Manufacturer's Specifications
Directional Pattern: Cardioid.
Acoustic Operating Principle: Symmetrical pressure-gradient.
Transducer Type: Condenser.
Frequency Response: 40 Hz to 20 khz
Sensitivity (Open-Circuit Voltage): 25 mV/Pa (8 mV/Pa with 10-dB attenuator), ± 1 dB.
Nominal Output Impedance: 150 ohms, balanced.
Minimum Load Impedance: 1 kilohm. 600 ohms minimum to 130 dB SPL (140 dB SPL with attenuator).
Equivalent SPL: 12 dBA rms (16 dBA with attenuator) per DIN 45 500: 21 dBA peak (26 dBA with attenuator) per CCIR 468.
Maximum SPL for Less than 0.5% THD at 1 kHz: 134 dB (142 dB with attenuator).
-3 dB Roll off: 40 Hz, 12 dB/octave with low-cut filter off: 120 Hz, 6 dB/ octave with low-cut filter on.
Power Requirements: 48 V, ± 4 V, 2 mA: phantom powering as per DIN 45-596.
Power-Supply Options: MZN 16 P48U a.c. supply for two microphones: MZA 16-P48U battery supply for one microphone, with switchable 20-dB pads and 20- (flat). 80-, and 140-Hz filters.
Supplied Accessories: MZQ stand holder: MZW 41 wind and pop screen: foam-lined carrying case.
Dimensions: 1 in. diameter x 5 1/8 in. L (2.5 cm x 15 cm).
Weight: Approximately 31/, oz. (100 grams).
Prices: Microphone, $839: MZN 16 P48U a.c. supply, $345: MZA 16 P48U battery supply, $339.
Company Address: P.O. Box 987, Old Lyme, Conn. 06371.
Sennheiser, a German firm, manufactures a wide variety of dynamic and condenser microphones, headphones, wireless microphone and headphone systems, and related electronics. The designation "MKH" refers to their line of studio condenser microphones, each of which employs what the company calls the "RF Principle." Instead of applying a high d.c. voltage to the capsule, about 10 V of 8-MHz r.f. bias is applied from a low-noise oscillator. The capacitance variations of the capsule modulate the oscillator, and this modulation is translated into an audio signal. Since the capsule is a capacitor, the impedance of its 8-MHz output is low compared to the audio-frequency output of an equivalent d.c.-biased microphone. Sennheiser says that this renders the MKH microphones less susceptible to popping and sputtering noises caused by high humidity, thus making them suitable for outdoor use. The MKH line includes an omnidirectional, a figure-eight, and a cardioid mike, short and long shotgun models, and a special-purpose omni unit which features flat response down to 0.1 Hz. All of these microphones, except for the last, feature very low noise levels.
I've had some previous experience with the MKH mikes; it came in 1980, when I was refurbishing the 1970-vintage RCA console in the Grand Tier broadcast booth at New York's Metropolitan Opera House. The console inputs came from a number of short MKH-416 shotgun mikes hung 40 feet above the orchestra and stage. (This was low, com pared to the reported 100-foot proscenium height!) The MKH shotgun mikes yielded excellent audio in spite of their long distance from the performers, and the very long cables between the mikes and the power supplies in the booth did not impair that quality.
The appearance of the MKH 40 is best described as "understated." The satin-black cylindrical housing is quite plain, save for the flat surfaces along two-thirds of its length. These surfaces serve the practical functions of providing secure mating to the swivel mount and acting as a "panel" for the 120-Hz filter and output attenuator switches. The microphone is very lightweight compared to many other professional condenser microphones.
The specifications for the MKH 40 indicate that it has a very wide dynamic range, uniform frequency response at all useful pickup angles (e.g., a polar pattern independent of frequency), and low distortion. According to the published frequency response curves, it also is flatter than the MKH cardioid that came before it (MKH-406) and has lower noise. It is recommended by the manufacturer for digital recording and, like the MKH-406, is said to be appropriate for outdoor as well as indoor use.
