Sound recording & playback

Transcription

1 Sound recording & playback Dynamic microphone Condenser microphone Carbon microphone Frequency response curves Sound recording Amplifiers Loudspeakers Sound recording & playback - 1

2 Dynamic microphone Dynamic microphones have a coil attached to a diaphragm suspended in the magnetic field of a magnet. Pressure changes cause the diaphragm to vibrate, changing the amount of magnetic flux threading the coil inducing a current in the coil to produce a signal. Sound recording & playback - 2

3 Condenser microphone Condenser microphones have two charged plates, the front one of which is very thin, and usually covered in a very fine gold layer. Pressure changes cause the front plate vibrate, changing the distance between the plates. This changes the capacitance of the system. A voltage is supplied to plates, and thus the amount of charge on the plates varies, causing a current to produce the signal. Sound recording & playback - 3

4 Carbon microphone A carbon microphone uses a capsule containing carbon granules pressed between two metal plates. A voltage is applied across the metal plates. The diaphragm vibrates with sound waves, applying a varying pressure to the carbon. This changes the resistance and the current through the microphone. Carbon microphones were once commonly used in telephones; they have low-quality sound reproduction and a very limited frequency response range. Sound recording & playback - 4

5 Frequency response curves microphones The response curve of a microphone shows how well a microphone records sound at different frequencies. Frequencies that have higher db values on the chart are reproduced at a greater intensity. A perfect microphone would have a flat curve over the whole range of frequencies. The response curve is determined by a number of factors, such as the dimensions of the microphone & the type of transducer used. Sound recording & playback - 5

6 Frequency response curves microphones Condenser microphones tend to have a fairly flat response curve. The reproduce well over a wide range of frequencies. Dynamic microphones have peaks designed to gain clarity with stage vocals. (1-5 khz) Sound recording & playback - 6

7 Sound recording There are a number of different methods that have been used to store recordings of sound. Vinyl records have a groove with sideways variations that corresponds to the sound wave. Magnetic tape has magnetic iron oxide particles aligned to represent the sound wave. Digital recordings (such as CDs and mp3s) are created by converting sound intensity into a digital number, at a defined rate. CD audio uses 16 bit quantisation (allowing for over 65,000 possible values), with the intensity sampled 44,100 times per second. The data is recorded as a series of 0s and 1s. MP3 files are converted to a lower bit rate and have some inaudible frequency combinations compressed to reduce the file size. Digital signals need to be converted through a Digital to Analogue Converter (DAC) for playback through a loudspeaker. Sound recording & playback - 7

8 Amplifiers Audio amplifiers are designed to increase the electric current that will be sent to the loudspeakers. Amplifiers are generally measured by their power output. Typical home stereo systems will be rated between 20W and 100W per channel. Clipping of the signal through the amplifier can cause distortion and overheating in speakers, so the amplifier is best chosen to be a higher power rating. Perfect amplifiers will increase the intensity of electric signals uniformly across all frequencies. For audio systems, an equaliser is also used so that selected frequencies can be made louder or quieter to suit the music or listener s tastes. A loudness button will increase the intensity of the lowest frequencies, to accommodate the ears reduced perception at low volumes. (Not so useful at higher volumes) Sound recording & playback - 8

9 Loudspeakers A loudspeaker works like a dynamic microphone in reverse. A coil of wire surrounds a magnet attached to the diaphragm. Changes in the current through the wire cause motion of the diaphragm following the same pattern. The movement of the diaphragm causes movement of air. Sound recording & playback - 9

10 Loudspeaker design The basic principle of a loudspeaker is that needs to move as much air as possible, in phase with the electric signal. Speaker baffles are designed to stop the destructive interference of waves in front and behind the speaker. Ports allow for low very frequencies to resonate and be reproduced louder. Two-way speakers have two individual speakers: - a small tweeter for high frequencies - a woofer for low frequencies - the crossover separates the signals The impedance of a speaker is a measure of how the speaker opposes the variations in alternating current produced. Most speakers are between 4 and 8 ohms. Sound recording & playback - 10