extension to a suitable mic input. The MP30 is intended for 200 ohm balanced mic inputs - the phantom powering necessitating balanced operation, of course. (£74 mic only, £84 with stand adaptor and carrying case).
I must describe the various miking experiments that I tried as fun. Initially I placed the mics on a coffee table and, in mono, listened to the pickup of conversation from several people in the room. There was a 'natural' quality about the sound, devoid of any off-axis colourations which seemingly, in mono, gave an impression of the distance each speaker was away from the table, as would be expected from the uniform frequency/polar response. The room was not a lively one and interestingly I did not feel that the room acoustic was emphasised or was making an unnatural contribution to the microphone output.
A view expressed by a colleague suggests that a PZM mounted on a wall, for example, should be seen as 'removing' that wall as far as the mic is concerned, but not as far as the room is concerned. In the latter case the wall will still, of course, feed reflected energy back into the room, but the mic does not directly receive 'colouring' reflections from it.
So much for the initial experiments. In the musical situation things became even more interesting, but at the same time, frustrating. It emerged that this type of microphone is certainly not the answer to all miking problems. This, it seems to me, is the impression created by the extensive write-ups originating from the Crown camp. Much more down to earth was the balanced view in the booklet supplied with the Sennheiser Boundary type.
Getting to the nub of my feelings about these types of mic, my views can be expressed as follows: sound quality - excellent. For instance, put one on the floor in front of a drum kit and there is, in mono, a very well balanced sound of the kit. Similarly, on the inside of the raised lid of a grand piano, there is a fine sound with excellent balance of the sound spectrum. The same for other instruments. But in multi-mic situations there are problems of leakage due to the hemispherical pickup. In a close-knit band situation this is very difficult to come to terms with. In larger set-ups, helped by absorbent acoustics, this is less of a problem.
For me the major problem was in stereo use. Spacing the mics out is not sonically satisfying for me, being totally conditioned to coincident directional crossed pairs! Sennheiser recommend not more than 1 metre of separation in any size of venue to keep a 'defined left and right' image. Any spacing produces very anomalous stereo as far as I am concerned.
The automatic restoration of low frequencies when used on walls, floor or ceilings means that where the mics have to be 'flown' (ie. hung from a ceiling), they must be mounted on a suitably sized baffle. To improve the appearance of any such set up, these baffles can be made of a perspex-type sheet. But again, one is still up against the need to space two such arrangements for stereo. An alternative is to have one sheet of perspex with the PZMs taped to either side of it. Pity, but I cannot accept the 'stereo' produced, compared to coincident crossed pair recordings.
The Amcron PZM reviewed has a slightly brighter sound than the other two. Interestingly, the Milab Hemi is now supplied with a small capacitor (0.01 uF) soldered on the print side of the PCB in the amplifier coupling unit. This lifts the high frequencies (HF) as this was often requested by those already accustomed to the 'bright sound of the competitor'. One of the Hemis supplied had a 0.022 uF capacitor installed and this I feel raised the HF a little too much. The smaller value is just right.
Now, here is perhaps a contentious point - the three mics are all very similar; and isn't it likely, given the basic principles involved, that they should be, as the cavity and mass resonances and acoustic labyrinth differences of other types of mics do not apply?
With this in mind and the prices of the competitors' models, the Milab Hemi-Mike MP30 must be considered rather excellent value for money at its price. Part of this lower price undoubtedly lies in the use of an existing phantom supply. The UK importers, AVM, can supply a circuit suitable for DIY allowing 12, 24 or 48 volt powering which also permits connection to unbalanced inputs. The DC source could easily come from batteries, as with the other two models.
Amcron have in fact, a range of PZM mics. Some of them do not have the characteristic HF lift and there is one for permanent installation in lecterns, pulpits etc. There are also a couple of tie clip versions and one for use as an acoustic instrumental pickup.
Finally, if I were to be asked to name the best sounding one irrespective of price (as heard via my Sony PCM F1 recordings and on my particular set of monitor speakers) - it is the Sennheiser Boundary that seems to pip the others.
(Amcron) HHB, (Contact Details).
(Sennheiser) (Contact Details).
(Milab) (Contact Details).
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Review by Mike Skeet
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