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The Pressure Zone Microphone Theory and Application Guide, published by Crown, is an excellent guide to using boundary microphones.
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Type: Electret condenser (back electret).
Frequency Response: 20 Hz to 20 kHz.
Polar Pattern: Hemispherical when lying on a large, flat surface.
Sensitivity: Battery power, 82 dBV/µbar (62 dBV/Pa); phantom power, 70 dBV/µbar (50 dBV/ Pa).
Output Impedance: 150 ohms; recommended minimum load impedance, 1 kilohm.
Self Noise: 21 dBA equivalent SPL.
Maximum Input SPL: 120 dB (battery power).
Polarity: Positive pressure on diaphragm produces positive voltage on pin 2 with respect to pin 3 of output connector.
Power: Battery, 1.5-V size N. E90 Eveready or 6-V A544 Eveready; 12 to 48-V phantom power optional.
Connector: 3-pin professional audio type.
Case: Carbon-filled nylon.
Accessories: Fabric windscreen supplied; PH4 power supply (powers up to four PZMs) optional.
Dimensions: 3-15/16 in. (10 cm) W x 7-3/16 in. (18.2cm) L x 7/8 in. (2.2 cm) H.
Weight: 2.4 oz. (68 grams) without cable and windscreen.
Prices: Microphone, $169; power supply, $179.
Company Address: 1718 West Mishawaka Rd., Elkhart, IN. 46514.
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The theoretical advantage of a boundary microphone mounted on the floor (for example), as compared to a conventional microphone on a floor stand, is the elimination of the reflected sound wave from the floor. This delayed reflection interferes with the direct sound wave, resulting in many dips and peaks in the frequency response of the input sound pressure to the microphone-the notorious "comb filter" effect. An equally important practical advantage is a uniform, hemispherical polar pattern versus frequency, which is related to the small size of the capsule and its proximity to the plane. What all of this means is that the boundary microphone potentially can offer cleaner, more natural sound than a conventional microphone of about 1 inch diameter, mounted on a floor stand 3 or more feet from a musical instrument. The off-axis high-frequency sound, whether direct or reverberant, is not attenuated, as it would be with conventional microphones (except very small conventional models, such as the new Brüel & Kjaer Studio Microphones).
Are boundary microphones, and PZMs in particular, the answer to all sound pickup problems? Of course not. The boundary microphone is just another type of omnidirectional microphone. (For those interested in more on the subject, I have covered these types in a lengthy tutorial.) The professional PZM models are priced in the range of $350 each, including the essential power supply. Amateurs having low- to medium-cost cassette recorders might find them beyond their budget. The new PZM-180 was designed for lower cost, with a plastic housing and backing plate, and powered by an internal battery and output transformer instead of the costly power supply included with the pro model. The trade-offs, apparent in the PZM-180's specifications, include reduction of maximum input SPL from 150 to 120 dB, and a 6-dB reduction in sensitivity; the latter deficiency, however, may be corrected by using optional phantom powering. It may seem odd that the PZM-180's frequency response is specified as 20 Hz to 20 kHz while the response of the professional Model PZM-30GP is specified as 50 Hz to 15 kHz, but the lower-priced model's response is specified within looser tolerance limits. Similarly, the self noise specification for the PZM-180 is 1 dB higher than the self-noise level currently given for the pro model.
A significant advantage of the PZM-180 is its small size and light weight. Thus, it is possible to fit a small stereo cassette recorder plus a pair of PZMs and cables into a briefcase. (Note that cables are not included with the PZM180.) The small, black-finished PZM-180 placed on floor or stage is less conspicuous than a conventional microphone on a floor stand, and obviously less objectionable to performers. This should encourage more audiophiles to record musical performances.
The two units tested were supplied by the manufacturer with frequency response curves; I do not know if the performance of off-the-shelf mikes will be similar.
The specifications indicate that the PZM-180 requires a 1.5-V N cell (a readily available hearing-aid battery) or an Eveready A544, which is a 6-V photo battery that will provide greater sensitivity. Optionally, a 12or 48-V phantom power source may be used, a feature which professional users will appreciate. Since there is no on/off switch, the battery is being drained constantly; still, Crown states that they expect a 2,500-hour battery life for the N cell. I found that the acoustical sensitivity decreased by 2 dB in one month (about 700 hours) with an alkaline cell, as compared to Crown's claim of a 6-month life. I removed the battery after each usage.
The battery stays in the mike for phantom powering, so in a permanent phantom-powered installation, the power should be permanently "on" to avoid battery drain and eventual corrosion. (It may be possible to replace the cell with a piece of metal, but I did not try this.) Crown indicates that a 6-V battery (Duracell PX-28) may be substituted for the N cell by using a piece of aluminum foil for filler, or by bending the clips. On one mike, a clip broke when bent, so foil is probably a better bet. A metal spacer could be included with the microphone; this might prevent noise due to loose contact.
