Microphone Symbol
Definition: The Microphone symbol represents a transducer that converts acoustic sound pressure waves into an electrical signal, used in circuit and schematic diagrams to indicate an audio input stage — most commonly an electret condenser microphone (ECM) requiring a bias voltage — as defined in IEC 60617-08 (signal transducer symbols).
Also known as: mic symbol, microphone transducer, electret microphone, ECM, condenser mic, audio input transducer, MIC symbol.
What the Microphone symbol means
The Microphone symbol denotes an electroacoustic transducer that converts variations in air pressure (sound) into a proportional electrical signal. In most modern circuit diagrams, and specifically in this symbol's context, the microphone is an electret condenser microphone (ECM) — a two-terminal device with a + (positive/bias) pin and a − (negative/ground) pin. The ECM contains a built-in JFET preamplifier that requires a small bias current (0.1–1 mA) supplied via a pull-up resistor on the + pin.
In schematic diagrams the microphone symbol marks the acoustic input point of a circuit. It indicates where audio signal capture begins — the source of voice, music, or ambient sound that is processed by amplifiers, filters, ADCs, or directly by a microcontroller's analogue input.
How to identify the Microphone symbol
The IEC 60617-08 microphone symbol is drawn as a circle (representing the transducer capsule) with a vertical bar or plate symbol inside or adjacent to it, representing the diaphragm. A single connection line exits the bottom of the circle for the signal/hot lead, and a ground connection or shield indicates the reference. In the schematic block symbol shown here, two pins are exposed: + (positive bias and signal output, anode-equivalent) and − (negative/ground/shield). The physical appearance of the symbol closely resembles the speaker symbol but with the arrow direction reversed (the microphone 'receives' rather than 'emits' sound).
Function in a circuit
An electret condenser microphone contains a permanently charged polymer film (electret) forming one plate of a capacitor. Sound pressure waves move the diaphragm, changing the capacitor gap and thus modulating the charge — generating a small AC voltage proportional to sound pressure. The built-in JFET converts this high-impedance signal into a lower-impedance output. The + pin receives the bias voltage through a 1–10 kΩ pull-up resistor; the audio AC signal is superimposed on this DC bias and is AC-coupled (via a series capacitor, typically 100 nF–10 µF) to the downstream amplifier. The − pin connects to the circuit ground and the microphone's metal case/shield.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-08 (transducers and electromechanical components) defines the microphone symbol as a circle with a perpendicular bar, representing the transducer capsule and diaphragm. The symbol designator is BM or MICRO. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315-1975 Section 15.5 defines the microphone symbol similarly as a circle with a bar, with the designator MIC or BM. The symbol is visually near-identical to the loudspeaker symbol but the electroacoustic conversion direction is opposite. |
| Key difference | IEC 60617-08 and ANSI Y32.2 / IEEE 315-1975 use essentially the same glyph — a circle with a vertical diaphragm bar — for the microphone. The primary distinction in both standards is the direction of the transduction arrow (microphone: acoustic to electrical) compared to the loudspeaker (electrical to acoustic). |
Terminals / pins
| Pin | Name |
|---|---|
| pos | + |
| neg | - |
Typical values
Operating voltage (ECM bias): 1–10 V DC (typical 3–5 V via 1–10 kΩ pull-up). Current consumption: 0.1–1 mA. Sensitivity: −42 to −30 dBV/Pa (typical ECM capsule). Frequency response: 20 Hz – 20 kHz (audio ECM). Output impedance: 100–2200 Ω (through JFET). Signal-to-noise ratio (SNR): 60–70 dB for good-quality ECM. Operating temperature: −20 °C to +70 °C.
Where the Microphone symbol is used
- Voice input for Arduino, ESP32, or Raspberry Pi projects (speech recognition, voice recording, decibel meters)
- Sound-activated circuits and clap-switch automation systems
- Audio amplifier input stages in portable recorders, intercoms, and hearing-aid circuits
- Ultrasonic detection circuits when paired with wide-bandwidth microphone capsules
- Environmental noise monitoring and decibel-level measurement instruments
- Telephony and VoIP interface circuits requiring a voice-grade microphone input
Example
In an Arduino sound-level meter, the microphone symbol shows its + pin connected through a 10 kΩ pull-up resistor to the 3.3 V supply rail and its − pin connected to ground. The audio AC signal on the + pin passes through a 100 nF coupling capacitor to the input of an LM358 op-amp configured as a non-inverting amplifier (gain = 101). The amplified audio signal feeds the Arduino's analogue input pin, and the Arduino computes the peak-to-peak voltage to display the approximate sound pressure level in dB.
