Microphone

A microphone is a transducing instrument that converts sound waves into corresponding electrical signals. Commonly known as a mic or mike, it is foundational to modern communication, recording, broadcasting, and sound reinforcement. Microphones are used in telephony, hearing aids, public address systems, live and studio audio engineering, motion picture production, two-way radio communication, megaphones, broadcasting, and a wide range of digital devices including computers and mobile phones. They are also essential in applications such as speech recognition, Voice over IP, ultrasonic sensing, and engine knock detection. Contemporary microphones employ several transduction principles, most prominently dynamic, condenser, and piezoelectric mechanisms.

Historical Development

Efforts to amplify the human voice began with acoustic megaphones, used as early as fifth-century BC Greece, where theatre masks incorporated horn-shaped mouth openings to strengthen sound projection in amphitheatres. Advances in sound transmission continued in 1665 with Robert Hooke’s “lover’s telephone”, a stretched-wire device that transmitted vibrations mechanically.
Electrical approaches appeared in the nineteenth century. Antonio Meucci produced a dynamic microphone in 1856 by moving a wire coil within a magnetic field to generate electric current, a principle later fundamental to telephony. Johann Philipp Reis followed in 1861 with a membrane-based transmitter capable of generating intermittent current. In 1876 Alexander Graham Bell and Elisha Gray developed liquid transmitters using diaphragms connected to conductive rods immersed in acid, though the sound quality remained poor.
The crucial breakthrough came with the loose-contact carbon microphone in the 1870s. Independently developed by David Edward Hughes, Emile Berliner, and Thomas Edison, this type became the first microphone to support practical voice telephony. Although Edison secured a patent in 1877, Hughes’ earlier public demonstrations have led many historians to credit him with the invention. Berliner’s design gained commercial significance when adopted by Bell’s telephone company, and Edison later improved the concept in his carbon-button transmitter of 1886. This technology was used in the first radio broadcast from the New York Metropolitan Opera House in 1910.
The next major development was the condenser microphone, invented by E. C. Wente of Western Electric in 1916. Soon after, in 1923, H. J. Round created the Marconi-Sykes magnetophone, the first practical moving-coil microphone, later refined by Alan Blumlein and Herbert Holman into the HB1A. The same year saw the introduction of the ribbon microphone, attributed to Harry F. Olson, who adapted the principle of a ribbon loudspeaker into a sensitive bidirectional microphone. Companies such as RCA advanced ribbon microphone design significantly, particularly in directional control.
The growth of cinema and broadcasting in the mid-twentieth century increased demand for high-fidelity and highly directional microphones. Electro-Voice met this requirement with its shotgun microphone in 1963. During the later twentieth century, further innovation included the development of iconic dynamic models such as the Shure SM58 and SM57, which became standards in live performance and recording.

Types and Classification

Microphones may be classified according to their transducer principle, directional properties, diaphragm size, or orientation (end-address or side-address). Among modern microphones, three principal transducer types dominate: condenser, dynamic, and contact (piezoelectric).

Condenser Microphones

Crescenting from Wente’s 1916 invention, the condenser or capacitor microphone operates using a diaphragm that forms one plate of a capacitor. Sound waves move the diaphragm, altering the distance between the plates and thereby modulating capacitance. These changes create the electrical signal corresponding to the sound.
Condenser microphones range from inexpensive consumer devices to precision laboratory and studio instruments. Their popularity stems from the low mass of the diaphragm, which enables excellent transient response and wide frequency capture. Because the capacitor requires electric charge, condenser microphones must be powered either by phantom power or by internal batteries.
Many designs incorporate dual diaphragms, enabling selectable polar patterns such as cardioid, omnidirectional, and figure-of-eight. Some models allow continuous variation of the polar pattern, offering greater flexibility for studio work. Two principal subtypes exist: DC-biased condensers and radio-frequency (RF) condensers.

DC-biased Condenser

In DC-biased condensers, a steady charge is maintained on the capacitor plates. Vibrations alter the capacitance and consequently the voltage across the plates. With typical capsule capacitances between 5 and 100 picofarads and high-value bias resistors, the audio signal forms a high-pass component while the bias forms a low-pass component. Since the voltage swing can be significant, internal electronics generally serve as impedance converters rather than amplifiers.

RF Condenser

RF condenser microphones use a low-voltage radio-frequency oscillator. The capsule either modulates the amplitude of the signal or forms part of a resonant circuit whose frequency varies with diaphragm movement. After demodulation, the microphone produces a low-impedance audio signal. The absence of high polarisation voltages makes the system more resistant to humidity, making RF condensers ideal for field recording and broadcasting.

Dynamic Microphones

Dynamic microphones operate by electromagnetic induction. A lightweight coil attached to a diaphragm moves within the field of a permanent magnet. The coil’s motion generates a voltage proportional to the sound wave. Dynamic microphones are robust, handle high sound-pressure levels well, and require no external power. These characteristics make them suitable for live sound reinforcement, percussion recording, and broadcasting environments.

Ribbon Microphones

Ribbon microphones use a thin corrugated metal ribbon suspended between magnetic poles. Sound waves move the ribbon, generating a voltage. This type is known for its smooth frequency response and naturally bidirectional polar pattern. Ribbon microphones were widely used in early broadcasting and continue to be valued in modern studios for their warm tonal qualities.

Contact (Piezoelectric) Microphones

Piezoelectric microphones use crystals that generate voltage under pressure. These are often used as contact pickups on musical instruments, in ultrasonic sensing, and in specialised applications where airborne sound is not the primary source of vibration.

Directional Characteristics

Microphones are frequently described by their polar patterns. Common types include:

• Omnidirectional, capturing sound equally from all directions.
• Cardioid, most sensitive at the front and least at the rear, suitable for isolating sound sources.
• Figure-of-eight, capturing from the front and rear while rejecting sound from the sides.
• Shotgun, providing extreme directionality through interference-tube design.

Directionality affects microphone placement and is central to recording and reinforcement techniques.

Modern Applications and Technological Integration

Microphones today are embedded in numerous devices and systems. Consumer technology uses miniature condenser capsules in smartphones, laptops, cameras, and voice-controlled devices. Studio environments rely on a wide range of microphone types for different tonal characteristics and recording strategies. In live sound, dynamic and electret condenser microphones ensure durability and clarity.
Advances in digital technology have expanded microphone capabilities through integrated digital signal processing, noise cancellation, beamforming arrays, and wireless transmission. In fields such as broadcasting, film, telemedicine, and virtual conferencing, high-fidelity and low-latency microphones are indispensable.