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Mono Compatibility, Is it still important. How does it work?

Updated: Sep 8, 2022

Do we still need mono compatibility?

I still regularly get asked about mono and mono compatibility and if it is needed. And if so, how to ensure good mono compatibility.

1. Is mono compatibility still relevant?

Mono compatibility for music and program is still very important in the modern age of audio production. A large amount of mono playback systems still exist despite the fact that most modern music production is stereo. Lets look at why mono playback exists, why it is utilized and how make considerations for this in audio production and engineering.

2. Where do mono playback systems still exist?

When I work I like to think about two overarching ways that music is consumed or utilized. 1. Personal playback

2. Shared playback

The reason I highlight this – is people might quite often mix with a specified outcome in mind for playback (this is not to say they are not mixing to sound optimal on all systems). IE. Mixing EDM mainly for DJ/Club playback or mixing an intimate indie record that is likely to be consumed via personal playback systems like headphones. Regardless of the ideal way you would like your music or audio productions to be consumed, mono compatibility is still important. 1. Personal Playback Mono compatibility In my opinion there is less need for personal playback mono compatibility (as much as for shared playback). But it is still important. A large amount of the play back systems that people will use to consume music personally are stereo systems. Such as headphones, Hifi systems and bookshelf speakers etc. Even most laptops now are stereo. However there are still mono personal playback systems, such as phone speakers and some Bluetooth speakers and radio (radio can be stereo or mono depending , but I won’t get into that too much here, links at the end for further reading). Personal mono playback back also exists in live sound, with Fold back wedges usually being a mono send (unless you’re really lucky) or Live IEMs Being mono.

2. Shared playback mono compatibility depending on the type of audio production you do there a much larger need for mono compatibility in shared playback systems. The first systems that come to mind here are PA systems in bars, clubs, venues and at festival stages. These PA setups are (Hopefully) usually always stereo systems. But they may have a mono component involved. With the most common occurrences being a mono sub setup and mono in fills and out fills. Dig a bit deeper and you start to find a lot more mono systems as well. If you ever look around a venue and see speakers dotted around the building these are quite often mono. Some common examples being out fills at clubs and venues, Speakers at table booths and PA systems at gyms.

For understanding the way that these mono systems might affect your music – we have to understand what type of mono signal they are using.

3. Mono in gear design and work flow. Mono compatibility is also very important when considering where mono is used in professional gear design. Analog compressors may still use a mono sum for their detection circuit for compression. A good example of this is most SSL Bus style compressors. Or side chain circuits in compressors.

3. Understanding mono Here I cover the different types of mono systems and how they are being fed their mono signal and how this will possibly affect the output of your program audio.

3.1 Mono Sum Stereo is a form of playback that consists of TWO separate channels of audio. A Mono Sum is a summation of those two channels. The Left and Right Audio channels. With a level adjustment to compensate.

So Mono Sum is (L+R)-3dB The reason why the Mono sum is adjusted by -3dB is to compensate for the level increase from adding waveforms together.

Adding the Left and Right channel together acts the same as adding any other wave forms to each other.

3.1.1 Adding and subtracting waveforms.

When looking at adding simple sine waves. If you add two of the exact same sine wave together you will get a peak and rms level increase of +6dB (Fig 1.01, 1.0, Fig 1.1)

Fig 1.01

If you add two signs waves that are completely out of phase with each other they will null to zero (fig 1.02, 1.2)

Fig 1.02

But of course audio can exist in varying amounts of being in or out of phase with each other. Based on how in or out of phase the audio is with each other it will either be additive (positive Correlation) or subtractive (negative correlation). Thankfully in most audio production we are not usually dealing with single sine tones, and especially (hopefully) not dealing with sine tones that have an inverted phase between the L and R channels. When dealing with complex wave forms, the addition of complex waveforms (in this case, our Left Channel + Right channel) will usually result in a +6dB increase in peak level and a +3dB increase in RMS level. (fig 1.3, Fig 1.4) This is why when we sum to mono there is a -3dB adjustment to compensate for this 3dB increase in RMS signal.

And this is where we can (hopefully) start to understand the potential issues with Mono summing – as shown above, some signals will sum perfectly together (+6db Peak//+6dB RMS), Some complex waveforms will sum to a lesser extent (+6dB Peak//+3dB RMS) and some waveforms may null entirely.

Fig 1.0 Sine tone

Fig 1.1 Sine tone 1 and 2 Add – Showing a 6dB peak and 6dB RMS increase

Fig 1.2 Sine tone 1 and sine tone two (phase inverted) added – Showing that these signals completely null

Fig 1.3 Pink noise 1 playing – showing a peak level of -20dBfs and an RMS level of -23dB

Fig 1.4 Pink noise one and pink noise two added together to show an increase of +6dB peak level (Now at -14dBfs) and an increase of +3dB RMS level (Now -23dB)

3.1.2 Issues that can arise with a mono Sum.

Phase – With Mono sums it is important to think about the correlation of the phase between the L and R channels. If your mix has low correlation – A lot of out of phase audio between the 2 channels. Then your mix will have less mono compatibility. The elements in the mix with less phase coherency will be quieter or extreme cases could disappear completely in a mono sum (as shown above) This may not be an even level decrease across the whole frequency spectrum or a consistent level decrease either and only certain instruments may be affected.

