What Are Audio Interfaces & Why Would A Microphone Need One?

Have you ever noticed that microphones need to be connected to other audio devices for their audio signals to be used properly? These mic inputs could be on preamplifiers, mixing consoles, recorders, or, as the name of this article suggests, interfaces.

What is an audio interface, and why would a mic need one? An audio interface is a device that allows a computer to interface with audio inputs and outputs. Audio interfaces are key components in the communication between analog and digital audio devices (including microphones) and a computer's audio I/O and digital audio workstation.

In this article, we'll talk more about microphones and their audio interfaces to help you better understand their functions and the situations that call for their use.

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What Is A Microphone Audio Interface?

As mentioned, a microphone interface is an audio interface. These interfaces essentially allow communication between various audio input and output devices and digital software.

Microphone interfaces, then, allow specifically for the communication between microphones and digital software and computers.

Note that the vast majority of audio interfaces on the market today have microphone inputs and are, therefore, microphone interfaces as well.

The term “microphone interface” is used much less often as an “audio interface,” though they often refer to the same piece of audio equipment.

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How Do Microphone Interfaces Work?

To better understand how mic interfaces function, we'll look at a single signal path. That is to say, we'll be discussing one microphone, one mic input, and the potential pathways for this mic signal.

It's important to know that many interfaces offer multiple mic inputs and have great flexibility in their signal routing capabilities.

So let's start with the microphone. A microphone is a transducer that converts sound (mechanical wave energy) into audio (electrical energy).

To learn more about how microphones work, check out my article How Do Microphones Work? (A Helpful Illustrated Guide).

This electrical audio signal is an AC voltage that effectively mimics the sound waves at the mic diaphragm (only in electrical form). The mic signal is outputted via the mic output and through the connected mic cable.

The mic cable is typically a balanced XLR cable and carries the mic signal to the microphone interface.

At the microphone interface, the signal is amplified by a mic preamp. This boosts the relatively low “mic-level” signal to a healthier “line-level” signal for use with other audio equipment (professional or consumer-grade).

So far, there is nothing unique or special about this signal path. Many mic preamps, mixing consoles, and analog mic inputs share this initial signal path.

However, the mic signal passes through an analog-to-digital converter at the audio interface and is effectively converted from an AC electrical voltage to digital information (1's and 0's).

This digital mic signal is then sent out of the audio interface to a connected computer or digital mixer.

The digital mixing/routing software is where things get really interesting. An inputted microphone signal can be sent to multiple channels within the software. It can be recorded and played back through the same interface.

The same digital cable (USB, Firewire, Thunderbolt, etc.) that carried the mic signal to the computer will carry digital audio out of the computer. This outputted digital audio is then passed through digital-to-analog converters before being sent out of the interface's outputs.

Related article: FireWire Vs. USB Vs. Thunderbolt Audio Interfaces

These outputs include monitor outputs, headphone outputs, and other bus outputs.

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Recapping How Audio Interfaces Work

The above was a simple explanation of how microphone audio interfaces function.

To recap, audio interfaces allow different digital and analog devices to transfer audio between one another effectively.

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A Hypothetical Audio Interface Example

A single interface could, for instance, connect a computer's digital audio workstation to several microphones, a directly injected bass guitar, and a keyboard. This same interface could output analog audio to several headphones (each with a potentially different mix) as well as a pair of studio monitors.

So, what do we have in the above hypothetical audio interface?

• 8 inputs (7 mic inputs for the 6 mics and 1 DI box and 1 high-impedance line/instrument input for the keyboard)
• Analog-to-digital converter
• Interface to computer/DAW
• Digital-to-analog converter
• 6 outputs (2 monitor outs and 4 headphone outs)

Note that, in this example, there may be other inputs and outputs that aren't in use. It's very common for an interface to have more outputs than it does inputs. This is simply an example.

The Inputs

Note that, in the above diagram, inputs 1-6 are mic level inputs. Inputs 7 and 8 are line inputs (7 is a low-impedance line input while 8 is a high impedance instrument-line input).

Each of the inputs has its own preamplifier that applied gain to the inputted signal. Adjusting these gain levels appropriately will bring the audio signals to healthy levels that are balanced with the other audio inputs.

Oftentimes these inputs will be combination jacks that offer both mic and line inputs. Some inputs (like input 8 in this example) can switch between low-impedance (for microphones and DI boxes) and high-impedance for many unbalanced instruments (like the keyboard, for example).

