Input Transducers (Microphones)

Microphones Transducers that convert sound waves into electrical signals There are several different type of microphones Dynamic Condensing Carbon granule What is the most common application for a microphone? Telephone receiver The main two are condensing and Dynamic, but we will look at Carbon granule because that is the type used in telephones. Also other types e.g. ribbon

Dynamic The diaphragm is moved by changing sound pressure This moves the coil, which causes current to flow as lines of flux from the magnet are cut Diagram - http://www.audio-technica.com/using/mphones/guide/micdoes.html

Dynamic Advantages Disadvantages Rugged Inexpensive to manufacture Tolerates extremely high sound pressure levels Requires no power supply Disadvantages Reduced high frequency performance Needs amplification the SM57 is the dynamic microphone we have in the labs it is also the standard microphone used on the podium of the U.S. President for over 30 years Why might it have poor high frequency performance? – the movement of the diaphragm is restricted by the mass of the coil – reducing high frequency performance. Effective up to around 16kHz after which efficiency tends to fall off significantly. Small output signal which requires amplification.

Condensing A pair of parallel metal plates separated by an insulator across which is stored an electrical charge Q=CV Moving the plates will change the voltage across them causing current to flow in or out of the capacitor through the resistor Also known as capacitor microphones Q= charge in coulombs C is the capacity in farads Capacitance is varied by moving the plates apart. By monitoring the voltage across the resistor with a high impedance (why) preamp a signal can be derived. Why – in order to follow the voltage changes without the allowing the electrical charge to leak away

Charging the Condensing Mic The microphone must have some means of charging the plates Battery Phantom powering Electret The majority of serious recording work is carried out with capacitor microphones

Condensing Advantages Disadvantages No coil and the diaphragm can be made very thin (few microns thick) thus light weight and less inertia so better high frequency response. Capacitor microphones can be made with virtually any response pattern. Disadvantages More complex thus more expensive

Carbon Granule When acoustical pressure is exerted on the carbon granules, the granules are pressed closer together which decreases the measured resistance. http://www.hooked.net/~gelliott/Microphone.html - by: Mark Elliott By running a current through the carbon, the changing resistance changes the amount of current that flows.

Pickup Patterns Microphones can also be classified by their pick up patterns Omnidirectional Unidirectional Bidirectional Omnidirectional – in all ways and all places Unidirectional – one way Bidirectional two-ways

Omnidirectional Pickup of sound equally well within a 360 degree radius around the microphone element The drawing above shows a simplified sketch of an omnidirectional microphone's pickup pattern. That pickup area is depicted in light blue. The closer the blue area is to the outside circle, the better the sound. As you can see, there is virtually no difference between the ability to pickup sound from the front of the mic and the back of the mic. http://www.ktsw.swt.edu/mc3309/L10_omni_drc.html

Omnidirectional Advantages Doesn't have to held directly in front of sound source to pick up adequate sound Gives a sense of the environment Is not affected by slight changes in mic-to-source distance Provides a natural sound in rooms with good acoustics Is less susceptible to handling noise and noise created by wind or popping Is not subject to the proximity effect

Omnidirectional Disadvantages Difficult to use in noisy locations Susceptible to feedback in locations where microphone is feeding a loudspeaker

Unidirectional (Cardioid) Primary pickup of sound from one side of the microphone's element Wide angle cardioid Supercardioid Hypercardioid The drawing above shows a simplified drawing of a unidirectional microphone's pickup pattern. That pickup area is depicted in light blue. The closer the blue area is to the outside circle, the better the sound. As you can see, there is significant dropoff in the microphone's ability to reproduce sound from the backside of the mic. Advantages Discriminates against unwanted sound Reduces danger of feedback in locations where microphone is feeding a loudspeaker picks up very little or no background sounds Disadvantages May be subject to the proximity effect Must be set correctly to avoid sound source being off-mic Less natural sound in rooms with good acoustics Must take care to avoid covering ports More susceptible to wind, popping and handling noises

Unidirectional Advantages Discriminates against unwanted sound Reduces danger of feedback in locations where microphone is feeding a loudspeaker Picks up very little or no background sounds

Unidirectional Disadvantages May be subject to the proximity effect Must be set correctly to avoid sound source being off-mic Less natural sound in rooms with good acoustics Must take care to avoid covering ports More susceptible to wind, popping and handling noises