The MKH 40 requires external phantom powering; not wanting to gamble on introducing noise from a lab power supply, I opted for the battery supply, MZA 16-P48U. This was a good choice because it includes a 20-dB attenuator and response-shaping filters-flat (20 Hz) plus 80 and 140 Hz positions-which supplement those on the mike. The power-supply switches offer flexibility in recording and are an advantage in any live concert setting where you can't access the mike to make changes. At $678 for a pair, these battery supplies are a bargain, at least compared to one that I've tested which cost in excess of $1,000.
Measurements
Fig. 1--Impedance vs. frequency. Top three curves show effects of power-supply
filter switches; actual impedances are approximately 100 ohms less than shown
due to use of isolation transformer in this test. Bottom curve, made direct
into test gear with power-supply filters off, is same for all mike filter and
attenuator switch positions.
Fig. 2--On-axis frequency response at three distances with microphone roll-off
and power-supply filters switched out. (0 dB =-36 dB re: 1 V Pa)
Fig. 3--On-axis frequency response at infinite distance for different filter
settings. Microphone roll-off as noted.
Readers of my previous reviews will recall that my tests include measurements of impedance, frequency response (both axial and at various angles off-axis), and (for condensers) noise level. With all of the switch selections available on this microphone and power supply, it was not practical to conduct all of these tests for every switch setting. Therefore, I planned the tests so as to sample all settings but minimize the total number of tests.
The curves of impedance versus frequency, in Fig. 1, give some idea of the maze that would result from trying every switch setting for every measurement. The bottom curve is the impedance of the direct output from the battery power supply into an unbalanced test circuit, with the supply's filter switched out (that is, with the switch at its "20 Hz" setting). All possible settings of the microphone's filter and attenuator switches were checked, with no effect on the result.
Impedance at 1 kHz remained near the rated value of 50 ohms, and the low-frequency peak remained at 125 Hz. This peak will not cause appreciable loading with most low-impedance inputs rated at 150 to 250 ohms.
For the upper three curves, which show the effects on impedance of changing the power-supply filter settings, a transformer was used for isolation from the test jig. (This was done because the instructions for the power supply warn against unbalancing the output, as was done for the bottom curve, even though no error was apparent.) The transformer added about 100 ohms to the impedance values, and this should be mentally subtracted from the values shown.
Unlike the filter and attenuator in the microphone, the filters in the battery supply did cause a considerable change in impedance at low frequencies. However, the maximum value probably remains below about 400 ohms, which most microphone inputs can easily tolerate.
Next, I measured on-axis frequency response, with all filters and attenuators switched off, at three distances from my sound source (Fig. 2). Readers of past reviews will recall that this unique source, whose driver has a 2-inch-diameter piston, can be used for accurate measurements from 30 Hz to 20 kHz at distances as close as 6 inches. These response curves reveal proximity effects which closely match those of an ideal cardioid microphone. The low-frequency response is probably flat above 30 Hz at some distance between 1 foot and infinity (1 meter would be a good estimate). Thus, when the MKH 40 is used at any distance closer than 3 or 4 feet, the bass boost should be equalized out, unless you desire it as a special effect.
The next test measured the effects of the various filters upon axial frequency response at infinite source distance (Fig. 3). This data will assist in selecting the desired filter when the microphone is used close to the source or when noise attenuation is desired. Unfortunately, the filters are not ideal for proximity-effect reduction since the curves in Fig. 2 are not precisely the inverse of the curves in Fig. 1. My recommendation is to use the mike's roll-off filter and switch the power supply (if used) to 20 Hz for distances of 1 or 2 feet, and to use external equalization for closer distances.
The 80- and 140-Hz settings will be more useful for noise reduction when recording sound sources that have no low frequencies. Note the significant attenuation at the high frequency end of Fig. 3 when the foam windscreen is in place. This windscreen should not be used without some thought as to its sonic effect.
The axial frequency response curves show a rise above 1 kHz; this rise reaches 3.5 dB in the region from 4 to 6 kHz.
The curve given on the microphone's catalog sheet indicates a flat trend with a ±2 dB tolerance envelope. In contrast, the instruction booklet accompanying the micro phone shows a curve which rises about 2 dB and a tolerance envelope that extends to +4 dB. I assume that the information in the instruction booklet is more accurate.
