Manufacturer's Specifications
Type: Combination pressure-gradient and line.
Transducer: Electret condenser.
Frequency Range: 60 Hz to 20 kHz.
Polar Pattern: Hypercardioid at low frequencies, lobar at frequencies above 1 kHz: symmetrical about axis.
Output Impedance: 150 ohms nominal, 100 ohms actual; recommended minimum load, 800 ohms; minimum permissible load, 150 ohms.
Output Level at 1 kHz: Open-circuit voltage, -53 dB re: 1 V/u bar (-33 dB re: 1 V/Pa).
Output Clipping Level and Maximum SPL Input (at 1 kHz for 0.5% THD): With 800-ohm load, -1 dBV (0.89 V) for 127 dB SPL input; with 150-ohm load, -12 dBV(0.25 V) for 119 dB SPL.
S/N Ratio: 79 dB re: 94 dB SPL.
Hum Pickup: Less than 3 dB equivalent SPL in 0.001-gauss (1 mOe) magnetic field at 60 Hz.
Output Noise: 16 dB equivalent SPL, A-weighted.
Polarity: Positive pressure on diaphragm produces positive voltage on XLR output connector pin 2 relative to pin 3.
Power Requirements: 11 to 52 V d.c. phantom power, 2-mA current drain.
Filter: Integral low-cut filter, switch selectable 15-dB/octave roll-off be low 60 Hz in "flat" position and be low 160 Hz in roll-off position.
Operating Conditions: -20° to + 135°F (-29° to + 57° C), 0% to 95% relative humidity. Temperatures to 265°F (74°C) and relative humidity up to 80% acceptable for storage.
Dimensions: 13/16 in. handle diameter x 20 5/8 in. long (2 cm x 52.4 cm).
Weight: 6.9 oz. (195 grams).
Supplied Accessories: Carrying case and foam windscreen.
Optional Accessories: Model A89SM shock mount; Model A57E swivel adaptor; PS1A power supply.
Prices: SM89, $900; A89SM, $90; A57E, $12.50: PS1A, $150.
Company Address: Shure Brothers, 222 Hartrey Ave., Evanston, Ill., 60202-3696.
Shotgun, or line, microphones are designed for high directivity in long-distance sound pickup. While early models (see sidebar) used tubes 5 or 6 feet in length, modern units like the Shure SM89 use slotted or perforated tubes, about one-third as long as those of early shotguns, coupled to high-sensitivity transducers. These line microphones use the principle of wave interference to achieve their high degree of directivity. When a line mike is aimed at the source of sound, all the sound waves from that source entering the slots or holes in the line arrive at the transducer simultaneously, and audio output is at its maximum. When the mike is angled away from the source, the sound waves reach the transducer at different times and cancel each other, minimizing audio output.
A line mike's directivity increases with frequency, so its pickup pattern is narrow at high frequencies and wide at low frequencies. This could result in nonuniform audio quality when picking up groups of performers or musical instruments. Shure, however, claims uniform frequency response throughout the SM89's sonic acceptance angle of 60°. They have minimized the problem of frequency-dependent directivity by designing this mike's line tube to have nonuniform acoustical impedance and by using a pressure-gradient transducer whose pattern at low frequencies is hypercardioid. This is indicated by the SM89's precisely machined slotted acoustic line, whose slots are spaced more widely at the transducer end.
Even before I got to that precise machining, the SM89 had impressed me as a high-quality product. That impression began with the sturdy, fabric-covered case the mike was packed in and continued with its tapered foam wind screen. The low-cut filter switch on the handle is semi-recessed and the microphone is very light in weight. How ever, the microphone does not come with any mounting accessories (though a shock mount and swivel adaptor are available), and the user must furnish a cable to mate with the three-pin XLR male plug on the microphone.
