What Are Omnidirectional Microphones

Omnidirectional microphones can pick up sound from virtually any direction. This design can be useful when a device needs to pick up ambient sound or is used in an environment where sound sources are moving and it is not practical to move the microphone along with them. It does have some drawbacks, however, including muddy sound quality in some cases.

Microphone Design

The distinctive rounded end is a well-known characteristic of an omnidirectional microphone. The look is created by the bulging mesh installed over the electronic pick-up, which protects the internal electronics and can limit interference like breath noises and pops. Some omnidirectional microphones take this mesh covering one step further, with a cover made from foam used as a protective sleeve over the head of the device. The foam does not prevent sound from entering the mic, and also acts as a shield against wind and explosive breath sounds. The extreme sensitivity of omnidirectional microphones requires meticulous design to keep sound as crisp and clear as possible.

Wireless omnidirectional microphones that transmit signals without the use of a cable are also available. Small wireless mics called lavaliers are usually clipped to the speaker's lapel or blouse and fed to either a concealed wireless transmitter or a channel on an audio mixing board. Omnidirectional lavalier microphones also have the distinctive round shape of their larger counterparts.

Uses

The most common uses of an omnidirectional microphone involve groups of singers or instrumentalists. A microphone can be suspended from the ceiling above a choral group or positioned between a vocalist and an accompanying piano or guitar. Solo performers can hold an omnidirectional microphone in various positions and still be amplified.

This equipment can also be useful for meetings and events where there may be multiple speakers but a single microphone, or where it is important to capture sound from several angles. As different speakers add to the conversation or change position, the microphone will still be able to pick up their voices. The alternative is tracking individual speakers with unidirectional mics, which can be time-consuming and expensive, especially for small organizations that don’t have a large budget for sound equipment.

Potential Advantages

Omnidirectional microphones can be very easy to set up and use, even by inexperienced people who may not have used one before. As a result, someone with little or no experience can usually manage the set up, helping to reduce expenses for an event. This can be helpful at events where attendees may need to use a microphone to speak, like town hall meetings and wedding parties.

The broad pickup abilities of an omnidirectional microphone also make it very usable in environments where wide coverage is needed or where the precise origins of sounds may not yet be known. For example, someone recording wildlife might use an omnidirectional microphone to pick up general sounds and background noises because he or she cannot predict how the subjects might move.

Low-cost options with relatively high quality are also available. This can be useful for organizations concerned about budget, or people starting to learn about sound systems who are not able to invest in expensive specialized equipment. An omnidirectional microphone can be used in a variety of applications, while a more focused unidirectional device is less flexible.

Potential Disadvantages

Ideally, an omnidirectional mic would pick up sound in a perfect circle around its center. The laws of physics make this somewhat challenging, however, and in real-world use, this type of microphone cannot pick up sound perfectly from every direction. It can also cut out some high and low frequencies, and sound coming from an extreme angle may not be reliably detected.

The inability to discriminate between wanted and unwanted sounds means that ambient noise can be picked up and amplified. Some performers may want the sounds of an enthusiastic audience to be included in the session, for instance, but others may want these noises blocked out. A unidirectional microphone may be better at keeping background noise out of the recording and amplifying equation.

Another risk with omnidirectional microphones involves the triangle between the microphone, the performer, and the speakers. If an omnidirectional microphone is placed too close to the speakers, it will pick up extraneous noise. This noise is then fed back into the system through the microphone and amplified again. The result is a very unpleasant phenomenon called a feedback loop. Great care must be taken to avoid putting an omnidirectional microphone directly in front of the speakers.

Unidirectional Microphones

The design of an omnidirectional microphone contrasts with unidirectional microphones, which only pick up sound from a more targeted source. There are several different types of unidirectional mics, each classified by its polar pattern or directionality — the shape created when the sound pickup is mapped on a flat plane. Some options can include a shotgun microphone, which is a highly directional device intended for pointing at a specific point source of sound; and a cardioid, which is named for the heart-like shape of its polar pattern. While multiple unidirectional microphones can offer better sound quality in some cases by capturing specific sources with less background noise, they can be expensive and more difficult to set up correctly.

