Microphones NYU（麦克风纽约大学）.pdfVIP

Microphones Juan P Bello Early recording • Early recording and reproduction was entirely acoustic • Sound was captured by a horn terminated on a diaphragm that vibrated in sympathy with the sound • The diaphragm was attached to a stylus which cut grooves on foil or wax • Performers had to clutter around the horn. • Little control of individual levels Sound in electrical form • Sound in electrical form can be amplified, mixed and recorded. • We can convert the acoustical waveform into an electrical waveform of the same shape • Amplitude becomes voltage (V), and air particle motion becomes electrical current (I) - electrons playing the role of air particles. • The current’s direction of flow changes with cycles of compression/rarefaction creating an alternating current (AC) • The flow of electrons in a conductor is impeded by a certain amount of resistance (R) • The relationship between V, I and R is regulated by Ohm’s law: V 2 2 = I×R. Their relationship with Power (W) is: W = I ×R = V / R • In AC systems resistance is replaced by impedance (which also includes reactance). Impedance is frequency dependent Electro-magnetic induction • Electromagnetic transducers convert mechanical motion (as produced by, e.g., an acoustic wave) into an electrical signal • An electrical current is induced (and voltage is produced) if : (i) a static conductor is situated in a changing magnetic field, or (ii) a conductor moves in a static magnetic field • This phenomenon is characterized by Faraday’s law of electromagnetic induction (/demo_faraday) Electro-magnetic wire field Magnet S N M