The terms bus or bussing are commonly used in the world of audio, whether we're working with live sound, broadcasting, recording or mixing. When learning about audio engineering, it's important to understand the terminology and how it applies to our work. If you're wondering about audio buses, you've come to the right place to learn more about them!
What is an audio bus? A bus, in audio engineering, is a signal path used to combine multiple signals together and route them to a specific destination. Bus signal paths are physical in hardware or virtual in digital systems. Buses are often used for routing to subgroups, auxiliary tracks, and to the outputs of a mixer.
In this article, we'll discuss buses in more detail, focusing on how they're used in audio.
A Primer On Signal Flow
To understand buses, we should have a good understanding of signal flow. As the name would suggest, signal flow refers to the flow of an audio signal from its starting point to its ending point. In other words, how is the signal routed from one device or signal path to another?
In audio mixers, where bussing is the most common, we will have audio tracks/channels fed by audio inputs. The audio on these channels can be live from microphones, directly-injected instruments, other playback devices, and more. Alternatively, in mixing and playback, the audio is either on the track itself (common in digital audio workstations) or fed into each track/channel input from a physical storage media (tape, for example).
Usually, the output of these audio tracks will be automatically routed to the main stereo outputs of the mixer. The summing of all these tracks' audio is generally done through the “mix bus”, a signal path that combines everything being sent to the mixer's main output.
That stated, there are other routing options available in mixers (hardware is a bit more limited, while DAWs offer much more flexibility).
The first worth noting doesn't have to do with bussing, though it's important for understanding signal flow. Most mixer channels will have insert slots (either physical or virtual) where processes can be inserted into the signal chain.
For example, if we inserted an EQ in insert 1 and a compressor in insert 2, the audio would come into the channel, flow out of the channel to the EQ (first processor), back to the channels, back out of the channel to the compressor (second processor), and then back into the channel for further routing.
For more information on inserts, check out my article Audio: What Are Inserts? (Mixing, Recording & More).
At the other end, the audio track outputs don't necessarily have to be routed (bussed) directly to the main outputs (mix bus). Rather, they can be routed (bussed) to their own subgroups. Subgroups are their own channels that host the summed audio from multiple, often similar, tracks. The subgroups can then be routed (bussed) on the mix bus to the main outputs.
For more information on subgroups, check out my article What Are Subgroups? (Audio Mixing, Recording, Live Sound).
Most mixer tracks will also have auxiliary send options, where we can route signal pre or post-fader to an auxiliary track. We can send independent levels (separate from the track output) of a combination of tracks to an auxiliary track. The aux track can then be routed back into the mix through the mix bus (or another bus). It can also be routed to a separate output to effectively create a different mix than the one routed to the main outputs.
Signal flow is a vital concept to understand when working with audio. A bus is a signal path that can carry audio from multiple sources from one place to another, which is essential for proper signal flow.
What Are Audio Buses?
An audio bus is a signal path capable of carrying audio from multiple sources from one place to another.
The term “bus” is accurate here, as we can imagine a passenger bus in a similar way. Passengers (audio signals) board the bus to be taken from point A to point B.
Buses, themselves, are not channels. Rather they are signal paths capable of sending audio from one channel to another.
So we can set buses as outputs and inputs, sending audio from one or more channels (outputting) to one or more channels (inputting).
A track/channel output or its auxiliary send can be set to a specific bus (Bus 1, for example), and a subgroup or auxiliary track's input can be set to receive that bus (Bus 1, for example).
Most mixers and DAWs automatically bus everything to the mix bus, which goes to the output of the mixer or DAW. However, we can use buses to route signals elsewhere in the mixer, like subgroups and auxiliary tracks, before routing those channels ultimately to the mix bus.
In general, mixers with more buses have more routing capabilities than those with fewer.
Note that buses themselves are not audio tracks, nor are they MIDI tracks that contain audio clips or MIDI information. Again, they are the signal paths (physical or virtual) that route audio within the mixer, console or DAW.
To reiterate, a bus refers to a physical point where multiple signals are summed together in analog devices. In digital devices, this is done virtually through digital means.
Busses are general-purpose and are, therefore, often vague in their description.
Much confusion about buses comes from how we discuss them in practice. For example, we'll often call our subgroups by their name followed by the word bus, i.e., drum bus, guitar bus, vocal bus, etc. When doing so, we often reference the subgroup channel on the mixer or DAW rather than the audio conduit that carries the audio signals themselves.
So if we want to be technically correct, it's better to call these “buses” subgroups.
How Are Audio Buses Used?
Routing is an important part of audio engineering, recording, mixing, etc. Combine this with the fact that buses are so broad in their service, and it's no wonder they're used so much in mixers.
Remember that a bus is simply a signal path that allows multiple signals to be summed together. With that in mind, let's discuss the most common uses for audio buses:
Using Buses: The Mix Bus
The most obvious use of bussing is the mix bus.
The mix bus is at the crux of mixing. It effectively sums together the audio of all the tracks that we want present in the mix. With the entire goal of mixing to create a balanced mix of multiple audio tracks, the mix bus is key.
Now, while many refer to the mix bus as its own dedicated channel, it's important to note that, technically speaking, the mix bus is only the signal path to send/route our audio to our main, typically stereo, mix.
Again, most channels will automatically be routed to the mix bus unless otherwise changed.
Using Buses: Subgroups
Another use of buses is to route tracks/channels to subgroups.
Subgroups are mixer channels that host the audio from multiple tracks/channels. The tracks/channels included in a subgroup are typically similar in nature (all the drum tracks to the drum bus or all the backup vocal tracks to the backup vocal bus, for example).
The audio tracks are set up with a specific bus as their output. The subgroup channel is then set up with that bus as its input. In other words, each audio track of the subgroup is routed directly to the subgroup channel via a bus.
These subgroups can streamline mixing by making it easy to process groups of tracks together rather than processing each track individually.
Using Buses: Auxiliary Tracks
Last but not least on the list of use cases for buses are auxiliary tracks.
An auxiliary (aux) track is a channel on the mixer but not an audio track (it doesn't record audio). Rather, it takes in audio from other tracks within the mixer from the “send” busses of those tracks.
In analog mixers, there are a set number of available auxiliary sends (buses) available on each channel. In digital mixers and DAWs, these virtual sends are typically much higher in number and more flexible in their usage.
These “send” buses allow us independent control over how much signal of a given channel is sent to the auxiliary track without affecting the output of the channel (wherever the output may be routed to). The volume control of an aux send can be dialled in to send the appropriate signal level on the “send bus”.
The aux sends can often be set as either pre- or post-fader, meaning the signal level being sent can be independent or dependent on the actual fader position of that track, respectively.
The auxiliary track being sent to will have that bus as its input.
Regarding aux track output routing, we can output auxiliary mixes to other mixer outputs if we need alternative mixes (common for monitor and headphone mixes for different musicians).
We can, alternatively, choose to have these aux tracks routed back into the mix, either via the mix bus or some other bus that eventually leads to the main mix outputs. In this case, we generally refer to the aux track as a “return” (as in part of the “send” and “return”).
Effects returns are extremely useful for giving us independent control over time-based effects (delay and reverb) and also prove invaluable for parallel processing.
I discuss how to use effects sends/returns and the benefits of doing so in this YouTube video.
For more in-depth info on using bussing in your mixes, check out my video below:
To learn more about auxiliary tracks, check out my article Mixing/Recording: What Are Auxiliary Tracks, Sends & Returns?