collecting sound
Apr 1, 1999 12:00 PM, Bruce Bartlett
A mic is a transducer, a device that changes one form of energy into another. Specifically, a mic changes sound into an electrical signal. A mic can be grouped into three types depending on its specific process for converting sound to electricity-dynamic, ribbon or condenser. A dynamic mic capsule, or transducer, is shown in Figure 1 along with condenser and ribbon mics. In a dynamic mic, a coil of wire attached to a diaphragm is suspended in a magnetic field. When sound waves vibrate the diaphragm, the coil vibrates in the magnetic field and generates an electrical signal similar to the incoming sound wave. Another name for a dynamic mic is a moving-coil mic, but this term is seldom used. Dynamic mics tend to have rougher, although quite usable, responses. They are rugged and reliable, tolerate heat, cold and high humidity and can handle high volume without distortion. Dynamics are preferred for guitar amps and drums, and those with a flat response can take the edge off woodwind and brass instruments.
In a ribbon mic capsule, a thin metal foil or ribbon is suspended in a magnetic field. Sound waves vibrate the ribbon in the field and generate an electrical signal. Ribbon mics are generally prized for their warm, smooth tone quality, but they can be rather delicate. The element is a piece of suspended metal film that can be easily damaged.
A condenser or capacitor mic capsule has a conductive diaphragm and a metal backplate placed close together, both of which are charged with static electricity to form two plates of a capacitor. When sound waves strike the diaphragm, it vibrates, which varies the spacing between the plates. In turn, this varies the capacitance and generates a signal similar to the incoming sound wave. Two types of condenser mics are the true condenser and electret condenser. In a true condenser mic (externally biased mic), the diaphragm and backplate are charged with a voltage from a circuit built into the mic. In an electret condenser mic, the diaphragm and backplate are charged by an electret material in the diaphragm or on the backplate. Electrets and true condensers can sound equally good, although some engineers prefer the more costly true condensers. Condenser mics usually feature wide, smooth frequency response with detailed sound and extended highs, and omnidirectionals have excellent low-frequency response. Transient attacks sound sharp and clear, and the condenser mic is often preferred for acoustic instruments and cymbals. Because of its lower diaphragm mass and higher damping, a condenser mic responds faster than a dynamic mic to rapidly changing sound waves (transients). This mic can also be miniaturized.
A condenser mic needs a power supply to operate, such as a battery or phantom power supply. Phantom power is 12 VDC to 48 VDC applied to pins 2 and 3 of the mic connector through two equal resistors. The mic receives phantom power and sends audio signals on the same two conductors. Many mixing consoles supply phantom power at their mic input connectors. Simply plug the mic into the mixer to power it.
Dynamics and ribbons, however, need no power supply. You can plug these types of mics into a phantom supply without damage, unless either signal conductor is accidentally shorted to the mic housing.
Admittedly, there are exceptions to a given mic type's general tendencies. Some dynamics have a smooth, wide-range frequency response. Some condensers are rugged and handle high SPLs. Specs of the particular mic are variable, even within a given mic type.
Polar pattern Mics also differ in the way they respond to sounds coming from different directions. An omnidirectional mic is equally sensitive to sounds arriving from all directions. A unidirectional mic is most sensitive to sound arriving from one direction-in front of the mic-and softens sounds entering the sides or rear of the mic. A bidirectional mic is most sensitive to sounds arriving from two directions-in front of and behind the mic-but rejects sounds entering from the sides. Figure 2 shows various polar patterns. A mic's polar pattern is a graph of its sensitivity vs. the angle at which sound comes into it drawn on polar graph paper. Sensitivity is plotted as distance from the origin.
There are three types of unidirectional patterns-cardioid, supercardioid and hypercardioid. A mic with a cardioid pattern is sensitive to sounds arriving from a broad angle in front of the mic. It is about 6 dB less sensitive at the sides and about 15 dB to 25 dB less sensitive in the rear. The supercardioid pattern is 8.7 dB less sensitive at the sides and has two areas of least pick up at 125degrees away from the front. The hypercardioid pattern is 12 dB less sensitive at the sides and has two areas of least pick up at 110degrees away from the front. The supercardioid and hypercardioid reject sound from the sides more than the cardioid, but they pick up more sound from the rear than the cardioid.