Key facts
- The Microphone symbol represents an electroacoustic transducer that converts sound pressure waves into an electrical signal; in most modern schematics this is an electret condenser microphone (ECM).
- Two pins define the ECM symbol: + (positive bias and signal output, requiring a bias supply via a pull-up resistor) and − (ground/shield).
- An ECM contains a permanently charged polymer electret film and a built-in JFET amplifier; it requires a DC bias current (0.1–1 mA at 1–10 V) supplied through a resistor on the + pin.
- IEC 60617-08 defines the microphone symbol as a circle with a diaphragm bar; the designator is BM or MICRO. ANSI Y32.2 / IEEE 315-1975 uses the same glyph with designator MIC or BM.
- The audio signal from an ECM is AC-coupled (via a series capacitor) to the downstream amplifier stage because the DC bias voltage must not reach the amplifier input.
- Typical ECM sensitivity is −42 to −30 dBV/Pa; a better (less negative) dBV/Pa value means higher sensitivity and a stronger output signal for the same sound pressure level.
- The microphone symbol is visually similar to the loudspeaker symbol in both IEC and ANSI standards; the direction of transduction (acoustic-to-electric vs electric-to-acoustic) distinguishes the two.
Diagrams that use this symbol
- subwoofer wiring diagrams
- speaker wiring diagram
- speaker connection diagram
- speaker diagram
- amplifier wiring diagram
- metra line output converter wiring diagram
- 4 channel amp wiring diagram
- power amplifier circuit diagram
Frequently asked questions
What does the microphone symbol mean in a circuit diagram?
The microphone symbol represents an electroacoustic transducer that converts sound pressure waves into an electrical signal. In most modern schematics it represents an electret condenser microphone (ECM) with a bias supply connection (+) and a ground connection (−), indicating the audio input stage of the circuit.
What does the microphone symbol look like on a schematic?
The microphone symbol is a circle (representing the capsule) with a vertical bar or plate inside, representing the diaphragm. It has two connection pins: + (positive bias/signal output) and − (ground/shield). The symbol closely resembles the loudspeaker symbol but represents the reverse transduction direction — converting sound to electricity rather than electricity to sound.
What do the + and − pins mean on the microphone symbol?
The + pin is the positive bias and signal output terminal. A pull-up resistor (typically 1–10 kΩ) connects this pin to the supply voltage, providing the DC bias current the internal JFET requires. The audio signal (AC) appears superimposed on this DC bias and is AC-coupled to the downstream amplifier. The − pin is the ground and shield connection.
How do I connect an electret microphone to an Arduino?
Connect a 10 kΩ resistor between the Arduino 3.3 V or 5 V pin and the microphone + pin. Connect the − pin to Arduino GND. Add a 100 nF–10 µF capacitor in series between the + pin and the Arduino analogue input (A0) to block the DC bias and pass only the AC audio signal. Optionally add a non-inverting op-amp stage (gain 100–1000) between the capacitor and the ADC input for better sensitivity.
What is the difference between a condenser and a dynamic microphone symbol?
Both condenser (ECM) and dynamic microphones use the same IEC 60617-08 / IEEE 315 microphone symbol (circle with diaphragm bar). The distinction is made by a text label or a note rather than a different glyph. Electrically, a condenser/ECM microphone requires a DC bias voltage (phantom power or pull-up resistor), while a dynamic microphone generates voltage from coil motion and requires no bias supply.
What standard defines the microphone schematic symbol?
IEC 60617-08 (transducers and electromechanical components) defines the microphone symbol as a circle with a perpendicular diaphragm bar; the designator is BM or MICRO. ANSI Y32.2 / IEEE 315-1975 Section 15.5 defines the same glyph with the designator MIC or BM. Both standards use an identical or near-identical glyph.
What is microphone sensitivity and what unit is it measured in?
Microphone sensitivity is the output voltage produced for a given sound pressure level, expressed in dBV/Pa (decibels relative to 1 volt per pascal of acoustic pressure). A typical ECM capsule has a sensitivity of −42 to −30 dBV/Pa. A sensitivity of −30 dBV/Pa means the microphone produces 31.6 mV for a 1 Pa (94 dB SPL) sound — higher (less negative) values indicate greater output for the same sound level.
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