Loopback’s idea is simple, as is its interface: Any time you have an option for a single audio input (for example, Skype’s Audio/Video preferences or GarageBand’s live-recording option), you can use a Loopback virtual audio device instead. Loopback lets you create any number of virtual devices, each of which has a unique set of audio sources. These can be the output of an app, an audio input device attached to the computer, or even another virtual device. OS X treats each virtual device exactly as if it were a legitimate physical or app source.
You might want to record multiple mic inputs, or mix sound playback (like background music or other audio) into whatever you’re sending into a recording or out to speakers at a live event where the Mac is the mixer. It works well with soundboards, like the eponymous Soundboard from Ambrosia Software ($50), which let you create an array of pre-recorded sounds or snippets you can drop into an audio stream (think drive-time AM radio).
Back in 2005, when I started to create podcasts with Skype as the conduit for remote guests, I used Audio Hijack Pro (AHP) and the free Soundflower extension to redirect (or “pipe”) all the different audio components I needed to the right place. I wanted to capture my mic input before it hit Skype but also let Skype pick it up while recording Skype’s output as a separate track. It was Rube Goldberg-like, but AHP and Soundflower let you do it. (You can still read my write-up and see the gory interface photos.)
Fast-forward to 2015 and Audio Hijack 3’s release, which was an elegantly simplified version of AHP that hid the guts well, but still had layers to dive into for more complicated features. With Audio Hijack 3, you can route and combine audio inputs, but you can’t route Audio Hijack’s output into other software. That’s where virtual audio devices come in handy.
The long-running Soundflower was a lovely hack that helped for many years. It came in 2-track and 16-track versions, and let you route audio output—including from applications—into any track, and then use that as an input to any program, or even the system. Soundflower eventually languished, and Loopback is Rogue Amoeba’s replacement for those who need these kinds of virtual audio endpoints to use with other software, including Audio Hijack.
A “+” popup menu lets you select among recent applications and all current system inputs, or you can choose any application. In the default configuration, all the audio sources are muted, so they don’t pass sound through to the system output, which can cause echoes and other trouble. You can also disable muting if you need to hear them. You can optionally select to monitor the virtual device through any system output available. (These devices can even be edited in Audio MIDI Setup to set sample frequency, like other audio devices.)
Loopback includes a more powerful version of Soundflower’s 16-channel option, too. Leave Channel Mapping to automatic, and the virtual device is a mono, stereo left/right output, or multi-channel input—whatever the source input produces. However, click the Manual button, and channel information appears for each device in the Sources list. You can then track mono and individual stereo tracks into mappings for output channels, up to 32 total. Of course, the app that’s accepting input from this multi-track virtual device needs to know how to cope with it, but many audio programs do just fine.
While most uses of Loopback are parallel—multiple devices ganged into one output—you can also use Loopback without any inputs selected as a “serial” pass-through. This lets you take the output of one program and select it as the input of another, chaining audio through intermediate steps. Pass-through audio is useful when recording in one program that’s aggregating or processing inputs (like Audio Hijack) that you want to send elsewhere. Rogue Amoeba’s example is pretty canonical: Recording a podcast that you also want to livestream through an online service.
An interesting use of bundling and pass-through would let you combine different online audio chat services, such as Skype and FaceTime, allowing everyone to hear everyone else even while using different networks, whether for a podcast or just a conference call.
The only drawback of Loopback is its price: At $99 regularly ($75 in an introductory sale), it’s not a casual purchase for most people. Audio professionals and those who regularly record podcasts will find the price perfectly fine, and the demand for such a specialized offering may be so low that Rogue Amoeba needs to price it high to make it viable to continue to develop. Loopback fits the bill nicely as an elegant way to better control audio routing.