Are Microphones AC Or DC Devices?

Microphones act as transducers, converting sound waves (mechanical wave energy) into audio signals (electrical energy). The movement of this electrical energy is called current, which happens in one of two ways: alternating current (AC) or direct current (DC).

Are microphones AC or DC devices? Some microphones require DC voltages to power their active circuitries (internal preamps, etc.) and polarize their mic capsules, making them DC devices. Yet other mics require no powering at all. Microphones output AC signals (analog audio signals), making them AC output devices.

So some microphones do not require power at all, while others need DC to function properly. Let's discuss, in more detail, why some mics are DC devices while others are not while also discussing the AC outputs of microphones.

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What Makes A Microphone A DC Device?

Let's first start by defining what a DC device is. A DC device is any electronic device that requires direct current electricity to function.

By that definition, microphones that require a DC voltage, however small the voltage may be, in order to function properly, are DC devices.

Some microphones, then, are DC devices, while others are not. Microphones that require a DC voltage to function properly are known as active microphones. Microphones that do not require any electrical power to function are known as passive microphones.

So what makes an active microphone a DC device? There are two main reasons why a microphone would require external DC power:

• Active circuitry: Nearly all condenser microphones and a few ribbon microphones are designed with active preamplifiers in the bodies of the microphones. The active amplification circuits of active mics act to not only boost the signal from the capsule, but also adjust the mic signal impedance so that it may be used effectively. Amplifiers require external electrical energy in order to amplify the applied input signal.
• Capsule polarization: Some condenser microphones (known as “true” or “non-electret” condensers) require an external voltage to properly polarize their capsules. Condenser mic capsules are basically parallel plate capacitors and require electricity to hold an electric charge.

The question of “is this microphone a DC electronic device or not” is essentially the same as “is this microphone active or passive?” When working with microphones, it's always good to know if they are active or passive.

The following are a few generalities about whether or not a microphone type is active or passive:

• Condenser microphones are active: Whether the external power is used to polarize the capsule, power the amplifier, or fix the signal impedance, condenser mics are all active. Both “true” condensers and electret condensers are active.
• Moving-coil dynamic microphones are passive: Moving-coil dynamic microphones have simple, robust designs and are all passive.
• Ribbon dynamic microphones are typically passive, though some are active: Ribbon mics are dynamic and do not require any power in order to function properly. Because of this, most ribbon mics are passive. However, because ribbon mics output such low mic signals, some have been designed with preamplifiers in order to output signal to healthier levels.

With the above generalities in mind, it's really only the ribbon mics that we should raise concern. That being said, it's always best to do your homework on whatever microphone you'll be using in a job.

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How Is DC Supplied To Microphones?

So far, we've gathered that all condenser mics and some ribbon mics are active and, therefore, require an external DC power source to function.

External DC voltage is supplied to microphones in a multitude of ways, depending on the specific microphone requirements. Below is a list of possible methods of powering active microphones:

• Phantom power: Phantom power (P48 or +48V) is the most common way to power standalone active microphones (ie: microphones not integrated in other electronics such as cellphones, laptops, etc.). P48 works by sending +48 volts up pins 2 and 3 (relative to pin 1) of a balanced XLR cable. Professional phantom power is a true 48 volts DC, however some phantom power supplies vary from this value (11 to 48 volts DC is acceptable). Microphones that require phantom powering are designed to take what they need from the 48 V DC. Phantom power is often supplied by mic preamps or audio interfaces.
• DC bias voltage: DC bias voltage is by far the most common way of powering active microphones when we take into account the number of tiny electret mics in laptops, cellphones, and other electronic devices around the world. DC bias voltage is also used to power many electret lavalier-type microphones. Bias voltages typically range from 1.5 to 9 volts DC and are carried on a single conductor so there is not need for a balanced audio line. DC bias's primary role is to power the field-effect transistors (FETs, JFETs, etc.) of electret microphones, which convert the impedance of the mic signal so that it can be used effectively. DC bias voltage is often supplied via batteries (for standalone mics) or from another internal power supply (in the case of electronic devices with integrated microphones).
• External power supply: Many vintage tube condenser microphones have dedicated power supplied that would provide the required DC voltage.
• USB power: Most USB microphones on the market are of the condenser variety. These active mics draw current from the connected computer via USB.

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Microphones Output AC Audio Signals

Some microphones require DC powering, and others don't. However, all microphones output AC audio signals. Let's discuss this point.

Whether we're using a passive microphone or an active one, the general principle of a microphone is the same: A diaphragm moves sympathetically with the sound waves around it, and a coinciding audio signal is produced.

The question here has to do with audio signals. Why are audio signals AC rather than DC?

Let's Start By Discussing Sound Waves

Sound waves are waves of compression and rarefaction that propagate through gaseous, liquid, and solid mediums. Sound waves vary the pressure in such medium. The waves themselves have peak points (maximum compression) and trough points (maximum rarefaction) and basically oscillate in between.

Now Let's Discuss Microphone Diaphragms

Microphone diaphragms react to the changing sound pressure caused by sound waves. The pressure variance against one or both sides of a mic diaphragm causes the diaphragm to move back and forth about its resting position. Like the sound waves that affect it, the diaphragm will experience a maximum displacement in both the front and back directions. The diaphragm will also oscillate back and forth, changing direction after reaching its full displacement to either side of the resting position.

Finally, Let's Talk About The Microphone Signal

Mic signals are electrical representations of the microphone's diaphragm movement. As the diaphragm oscillates, its direction of movement changes rapidly. The resulting microphone signal current does the same. Rapid changes in the direction of current mean the mic signal is an alternating current!

As an easier visual example, take a look at a large loudspeaker projecting low bass frequencies. At these lower frequencies, we can actually see the loudspeaker diaphragm oscillating back and forth, pushing air in a way that represents the audio signal being sent to the speaker.

So microphones are not AC devices by the definition that they do not work on AC voltage. That being said, microphones are indeed AC output devices.

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A Note On USB Microphones

The popular caveat here would be that USB microphones output digital audio data rather than electrical AC audio signals.

The truth is, the mic capsules of USB microphones do, in fact, create AC voltages. However, these microphones have built-in analog-to-digital converters that convert these AC voltages into digital data before the audio is outputted from the microphone.

For more information on analog and digital microphones, check out my article Are Microphones Analog Or Digital Devices? (Mic Output Designs)“.

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Related Questions

Will phantom power damage my microphone? Phantom power may damage the ribbon diaphragms of vintage ribbon microphones that are not designed to receive +48V. Modern microphones either require phantom power or have transformers at their outputs that will effectively block DC and will not be damaged if phantom power is applied.

For more information on phantom power and ribbon microphones, check out my article Will Phantom Power Damage My Ribbon Microphone?

Are dynamic microphones active or passive? The vast majority of dynamic microphones are passive. Dynamic mics transduce energy via electromagnetic induction and do not require power to function. That being said, there are some dynamic mic designs (particularly ribbon mics) that have built-in preamplifiers and are therefore active.

For more information on moving-coil dynamic mics, check out my article Moving-Coil Dynamic Microphones: The In-Depth Guide.