For example if you have used phase or phase effects to create a stereo bass-line – Some notes will be louder or quieter than others depending what notes are more in phase or out of phase with each other across the Left and Right channels.

One of the main issues that can arise that I think is especially important is how the mono sum will affect the relationship between the low end in the kick and bass tracks. An example of this would be;

You have a mono kick – With perfect phase correlation between the Left and Right Channels. When this kick sums it will retain the same relative level as it did in the stereo mix If you also had a bass with perfect phase correlation between the Left and Right Channels then this would also sum perfectly and the level relationship between the kick and the bass would also be the same as in the stereo mix. But say you have a Kick With perfect phase correlation but a bass-line with effects or otherwise that give it a lower correlation reading. Because of it having a lower phase correlation it will not sum perfectly and will give you a lower relative level to what it had in the stereo mix. So now the level relationship between the kick and the bass in the mono sum has changed. Phase Based Effects

With a mono sum – phase and time based effects will present themselves as a level and frequency change depending on which frequencies are summing more and which frequencies are cancelling each other out because of phase.

3.2 Side selective playback

Some mono systems will just take the left side of the audio and use that for playback. This is less than nominal and is usually a big flaw of system design that may be done for any reasons, IE. Lack of channels on the processor in a club so active speakers are daisy chained throughout a club off the Left output of a stereo processor. Getting only one side of the stereo signal means that you will still get all your centre panned instruments and vocals coming through, but if anything is hard panned to one side this may be lost depending which side is being used for playback.

Depending on your age, your most listened too side selective playback may have been listening to one ear bud each of your iPod(or Walkman, Or cassette player) on the bus with your partner. Would be a bit odd if you were just listening to the drums of the song and your partner was only listening to the vocals and guitars.

Side selective playback can also exist to an extent because of the audience’s placement in the crowd. Audience standing in front of the Left PA stack, is very unlikely to hear the Right PA stack and will therefore not hear much audio information that is panned right or hard right. If something is very important in a mix and in a song, remember that if you hard pan this. It may be lost in mono systems that are not a mono sum.

3.2.1 Issues that can arise with side selective playback are Hard Panning – Anything that is hard panned to the opposite channel being used will be completely lost and any elements of the mix panned various amounts to the opposite channel being used will appear quieter in the mix. Pan based Effects are also an issue for this reason – Here is an example

Let’s say we have a mono speaker setup And we are playing back a track that has just a stereo guitar panning from left to right. If the speaker was just fed of the Left channel. Then the Panning effect of the guitar would just appear as a tremolo effect or a constant increasing and decreasing of volume

However, if it was a Mono sum of the Left and right channels. You would be just getting an almost constant level and the guitar would not have a changing volume in the mix as it did on the L channel only setup. Mono sum is far better Equip to deal with panned content as regardless of the position of the audio in the stereo field it will still get represented in the mono signal However there might be some changes in level and placement in the mix dependant on phase and pan law.

3.3. Non spaced stereo systems There are Bluetooth speakers and other speakers that claim to be stereo. And even though they may claim to be stereo, true stereo exists in how sound hits out ears and where that sound comes from (like optimal listening being in triangulation with your studio monitors. These playback systems may have separate speakers producing the L and R channels of the audio. But they are both originating at the same location. These devices are not true stereo, as true stereo requires two SPACED loudspeakers. These speaker designs can be harder to equate for or understand the translation of, as it is not a single speaker producing a mono sum. And with multiple speakers in a small space producing a “stereo” sound there will be a lot of phase cancellation.

3.4 Mono Formats

Most formats that we use to carry audio now are all stereo. IE Digital Wav files, CD, 2Tk Tape.

However some playback formats are mono. Although most of these are obsolete or have been obsolete for many years – Unless you plan on releasing your next single on grammar phone. Vinyl is a playback medium that is stereo. However because of the constraints of the physical medium the bass on vinyl is often Mono’d. From experience usually anywhere up to around 150Hz.

4. Check your mix in mono//checking mono compatibility.

Because of the playback systems and formats stated above it is very important to check your audio productions for mono compatibility

This point may be slightly self-explanatory. But is very important. First think about the type of mono compatibility you are checking for. Are you checking compatibility for the mono sum?

Or for side selective playback?

