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Topics on this page: ...are characterized by flat frequency response down to the lowest audible frequencies. This is possible only with condenser microphones. Also, due to their operating principle, there is no low-frequency emphasis (proximity effect) in near-field, close-speech use. For reasons of capsule geometry, the omnidirectional pattern can be maintained in its ideal form only up through the midrange frequencies. At higher frequencies, sounds arriving on axis are progressively emphasized by the interaction of the capsule housing with the shorter wavelengths. The larger the diameter of the housing, the greater the difference in high-frequency response between on-axis and off-axis sound. This effect can be seen clearly in the capsules’ polar diagrams. When the high-frequency emphasis is corrected so that the response measures flat on axis, the result is a pressure transducer type such as the CCM 2. These microphones are ideally suited to picking up acoustic sources in the near field where the lack of brilliance in sound arriving off axis is of less importance. If this type is placed in the reverberant sound field, where reflections from walls, ceiling, floor, etc. predominate, there is a loss of overall brilliance. Most of these sounds, often with their high-frequency content attenuated by surface absorption, reach the microphone at oblique angles of incidence and suffer additional losses as compared with sounds picked up on axis. Here a microphone with some high-frequency emphasis (CCM 2S, CCM 2H, or CCM 3) is required so that at high frequencies there will be balanced sound rather than a roll-off. This, of course, adds brilliance to sounds picked up at close range and on axis, an effect which may be desired in some circumstances. The “ideal” pressure transducer for all situations obviously does not exist. The user must consider the nature of the pickup in order to make an appropriate selection. However, the design of the CCM 2S achieves a technically sophisticated compromise between the requirements of working in the direct and the reverberant sound fields (in the region of the reverberation radius). Particularly for two- and three-microphone stereo pickups, which are usually made near the reverberation radius (where the direct and reverberant sound fields are of equal intensity), the “2S” type has become a
favorite of many sound engineers. This is also true for the “2H” type. The characteristics of the CCM 2H come somewhat closer to those of the free-field models CCM 2. Schoeps makes many different types of directional capsules and microphones, each having specific features and a range of typical applications. As can be seen by examining their polar patterns, all have in common that their sensitivity depends on the direction of sound incidence. As a result they may be placed at a greater distance from the sound source than an omnidirectional microphone of equivalent sensitivity, while still picking up the same balance of direct and reverberant sound. The bidirectional CCM 8 is a pure pressure-gradient transducer. Our other directional microphones use combinations of the pressure and pressure-gradient principles; their various directional characteristics result from differing proportions of these ingredients. All our microphones, including the multi-pattern ones, are single-diaphragm - a feature unique to Schoeps. This results in polar patterns that are less frequency-dependent than any dual-diaphragm design can offer, as well as low-frequency response (with our single-pattern omnidirectional capsules or in the omnidirectional setting of our multi-pattern capsules) that is essentially perfect. One advantage of small pressure-gradient transducers is that their directional pattern can be kept constant across a wider frequency range than with a pressure transducer. On the other hand, their low-frequency response in a free sound field is not as extended as that of a pressure transducer. Placement in the near field can compensate for this bass roll-off, but there is also a risk of overcompensation. Proximity effect may also be used to suppress environmental noise by choosing a microphone type having a large bass roll-off (and/or by the use of a corresponding electronic filter). A cardioid microphone at a distance of less than 40 cm, for example, will pick up a speaking voice quite clearly, while environmental noise will be suppressed due to the directivity of the cardioid pattern and its bass roll-off. At the same time, the lower frequencies in a person’s voice will be restored to normal by virtue of the proximity effect, resulting in a clear and full sound. By choosing a microphone of high directivity it is also possible to avoid acoustic feedback. If a loudspeaker is set up within the reverberation radius, it should, for obvious reasons, be positioned where the microphone has its minimum sensitivity. If the loudspeaker is beyond the reverberation radius, its radiated sound will reach the microphone after being reflected by the walls, floor and ceiling of the