The impedance versus frequency (Fig. 1) of both PZM180 samples was identical. The 66-ohm value at 1 kHz is very similar to the values measured on the professional models. The rated 150-ohm value is obtained below 30 Hz, where the impedance rises, perhaps indicating a coupling capacitor. Because of this effect, the recommended minimum load of 1 kilohm should be observed or some attenuation of the region below 50 Hz may occur. Unlike the professional model, the PZM-180's impedance does not rise above 10 kHz; this may indicate that the transformers in the 180s have lower leakage inductance, a desirable improvement. The constant (resistive) impedance above 50 Hz means that loading will not affect frequency response in this region.
The PZM, like most boundary microphones, is not amenable to frequency response testing in strict accordance with EIA standards, which relate to microphones used in free space. My articles in the April 1977 and September 1978 issues detail my test procedures following these standards, which define the frequency response as the open-circuit voltage resulting from a constant sound pressure at the microphone location before the microphone is introduced.
The microphone, with its integral backplate, is easily tested according to these rules, and Crown calls this the Free-Field Frequency Response. The ideal test of the microphone on a boundary would be an infinite baffle. Neither I nor Crown had such a test facility, so we used reasonable substitutes.
The 1983 PZM review describes my "pseudo-infinite baffle" test, for which the PZM is mounted on a 2-foot-square piece of plywood placed 6 inches from a 2-inch-diameter sound source. The SPL is calibrated before the baffle is introduced. Bruce Bartlett, a Crown engineer, described their setup: "We measure PZM microphones at 30° vertical incidence on a 4-foot-square plywood/metal boundary, at 2 feet from the sound source. A flush-mounted B & K 1/4-inch, pressure-calibrated microphone is used as a reference. Its response, when mounted in the boundary, is subtracted from the response of the microphone on the boundary. This results in a pseudo-infinite plane measurement." Crown supplied frequency response and sensitivity data with the microphones, obtained on a Crown Tecron TEF-10 acoustical computer by Time Delay Spectrometry (TDS). These curves indicate a false roll-off below 200 Hz because, in a room of moderate size, the time window cannot admit a full cycle of the direct sound wave and still reject reflected waves from the room boundaries.
Crown's TEF-10 printouts are greatly compressed on the frequency axis compared to the figures presented here, but an eyeball check showed generally similar curves, save for the TDS roll-off below 200 Hz. The sensitivity values I obtained are systematically 6 dB higher than Crown's because of the difference in our method of calibration. I think my results show what really happens when a PZM is used in place of a conventional mike, and, in my opinion, more closely conforms to EIA and ANSI standards.
My 1983 review shows frequency response curves at various angles of incidence for the PZM-30GP mounted on 2and 4-foot-square baffles, measured at 6 feet outdoors.
These curves may be presumed to relate to all PZM models, if differences in the pseudo-infinite-baffle response are taken into account.
Figure 2 shows the pseudo-infinite-baffle frequency responses of the PZM-180s, which are generally similar to my data for the pro model but with two differences: First, the low-frequency responses are flat to 40 Hz, whereas the PZM-30GP's response rolled off to-6 dB at that frequency.
The response of the 30GP crosses 0 dB at 17 kHz, but the 180s' extend to 20 kHz. The peak at 13 kHz is probably an artifact because, with finite baffles, the 180 is likely to exhibit a flat trend over the entire hemisphere, similar to the 30GP. Both PZM-180s show similar responses, with only 1to 2-dB differences in output below 4 kHz. The response with phantom power is identical, with 10-dB higher sensitivity. With the 6-V battery, output was up by 4.5 dB. I did not do further testing with this battery.
Figure 3 shows frequency response versus angle of sound incidence, and indicates that the PZM has very uniform response over the entire hemisphere. Observe that the 45° response is within about ±3 dB from 35 Hz to 17 kHz, the best response curve obtained so far from these hard-to-measure microphones.
Figure 4 shows that the PZM-180's response is acceptable even without a baffle, which was not the case with the pro model, probably because of the 180's smaller back plate. The low-frequency response is very flat, and the bright high-frequency response may be desirable for many pop vocal and instrument pickup applications. My 1983 review indicates that the output should fall by 6 dB below the cutoff frequency of this baffle, which is on the order of 1 kHz, but the actual drop is only about 3 dB. Figure 5 shows the effect of adding the cloth windscreen.
(It is to be used with its logo on the bottom side of the mike because the paint blocks the pores, even though logic might dictate that the logo be facing up.) This screen has a far greater attenuation effect than most foam screens, and it can be used with impunity only for speech, vocals, or low frequency instruments. Since it tends to flatten the peaks in the curves of Fig. 4, the screen may be used to provide high-frequency equalization. All in all, I'd prefer a block of foam, such as that supplied with the pro models.