Mono Sum is the type of mono compatibility that is usually most often checked by engineers. Mono your mix and listen to what elements disappear or change position in the mix, and understand why this may be happening. If there is an element that is very important to the mix that is disappearing in the mono sum – try find new ways to achieve the sound you want in the stereo mix, that has better mono summed compatibility. Eg using Reverb for width instead of stereo wideners.

There are a few handy tools that can help with looking at mono sum compatibility outside of just using your ears – Such as correlation meters I Personally use the correlation meter on voxengo span for looking at entire mix correlation

But for a multiband Correlometer there is a great free option with the Voxengo correlometer.

As well as checking what your mix sounds like as a mono sum it can also be important to listen to what your mix would sound like when side selective playback may be used.

Listen to just the Left or the Right side of the mix and see if the mix still stands up well when just one channel is played. If the mix sounds thin or is missing an element because it is panned to the other channel not being listened too – then consider how to change this or fill this out – Maybe adding in another element to support the hard panned elements. It can also be wise to think about mono compatibility in certain frequency ranges over others dependant on the expected use of your mixes. If you are making music for being played By DJs at festivals then checking the mono compatibility on the low frequencies might be far more important than the other frequency ranges.

Keeping your sub instruments panned centre also makes sense as humans have no sound localisation skills under around 80-120Hz depending on head size. I will be writing more on sound localisation on another article in the future – so if you want to learn more here, keep an eye out.

5. Compose/record for good stereo image and mono compatibility

Good mono compatibility in audio production starts at the composing and production stage The choices you make here will be one of the biggest factors in how well your mix will translate to mono.

Considerations here can include

1. Instrumentation

2. Effects

Instrumentation If you want a nice wide mix that still has a solid centre signal and good mono compatibility make sure you have instrumentation to suit that outcome. Listening to most modern productions you will notice that instruments with low frequency content and especially instruments with sub bass frequency content are usually always panned centre. This is also true for some of the most important instruments to the song. Ie Kick, snare, voice, Lead Guitar. This is for the reasons listed above about making sure the most important instruments to the song will be heard regardless of Monoing or side selective playback (If you skipped to hear hoping for some easy tricks, go back and read above) I say most modern productions – as around the time of the birth of stereo audio and in its popularisation throughout the 60s there was some interesting panning going on in record productions back then with some elements that we are used to hearing panned centre being panned throughout the stereo Field. Eg Jimmy Hendrix - The Wind Cries Mary, Where the drums are panned to the right and the vocals are panned to the left. (again important to note – that panning an element to one side does not necessarily reduce phase coherency – so these mixes could have still a great mono sum compatibility, but not great side selective mono compatibility, If you were to listen to just the right side of this recording, you would hear almost no vocals – which wouldn’t be ideal as it is the driving force of the song)

Use instruments that have a natural stereo field.

If we think about acoustic instruments (or samples of acoustic instruments) Some Acoustic instruments are a mono source instrument – such as solo brass and woodwind instruments. However other instruments naturally have a stereo field as they produce sounds from multiple sources, such as Pianos and Organs. Some instruments that seem like a mono source instrument may not be in a recorded sense. For example – an acoustic guitar may seem like it produces sound from a mono source But when listened to very closely – depending on where you listen on the instrument there are slight variances in sound. On the acoustic guitar we can utilize the small changes in tone and augment it with mic placement to create a natural stereo field. Now as said above – a solo brass or solo woodwind is a mono instrument. But a well spaced ensemble recorded has a great stereo field. However – do note – that a stereo recorded instrument or ensemble may not necessarily have good mono compatibility or coherency depending on how well they have been recorded or what microphone techniques were used.

It tends to be much more effective to create stereo width with good correlation using multiple mono instruments instead of trying to force a single mono instrument to be stereo. For example of you want a stereo guitar that is still mono compatible. It is far better to compose or arrange song with multiple guitar parts panned out in the stereo field. As opposed to taking a single guitar track, duplicating it, and adding effects or delaying it to give the illusion of stereo guitars – as this will case phase issues and a mix with low coherency.


Think about the effects you use or may plan to use in the producing/writing stage as these can be a large deciding factor in how coherent your mix will end up. Some effects mess with phase and timing which can affect mono compatibility. So, think about how you use these effects, and across what frequency ranges they affect. Think very hard about effects that you may be using on instruments with low frequency content.

6. Mid Side/Stereo Processing

Mid Side Processing can be a great tool for use in Audio production. A small recap or for those that don’t know Mid Side processing is the ability to be able to process the Mid signal and side signals of a stereo audio signal separately. Or is it? There is slight issue in calling it Mid Side Processing Even though it is the favoured terminology and I’m using here this naming construct can cause issues with how people thing about M/S and how they understand the process. Mid Side processing should more correctly be called Sum and Difference Processing As the two Channels you end up with in Mid Side are L+R (sum) L-R (Difference) Or even better to be though of as L + Right Phase inverted It is important to make this definition as this is one of the topics that I find the most misinformation and misunderstanding about online – and it took me a long time to understand this correctly (or more correctly than I used too). Side Processing will no affect everything that has been panned to the sides. But it will only affect the difference between these two channels. 3 Examples for how it will sum to difference (L+ Phase inverted R)

Mono signal panned centre.