Omnidirectional mics feature all-around pick up, and consequently, they pick up the most room reverberation. They do not offer much isolation unless you mic close. They have a low sensitivity to pop (explosive breath sounds) and a low handling noise. With an omnidrectional mic, there is no up-close bass boost (proximity effect). An extended low-frequency response in condenser omnis makes them great for the pipe organ or bass drum in an orchestra or symphonic band. Generally lower in cost, they are acceptable when feedback is not a problem.
Unidirectional mics (cardioid, super-cardioid and hypercardioid), on the other hand, have selective pickup and reject room acoustics, background noise and leakage. Their good isolation yields a high degree of separation between instruments, and they feature up-close bass boost, except in mics with holes in their handles. They provide better gain-before-feedback in a sound-reinforcement system and have coincident or near-coincident stereo micing.
Cardioid, supercardioid and hyper-cardioid polar patterns are somewhat similar. Cardioids support a broad-angle pickup of sources in front of the mic with maximum rejection of sound approaching the rear of the mic. Super-cardioids, however, provide the maximum difference between front- and rear-hemisphere pickup (good for stage-floor applications), with more isolation than a cardioid and less reverb pickup. Hypercardioids have maximum side rejection in a unidirectional mic, with maximum isolation and maximum rejection of reverberation, leakage, feedback and background noise.
Bidirectional mics allow front and rear pickup with side sounds rejected, which is useful for across-table interviews or two-part vocal groups. These mics allow maximum isolation of an orchestral section when placed overhead and can perform Blumlein stereo micing (two bidirectional mics crossed at 90degrees). They are seldom used in installed sound.
A good mic's polar pattern should be about the same from 200 Hz to 10 kHz. Otherwise, you will hear off-axis coloration; the mic will have a different tone quality on and off axis. Small-diaphragm mics tend to have less off-axis coloration than large-diaphragm mics.
Frequency response As with other audio components, a mic's frequency response is the range of frequencies that it will reproduce at an equal level (within a tolerance, such as +/-3 dB). The mic frequency response that is adequate to reproduce the source with high fidelity is 80 Hz to 15 kHz for most musical instruments, 40 Hz to 9 kHz for bass instruments, 80 Hz to 12 kHz for brass instruments and voice, and 40 Hz to 15 kHz for an orchestra or symphonic band. A wider-range response works, too.
If possible, use a mic with a response that rolls off below the lowest fundamental frequency of the sound source. For example, the frequency of the low E-string on an acoustic guitar is about 82 Hz. A mic used on the acoustic guitar should roll off below that frequency to avoid picking up such low-frequency noise as rumble from trucks. Some mics have an integral low-cut switch for this purpose, or you can filter out the unneeded lows at the mixer.
A frequency-response curve is a graph of the mic's output level in decibels at various frequencies. The output level at 1 kHz is placed at the 0 dB line on the graph, and the levels at other frequencies are so many decibels above or below that reference level. The shape of the response curve suggests how the mic sounds at a certain distance from the sound source. If the distance is not specified, it is probably 2 feet to 3 feet (610 mm to 914 mm). For example, a mic with a wide, flat response reproduces the fundamental frequencies and harmonics in the same proportion as the sound source, so a flat-response mic tends to provide accurate, natural reproduction at that distance. Note that mic placement can greatly affect the recorded tone quality. A flat-response mic does not always guarantee a natural sound because mic placement has such a strong influence.
A rising high end or a presence peak around 5 kHz to 10 kHz sounds more crisp and articulate because it emphasizes the higher harmonics (see Figure 3). Sometimes this type of response is called tailored or contoured. It is popular for guitar amps and drums because it adds punch and emphasizes attack. Some mics have switches that alter the frequency response.
Most unidirectional and bidirectional mics boost the bass when used within a few inches of a sound source. You may have heard how the sound gets bassy when a vocalist sings right into the mic. This low-frequency boost related to close mic placement is called the proximity effect, and it is often plotted on the frequency-response graph. Omni mics have no proximity effect; they have the same tonal balance at any distance.
