For live sound, PA, and student sound crews

Find feedback-prone frequencies before the show

Check1.2 listens through your iPhone mic while you check a stage monitor and shows, in Hz, the bands most likely to feed back. Use it when you ring out monitors: it tells you where to look on the EQ, and you make the call.

Get Check1.2 free on the App Store Free to download. No external mic needed. iPhone with iOS 17.0 or later.

Sound familiar?

The band is about to walk on, a monitor starts to ring, and you can't tell which frequency to pull down.

A speaker may be wired with reversed polarity, and polarity is hard to hear by ear.

You're teaching students or new crew to run sound, and they can't yet hear which band is ringing.

How Monitor check works

First it measures the room's ambient sound for 15 seconds. Then you talk into the mic through the monitor. After you stop, it keeps listening: a tone that stays at the same frequency is close to feedback, which tells it apart from the room's reverb. It lists the bands most likely to feed back as a number and a word, such as "about 250 Hz (Muddy)." While you record, a green, yellow, or red lamp warns you when feedback is close.

You make the fix

Check1.2 does not touch your mixer or EQ. Pull the level down first. Move the mic or the monitor if you can. If it still rings, make a small EQ cut at the band it found, then listen again. Results depend on the room and the system.

Who it's for

Students learning live sound, and the teachers who run school bands and sound crews. Rehearsal studios and small venues where the band or the staff run the PA without a dedicated engineer. Anyone training new live-sound staff. All Monitor check needs is your iPhone mic.

Also in the app

Analyzer: 50 Hz to 18,000 Hz. The bands most likely to feed back get a solid red line.

Speaker check: play Left, Right, Center, and reversed polarity and check by ear (free). With the Full version, the iPhone mic measures polarity and left/right wiring from the center of the audience area in about 6 seconds.

Frequency quiz for ear training. Free: Beginner, 2 sets a day.

Stereo recording with no compressor or limiter. Free: 5 minutes per take, 2 saved.

Voice profile, Metronome, and Tuner.

Free and Full

Free: Monitor check, Analyzer, Voice profile, Metronome, Tuner, and Speaker check by ear. No account needed.

Full version: $6.99, one-time, not a subscription (U.S. App Store, October 2026). Prices can change, so check the App Store. Keep unlimited Monitor check results (free keeps 3), compare with your last visit to the same venue, and keep a note of what you changed.

Try Full free for 7 days. When the trial ends, the app goes back to the free version. You are never charged.

Know the limits

Check1.2 is not a measuring instrument, and its dB readings are not calibrated. The iPhone mic can't measure accurately below 150 Hz, so feedback detection is off below 150 Hz (the Analyzer still shows that range). Start at a low level and bring it up slowly.

Common questions

Does Check1.2 stop feedback automatically? No. It shows where feedback is likely. You make any level, placement, or EQ changes on your own system.

Do I need an external mic? No. Monitor check uses the iPhone's built-in mic.

Is it a subscription? No. The Full version is a one-time purchase.

Which iPhones does it run on? iPhone with iOS 17.0 or later.

Privacy

Recordings, analysis, your voice profile, results, and venue notes stay on your iPhone. The app connects only for Apple's in-app purchase. No ads, no tracking. The App Store privacy label reads "Data Not Collected."

Try Monitor check free at your next soundcheck

Ring out your stage monitors with the iPhone in your pocket, and see in Hz where to look on the EQ.

Get Check1.2 free on the App StoreFree to download. Full version $6.99, one-time, with a 7-day free trial.