In an article on the SM89 ("Reducing Off-Axis Comb-Filter Effects in Highly Directional Microphones," Journal of the Audio Engineering Society, June 1987), the author, Shure's Yuri Shulman, indicated that the mike's design also avoids the high-frequency roll-off and the comb-filter effects that can produce holes in the frequency response for sounds outside a line mike's narrow frontal angle of acceptance.
Shure has conducted field tests in television studios and at location recording sessions that indicate response uniformity throughout a frontal angle of 60° is desirable so that aiming would not be overly critical.
The rated sensitivity of the SM89 is about 20 dB greater than that of an efficient dynamic microphone. Therefore, the SM89 will produce the same output at 10 feet from a person speaking that a dynamic unit will give at 1 foot. This output level is remarkably high for a small-diameter (less than 3/4 inch) condenser microphone. To achieve this high sensitivity, the electret is charged to an equivalent potential of 180 V. The diaphragm, of gold-coated Mylar, is spaced 0.002 inch from the electret element on the backplate. According to Shulman, a charge of 100 V for each 0.001 inch of spacing is safe from electrical pop noise.
Measurements
As always, I first tested the SM89's electrical output impedance. For this and other tests, the microphone was powered by Shure's PS1 power supply, a 21-V unit listed on the SM89 data sheet as an acceptable option.
The constant-current test setup for impedance measurements (see "The Compleat Microphone Evaluation," Audio, April 1977) requires one of the microphone's output conductors to be grounded. To avoid shorting the d.c. on the mike leads, a heavily shielded 1:1 isolation transformer was used between the PS1 and the impedance test circuit. This introduced a small series resistance, which was measured by substituting a precision 150-ohm resistor for the mike and subtracting 150 from the measured total impedance. This value was then subtracted from the reading obtained with the microphone connected.
Fig. 1-Output impedance vs. frequency with response switch set for "flat" (solid
curve) and low-cut (dashed curve).
Figure 1 shows the actual impedance of the SM89. In the midrange, it was exactly 100 ohms, as specified. The rising impedance above and below this region suggests that the SM89 should be connected to a preamp whose input impedance is much greater than 100 ohms; otherwise, some bass and treble roll-off will result. Note that the low-cut equalizer setting does not affect the impedance of the mike.
Shure's specifications indicate that the SM89 will clip at lower input levels with low-impedance loads. However, many contemporary mixing boards, including several from Shure, will provide high enough input impedance and adequate powering for this mike.
Next, the frequency response in a free field was tested. Instead of measuring polar patterns at fixed frequencies, I measured frequency response at various angles. These curves are more meaningful than polar plots, whose curves are adjusted in size (normalized) so that the on-axis (0°) output is at the 0-dB reference point for all test frequencies.
For this highly directional microphone, responses were measured at more angles than are used in testing an omni directional or cardioid 'mike.
The choice of distance between test source and mike is critical. The object is to select a distance where the arriving sound will be a plane wave (i.e., will provide a uniform sound level all over the microphone). Additional requirements are that this condition should apply down to the lowest frequency of interest (f) and that the testing distance should be at least four times the maximum dimension of the microphone. L. J. Anderson used a 28-foot testing distance for his early, 6-foot long mike ("A Line Type of Microphone for Speech Pickup," Journal of the Society of Motion Picture Engineers, March 1941). Shure and I both used a distance of 6 feet in testing the SM89, whose acoustical line is 16 inches long. At this distance, my large spherical sound source, which uses an 8-inch speaker, looks like a point source. Also, Leo L. Beranek's book, Acoustic Measurements, quotes Anderson as stating that the testing distance shall be at least 350 divided by f. If f equals 60 Hz, as it does here, 6 feet is an adequate distance for establishing plane-wave conditions.
I used my large sound source outdoors and calibrated it with the SPX Laboratory Ribbon microphone, as described in the September 1978 issue of Audio. This setup was used for the review of the Nakamichi mike system in that issue because it included a shotgun attachment. Later, in the March 1983 issue, this outdoor setup was used to test an early Crown PZM microphone. At that time, I added an RC equalizer to flatten the speaker response somewhat, making for easier interpretations of microphone curves. All tests with this source have been conducted at the 6-foot distance. I had to calibrate with the SPX mike as it has a figure-eight pattern, which cancels potentially troublesome reflections from the ground, just as the shotgun mike does.
