How To Tell If An Audio Signal/File Is Mono Or Stereo

Mono and stereo are the two most popular formats for audio files and signals. It's worth learning how to tell the difference for compatibility issues between audio devices and audio recording and playback.

In this article, we'll discuss each of these four methods to understand better how to find out whether an audio signal is mono or stereo.

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A Brief Discussion On Mono & Stereo Audio

In audio mixing and music production, it's important to know whether your sound recordings are mono or stereo. For the uninitiated, mono and stereo refer to the number of channels in a sound signal.

In mono (monophonic) sound, the audio is confined to one channel. When a mono recording is played through stereo speakers, the singular mono channel is replicated, and the same exact signal is played through both the left and right speakers.

With stereo signals (stereophonic), your sound is split into two channels: left and right. The stereo effect allows you to perceive width. This allows the listener to ‘sense' where their soundstage the specific sound (whether that's vocals or an instrument) is coming from. This is called sound source localization.

Producers have used this trick for decades to create rich and full sounds in music and other recorded media.

We naturally hear in stereo since we have two ears. The differences in timing and level between sounds at either ear give us a sense of dimension and direction. Stereo audio takes advantage of this biological fact to provide us with an arguably better and more realistic listening experience (though the stereo field can also be manipulated for special, otherworldly effects).

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Method 1: Looking At The Information Of The Audio File

One preliminary method of finding the number of channels is to find the basic info of a digital audio file on your computer. Of course, even 2-channel audio files can render mono results if both channels are identical. However, if we find that the audio has only one channel, we know for certain that it's mono.

In macOS, open Finder and locate the audio file in question. Right-click and select “Get Info” (the hotkey is “Command + i”). There will be information regarding the number of audio channels in the pop-up window under “More info:”.

Unfortunately, WindowsOS has removed this feature. The analogous “Get Info” requires us to open Windows/File Explorer and locate the audio file in question. Right-click and select “Properties” (the hotkey is “ALT + Enter”). However, there is no information in the pop-up window regarding the number of audio channels.

Audio playback programs like iTunes and Windows Media Player will be able to give this information on a per-file basis.

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Method 2: Audio Editing Software

Our first way of recognizing whether or not an audio signal is mono is by checking to see what the sound input is showing up as in your digital audio workstation (DAW).

A digital audio workstation will recognize how many channels any sound source has, and this information should be easily retrievable.

This is simple in the freeware DAW Audacity (available for Mac, Windows and Linux). Ensure you've got the right input selected and check the channel selector. If your sound source is mono, only 1 recording channel will be available.

In a more complex DAW like Adobe Audition, dive into your Preferences to change your Audio Channel Mapping. If your source is stereo, there should be an option to select both the Left and Right channels.

If you're working with a mono signal (a microphone, for example), you'll only see an L channel.

When importing an audio file, you can easily tell if a track is mono by counting the number of channels. If, upon importing a record, there's only one channel (see below).

If there's only one channel, the sound file must be mono. However, even if there are two tracks, the recording may still be mono. Try the remaining two methods to test whether a recording is a true stereo or a mono recording mixed into a stereo file.

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Method 3: By Waveform Or By Ear

Sometimes, a mono recording is mixed into a stereo track. This is usually achieved by replicating the same exact signal of the mono track into the right channel.

In music production and sound mixing, you'll need to know whether the audio track you're working with was recorded in mono or stereo. It may appear when you've imported as two channels, but these may be mixed after the recording to appear stereo.

If this has happened, it's fairly easy to recognize whether or not a track is true stereo. This method is split into two tricks.

Our first trick is to look at the waveforms of each channel. If the waveforms are exactly the same, chances are the recording was mono. If there are obvious differences between the waveforms of each channel, you're dealing with a stereo file.

For example, the image below shows a stereo mix of a sound recording from a mono microphone. We can see that the waveform of the left channel is identical to the right channel.

The next example screenshot is the waveforms of a stereo recording – this time, a song (most modern music is mixed in stereo). We can see the differences between the two channels through the different waveform shapes. This difference will be more subtle in some tracks and perhaps just a question of amplitude changes.

You can also recognize whether the track is stereo or mono by listening to it. This is the “by ear method.”

This will take a bit of practice, but you're listening to hear where the sound is coming from or changes in the track between the left and right channels.

Mono sound recordings will have no differences between each channel. A product of mono audio also means that recordings will appear to come from the dead center of the soundstage.

Sound professionals can often tell whether a track is mono or stereo by just listening to it, and it's possible to train your ears to tell as well.

Hearing the differences is easiest with headphones since there's greater separation of the channels at the ears. Listening with monitors or stereo speakers can work as well, though the right ear will hear the left speaker to a greater degree (and vice versa) than is the case with headphones.