The warmth created by proximity effect adds fullness to drums. In most situations, however, the proximity effect lends an unnatural boomy or bassy sound to the instrument or voice the mic picks up. Some mics-multiple-D or variable-D types-are designed to reduce this low-frequency boost. These types have holes or slots in the mic handle. Some mics have a bass-rolloff switch to compensate for the bass boost, or you can roll off the excess bass with the mixer's EQ until the sound is natural. By doing so, you will also reduce any low-frequency leakage the mic picks up.
Other specs Impedance is the mic's effective output resistance at 1 kHz. A mic impedance between 150 V and 600 V is low; 1,000 V to 4,000 V is medium, and above 25 kV is high. Almost all mics these days are low impedance, which lets you run long mic cables without picking up hum or losing high frequencies. The input impedance of a mixer mic input is about 1,500 V. If it were the same impedance as the mic, about 250 V, the mic would load down when pluged in, which could result in loss of level, distortion or a thin sound. To prevent this, a mic input has an impedance much higher than that of the mic, typically 1,500 V, but it is still considered a low-Z input.
Another important spec is maximum SPL (sound pressure level), and if it is 125 dB SPL, then the mic starts to distort when the sound source generates 125 dB SPL at the mic. A maximum SPL spec of 120 dB is good; 135 dB is superior, and 150 dB is excellent. Dynamic mics tend not to distort, even with loud sounds. Some condensers are just as good, and some have a pad you can switch in to prevent distortion in the mic circuitry. Because a mic pad reduces S/N ratio, use it only if the mic distorts.
Sensitivity tells how much output voltage a mic produces when driven by a certain SPL. A high-sensitivity mic generates a stronger signal (higher voltage) than a low-sensitivity mic when both are exposed to an equally loud sound. Typical sensitivity specs for three transducer types are 5.6 mV/Pa (high sensitivity) for condensers, 1.8 mV/Pa (medium sensitivity) for dynamics and 1.1 mV/Pa (low sensitivity) for ribbons or small dynamics. The louder the sound source, the higher the signal voltage the mic puts out. A louder instrument, such as a kick drum or guitar amp, can cause a mic to generate a signal strong enough to overload a mixer's mic preamp, which is why most mixers have pads or input-gain controls.
Self-noise or equivalent noise level is the electrical noise or hiss a mic produces. It is the dB SPL of a sound source that would produce the same output voltage that the noise does. Usually, the self-noise spec is A-weighted-the noise was measured through a filter that makes the measurement correlate more closely with the annoyance value. The filter rolls off low and high frequencies to simulate the frequency response of the ear. An A-weighted self-noise of 18 dB SPL or less is excellent (quiet); around 28 dB SPL is good, and around 35 dB SPL is fair. Because a dynamic mic has no active electronics to generate noise, it has low self-noise (hiss), so most spec sheets for dynamic mics do not specify self-noise.
The S/N ratio is the difference in decibels between the mic's sensitivity and its self-noise. The higher the SPL of the sound source at the mic, the higher the S/N ratio. Given an SPL of 94 dB, an S/N ratio of 74 dB is excellent; 64 dB is good.
The higher the S/N ratio, the cleaner the signal and the greater is the reach of the mic. Reach is the clear pickup of quiet, distant sounds due to a high S/N ratio. Reach is not specified in data sheets because any mic can pick up a source at any distance if the source is loud enough.
Output level can be mic level or line level. Most mics have a mic-level output (roughly 2 mV), but some have built-in preamps to provide a line-level output (about 1.4 V). They can be plugged directly into VCRs for such applications as security recordings.
Boundary mics Conventional mics are designed to be placed on mic stands, held in the hand or hung from the ceiling. Boundary mics are designed to be used on surfaces. Such mics are often placed on conference tables, taped under piano lids or mounted on stage floors. A boundary mic uses a mini condenser mic capsule mounted near a sound-reflecting plate or boundary. Because of this construction, the mic picks up direct sound and reflected sound at the same time-in-phase at all frequencies-so you get a smooth response that is free of phase cancellations. A conventional mic near a surface sounds colored; a boundary mic on a surface sounds natural.
