Frequency sweep speaker test on iPhone: find rattles fast

What a frequency sweep test is for

A sweep plays one tone that slides smoothly from a start frequency to an end frequency. Every rattle has a specific note that sets it off — a loose screw, a speaker surround, a phone case, a picture frame on the wall, you name it. Stepping through frequencies by hand takes forever. A sweep crosses all of them in a matter of seconds, and you just listen for the exact moment something else joins in.

How to run a frequency sweep, step by step

1. Open Tone Generator, choose Frequency Sweep. Free version shows Pro Feature and an Unlock Now button here instead.
2. Set Start Frequency and End Frequency — each runs 20 Hz to 20,000 Hz in 10 Hz steps, and the end can be lower than the start if you want a downward sweep.
3. Set Duration, 1 to 60 seconds, and pick a Waveform. Sine is the right call for finding rattles.
4. Phone volume to something moderate. Status reads Ready to Sweep.
5. Tap Start Sweep. It switches to Sweeping... and stops on its own at the end, or tap Stop Sweep to cut it short.

Worked examples: where the sweep is at each moment

It's linear — frequency climbs by the same number of hertz every second. If you want the formula: frequency at any moment = start + (end - start) x elapsed time / duration.

That's 360 Hz per second — fast enough to hear the whole range in one pass, but a rattle will only last a fraction of a second when it does show up, so pay attention.

Here's the catch: a linear full-range sweep spends only half a second below 1,000 Hz, which is exactly where most rattles actually live. For speaker and bass work specifically, a narrow range beats the full sweep every time.

Only 20 Hz per second here. So if something buzzes 12 seconds in, you already know it's around 340 Hz without doing any more math.

Finding the exact rattle frequency

1. Run a wide sweep first, the default's fine, and note roughly when the rattle starts.
2. Work out the frequency at that point with the formula above.
3. Sweep again over a narrow range around it — 100 Hz either side is a reasonable start — with a longer Duration.
4. Switch to Manual Tone, hold that frequency, press on parts of the case or object until the buzz stops. Whatever you're touching is your loose part.

Choosing a Duration: fast scan or easy timing

The same range runs faster or slower purely based on Duration. The default 200 to 2,000 Hz covers 360 Hz per second over 5 seconds; stretch that to 20 seconds and it drops to 90 Hz per second, four times slower, which makes timing a rattle's start a lot easier. My usual approach: a short, fast sweep to find roughly where the problem lives, then a slow, narrow one over that spot to pin it down exactly.

Limits worth knowing

• Phone speakers put out little deep bass, so sweeps below about 100 Hz come out faint.
• Output's mono — no sweeping one speaker at a time.
• The app never measures the speaker's actual response; everything here is by ear.
• Pitch can run a few percent below the numbers shown right now, and a sweep might run slightly longer than set. Fine for rattle-hunting; not fine for anything you'd call a measurement.

Questions and answers

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