1. Input Transducers
(Microphones)

2. 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

3. 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 -

4. 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.

5. 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

6. 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

7. 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

8. Carbon Granule
When acoustical pressure is exerted on the carbon granules, the granules are pressed closer together which decreases the measured resistance.
- by: Mark Elliott
By running a current through the carbon, the changing resistance changes the amount of current that flows.

9. 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

10. 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.

11. 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

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

13. 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

14. 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

15. 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

16. 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

17. 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.

18. Condenser Frequency Response
Omnidirectional condenser microphone
Cardioid capacitor microphone

19. 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.

20. 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)

21. 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

22. 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.

23. 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

24. 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.

25. 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.

26. 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.

27. 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.

28. 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

29. Fin
Fin