Unidirectional boundary mics have a half-cardioid or half-supercardioid polar pattern. They work well on a conference table or near the front edge of a stage floor to pick up drama or musicals. Some conference mics of this design have elegant styling and push-to-talk switches. Miniature, unidirectional boundary mics are no bigger than a silver dollar. The electronics for these mics is in a separate chassis under the table.
Theater stage Having established a good foundation of mic knowledge, we will now consider various fixed-installation applications. Plays and musicals in an auditorium are a challenge to pick up and amplify well. First, try to solve the problem at its source. The director should ask the performers to speak loudly and clearly. If the music drowns out the actors, tactfully ask the musical director to have the pit orchestra play more quietly. Good mics can help, too. Three types of mics are used for stage applications: floor mics, hanging mics and miniature wireless mics.
For floor mics, omnidirectional boundary mics work well for area pickup of actors. Such mics are rugged enough to withstand kicks by dancers and can be stepped on without damage. Placed on the stage floor near the footlights, they are almost invisible. You might wonder if the mics pick up footsteps because they are on the floor. Unidirectional boundary mics are not sensitive to floor vibrations, but, like your ears, they do hear footsteps acoustically. This is normally not a problem because the audience associates the sound with the actors walking across the stage. Because of their cardioid or supercardioid polar pattern, floor mics reduce feedback and attenuate the pit orchestra. Typically, space three floor mics evenly near the edge of the stage (see Figure 4). Place them as closely as possible toward the actors. One or two mics might be enough for a small stage.
The more mics that are on, the muddier the sound, and the poorer the gain before feedback will be. To aid clarity and reduce feedback, have the sound person turn up as few mics as possible. For example, suppose an actor walks across the stage from left to right while talking. The left mic should be turned up first, then turned down while turning up the center mic. Next, the center mic sould be turned down while turning up the right mic. The sound mixer should follow cues in the script to know when to turn various mics up and down.
Hanging mics are sometimes used because of the limitations of floor mics, which can typically reach about 20 feet (6.1 m). If it is hard to hear actors farther upstage, hang one to three mini cardioid mics upstage or tape omnidirectional boundary mics to the set.
The preferred way to pick up the main actors on stage is with lavalier wireless mics. You can clip a miniature omnidirectional mic on clothing or hang it over the actor's forehead with the cable running through the hair. To prevent sweat damage, you might want to cover the mic in a thin plastic sandwich bag or plastic wrap. The plastic should be loose, not stretched taut. Wire on the appropriate connector, and plug the mic into a belt-pack transmitter of your choice.
Because the mic is close to the actor, the sound will be louder and clearer than with a floor mic. If the budget permits, use mini wireless mics for the main actors and floor mics for group pickup. Adjust the audio trim pot in each transmitter as high as possible without distortion when the actor is yelling.
Lecterns and pulpits Two ways to pick up speeches at a lectern are with a gooseneck mic or with a clip-on mini mic (lavalier). Current gooseneck mics have a slim, elegant design that does not detract from the person speaking. The goosenecks adjust without creaking. Some models plug into a female XLR-type chassis mount connector in the lectern; others have circuit modules with screw terminal connectors. Need extra ruggedness? One model has a ball-and-socket swivel instead of a gooseneck. The swivel operates silently.
Have users talk about 8 inches (203 mm) away and over the mic to prevent breath pops. Consider using a lectern-mounted shock mount to reduce lectern thumps. Some lectern mics have an internal shock mount. If the person talking wanders around while speaking, a miniature omnidirectional mic will need to be clipped to his clothing. Do not have the gooseneck mic and the lavalier mic on at the same time, or comb filtering will be audible.
Conferences and teleconferences Suppose the client needs to pick up speech at a conference table. A simple mic setup that works well for recording is a single omnidirectional boundary mic on the table top at the center of every eight people. This mic, however, will pick up more room acoustics and speaker feedback than individual closeup mics. You may need to use several unidirectional boundary mics. Place one mic near each person, or one mic between every two people, an arm's length away.