Figure 6 shows the noise of Unit No. 1. Unit No. 2 was found to have a considerable amount of low-frequency noise, plus impulse spikes. (As my noise tests were conducted long after the response tests, it is uncertain whether Crown shipped a noisy mike.) As you'd expect, the PZM180's equivalent noise level of 24 dBA is higher than the 19 dBA measured for the 30GP. I find that acceptable, considering the relative price of the mikes, although Crown's specifications for the two mikes show the 180 to have only 1 dB more noise than the 30GP. I found that the equivalent noise SPL was the same with N-cell or 36-V phantom power.
The maximum input SPL for the PZM-180 is rated at 120 dB for N-cell power, but I observed clipping on the 'scope to be 124 dB for speech. This increases only to 127 dB with 36-V power.
The phasing of the PZM-180 as measured on my EMT 160 polarity tester was, as specified, pin 2 positive, in accordance with standards.
Fig. 5-Effect of windscreen (logo side down) on frequency response, with microphone mounted on pseudo-infinite baffle.
Use and Listening Tests
The first series of tests compared the PZM-180 to a Nakamichi CM-700 condenser microphone with omnidirectional capsule. With their windscreens, both mikes had equal resistance to breath "pops." The PZM-180's 60-Hz magnetic hum was 20 dB lower, which seems remarkable since its transformer is unshielded. Close-up voices sounded identical, at any angle of incidence, and vibration sensitivity was equally low with either mike.
My first recording with the PZM-180 was of a tenor singing and playing the piano in a large church. I used a "bipolar pair"-two PZMs slightly off-center and on opposite sides of a 2-foot-square baffle made of 1/4-inch Masonite. I mounted this array on a Shure 14-foot stand (using 1/2-inch pipe fittings and an Atlas AD-1 adaptor) and placed the stand opposite the keyboard end of a Steinway "B" piano whose lid was open on half stick. The mikes were about 7 feet from the floor, and the baffle edge pointed at the music holder. I used a Sanyo RD S30 cassette deck with Type II Maxell tape and Dolby B NR. Playback in my listening studio showed good imaging, with the vocalist on the left and the piano smoothly spread from left to right. I thought the sound quality was perfectly natural, and there seemed to be no coloration from the measured high-frequency sound peaks (but note that the sound incidence angles were about 45° to 90°).
At this recording session, I also had an opportunity to record a pipe-organ rehearsal. Again, the imaging and sound quality were very realistic. Later on, I recorded the same organ with an AKG C422 stereo microphone used in X-Y stereo with cardioid and figure-8 polar patterns (though it seemed absurd to use a $2,800 mike with a cassette recorder costing less than one-tenth as much). The sonic differences between the C422 and the PZM-180 fall within the 30-Hz to 15-kHz response of the recorder. I was pleased to find that sound quality and imaging were fairly similar with both mikes, the PZM sounding somewhat brighter. The AKG had no bass roll-off, and it captured very deep and fundamental pipe-organ bass sound. The PZM-180 had equally excellent bass sound, which is quite remarkable considering its low cost.
Next, I worked with an associate, Carlton Read, to record a brass quintet in a church having rather dead acoustics.
The first half of the concert was recorded using a pair of RCA 77-DX ribbon microphones on floor stands about 4 feet apart, the ensemble being in a tight semicircle. The second half was recorded with the PZM-180s on the carpeted floor, exactly where the 77-DXs had been. A battery-operated Marantz cassette recorder was used. Although the 77-DX is preferred by many recording engineers for recording brass, in this case the PZMs were superior because of improved stereo imaging. Their sound imaging was accurate and smooth from left to right, whereas the 77-DX sound tended to have a "hole in the middle." The sound quality was somewhat brighter and clearer with the PZMs, too.
The PZMs were used unsuccessfully to pick up an orchestra from the edge of the stage. Here, the balance between close and distant instruments was bad; I suspect that a bipolar pair over the conductor's head would have been better. A more successful application was sound reinforcement of downstage action in a school play, with the PZMs at the edge of the stage.
The PZM-180 microphones tested have somewhat better frequency response than the PZM-30GP professional model, but a dynamic range of only 100 dB compared to 141 dB for the pro model. They would probably be excellent for most audiophile recording applications and for professional applications not involving high sound levels. The bipolar pair is a good substitute for a costly stereo microphone. A pair of PZM-180s and a small, battery-operated stereo cassette recorder can make professional-quality, on-location recordings without the size and weight drawbacks associated with conventional mikes and floor stands.
I hope that Crown will provide an adaptor for the 6-V battery, and possibly a dummy battery for permanent phantom-powered installation. The windscreen design needs improvement. Experience with the PZM-30GP shows that a semi-pro model with somewhat flatter high-frequency response may be desirable for classical music recordings.
I rate the PZM-180 a best buy for audiophiles who have low- or medium-cost stereo cassette recorders and want to make professional-sounding tapes.
-Jon R. Sank
1. Sank, Jon R., "Microphones," Audio Engineering Society, Paper C1000, presented at the 2nd AES International Conference: The Art and Technology of Recording, May 1114, 1984; Anaheim, Calif.
(Source: Audio magazine, March. 1985)
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