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Objectives

By the end of this lesson participants will:

  • Select appropriate audio equipment
  • Differentiate between line and microphone inputs
  • Handle microphones effectively in news gathering

Elements of Sound

What we humans know as sound reaches our ears as vibrations of the air set of by vibrations of something at the source of the sound -- a drum beat, a dropped plate, a honking horn, a clap of thunder, some one speaking. The force of those vibrations at the source sets the nearby air in motion. That motion creates a wave -- like the waves on water -- that moves away from the source. The eardrum receives the vibrations and the other elements of the inner ear translate that vibration into signals the brain interprets as sound.

In the process, the human ear acts as a transducer -- translating the sound waves into signals the brain can interpret. There are other transducers in the audio production process, too. A microphone translates the vibrations of speech or other sounds into electrical signals that are sent along the audio cable to a recorder or other production device. From there, the sound is distributed to another transducer -- a monitor, a TV or radio speaker, headphones, etc. -- that turns the electrical signal back into sound waves that humans can hear.

Two other terms are useful in understanding sound waves. Amplitude is the force of the wave. Think of higher amplitude as louder sound. Frequency is the number of sound waves in a second -- the greater the number, the higher the pitch. A high pitched sound like a singer's high note or a fire siren will produce more vibrations in a given time period than the beat of a large bass drum. Those high sounds are produced by faster vibrations than are produced by the sources of lower pitches.

Microphone Characteristics

Microphones function as tranducers to translate the vibrations of the sound waves into electrical energy that can be stored on a recorder or processed in audio production. Each microphone will have a diaphragm that vibrates from the sound waves and passes that vibration to other parts of the microphone that translate the force into electrical signals. There are two primary types of microphones in wide use. They are known as dynamic and condenser microphones.

Dynamic microphones can be considered the work horses of the audio recording. They contain a diaphragm, wire coil and magnet to translate the sound waves to electric signals. The movement of the diaphragm set in motion by the sound waves creates movement of the wire coil. As the coil passes by the magnet, electric signals are produced.

The condenser microphone requires a power source. The power energizes two metal plates. One of the plates serves as the diaphragm. The vibrations change the distance between the plates and that difference translates into electrical signals that can be distributed to the recorder or audio production unit. The power for the condenser microphone can come from a microphone battery or from what is called phantom power. This is direct current (DC) that is distributed from an external power source or from a camera or other audio recorder. It reaches the microphone through the microphone cable. Since it is DC power, there is no conflict with the sound signals traveling in the opposite direction to the camera or recorder because those signals travel as alternating current (AC). Since the condenser microphone is powered, it is generally more sensitive and can pick up a wider range of sound than a dynamic microphone that is stimulated by the force of the sound wave.

Directionality. Microphones are constructed to record sound in specific directions. Omnidirectional microphones pick up sound in all directions. A unidirectional microphone picks up sound primarily from a single direction. Examples are the shotgun and boom microphones that video crews sometimes use to pick up sound from a distance when a microphone can not be placed near the subject. These microphones are often classified as hypercardioid. Its cousin the cardioid microphone. Its pick up pattern is like a valentine heart, picking up sound mostly in front of the microphone with a bit of pick-up from the sides. A bidirectional microphone picks up sound from opposite directions. Some microphones may allow users to select the directionality.

Impedance. This is a measure of a microphone's resistance to the AC current. Manufacturers will list the impedance of a microphone. Generally, low impedance is better than high impedance. Consumer grade microphones are likely to have higher impedance than professional equipment. The performance of those consumer microphone is best with short cords. As the length of the cable increases the quality of the audio signal declines. Cables themselves attract electrical interference.

Frequency response. This is a report of the range of frequencies that the microphone can process effectively. Even if the microphone can process a wide range of sound frequencies, it will also process some of those frequencies more effectively than others. For example, a microphone that favors high frequencies will produce sounds that seem to be at a higher pitch than the original sound.