Play a stereo recording and try and pick up on the subtle differences between the left and right channels. Pay attention to the direction that certain instruments are coming from.

Though vocals are often mixed in the centre, vocal delay effects are often stereo. Bass elements, including the kick drum, are also often panned to the centre.

Guitars, keyboards, drums and cymbals are often panned gently or aggressively to the left or right. Listen to a few stereo recordings to hear the directionality of such elements.

Modulation effects like phaser, chorus and flanger, especially when applied to synthesizers, are often bigger and better in stereo. Listen for time-based effects like delay and reverb as well.

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Method 4: Phase Inversion

If you're looking for a more technical method of recognizing whether a sound signal or file is mono or stereo, try phase inversion.

To do this, you'll need to import the track into your chosen DAW. For simplicity, we'll use Audacity for our demonstration. I've imported a stereo mix of a sound recording from my MacBook Air's Microphone. I know this is a mono recording, but we'll see how this can be proven.

First, split the stereo track into two separate tracks: one for the left channel and one for the right.

Then, select one channel and invert it. In Audacity, this is achieved by Effects > Invert. Other DAWs should have a simple process for achieving the same thing.

Then, we'll need to playback the track in Mono. Some DAWs will have a switch for this, or you can use VST plugins to achieve this “collapse to mono” process.

The Waves InPhase is a great example of an audio plugin with collapse-to-mono and phase inversion capabilities.

A straightforward method is to adjust the panning of each individual track to be in the centre.

Then, hit Play to listen to the track. If you can't hear anything, it's a mono recording. This is because the inverted track cancels out the other channel through phasing.

If the track is true stereo, you'll still be able to hear something: the differences in audio between the two channels. Pretty neat, right? Note that the audio will sound thin since most of the low-end bass frequencies are mixed in the centre with stereo recordings.

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What About Analog Audio?

Before we wrap this article up, it's important to discuss analog audio formats such as tape cassettes and vinyl records. Digital audio formats have tonnes of embedded information that can be accessed with computers and software, but analog formats do not share such a luxury.

The most straightforward method, once mastered, is to listen and hear whether the left and right channels are different. Again, this is easier with headphones than speakers.

However, there are physical markers to consider to help us determine whether tape or vinyl records are mono or stereo. The most obvious marker is an indication on the cover of the vinyl or tape itself to tell us whether the recording is mono or stereo. Let's now consider the other clues.

Is My Vinyl Record Mono Or Stereo?

A vinyl record contains grooves in which the stylus/needle reads as the vinyl spins. The grooves, like valleys, naturally have two sides, with the needle fitting within the groove.

Mono vinyl records only have lateral cut grooves (no vertical components). The needle reads information from side to side within the groove but not up and down. These grooves look smooth.

Stereo vinyl records have lateral and vertical cut grooves with different information on each groove “wall.” These two walls meet at a 45-degree angle, and the needle reads information from side to side and up and down. These grooves look jagged.

You may need a magnifying glass and a good amount of light to see the differences in the grooves.

Mono phono cartridges are the best way to enjoy mono vinyl records since they only produce one channel. However, having an all-purpose stereo cartridge is likely the best bet if you're only going to use a single cartridge.

The small inconsistencies (crosstalk, phase errors, and tracking errors) between the two channels of a stereo cartridge on a mono record may or may not be audible.

However, a mono cartridge should not be used on a stereo vinyl record unless it's designed with horizontal and vertical compliance. Otherwise, the sound will be distorted, and the record may even suffer damage in its grooves.

Is My Cassette Tape Mono Or Stereo?

Cassette tapes, like other magnetic tapes, hold a magnetic imprint of audio signals. The tape is made of plastic film coated with tiny magnetic particles, which form the audio signal. As the tape moves past a playback head, the imprinted waveforms are converted into electrical audio signals, which are then amplified for our listening pleasure.

The full tape width of the cassette format is 3.8mm, and it can hold two mono recordings, two stereo recordings, or one of each, both taking up 1.5mm of one side of the tape. As the tape runs one way, the first recording is played back. As the tape runs the other way, the second recording is played back.

For mono recording, the track width is 1.5mm. For stereo recording, each channel has a width of 0.6mm with a 0.3mm separation to avoid crosstalk.

The magnetic waveforms on the tape are practically invisible to the naked eye. However, using a viewer like the Magnet Source Magnetic Field Viewer Card can grant us a better idea of how the tape has been recorded.

If we see two “thick” 1.5mm waveforms, we know there are two mono recordings on the tape. If we see four 0.6mm waveforms, we know that there are two stereo recordings.