Bidirectional Optimum pickup of sound from two sides of the microphone element Advantage - can be used to reduce number of mics in a circuit when two voices are involved in an interview or performing a voice-over Disadvantage - no separate control over levels of talent, so they must maintain proper mic-to-source distance and remain on-mic In this simplified drawing of a bi-directional microphone's polar pattern, the working area is shown in light blue. The closer the blue area comes to the outside circle, the better the sound reproduction. As you can see, a bi-directional microphone is not designed to pick up sound from the sides.   Advantage - can be used to reduce number of microphones in a circuit when two voices are involved in an interview or performing a voice-over Disadvantage - no separate control over levels of talent, so they must maintain proper mic-to-source distance and remain on-mic

Proximity Effect The closer the sound source is to the microphone, the greater the emphasis on the bass frequencies in the sound Many microphones are equipped with a bass roll-off switch to nullify the proximity effect Occurs in cardioid and bidirectional (figure of eight microphones)- boosting of low frequencies. Typically happens at mic-source distances of a couple of inches or less.

Condenser Frequency Response Omnidirectional condenser microphone Cardioid capacitor microphone http://www.audio-technica.com/using/mphones/guide/pattern.html#omni

Dynamic Frequency Response Handheld omnidirectional dynamic microphone Handheld hypercardioid dynamic microphone with Hi-ENERGY® neodymium element – explain neodymium Compare response with condenser and you will see why they are used more often.

Impedance One important characteristic of a microphone is its output impedance This is a measurement of the AC resistance looking back into the microphone Low (50-1,000 ohms) High (5,000+ ohms)

Low Impedance Most microphones used in broadcast applications are wired for low-impedance circuits Low-impedance circuits have three wire connectors: a shield and two other wires (hot and cold) to conduct the current                                 Low-impedance circuits allow the use of long microphone cables. Low-impedance circuits are less susceptible to noise picked up from electrical wiring

Balanced Systems Balanced lines are much less susceptible to RFI (Radio Frequency Interference) and the pickup of the other electrical noise and hum In a balanced line, the shield of the cable is connected to ground, and the audio signal appears across the two inner wires which are not connected to ground Because signal currents are flowing in opposite directions at any given moment in the pair of signal wires, noise which is common to both is effectively cancelled out ("common mode rejection"). This cancellation can't occur when only one signal wire plus the shield is used. Of course, it is possible to wire a low-impedance microphone directly to an unbalanced low-impedance input, but the noise-cancelling benefit will be lost. This should not be a problem with short cable runs, but if longer cables are used, a balanced input is preferred. We will be using balanced systems.

Balanced Systems Output signal is split into two opposite phases Positive and Negative or Hot and Cold When returned to the same phase at the amplifier input noise is cancelled. Draw waves and phases on board. This is known as common mode rejection

Phantom Powering The standard power supply for microphones is 48v DC power is supplied along the signal leads, which means no extra cables All capacitor mics and electret mics require powering to operate.

Connectors Two types of connectors are commonly used with balanced systems Non balanced microphones can be connected to an XLR lead by linking the earth and cold pins. XLR connectors lock, but ¼ inch stereo jacks don’t. Wiring the hot and cold up the wrong way round doesn’t make much difference when using one mic, but with two it will cancel out a lot of the sound.

High Impedance High-impedance circuits have two wire connectors: a shield and one inner wire Used in budget systems High impedance makes the signal more susceptible to electromagnetic interference Signal is affected by cable capacitance in long runs of cables They have a relatively high output voltage (compared to low impedance types). Which means that the preamplifiers need not be so complex.

Feedback Feedback occurs when the sound from the loudspeaker arrives at the microphone as loud or louder than the sound arriving directly from the original sound source (talker, singer, etc.) Feedback is a condition in a sound-reinforcement application when the sound picked up by the microphone is amplified, radiated by a speaker, then picked up again, only to be re-amplified. Eventually the system starts to ring, and keeps howling until the volume is reduced.

Microphones and Stereo Signals Coincident or XY pair Right hand microphone picks up sound form left stage and visa a versa No phase problems as minimal space between microphones Sound source Most of what we hear on pop records is not true stereo but a collection of separately recorded mono sounds panned to different positions in the sound field. Cardioid Microphones

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