Dynamic Microphones: From Sound Pressure to Electrical Voltage
Dynamic microphones represent the most straightforward implementation of electromagnetic transduction principles, using a moving coil attached to a diaphragm to generate electrical signals proportional to sound pressure variations. When sound waves strike the diaphragm, they cause it to vibrate, moving the attached coil within a permanent magnetic field and inducing voltage according to Faraday's law.
The basic construction consists of a thin diaphragm (typically 10-50 micrometers thick) made from materials like polyester, polypropylene, or metal foil, attached to a lightweight coil of fine wire wound on a cylindrical former. This coil sits in the gap of a permanent magnet structure designed to provide uniform magnetic field strength across the coil's range of motion. The magnetic circuit typically uses high-energy permanent magnets like neodymium or samarium-cobalt to achieve strong fields in compact geometries.
The sensitivity of dynamic microphones depends on several key parameters:
S = Bl/(R_e + R_load) × A
Where B is magnetic field strength, l is the total length of wire in the magnetic field, R_e is the microphone's electrical resistance, R_load is the load impedance, and A is the effective diaphragm area. Higher magnetic fields, more wire turns, and larger diaphragms increase sensitivity, but these improvements often come with trade-offs in frequency response, size, and cost.
Frequency response characteristics of dynamic microphones are determined by the mechanical resonance of the diaphragm-coil system and the acoustic properties of the microphone housing. The fundamental resonance typically occurs in the 50-100 Hz range, below the intended operating frequency range. Above resonance, the response ideally remains flat until high-frequency rolloff begins due to diaphragm breakup modes and acoustic effects.
The mass-controlled region above the fundamental resonance provides the flattest frequency response, where the microphone output is proportional to acoustic pressure. At very low frequencies, the response becomes stiffness-controlled and decreases at 6 dB per octave. At high frequencies, the finite size of the diaphragm creates directional effects and response irregularities as the acoustic wavelength becomes comparable to the diaphragm dimensions.
Directional characteristics of dynamic microphones depend on the acoustic design of the microphone housing and the presence of ports or openings that allow sound to reach both sides of the diaphragm. Omnidirectional patterns result when sound pressure is equal on both diaphragm sides, while directional patterns develop when acoustic delays or phase differences exist between front and rear sound paths.
Cardioid (heart-shaped) directional patterns are achieved by providing an acoustic pathway from the rear of the microphone to the back side of the diaphragm through a carefully designed port system. The rear pathway includes acoustic resistance and mass elements that create the proper phase and amplitude relationships for maximum front sensitivity and rear rejection. The resulting polar pattern follows:
R(θ) = 0.5(1 + cos θ)
Where θ is the angle from the microphone axis and R is the relative sensitivity. This pattern provides good rejection of sounds arriving from behind the microphone while maintaining sensitivity for sounds in front.
Dynamic microphones offer several advantages that make them popular for live performance and recording applications: they require no external power, have good rejection of electrical interference, can handle high sound pressure levels without distortion, and provide robust mechanical construction that withstands physical abuse. However, they typically have lower sensitivity than condenser microphones and may exhibit less detailed high-frequency response due to the mass of the moving coil system.