The sensitivity, with the attenuators in the microphone and power supply switched out, is-36 dBV/Pa. My rule of thumb is that an unamplified microphone should have a sensitivity of at least-60 dBV/Pa so that contemporary mixing boards will not add noise to its signal. Thus, the MKH 40 has more than 20 dB of additional output available, which may be used to advantage in picking up low-level sounds. With loud sounds, however, such as full orchestra at loudest level or closely miked instruments, it will probably be necessary to switch in the attenuators on the micro phone, power supply, or mixing board. According to my test results, these attenuators do not introduce any frequency discrimination.
Fig. 4--Frequency response vs. angle for plane wave, with microphone roll-off
and power-supply filters switched out.
Fig. 5--Noise spectrum (third-octave bands) with microphone attenuator, roll-off,
and power-supply filter switched out.
The polar pattern was assumed to be the same in all planes of rotation, and one set of curves was drawn to represent the entire family of polar responses. The pattern was also assumed to be symmetrical in the half-plane not tested. Figure 4 shows the results: The polar pattern is quite uniform from 0° to 90° at 10 kHz. It is probably uniform out to 15 kHz at 60°, so an X-Y stereo pair angled at the recommended 120° will have uniform frequency response for all sources within its included angle. The rejection at 135°, about 14 dB at 1 kHz, is good. The rejection at 180° is acceptable, averaging about 15 dB. The manufacturer's data in the form of a polar pattern indicates about 14 dB rejection at 135° and more than 25 dB rejection at 180° up to 1 kHz. I do not count this as an important difference be cause the measurement is difficult, and significant differences between laboratories are possible.
The noise test revealed a remarkable feature of this micro phone: It can pick up sounds that cannot be heard by the unaided ear! Figure 5 shows the equivalent sound pressure level of the microphone's own noise, measured via 1/3-octave band filters, with the microphone in a sound-retardant box. The measurement was troubled by a strange lump at 75 Hz, yet I could hear no sound outside of the box at such a low frequency and level. By listening to the input and output of the filter in this band, I deduced that the lump was the sound of distant traffic, "tuned" by the natural resonance of my frame building. The SPL of this sound was 5 dB, which is 25 dB below the normal threshold of hearing at this frequency! The overall noise level was in agreement with the rated 12 dBA. The MKH 40 is therefore well suited to pickup of musical sources at low sound levels, but a very quiet location is needed. In many locations, even seemingly quiet studios, I expect this mike will reveal noise.
I found that the 9-V alkaline battery in the power supply measured 7.69 V after 12 hours of continuous operation.
Rated battery life is 20 hours, and the battery test light is calibrated at 6.5 V. This compares to 200 hours for a consumer-type electret mike with integral battery. To be safe, a fresh battery should be installed prior to any critical recording session.
Use and Listening Tests
Using my low-noise microphone amplifier, I compared the Sennheiser MKH 40 to another condenser microphone with a wide frequency range, a Nakamichi CM-700 electret. The CM-700, a premium consumer-grade microphone, sounded very hissy in a quiet room, whereas the MKH 40 reproduced the room noise with no audible hiss. Very impressive! Clipping level with close, loud speech was seen on a
'scope to be approximately 134 dB, as rated. Vibration noise pickup was less than with the CM-700, and magnetic hum pickup was lower too. On the other hand, the MKH 40 had somewhat more "pop" sensitivity with its windscreen mounted than the Nakamichi did. With voice as a source, the Sennheiser sounded identical to the Nakamichi at angles from 0° to 135°. This was quite remarkable, as the Nakamichi has exceptionally uniform polar response because of its small diameter (16 mm, compared to 25 mm for the Sennheiser).
I loaned the Sennheiser mike and battery supply to my associate Jack Shaw, who used them in recording piano and voice on videotape for a cable TV show. He reported that the resulting tape sounded "clearer, cleaner, and had less room noise than a tape made with the Nakamichi." However, after a few selections were recorded, the battery supply became noisy, and he had to switch to the Nakamichi. Later, I sprayed the battery contacts with contact cleaner, and the noise ceased I had the good luck to be able to record a concert which featured 18 brass and percussion players from the Philadelphia Orchestra, plus pipe organ and large choir. The con cert took place in the United Methodist Church in Haddon field, N.J. An AKG C-422 poly-directional stereo condenser microphone is permanently flown about 16 feet above the place where musicians perform in this church. In order to make a good comparison between the Sennheiser and AKG mikes, I mounted an X-Y pair of MKH 40s on a tall Shure S 15 stand, which raised them to a level only about 2 feet below the C-422.