room, arriving as reverberant sound from many directions. The microphone will pick this up less strongly than the direct sound from the source on the main axis. The off-axis attenuation increases with the directivity (random energy efficiency) of the microphone. The greater this is, the less danger there will be of acoustic feedback. When dealing with pressure-gradient transducers, their greater sensitivity to wind and vibration should be kept in mind. It is highly recommended to use a suspension that damps solid-borne sound (elastic suspension and/or sound-isolated stands), as well as popscreens and windscreens wherever appropriate. Note: Usually, any directional microphone is referred to as a “pressure-gradient transducer” even when it has only a certain pressure-gradient component (e.g. a
cardioid). This is not quite correct, since a pure pressure-gradient transducer necessarily has a figure-8 characteristic. Nevertheless, we have adopted this common practice. Certain types of recording are frequently made with “shotgun” microphones. In some particular situations, as when recording outdoors from a distance, shotgun microphones can offer certain advantages over other directional microphones such as the supercardioid CCM 41. But one should not be deceived: at low and middle frequencies, most “shotgun” microphones are no more directional than a supercardioid. Especially when used at close range, the directional pattern of a long tube is far from constant across the audio frequency spectrum. Instead, beginning at some frequency (which depends on the length of the tube), the directional effect will begin to increase. This results in a very undesirable and unnatural coloration of any sound arriving from off-axis. As a consequence, the microphone must be precisely aimed at the sound source at all times–which is not always simple if, for example, an actor moves rapidly. But even if one actor can be followed with complete success, the pickup of other actors nearby and the sound of the surrounding environment will be marked by false coloration. For indoor settings there are additional reasons to avoid using shotgun microphones. First, there is an acoustic consideration: at some definite distance–usually just a few meters–from any sound source, the diffuse, reflected sound energy will equal the direct sound energy. That distance is the reverberation radius. For acoustic reasons, shotgun microphones are less effective when used beyond the reverberation radius of a sound source. Second, comb-filter-like effects can result from motion of either the sound source or the microphone, because of the rear and (multiple, irregular) side lobes of the microphone. Third, low ceilings can often pose problems for long microphones. Stereophonic recording with shotgun microphones is also fraught with compromise because of the irregular polar patterns at different frequencies. And last but not least , shotgun microphones require large, and therefore rather heavy, windscreens. These can place a high “wind load” on a boom. For these reasons the user should consider carefully whether a supercardioid might be a preferred alternative. A trial comparison between a supercardioid and a shotgun is often quite surprising and enlightening. Schoeps have introduced a ‘shotgun’ type microphone that uniquely addresses many of the problems highlighted above. For details see the CMIT 5 U. Schoeps is often asked to recommend a microphone for a particular instrument or application. Some microphone manufacturers readily answer such requests. That might make sense if their microphones have a frequency response that is tailored to the characteristic sound of a given instrument. But the possible applications for such microphones would then be restricted significantly. In Schoeps opinion a good microphone ought to sound natural, and thus should be suitable for any instrument. This requires flat frequency response and a directional characteristic that is independent of frequency; then there will be no difference in sound quality whether the pickup is on- or off-axis. Obviously this ideal can be achieved only to a finite degree. With directional microphones, proximity effect affects low-frequency response significantly; with omnidirectional microphones the polar pattern is rarely ideal at the highest frequencies. Only in rare cases can the “correct” microphone be chosen unequivocally, since one must also consider the aspects of taste, recording location, position of sound sources and the microphone, and the atmosphere of the music or other program material. Any absolute prescriptions would thus be of limited value at best. However, Schoeps would like to offer some ideas to help orient the choice that must be made. The microphone type coming closest to the theoretical ideal is the omni. However, in practice the most commonly used microphone at medium distances is the cardioid CCM 4V. Thus the user should regard this as a starting point, but then ask whether there might be grounds for a different choice. These could include:
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