What is a Multiplexer

A multiplexer, sometimes referred to as a multiplexor or simply a mux, is an electronic device that selects from several input signals and transmits one or more output signals. In its simplest form, a multiplexer will have two signal inputs, one control input and one output. One example of an analog multiplexer is the source control on a home stereo unit that allows the user to choose between the audio from a compact disc (CD) player, digital versatile disc (DVD) player and cable television line, for example.

Multiplexers also are used in building digital semiconductors such as central processing units (CPUs) and graphics controllers. In these applications, the number of inputs is generally a multiple of two, the number of outputs is either one or relatively small multiple of two, and the number of control signals is related to the combined number of inputs and outputs. For example, a two-input, one-output multiplexer requires only one control signal to select the input, and a 16-input, four-output multiplexer requires four control signals to select the input and two to select the output.

Types of multiplexers also are used in communications. A telephone network is an example of a very large virtual multiplexer that is built from many smaller, discrete ones. Instead of having a direct connection from every telephone to every other telephone — which would be physically impossible — the network muxes individual telephone lines onto a small number of wires as calls are placed. At the receiving end, a demultiplexer, or demux, chooses the correct destination from the many possible destinations by applying the same principle in reverse.

There are more complex forms of multiplexers. Time-division multiplexers, for example, have the same input/output characteristics as other multiplexers, but instead of having control signals, they alternate between all possible inputs at precise time intervals. By taking turns in this manner, many inputs can share one output. This technique is commonly used on long-distance phone lines, allowing many individual phone calls to be spliced together without affecting the speed or quality of any individual call. Time-division multiplexers generally are built as semiconductor devices, or chips, but they also can be built as optical devices for fiber optic applications.

Even more complex are code-division multiplexers. Using mathematical techniques developed during World War II for cryptographic purposes, they have since found application in modern code division multiple access (CDMA) cellular networks. These semiconductor devices work by assigning each input a unique complex mathematical code. Each input applies its code to the signal that it receives, and all signals are simultaneously sent to the output. At the receiving end, a demux performs the inverse mathematical operation to extract the original signals.

What is a Potentiometer

A potentiometer is a manually adjustable electrical resistor that uses three terminals. In many electrical devices, potentiometers are what establish the levels of output. For example, in a loudspeaker, a potentiometer is used to adjust the volume. In a television set, computer monitor or light dimmer, it can be used to control the brightness of the screen or light bulb.

How It Works

Potentiometers, sometimes called pots, are relatively simple devices. One terminal of the potentiometer is connected to a power source, and another is hooked up to a ground — a point with no voltage or resistance and which serves as a neutral reference point. The third terminal slides across a strip of resistive material. This resistive strip generally has a low resistance at one end, and its resistance gradually increases to a maximum resistance at the other end. The third terminal serves as the connection between the power source and ground, and it usually is operated by the user through the use of a knob or lever.

The user can adjust the position of the third terminal along the resistive strip to manually increase or decrease resistance. The amount of resistance determines how much current flows through a circuit. When used to regulate current, the potentiometer is limited by the maximum resistivity of the strip.

Controlling Voltage

Potentiometers also can be used to control the potential difference, or voltage, across circuits. The setup involved in utilizing a potentiometer for this purpose is a little more complicated. It involves two circuits, with the first circuit consisting of a cell and a resistor. At one end, the cell is connected in series to the second circuit, and at the other end, it is connected to a potentiometer in parallel with the second circuit.

The potentiometer in this arrangement drops the voltage by an amount equal to the ratio between the resistance allowed by the position of the third terminal and the highest possible resistivity of the strip. In other words, if the knob controlling the resistance is positioned at the exact halfway point on the resistive strip, then the output voltage will drop by exactly 50 percent, no matter what the input voltage is. Unlike with electrical current regulation, voltage regulation is not limited by the maximum resistivity of the strip.

Rheostats

When only two of the three terminals are used, the potentiometer acts as a type of variable resistor called a rheostat. One end terminal is used, along with the sliding terminal. Rheostats typically are used to handle higher levels of current or higher voltage than potentiometers. For example, rheostats might be used to control motors in industrial machinery.