Figure 2 shows the axial response of the SM89. Note that the sensitivity is only 1 dB lower than the specified value! I trust that this reflects excellence in manufacturing as well as accuracy of measurements. The slight rising response above 3 kHz is desirable in long-distance sound pickup to offset losses due to absorption by room materials and air.
The low-frequency response below 60 Hz is flatter than Shure shows and would seem to indicate that the 60-Hz cutoff filter was not working when the switch was set to "flat." This is uncertain, as spherical-wave conditions below 60 Hz could have boosted the bass response. The roll-off setting does provide a rapid cutoff below 160 Hz, as specified. This should reduce low-frequency noise and reverberant bass boost in rooms without affecting speech quality.
Figure 3 shows just the front response curves of the SM89. The differences between the 0° and 30° curves are actually less than those seen in Shulman's article. However, the difference is 9 dB at the highest speech frequency, 6 kHz, and I was curious as to why Shure feels that these results represent uniform front response characteristics. For comparison, I measured the only shotgun on hand, a con temporary Japanese shotgun mike that is battery-powered and sells for perhaps one-third the cost of the Shure. On axis, its response rose steeply above 1 kHz, and at 30° off axis, the curve went downhill rapidly, developing a hole 25 dB deep at 8 kHz. I also noted in my September 1978 review of the Nakamichi CM-700 that its response with its shotgun attachment fell steeply above 7 kHz at 0° and at 30° fell rapidly above 4 kHz. All of this leads me to the conclusion that the test data on the SM89 probably represent adequate uniformity of frontal response. It is remarkable that Shure has been able to achieve good response to 15 kHz with the interference tube as an obstacle in front of the diaphragm. This is the first shotgun mike I have tested whose response seems suitable for music pickup.
Fig. 2--On-axis frequency response and sensitivity. (The 0-dB point is 54
dB below 1 N./u bar.)
Fig. 3--Frequency response vs. angle for sounds originating in front of the
microphone.
Fig. 4--Frequency response vs. angle for sounds from the side and rear.
Fig. 5--Third-octave noise spectra of SM89 plus PS1 power supply. (The 0-dB
point is 20 µPa.)
The curves for side and rear frequency response are shown in Fig. 4. In this region, a desirable goal would be 15 dB (or more) rejection of sound, as compared to the axial reference curve. I measured the SM89's 90° rejection as being less than 15 dB. However, reflections in the test setup can affect the results, and when 90° rejection is measured in my setup, the microphone faces a building that reflects sound. I note that in my 1978 review of the Nakamichi mike, the 90° rejection was also a little less than 15 dB. At low frequencies, I would accept as little as 10 dB of rejection, but the 180° rejection of the SM89 is less than 10 dB below 250 Hz. Shulman's test data show a similar trend at 180°, so my results are probably indicative of this mike's hypercardioid-style rear lobe. (I note that both the Nakamichi shot gun mike reviewed in 1978 and the contemporary shotgun mike used as a comparison here show 10 dB or more of rejection in this range. Thus, I do not think the test setup suffers from a reflection.) This is a minor deficiency in the SM89, as the results for rejection at 90° and 135° are acceptable. The 180° rejection is of less importance. If you visualize a microphone's directional pattern in three dimensions, you'll see that while all points 90° and 135° off axis form circles, 180° is just a single point, directly to rear of the Microphone.