Some of these mics come with push-to-talk switches so that users can control their own mics. These mics are run into a standard mic mixer. If the mics have no switches, the mixer should be automatic or gated. When someone speaks into a mic, the mixer turns on that mic and turns off all the other mics. By reducing the number of open mics, this gating makes the sound clearer and reduces feedback. Automatic mixers also have such features as chairperson override and number-of-open-mics (NOM) compensation. Some conference mics include three or four unidirectional boundary mic capsules in a single housing; simply place the mic in the middle of the table.
Distance learning In this application, the teacher wears a clip-on mic. Each student or pair of students has a desk mic. Students can switch on their mics when they want to ask the teacher a question. The desktop mic can be a unidirectional boundary mic or gooseneck. If there are no desks, goosenecks or switchable boundary mics can be mounted on the backs of seats. You can also hang mics about 8 feet apart and 8 feet (2.4 m) overhead. Angle the mics 45degrees downward to reject the loudspeaker in the front of the room (see Figure 5), and run the mics through an automatic mixer.
Courtroom In courtroom installation (see Figure 6), the judge should be miced by mounting a gooseneck or a unidirectional boundary mic on the judicial bench. If you want this mic to pick up both the judge and people standing at the bench, use an omnidirectional boundary mic.
If the lawyers wander around while speaking, have them clip a lavalier mic to their clothing about 8 inches (203 mm) under the chin. You might prefer to go wireless. If the lawyers speak mostly from a table, place a unidirectional boundary mic on their tables.
For the witnesses, install a lectern mic, ideally with a foam windscreen to reduce breath pops. A typical mic distance is 8 inches (203 mm) from the mouth.
Hang a mini cardioid mic over and in front of the jury. Place the mic 18 inches in front of the front row and 18 inches (457 mm) over the head-height of the back row. If you prefer a mic stand, use a handheld cardioid mic, or mount a unidirectional boundary mic on the short wall in front of the jury.
Athletic events These events require a mic that picks up the announcer without picking up the crowd noise. A noise-cancelling or differential mic does the job. Because it cancels sounds a few inches away, this mic must be used with lips very near, or touching, the mic. A common mic to use here is a headworn mic. Some are designed with the mic directly in front of the mouth for maximum isolation. Models that mount the mic at the side of the mouth sound more natural. Some announcers prefer to use a handheld mic with a foam windscreen. This mic can be either a cardioid dynamic type or a condenser differential type.
Houses of worship In this application (as in others) we want to hear the words loudly and clearly and hear music reproduced with full fidelity. Here are some suggestions. For the minister, refer to the tips about lectern or pulpit micing.
Some mics are specially designed for hanging over a choir; these mics are almost invisible and sound natural. Use one mic in the center of every 20 foot (6.1 m) to 30 foot (9.1 m) span. A choir of 30 to 45 voices should need only two or three mics.
If the choir mics are used for sound reinforcement, place them close to the choir to minimize feedback-about 18 inches (457 mm) in front of the front row of singers and about 18 inches above the head height of the back row (see Figure 7). If the mics are used only for recording or broadcast, place them about 10 feet (3 m) to 20 feet (6.1 m) away to pick up the acoustics of the sanctuary.
In some venues, hanging mics is inconvenient. Try placing them on mic stands. Extend the stands to full height, and aim the mics up at the choir. As an alternative, make the mic stands taller by adding baby booms, and aim the mics down at the choir. Some churches make mic stands of clear Lexan to reduce visibility.
For the baptismal, hang a miniature choir mic overhead, or mount an omnidirectional boundary mic inside on the glass (see Figure 8). If the baptismal is shallow, you can use a wireless lavalier mic because it eliminates the electrical hazard of dropping a mic into the water. The wireless mic uses a battery of up to 9 V and thus poses no risk of shock.
You will not need to mic the organ for sound reinforcement, but you might need to do so for recording or broadcast. Omni condenser mics are recommended because they reproduce the low notes with richness and depth. Hang one or two mics 10 feet (3 m) to 20 feet (6.1 m) from the organ pipes or organ loudspeaker, 3 feet (914 mm) apart for stereo.
