It's been a little while since I dropped some music gear recommendations on you, but trust me on this...this one is well worth the wait!
I remember being ecstatic when metronome apps first started surfacing on iOS, and then quickly realizing that I needed to play everything ppppp with it if I stood any chance of actually hearing the click. Even some of the more robust metronomes (in terms of custom subdivisions) like Metronomics are essentially useless in an ensemble setting because they are about as audible as:
So the options are, 1) cart around a Dr. Beat DB-90 and its requisite power adapter or 2) plug the iPhone into an existing stereo system or speaker and be constantly bending over or running across the room to adjust settings and turn it on/off. Both options are lousy.
Then it hit me: Bluetooth speaker, son!
I had ignored these when they were first introduced because of their dubious audio quality, but for a metronome, this strident mono sound would be perfect. After much deliberation, I decided on the TekNmotion Air Capsule because it pumps out good volume and (full aesthetic disclosure), it is housed in sexy brushed aluminum. It's small enough to fit in my messenger bag, isn't tethered by any cabling (BT connection to my iPhone) and runs for days on an internal rechargeable battery. This thing is changing my musical life, for real, and it doubles as a hands-free device in the car. While it isn't loud enough for a string quartet going full-tilt fffff, it is more than enough for most sonic scenarios. If you want even more decibels, some of these speakers can be daisy-chained (TekNmotion cannot).
Now, your metronome app of choice is ready for business and your phone can remain on the stand with you. Time to go get groovy...