The noise of the microphone and power supply was measured as described in the September 1978 issue. The microphone was put in a sound attenuating box, which in turn was placed in my quiet listening room (sound level less than 30 dBA). In this test, the isolation transformer had to be used, as my ultra-low-noise amplifier (the old vacuum-tube RCA OP-6 broadcast remote amplifier) has a grounded center tap. Figure 5, the third-octave spectrum, shows small peaks at the power-line frequency of 60 Hz and its harmonics, which were much attenuated by the A-weighted filter used for overall level measurement. Otherwise, the noise spectrum of the SM89 is uniform with frequency. The overall noise level was 17 dBA (equivalent SPL re: 20 1.153a), which is incredibly close to the specified nominal value of 16 dBA. With no weighting (flat meter response), the noise was a quite acceptable 22 dB.
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Historical Notes on Shotgun Mikes
The earliest commercial shotgun microphone I am aware of was introduced in 1937. It was 5 feet long, "in the form of an acoustic impedance element designed for easy attachment to the Western Electric 618A or 630A moving-coil microphone,", according to W. P. Mason and R. N. Marshall of Bell Labs ("A Tubular Directional Micro phone," Journal of the Acoustical Society of America, January 1939).
Dr. Harry Olson of RCA developed an experimental mike, 10 feet long and with five ribbon elements, at about the same time. In 1940, L. J. Anderson developed the RCA MI-3042, a 6-foot shotgun with multiple moving-coil transducers. Les Anderson recently told me that although the Olson mike was never produced, about 10 of the 3042s were manufactured. One 3042 was successfully used for long-distance pickup at a political convention.
These early mikes used multiple pipes and were referred to as rifle or gun microphones.
Because they are used relatively far from the source, shotgun mikes need to have high sensitivity. One way to achieve it is with condenser elements, as used in RCA's Varidirectional con denser mike, introduced in the 1950s, which had a detachable line consisting of many small tubes of varying length.
While moviemakers were one of the prime markets for which shotguns were designed, the Varidirectional was not very popular with them. Olson, inventor of this microphone, told me that it was "a law of physics" that a condenser microphone would, at some point, emit a pop sound. He indicated that the film people were afraid such an inevitable pop would occur during a long dialog scene.
The popular Electro-Voice 642, produced about the same time, used a dynamic element with heavy magnets to achieve high output. However, the weight of this mike made it difficult to use on a boom or pole.
Apparently, the early mikes were not successful because they were too large and heavy to use on a crowded movie set. The Shure SM89's half-meter length and its light weight are more suited to modern television and movie studio use, as well as for amateur and professional use with video or audio equipment.
-J.R.S.
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Use and Listening Tests
With some advice from the editor, I prepared a list of applications that Audio readers might have for a shotgun mike. In some of these cases, the mike would likely be used with a video recorder. The applications list includes: Re cording music at long distances in auditoriums; recording indoor or outdoor events (a speech or concert); recording outdoor sounds (such as birds, animals, trains, or planes), and sound amplification of theatrical events at schools and churches.
To check out the mike's general characteristics before doing any serious taping, I set up the SM89 and a Shure mixer outdoors, monitoring the output through an amplifier and a pair of noise-excluding circumaural headphones. My nearby sound source was a five-year-old child, and there was plenty of ambient noise from traffic and other children.
(There weren't any birds around-it was winter.) With the child 25 feet away and the mike pointed at him, his voice was heard clearly, with excellent quality, and was well above the ambient noise. At 30° off axis, sound quality and S/N were both good, supporting Shure's claims and my test data. When I pointed the mike 90° off axis from the child, I could only hear echoes of his voice from the building the mike was aimed at, plus ambient noise, Rejection of the sound source was good all over the rear hemisphere (90° to 270°). This simple set of tests indicated that the SM89 has good sound quality throughout its 60° angle of acceptance and that it has adequate rejection of off-axis reflections and unwanted noise.
The high output of the mike was astonishing-the mixer gain had to be reduced to a low setting for loud speech at 25 feet! At 200 feet, I was able to hear children's (raised) voices fairly well above the ambient noise. The windscreen prevented noise from the 10- to 15-mph wind, and fast motion of the microphone did not cause any wind noise.
I recorded a concert with pipe organ, synthesizer, and piano in the United Methodist Church in Haddonfield, N.J.