I had to position the Sennheisers for an included angle of 90° to match that of the C-422. The AKG, which is normally set for figure-eight (Blumlein) stereo, had to be set for cardioid patterns to match the Sennheisers. There was thus a potential sacrifice in stereo perspective, because the included angles were less than the usual 120° recommended for crossed cardioids.
Not having a 4-track recorder on which I could tape the outputs of these mikes simultaneously, I used two cassette machines from Aiwa, the AD-F990 and the AD-3700, with metal-particle tape and Dolby C noise reduction. Because the 990 has the HX-Pro headroom-extension system but the 3700 does not, I switched the units at intermission, to record with each mike on each machine.
The tapes were auditioned on two systems. One, in my home listening room, includes modified and equalized Altec 604C duplex studio monitors housed in large sealed boxes.
In audio quality and stereo perspective, the tapes made with the MKH 40 mikes sounded almost identical to those recorded with the C-422. The bass notes of the organ sounded exactly the same on each tape, proving to me that the slight measured roll-off the Sennheiser is insignificant.
The C-422's response is very uniform from 20 Hz to 20 kHz.
The sound of trumpets and cymbals was slightly brighter with the Sennheiser, indicating that the measured brightness was not an artifact. I should note that some commercial recordings do sound a little bright on my system, where as the many recordings I've made using the AKG sound neutral.
The second listening test was conducted in the living room of my friend Walter Harris, using three Klipsch horns. (Yes, I did say three.) Mr. Harris is an engineer who has known Paul Klipsch since PWK made his first horns 43 years ago. I was treated to a videotape of a visit to the newly assembled Klipsch Museum, conducted by the master him self, but that is another story. The Klipschorn was reviewed by Richard Heyser in the November 1986 issue, and a visit with Klipsch was described in the August 1980 issue. The center horn of Mr. Harris' system is a model which does not require placement in a corner, whereas the left and right horns are the corner types described by Heyser. In keeping with Heyser's results, I found that the sound of tympani was better on the horns than on my direct-radiating 15-inch speakers and that it held up very well at higher playback levels (100+ dB). At first, the stereo perspectives seemed astoundingly wide, because the speakers, as recommended by Klipsch, were set along the long wall of the room. The speakers subtended a 90° angle at the listening position, compared to 60° in my own listening room. However, a very even spread of sound was maintained.
In the bass, both microphones sounded identical. In the treble, the MKH 40 sounded neutral, whereas the C-422 sounded a little dull; trumpets and cymbals sounded brighter with the Sennheiser. Referring to Fig. 4 in Heyser's Klipschorn review, I see a gentle downward trend in the speaker's response from 1 to 15 kHz, as compared to my Altecs, which are equalized to measure flat to 10 kHz and to be 5 dB down at 15 kHz. Obviously, Klipsch has adjusted the response to sound well balanced with commercial re corded material. I confirmed this by playing a few CDs. This leads to the conclusion that recordings made with the Sennheiser will have response balance comparable to that of commercial material, without additional EQ.
At the risk of repeating a statement from a previous review, I find that an X-Y pair of MKH 40 microphones is an excellent portable substitute for the reference AKG, which is more often permanently flown than used on stands, and which costs more. The Sennheiser's 122-dB dynamic range makes it suitable for use with digital or dbx-encoded analog recording systems.
-Jon R. Sank
(Source: Audio magazine, Jan. 1988)
Also see:
Sennheiser HD-540 Reference II headphones (AURICLE, Jun. 1992)
Sennheiser Electronic Corporation (ad, Nov. 1984)
Beyerdynamic DT-911 headphones--review, teardown and analysis (Jan. 1993)
Shure SM91 Microphone (Jun. 1986)
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