Balanced and Unbalanced. Technically, the topic applies to microphone cables, not the microphones themselves; however, it is appropriate to consider the topic here. Microphones that are connected to their recorders and cameras with a small, 1/4-inch plug will be unbalanced. The cable has a single strand of metal that carries the signal to the recording or processing device. A second wire serves as the ground. These work well for many situations; however, the cable itself can attract additional electronic noise and in that way depreciate the sound quality. The longer the unbalanced cable the poorer the sound quality, other things being equal. A balanced cable will have a three prong connector. One line carries the signal from the microphone with a positive charge. The second line carries the same signal with a negative charge. The two are merged to produce a stronger final signal as it enters the recording or processing device. Along the way, any additional noise the cable may attract is canceled by the positive and negative flows. The third line in this configuration is the ground. Obviously, it is better to work with a balanced system.

Extraneous noise. The diaphragm of a microphone is sensitive to sound waves -- any sound waves. It is not able to differentiate from a speaker's voice and wind or from a musical instrument and the vibrations set off by handling the microphone. As noted above, microphone cables can add unwanted noise to the product. So can nose created by mishandling a microphone. A hand-held microphone might ap against the ring on the holder's finger and produce a tapping sound. The holder can "message" the microphone and produce unwanted sound. Clothing can brush against a lapel microphone, etc. The admonition: Handle microphones with care.

Inputs. The strength of a microphone's electric signal is weak. It needs to be boosted before it can be translated back to sound waves that humans can hear. The process is done by an amplifier (See the concept amplitude.). Amplifiers often have two types of input plugs. One set will accommodate microphone cables (mic level inputs). These inputs are expecting weak signals. The amplifier will have processes to boost the signal. The dials to change the bass or treble quality of sound on an audio speaker is an example of amplifier processing. The second set of inputs on an amplifier is known as line level inputs. From these the amplifier expects strong signals. As a guideline one plugs sound sources rather than microphones into line level inputs -- the output of an audio recorder, the output from the earphone jack in a radio or TV set, etc.

Position. Position the microphone as close to the sound source as you can reasonably get without its being so close that the force of the sound waves distorts the recording. Remember microphones are designed to process sound waves, not wind blasts. While the primary task in placing the microphone is to get good quality sound from the person speaking or from some other sound source, one also wants to obtain some of the surrounding sound (ambient) to place the primary sound in its context -- that additional sound helps create a sense of reality for the primary sound. For an interview, a microphone six to 10 inches (15-20 cm) should be appropriate. But test it. If the microphone is hand-held, hold it the same distance from the subject as from the interviewer. Point the microphone at the person who is talking or expected to talk -- subject or interviewer. If the recording must be down in a windy situation, attempt to shelter the microphone. Wind is devastating on sound recordings. Devise a wind screen for the microphone if one is not supplied. Craft fur from a fabric store can be fashioned into a wind screen.

Levels. A frequent problem with audio recording is inadequate levels (loudness) of the primary sound. The microphone was too far away from the person speaking or the sound source. It picked up an echo-like primary sound and too much other ambient noise. A second cause -- the microphone was too close to the sound or the sound was too loud to be handled by the microphone. One must test the microphone set-up whenever possible before the recording or broadcast session. One step in the checking process is to set the level expected from the sound source. The recording device into which the microphone cable is to plugged will often have a volume control. Use it to set the appropriate level. There may also be a meter, commonly a VU (volume unit) meter, that will provide a visual image of the loudness of sound. On the VU meter the ideal sound should register at zero -- approximately the mid point on the scale.

Multiple microphone set-up. Occasionally, the news event will require the use of more than one microphone -- recording a panel discussion, for example. In this situation it is necessary to provide each participant or primary sound source (sections of a band, for example) its own microphone. Each of the cables will be connected to a microphone mixer. That device will enable the producer to control the levels of the several microphones separately. In a complex production, the output of each microphone can be recorded separately for later production. In other situations, the mixer can combine the sound from all of the microphones into a single output that can be delivered to a recorder or camera. It is important to know if the output of the mixer is at line level or mic level to determine into which input it can be plugged on the recorder or camera.


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