Improve Your Mac’s Audio by Combining Speakers
Follow these steps to create a multi-output device:
In Audio MIDI Setup, click the + button under the list of audio devices, and choose Create Multi-Output Device.
Click the Use checkbox for each of the audio devices you want to combine.
As far as I can tell, it makes no difference which device is selected in the Master Device pop-up menu, although the Drift Correction checkbox will automatically be selected for devices other than the master device.
Unless you have a DAC (digital-to-analog converter) that provides higher sample rates, leave the Sample Rate pop-up menu at 44.1 kHz. If you do have a DAC, choose the highest sample rate at which you can hear an audio quality difference.
You should see a little speaker icon below and to the right of the Multi-Output Device entry in the list. If not, Control-click Multi-Output Device and choose Use This Device For Sound Output. (That has the same effect as selecting it for output in System Preferences > Sound > Output.)
To ensure that you don’t blast your ears with too-loud music the first time you use your Multi-Output Device, click the entry for your master device in the list and adjust the Master slider in the Channel Volume pane to something fairly low. Repeat the process with your secondary output device—as you can see below, my Thunderbolt Display has separate sliders for Front Left and Front Right.
Switch to the Music app and start playing a track with which you’re intimately familiar, preferably one with good stereo separation. I always default to the Dire Straits Brothers in Arms album—it was also my first CD back in the late 1980s.
Back in Audio MIDI Setup, work back and forth between the individual output devices to set the volume to a good level and adjust the left/right balance. How you do that may vary by device. For my iMac’s speakers, there’s a Balance slider that I’ve tweaked to the right to favor its right-hand speaker. For the Thunderbolt Display, there’s no Balance slider, so I had to increase the Front Left volume to favor its left-hand speaker. Because the iMac’s left speaker and the Thunderbolt Display’s right speaker sit right in front of me and work together, I’ve de-emphasized them.
Pay attention to the Mute checkbox to the left of the volume sliders. I found it extremely helpful to mute one set of speakers and then the other as I worked to set the volumes and balances appropriately. (For a more precise way of determining the ideal volumes, you could use a sound pressure level app like Decibel X to measure the sound from each speaker in turn.)
If you’re configuring a surround-sound system with more speakers, click the Configure Speakers button to bring up an interface for connecting speakers with channels. I don’t have the hardware to test this, so I recommend clicking the ? button in the Configure Speakers interface for more information.
Finally, if you would prefer to see a more descriptive name, click the name of the Multi-Output Device entry and edit it as desired.
You’re done, but now it’s time to investigate the gotchas.
Gotcha #1: No Volume Control for Multi-Output Devices
Unfortunately, as you’ll quickly discover, a multi-output device has no master volume control in the Sound preference pane, the Volume menu bar item doesn’t work, and macOS’s built-in keyboard shortcuts are disabled. Needless to say, this is a problem, albeit one that has numerous workarounds.
Sound Source: The best solution to this problem is the Sound Source utility from Rogue Amoeba (a recent TidBITS sponsor), which provides menu bar access to system devices (including multi-output devices), lets you control volume on a per-app basis, and can add effects to any device or app. Even better, if you take a quick trip to Sound Source > Preferences > Audio, you can select Super Volume Keys, which enables the system volume keys to control otherwise uncontrollable audio devices. Sound Source costs $39, though TidBITS members can save 20%, dropping the price to $31.20.
Music: Although I always think about controlling volume at a system level, I was reminded that the Music app has its own volume control and keyboard shortcuts. If you play audio only through Music, you could manage volume entirely within the app.
Keyboard Maestro: It’s always worth trying to solve problems with Stairways Software’s Keyboard Maestro, though in this case, it’s not a complete solution. Although Keyboard Maestro has actions that can increase, decrease, and mute the system volume, those actions won’t work unless you also have SoundSource’s Super Volume Keys enabled. More effective are Keyboard Maestro’s actions for controlling volume and playback in the Music app. With those, you can create macros that let you increase or decrease the volume of currently playing music, or just pause playback. Keyboard Maestro costs $36, and TidBITS members can save 20%, dropping the price to $28.80.
Boom 3D: I’m quite fond of the 3D sound effect provided by Boom 3D from Global Delight (another recent TidBITS sponsor). It provides some features that intersect with this problem, though it’s not entirely satisfactory. Most notably, Boom 3D has a volume control in its menu bar interface, and you can assign hotkeys to control it within any other app. Plus, Boom 3D enables per-app volume settings, much like Sound Source. The problem is that the main volume control doesn’t go to zero, rendering it useless as a mute option. The per-app volume does go to zero, so you can mute a particular app, but that requires an extra click and slider manipulation—there are no hotkeys for per-app volume. Boom 3D costs $14.99, and TidBITS members can save 25%, dropping the price to $11.24.
Realistically, since I have all these apps at my disposal, I’m going to employ all of them. SoundSource provides a nice dashboard to all sound-related activities on my Mac and enables the system volume controls, Keyboard Maestro lets me control audio from the keyboard, and Boom 3D enhances the music.
You can add an AirPlay device to Audio MIDI Setup by clicking the + button and choosing it from the Connect AirPlay Device menu. Once you do that, an AirPlay output becomes available for selecting as part of the multi-output device. Unfortunately, as soon as you make the multi-output device the default audio output, the AirPlay device disappears, and no audio is sent to it by the multi-output device.
However, there is a workaround, at least for those using the Music app. If you click the AirPlay button in the Music toolbar, you can select multiple AirPlay destinations and set their volumes independently. The Computer destination uses whatever audio output is selected in the Sound preference pane, so in this situation, it would be my Multi-Output Device. When I combined that with the Bedroom HomePod that I borrowed temporarily, my office was filled with sound.
There are two downsides to this workaround:
It works only in Music. That may be obvious, but if you want to listen to music on YouTube through Safari, say, you’re out of luck.
The only volume slider that controls all the selected speakers is in the Music app itself. You may find yourself, like me, thoroughly confused when other apps seem to be controlling volume without making any appreciable difference. (The HomePod was loud enough that I could barely tell when my iMac and Thunderbolt Display speakers weren’t contributing to the overall sound.)
The solution to those problems is Rogue Amoeba’s Airfoil. It can distribute audio from any Mac app—or all system audio—to AirPlay and non-AirPlay audio devices, working around the Music limitation. You can even independently choose which speaker through which to play “computer” audio. And it provides keyboard shortcuts for controlling volume and muting audio. Airfoil costs $29, or $23.20 for TidBITS members after the 20% discount.