A frequency response plot from your phone.

1. SweepLog chirp, 20 Hz → 20 kHz, 2 s
2. Speaker + roomLeft, right, or both
3. MicrophoneBuilt-in or USB
4. AnalysisFFT, 24 points per octave
5. Curve0 dB at 1 kHz, smoothed

The sweep is logarithmic, so every octave gets the same amount of time and energy and the bass isn't shortchanged. The app plays it at the phone's native output rate while recording, finds where each frequency landed in the recording, and reads the level there. The result is normalized so 1 kHz sits at 0 dB, which makes two curves directly comparable even if one was measured louder than the other.

A phone is not a measurement rig, and this app won't pretend otherwise. It's good at relative answers: is the left side the same as the right, did the repair change anything, is that tweeter alive. Here is what affects the curve, and what to do about it.

It's relative, not SPL

Everything is referenced to 1 kHz. The curve tells you the shape of the response, not how loud the system plays.

Phone mics roll off the bass

Built-in mics are made for voices, and most drop off steeply in the low bass. A lot of the low-end slope in Fig. 1 is the mic, not the speakers. A USB measurement mic with its calibration file loaded fixes most of this. With an external mic, the app records from Android's unprocessed input, so voice processing stays out of the way.

The room is in the measurement

At the listening position you're measuring the room as much as the speaker, and that's where the deep dips come from. To check a driver, put the mic a few centimetres from it, on axis.

Smoothing hides things

1/3 octave is easy to read and good for overall balance. Switch to 1/12 when you're looking for a narrow peak or notch.

Noise adds up

Measure in a quiet room and average three to five runs. Run-to-run wobble mostly averages out.

1. Left against right

   Stereo mode measures each channel in turn and overlays them. Two of the same speaker should track each other closely. When one curve wanders off from the other over a whole range, something in that speaker or its wiring is different.

2. Finding a dead driver

   A tweeter that has quit shows up as one channel falling away above a few kilohertz while the other carries on. A woofer with a damaged cone or a rubbing coil tends to show as a broad loss low down. Measure up close to the suspect driver to be sure.

3. Before and after PRO

   Measure before a refoam, a crossover repair or an EQ change, lock that measurement in history, and overlay the new one when you're done. Up to three can go on one graph. Same mic, same spot, same settings.

4. Checking a tone by ear and by meter

   The Tone Test tab plays a steady tone and shows what the mic actually picks up, frequency and level. Listen Only turns the generator off and analyses whatever is playing, which is handy for checking another source.

• Result after a stereo sweep
• Built-in help
• Chart scale and smoothing
• History: pick runs to compare
• History: two runs overlaid
• Tone Test: live readout
• Tone Test: tone and waveform

Measuring, exporting and calibration are all free. Pro opens up measurement history and comparison, removes the banner ad, and is a single purchase in the app. You can also watch an ad to open history for 24 hours.