Same signal In Both channels. When the right is phase inverted and added to the left. The two waveforms are completely out of phase and null to nothing in the Difference (side) Channel. Side Processing will not affect this signal at all Left + Right Sums perfectly so Mid (Mono Sum) processing will affect this signal.

Mono signal panned hard left, Phase inverted signal Panned Hard Right

When the right is phase inverted and added to the left, they are now perfectly in phase so they sum perfectly Side processing will affect this signal. Left + Right are out of phase so they cancel each other out so there is no signal in the Mid (mono Sum) So Mid processing will not affect this signal

Mono signal panned hard left

Signal only in the left channel. When the right is phase inverted and added to the left There is so summation or cancelation as there is no audio on the right channel. This signal will be affected by the side processing Mid (Mono Sum) processing will also affect this signal as Left + Right Sums as there is no audio on the right channels so there can be no summation or cancelation (L+R = L)

Mono signal panned slightly left

This means there is signal in the left channel and also some of the signal at a lower level in the right channel. When the right is phase inverted and added to the left there is some cancelation as there is the same signal at a reduced level in the right channel that has now been phase inverted. So there is side (difference) at a lower level than from the original stereo file This signal will be affected by the side processing Mid (Mono Sum) processing will also affect this signal as Left + Right Sums as there is audio on both channels that will sum to a higher level on the mid (sum) channel

This is important to think about and keep in mind when doing M/S processing. It can help breaking down you mix into all its individual elements in your head and thinking of how M/S processing will affect each of them individually.

If elements are Present in both the sum (mid) and difference (side) channels and processing like EQ is only applied to one of these sides, the phase change between the two can cause issues in mixes and affect phase coherency in a mix.

If you have a plugin that can do M/S or otherwise where you are able to separate the Mid and side Signal. Try monitoring in mono and raising the side signal – You will notice that there is no level change when monitoring in mono. As raising the side signals is only increasing the out of phase information between the Left and the Right Channels. This helps to show an issue with increased MS processing or program with a low correlation. Lets say you have a track and it gets processed in slightly different ways Opt A is kept with a high coherency and a positive phase correlation Opt B has the side signal increased with M/S processing and has a lower correlation Opt B will naturally now have a higher LUFS reading than Opt A because of the increase in side signal. But Both Opt A and Opt B would still have the same level of Mono signal So therefore If you were to normalise the loudness of Opt A and Opt B to the same LUFS level Opt B – which had mid side processing, would now have the same loudness as Opt A But Its mono Sum would be lower in Level. As detailed below

Fig 2.0 Pink noise with no Mid Side Processing. Note it is peaking at -20dBfs and has an RMS o of around -23dB

Fig 2.1 Pink noise with No Mid side Processing that has been Mono’d Note that the Signal has a Peak of 20dB and an RMS of -26dB

Fig 2.2 Pink Noise with a large amount of M/S processing (done on the BX solo) Note that the signal is now peaking at 7.9dBfs and has an RMS of 13.4dB And the signal has a negative correlation (See bottom right of Voxengo Span)

Fig 2.3 Pink Noise with a large amount of M/S processing (done on the BX solo) That has been Mono’d The signal is now peaking at 20dBfs and has an RMS of -26dB Which is the same as the levels shown in the mono’d signal without the M/S processing

Fig 2.4 Pink Noise with the M/S processing that has been loudness Normalised to match the pre M/S processing levels (this was done using MeterPlugs Perception and checked with Izotope RX)

Fig 2.5 Loudness Normalised Pink Noise with M/S processing that has been Mono’d Note that despite this being the same Level/Loudness as the signal without the M/S processing it has a mono signal that is far lower in level.

Thank you for making it this far in my article Been a little bit since writing articles so hopefully this doesn't read too jarring

If you have any questions on what I've written here or notice any issues with my writing/Wording don't hesitate to reach out to me Posted 01/08/22 Amendments List 08-09-22 Added mono side chain, compressor detector points

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1 Comment

David Williams
David Williams
Nov 29, 2023

Excellent article. Another VERY important shared listening condition is commercial background music systems (retail stores, office buildings, etc.). Typically, these systems are mono and playing out over 70v ceiling speakers. You definitely don't want your mix to sound bad and/or have missing elements due to a lack of diligence at the mixing phase. I'm still surprised at how often I'll be sitting at the dentist office and find myself listening to a "song" that is mostly drums, weird/phased keys, and intermittent vocals. Always makes we think....WOOPS...someone forgot to check their mix in mono.

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