The SM89 was located at the rear balcony rail, which is 100 feet from the synthesizer and piano and about 120 feet from the organ pipes. Video cameras are frequently set up here, and videophiles report horribly reverberant audio using their camera-mounted mikes. I usually provide audio from a Soundcraft mixing board to the video recorders, derived from a permanently flown AKG C-422 stereo microphone, 20 feet overhead.
For this concert, the AKG was set for figure-eight patterns, and its capsules were angled 90° apart. The SM89 was set for flat response and connected directly to an Aiwa F990 cassette recorder. A simultaneous Beta Hi-Fi videotape was recorded from the console. I did not expect very good results from any mike at the balcony location, but the SM89 recording surprised me. The audio was fairly comparable to the AKG's with respect to high and mid-frequencies and reverberant quality. The principal difference was heavy and almost subterranean bass from the SM89 on pedal notes. Reasons for the bass boost include reverberant sound buildup at the long distance and reduced directivity of a shotgun at low frequencies. This is easily remedied by rolling off some of the extra bass with equalization in the recording mixer or by setting the SM89's filter switch to its roll-off position.
---------- A89SM accessory shock mount
Next, an SM89, along with several old Nakamichi CP-703 shotguns, was set up in a high school auditorium for rein forcing a show. In this case, the mikes were aimed upward from the orchestra pit, and the reinforcement speaker over head turned out to be only 60° off the microphone axes. In this test, all of the mikes gave useful reinforcement with adequate audio quality, but aiming was less critical with the SM89. Because the Nakamichi mikes had a narrower acceptance angle, they allowed more gain before the onset of feedback than the Shure did. To get full coverage of the show, the narrow pattern of the Nakamichis had to be overcome by using several of these low-cost mikes. Had SM89s been used instead, their broader pattern would have permitted fewer microphones to be used.
I highly recommend using Shure's accessory shock mount. It includes two of Shure's "half mounts" (which are the best mounts I've found for vocal mikes on stands) plus an all-important snubber to prevent cable noise. Prior to receiving the shock mount, I had mounted the SM89 on a desk stand for the concert taping. During the recording, I accidentally hit the table supporting the mike and stand.
The resulting noise was very loud, as the mike was far from the sources of sound and the SPL at the mike was low. I suspect that this would not have happened had the shock mount been attached.
Regarding the possibilities for electrical pop noise, I did not hear a pop in several hours of listening. However, I did not test the microphone under adverse temperature and humidity. These conditions might have increased the chances for electrical popping.
The user may encounter some difficulty in removing the cable plug due to a rubber pad inside the barrel that prevents rattling of the connector when the mike is mounted on a boom.
I conclude that the SM89 is excellent for making high quality recordings of concerts and events, particularly from the frequently unfavorable location of a video camera. Out door sound pickup is no problem, and collectors of nature sounds and other outdoor sounds will appreciate the mike's excellent fidelity and flexible-aiming capability. Because of its light weight, high fidelity, and remote powering, the SM89 can be used advantageously as a permanently flown mike above vocal groups and musicians.
--- Jon R. Sank
Acknowledgements
I am grateful to Les Anderson for providing first-hand information on early shotgun microphone developments; to Jim Webb, Hollywood sound mixer, for digging out some early RCA and Bell Labs literature on shotgun mikes; to George Somes of San Diego, who provided some of that information; to A. J. May for trying the SM89 in the show application, and last (but not least) to my grandson Jonathan Seladones for his tireless efforts in generating sound outdoors.
-J.R.S.
(Source: Audio magazine, Dec. 1990)
Also see:
Shure SM91 Microphone (Equip. Profile, Jun. 1986)
Sennheiser MKH 40 Microphone (Equip. Profile, Jan. 1988)
Sennheiser HD-540 Reference II headphones (AURICLE, Jun. 1992)
Sennheiser Electronic Corporation (ad, Nov. 1984)
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