If the guitarist uses a pickup, connect its phone plug to a direct box. Set the ground-lift switch on the direct box to the position that produces the least hum (probably not lifted). There are also several ways to mic the guitar. One is to tape a mini omnidirectional mic onto the guitar body halfway between the sound hole and the bridge (see Figure 9). Another method is to place a cardioid condenser mic about 6 inches (152 mm) from where the fingerboard meets the guitar body, and aim the mic toward the sound hole.
With a grand piano, tape two omnidirectional boundary mics to the underside of the raised lid, one over the treble strings and one over the bass strings (see Figure 10), or use two cardioid condensers about 8 inches (203 mm) above the hammers and 8 inches horizontally from the hammers. If feedback is a problem, close the lid and adjust the mixer EQ (tone controls) until the sound is natural. You will typically need to cut a few decibels around 250 Hz to reduce boominess. An upright piano needs two cardioids a few feet apart near the soundboard.
Other specific applications have relatively simple solutions. A soloist or reader can be covered with a stand-mounted handheld mic. Be sure to place a foam pop filter (windscreen) on the mic to prevent breath pops. Use a baby boom on a mic stand to reach a person seated in a presider's chair. For the singer, the choice is a cardioid handheld mic, either condenser, dynamic or ribbon. It can be wired or wireless. Try a unidirectional boundary mic on the floor near the bench for the predue (kneeling bench for weddings). For the altar table, place a unidirectional boundary mic on the table aiming at the people speaking. Some mic models are available in white to blend with a white tablecloth. You may need to mic the congregation for recording or broadcast. To do so, hang one or two mini cardioid mics several feet over the front row of the congregation, aiming toward the rear of the hall.
Paging, security and surveillance The usual type of mic for paging applications is an omnidirectional dynamic on a gooseneck with a built-in switch. Another choice is a condenser type with a slim gooseneck and a push-to-talk switch in the base.
For security and surveillance, however, most mics are boundary types that install in an electrical outlet box, wall or ceiling. Some are designed not to look like mics so that they are nearly invisible. Look for a mic that filters out the lows to reduce rumble and boosts the highs for articulation.
One way to cover a large room is with omnidirectional boundary mics on the ceiling. Use the 2 to 1 rule for even coverage. The spacing between mics should be twice the distance from mouth to ceiling. For example, if the ceiling is 5 feet (1.5 m) above the talkers' mouths, the mics should be 10 feet (3 m) apart. Then, the mics will pick up everyone about equally.
In general, use as few mics as possible that will do the job. Remember that if several mics are on simultaneously, the sound might be reverberant or muddy. It helps to run all the mics into a gated mixer (automatic mixer), which keeps the sound clear. If the ceiling is high, mount the mics on support posts or walls about 10 feet (3 m) off the floor (out of reach). Medium-sized rooms are best covered with an omnirectional boundary mic on the ceiling near the middle of the room.
For a police interview room or psychiatrist's office, mount an omnidirectional boundary mic in the ceiling directly over the interview table. Another placement is in a wall close to the table at mouth height. If possible, deaden the room acoustics by adding curtains, carpet or acoustic-tile ceiling. This reduces room reverb, yielding a clearer sound.
Mount omnidirectional boundary mics in posts, in the ceiling or in walls for subway stations. Place the mics 10 feet (3 m) above the floor and out of reach. If the mics will be 10 feet from people on the average, mount the mics 20 feet (6.1 m) apart to pick up everyone about equally. That is, the mics should be no more than twice as far apart as they are from the people.
For points of entry, mount an omnidirectional boundary mic in an overhang, or mount a unidirectional boundary mic aiming down over the entry. It is best not to expose the mic to rainfall.
Feedback Here are some suggestions for reducing the likelihood of feedback in any application: use as few mics as possible; keep loudspeakers and mics as far apart as is practical; and place speakers behind mics. Turn down mics not in use. Keep mics as close to their sound sources, but no closer than necessary to achieve enough volume before feedback occurs. Finally, use directional mics-cardioid, supercardioid, or hypercardioid.
Mics are an integral part of many fixed installations. Familiarization with basic mic characteristics is a large step toward a smooth project. Determining the proper mic for a given application is crucial; it can avert a multitude of unnecessary hassles. Follow the tips in this guide, and look forward to better sound in your installations. Your clients will thank you.
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