Microphones

1. Microphones Dynamic and Condenser Polar patterns Frequency response Wireless

2. How a microphone functions • Dynamic microphones • Based on a moving coil of wire • Think of the reverse of a loudspeaker • Sound waves push on a diaphragm which in turn moves a coil of very fine wire through a magnetic field • Produces current proportional to the sound waves

3. Dynamic microphones • Usually very robust and durable • Very often used on stage where rough handling or dropping is common • Damage is most often caused by breaking suspension wires • Two wires which hold the diaphragm and coil in place • Often do not produce a flat frequency response and not very sensitive to low SPL • The relatively massive coil of wire takes a great deal of power to make move

4. Condenser microphones • Creates a capacitor using the diaphragm as one plate. • Capacitor is used to store an electric charge. Changing the distance between plates changes it’s ability to do that. • Requires the plates to be charged by an external source • Battery, phantom power… • As the diaphragm moves, it causes a ripple in the current from the phantom power. This ripple is proportional to the sound waves

5. Condenser microphone • Due to the lightweight diaphragm, only small pressure changes are required to produce an audio signal. • Very sensitive to small SPL • Much flatter frequency response than dynamic • Much more susceptible to damage • Requires external power supply to function

6. Frequency Response • A graph showing how a specific microphone responds to different frequencies. • No microphone is perfectly flat • Usually not desirable anyway • Microphones are designed for specific purposes and frequency responses are tailored for each use • Proximity effect • Usually only seen on dynamic microphones • As you move the mic closer to the sound source, you get an increase in the low frequency response. • Trained vocalists will make good use of this effect, the untrained will make themselves unintelligible.

7. Frequency Response Dynamic Vocal Mic Condenser Vocal Mic Live Instrument Mic Studio Instrument Mic

8. Directional properties • Every microphone will have a polar pattern • Description of the sensitivity of the microphone related to the direction the sound is coming from • Omni-directional • Sensitive from all directions • Used for ambient noise recording or where the sound source is moving and the mic cannot • Often on lavaliere mics • Sound is often general and unfocused. • Feedback more likely Omni-directional polar pattern

9. Directional properties • Uni-directional • Mostly sensitive from one direction and less from all others • Cardioid pattern • Sound is mostly picked up from the front, but some from the sides as well to allow for some movement of the mic off axis • Most common pattern, good for general use • Most common on hand held mics • Super and Hyper Cardioid pattern • Less sensitive to the sides, more sensitive to the rear • Sometimes called a “shotgun” mic • Very directional Cardioid polar pattern Super cardioid polar pattern

10. Directional properties • Bi-directional • Sensitive from the front and back and much less to the sides • Not much use on stage • Variable directionality • Polar pattern is adjustable to different patterns • Seems like a good idea, but rarely works perfectly well Bi-directional pattern

11. Specialty Microphones • PCC and PZM microphones • Collectively known as Boundary microphones • Trade names of Crown International • http://www.crownaudio.com/mic_web/pcc.htm • http://www.crownaudio.com/pdf/mics/101062.pdf • http://www.crownaudio.com/mic_web/pzm.htm • Often used as a floor mic to reinforce a large group of people who are moving • Musical chorus for example • Choir microphones • Small sensitive condenser mics usually hung overhead • http://www.shure.com/microphones/models/mxoverhead.asp

12. RF (wireless) microphones • A microphone which transmits it’s audio signal using radio frequencies rather than a cable • The mic has a radio transmitter built into it which transmits to a receiver off-stage. The receiver then sends the audio signal to a mixer • Most often in the form of a wireless • Vocal mic – transmitter is built into the body of the mic • Lavaliere – small mic capsule is clipped onto clothing or taped to the head (preferred). The mic capsule is wired to a small pack with the transmitter and batteries.

13. RF (wireless) microphones • The frequency that the packs transmit on are very important • Usually transmit in the VHF or UHF range and are susceptible to interference from other radio sources • Diversity receivers • MUCH more interference resistant than non-diversity • Involves two separate receivers spaced apart from each other • The system will select the strongest signal

14. RF (wireless) microphones • Lavaliere mic mounting • Usually mounted on the center of the forehead or over the ear • Toupee clips, bobby pins, high quality surgical tape. • Spirit gum only if ALL else fails • Occasionally mounted on clothing • Traditional use. TV interviews for example • Poor sound quality • Low frequency boost from being near the chest • Wind or clothing noise likely • Sound level changes as performer moves their head • Boom mounting – a.k.a. “Madonna Mic” • Mounted on a rig over the ear which puts mic very close to the mouth. • Best sound. Poor aesthetics.

15. Additional Readings • http://electronics.howstuffworks.com/framed.htm?parent=question309.htm&url=http://home1.pacific.net.sg/~firehzrd/audio/mics.html • http://www.mediacollege.com/audio/ • Interesting discussion on mic techniques • http://www.informatik.uni-bremen.de/~dace/guitar/recording/