The Analog-To-Digital Converter

An ADC effectively converts the analog audio signals from the inputs into digital audio information that can be sent from the interface to the connected computer and the computer software (including the digital audio workstation).

The Digital Interface And Computer Software

Once the audio is converted into digital information, it is sent to the computer for use in various software.

With larger interfaces (like in our example), there is typically an I/O software where the user can monitor levels and send individual inputs to different outputs.

Within the computer's settings and the computer's digital audio workstations, we must select the intended interface to be our input and output device. This allows for proper communication between the computer and the interface's inputs and outputs.

Once the interface is properly selected as the input and output device, the digital audio workstation will show the inputs/outputs as selectable.

More routing can be done within the DAW for various headphone sends and mixes.

Note that latency may be an issue when routing this way since the audio must pass through the ADC, the computer, and the DAC before being outputted. Depending on the computer's processing power, this may cause a slight delay in the signal flow.

The Digital-To-Analog Converter

Speaking of DACs, the digital-to-analog converter within the audio interface effectively takes the digital audio from the computer. It converts it to analog audio signals for the interface outputs.

The Outputs

The audio interface in the above example has multiple outputs.

2 of these outputs (1 and 2) are the typical line outputs for the left and right studio monitors. Outputs 3-6 provide 4 different headphone sends (mixes) that can be tailored for different musicians or performers during a recording session.

Getting Creative With Routing

The above example is but one of many ways an audio interface could be set up. Depending on the number of inputs and outputs in an interface, we can get very creative with the routing.

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Some Audio Interface Examples

Microphone audio interfaces range in complexity from super simple to very complicated, capable of intricate routing. Let's take a look at a few examples.

Single-Channel Audio Interfaces

Single microphone audio interfaces can be as simple as a one-channel analog-to-digital converter. One example is the Shure X2U:

Double-Channel Audio Interfaces

Popular two-channel microphone audio interfaces include the Focusrite Scarlett 2i2 and the UAD Apollo Twin:

Larger Audio Interfaces

There are many, many interfaces to choose from. They range from single or double inputs (like those above) to 8-channels and above. To learn more about my recommended audio interfaces for microphones, check out My New Microphone's Best Microphone Audio Interfaces.

One such example is the UAD Apollo 8 Quad, which has 8 inputs and 14 outputs:

As we see below, the UAD Apollo 8 Quad has its own software for routing purposes. More routing is possible within the digital audio workstation of the computer, but this console software covers all the necessary hardware routing:

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Why Would A Microphone Need An Audio Interface?

A microphone needs some sort of interface if its signal is to be connected to a digital mixer or any kind of digital audio software (including DAWs).

Common microphone connections that require audio interfaces include:

• Computers
• Digital mixers
• Digital recorders

Otherwise, a microphone (which is inherently analog) requires only a preamplifier to boost its signal to line level for use with other analog audio devices and equipment (like analog mixers and recorders).

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Built-In Microphone Interfaces

Many digital recorders and mixers will have built-in microphone interfaces that convert the analog microphone input signals to digital audio that the mixer/recorder can effectively use.

For example, the popular Zoom H4n Pro has a stereo pair of analog microphones in its design with a built-in audio interface (with analog-to-digital converter) to allow it to record the mics in a digital audio format.

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Related Questions

Can you plug a mic into a line input? It is possible to connect a microphone into a line input physically (this is typically done via XLR connectors). However, mic level signals for microphones are too weak for inputs expecting line level and plugging a mic into a line-level input results in a poor signal-to-noise ratio.

To learn more about XLR connectors, mic and line-level signals, and signal-to-noise ratio, check out the following My New Microphone articles (in order):
• Why Do Microphones Use XLR Cables?
• Do Microphones Output Mic, Line, Or Instrument Level Signals?
• What Is A Good Signal-To-Noise Ratio For A Microphone?

Can you plug a mic into an aux input? Though connecting a microphone into an aux input is possible, it is generally a bad choice. Auxiliary inputs are designed for an amplified signal (like headphone or speaker signals), so relatively low amplitude mic level signals are too weak to load the aux inputs effectively.

To learn more about microphone connections and audio levels, please consider reading my articles What Do Microphones Plug Into? (Full List Of Mic Connections) and Do Microphones Output Mic, Line, Or Instrument Level Signals?