Lecture 4 Flashcards
Define “transduction”
The transformation of energy from one form to another
What is the “frequency response”?
The range of frequencies over which the transducer is sensitive
What is a flat frequency response?
When a microphone is EQUALLY sensitive to a certain range of frequencies
How do dynamic microphones work? Are they passive or active?
- Sound vibrations strike a diaphragm.
- Attached is a copper coil, surrounding a magnet. The movement of the coil around the magnet induces an electrical current.
- Voltage level is proportional to the rate of sound pressure change.
- They need amplification for recording.
*Passive - do not require external power
When are dynamic microphones used?
Commonly used in live band situations
What is the quality of dynamic microphones?
Quality varies - can be high or low impedance. Generally they are sensitive to the range of frequencies corresponding to the human voice.
How do condenser microphones work?
- Consists of a diaphragm and a positively charged metal plate - together called a capacitor.
- The positive charge is provided by an external power source.
- The air gap between the diaphragm and the plate creates a constant electrostatic field.
- When the diaphragm vibrates after being struck by a sound wave, the charge across the capacitor changes, inducing an electric current.
When are condenser microphones used?
In recording studios. They are very expensive
Quality of condenser microphones
Very high quality - have a flat frequency response from 20 to 20,000 Hz, providing accurate transduction of the original sound.
Limitation: too sensitive, and therefore pick up a lot of background noise.
What are electret microphones?
A type of condenser microphone that is generally smaller, and has a power supply built in (i.e. a battery). They are generally cheaper to buy and still have good quality recordings. Examples include those in iPods.
What does it mean to have a high signal to noise ratio without peaking?
- High signal to noise ratio means that the level of background noise is LOW compared to the desired signal (speech).
- Peaking occurs when the input voltage is too high (i.e. the sound signal is too loud)
- This results in the sound becoming distorted and noisy.
- Therefore, a microphone should be positioned so that the sound signal is at a much higher level than background noise, but not so high that it distorts.
What are 4 considerations for high quality recordings?
Record in a QUIET place
Reduce REVERBERATION from hard surfaces
Keep mouth to mic DISTANCE CONSTANT
Place mic in an appropriate place for the context of the recording (i.e. is it acoustic analysis, or language sampling from a conversation?)
What is meant by sampling/ the sampling rate?
In A-D conversion, a CONTINUOUS variation in voltage is transformed into a sequence of DISCRETE digital numbers or values.
The sampling rate is the NUMBER OF TIMES per second that the voltage level is “sampled” or converted to a digital value.
What is Nyquist’s Theorem?
There must be AT LEAST TWO samples per cycle. This means that only frequencies up to HALF the SAMPLING FREQUENCY (SF) can be reproduced.
What is “under-sampling”? What does it introduce into the signal? How can it be reduced?
The process of sampling frequencies that are MORE than half the SF. It introduces artefacts/aliasing noise into the signal. It can be reduced by low pass filtering prior to conversion
