Summary Modern Recording Techniques

Microphones: Design and Application The Microphone: An Introduction - Usually the first device in a recording chain - A transducer that changes one form of energy to another - In the mic’s case, it would be from sound waves to electrical signals - The quality depends on certain external variables and also design variables - Some external variables are placement, distance from sound source, and environment - Some design variables are the mic’s operating type, design characteristics and quality - For the best microphone choice and placement, follow these “rules”: - Rule 1: There aren’t any rules. These are more like guidelines. There might be a placement that doesn’t follow the “rules” but fit your tastes so experiment - Rule 2: The overall sound of an audio signal is no better than the weakest link in the signal path. Basically, put the mic where the best sound signal is. - Rule 3: Use the “Good Rule”. If everything is good, like musician, instrument, mic, etc, then the recording will be good - Overall, use common sense and trust your instincts Microphone Design - There are three main types of microphones that are used: The dynamic, ribbon, and the condenser - - - Dynamic Microphone - Operates by using electromagnetic induction to generate an output signal - They usually have a stiff Mylar diaphragm. Attached to the diaphragm is a wire, called the voice coil. - When the acoustic pressure wave hits the diaphragm, the voice coil vibrates to the amplitude and frequency of the voice wave. Ribbon Microphone - Also uses electromagnetic induction - Older microphones used very thin aluminum ribbon (2 microns thick) as the diaphragm - Are very sensitive. If they are exposed to too much sounds, the diaphragm could break, rendering the microphone useless Condenser Microphone - - - Operates on an electrostatic principle Contains two plates called the condenser (in many parts of the world) or capacitor (in the US) - One is very thin, and acts as the diaphragm - The other is a fixed backplate The capacitor is capable of storing an electrical charge - The capacitance value and the voltage applied to it finds how much charge the capacitor can hold. - Q = CV, where Q is the charge in coulombs, C is the capacitance in farads, and V is the voltage in volts Most condenser microphones use phantom power, which is getting power supply from the console rather than from batteries inside the mic. Microphone Characteristics - - Directional Response - Refers to the sensitivity at various angles - Microphone directionality can be classified into two categories: omnidirectional polar response and directional polar response - Omnidirectional mic are responsive to sounds in all directions - Directional have certain areas where the mic is sensitive - Bidirectional has a figure eight polar pattern, where the microphone is sensitive in the front and rear axis. - Cardioid response, which are shaped like hearts, are sensitive mostly in the front axis and doesn’t pick up sounds in the rear axis. - Certain condenser mics can be electrically switched from one pattern to another Frequency response - Measurement of its output over the audible frequency range when driven by an input signal - The response curve is usually plotted on a range from 20Hz to 20kHz - A flat frequency response means that it responds equally to all frequencies - Rumble, which are vibrations that occur in the 3 to 25 Hz region, can be present in low frequencies through the floor from nearby sounds, which could mess up the recording. These are ways to prevent rumble: - Use a shock mount to isolate the mic from the mic stand - Choose a mic that restricts low frequency responses - Proximity Effect is the increase of bass response when brought within a foot of the sound source. As distance decrease, the bass increases - Using an omnidirectional mic could help with reducing the proximity effect - - Transient Response - Measures how quickly a mic’s diaphragm will react when hit by an acoustic wavefront - Each mic is different, which is why mics sound different Output Characteristics - Sensitivity Rating - The output level in volts a mic will produce, given a specific and standardized acoustic signal - Equivalent Noise Rating - Device’s electrical self-noise - The noise is produced within the diaphragm - Overload Characteristics - The mic’s limit at high sound-pressure levels - Microphone Impedance - Rating that’s used to help match the output resistance of one device to the rated input resistance requirements of another device - Measured in ohms - Most common are 50,150,250 for low, and 20 to 50k for high - Balanced/Unbalanced Lines - Balanced line uses three wires to properly carry the audio signal - High-impedance mics and most line-level instrument lines use unbalanced lines Microphone Preamps - Mic Preamps are used to boost the signal to acceptable levels since most microphone levels are too low to drive the line-level input stage of most recording systems.