SPL (deciBel) Meter featuring RT60, Leq, Sone, Spectrum Analyzer, Spectrogram, Chart Recorder, Signal Generator, Polarity Checker, and Mic calibration.

Recommended by "Sound and Vision" magazine. Loved by Audio Engineers worldwide: check out the Reviews!
 
Features: 1/1, 1/3, 1/6 & 1/12 octave RTA modes, Leq (wideband, Octave, variable duration), Sones (Loudness), RT60 (wideband, Octave), Peak Store, Spectrogram, Waterfall, Peak frequency, Impulse, Fast, Medium & Slow filters, Flat & A/C Weighting and film industry X Curve, Averaging, SPL Chart Recorder, Noise Criteria (NC and NR), White/Pink Noise, Sine, Square, Triangle, Sweep, Log Sweep, Warble, Ramp and Impulse signals & loudspeaker polarity checker, L/R select. RTA Store & Load, Hann windows, pinch to zoom, scroll to center.

Examples of recent updates (April 2019):
v8.0    New Feature: Load one or two stored spectrum files and display both alongside the realtime data.
v8.1    New Generator option: swap phase of Right stereo signal
v8.2    New Menu option "Set Resolution" allowing to set 4096, 8192, or 16384 sample lengths
v8.3    New 1/12 Octave RTA mode

Exclusive ISO 1/3 Octave Calibration - can correct for your phone's microphone response. Octave calibration discs are moved up or down to obtain a flat response, and match the overall SPL to that of an external meter. Save/Restore of Cal files (popular Dayton Audio iMM-6, MicW i436, also supported).

Use: gig measurements, home theater, acoustics, car, etc.

AudioTool FFTs sets of microphone samples. Aliasing reduced by Hann windowing. SPL calculated in realtime. Spectra may be saved, then loaded and displayed with the live spectrum. The "Store" button stores the current live spectrum - "Load" shows a list of stored spectra to select.

To hide the buttons, tap the screen. To restore them, tap again. To zoom the scale, pinch the screen. To move the scale, drag it up or down (left or right).

A pair of cursors can be shown: each can be moved to any position, and will display the SPL & time/frequency values there. The cursors can be turned on and off in the Menu.

The loudspeaker polarity checker can be used to check whether a loudspeaker is wired in phase: connect the AudioTool output to the speaker under test, select "Polarity" from the signal generator screen, then return to the RTA screen. If the speaker is out of phase (polarity reversed) AudioTool will show "Pol ---", otherwise "Pol +++" if the speaker is in phase.

The Noise Criteria function shows  a set of NC contours overlaid on the 1/1 Octave RTA display and the calculated real time current NC value is also shown.

The Chart Recorder shows a moving trace of the SPL measurements over the last few minutes.

RT60 measurements (how much reverberation or echo there is in a room or hall) can be made using the "RT60" function, triggered using a loud clap (or similar) to or with Pink Noise from the AudioTool generator.

The Signal Generator generates White and Pink Noise, Sine, Square, Triangle and Ramp waves, Sine Linear and Log Sweeps, Warbles and Impulses. The generator uses multiple cycled buffers, refreshed randomly so ensuring truly random White and Pink noise. 

The frequency accuracy of the Signal Generator is typically around 1%. Sine wave fidelity is good throughout most of the audio range. Other signals' leading and trailing edges will show ringing and decreasing frequency response artifacts in the upper ranges, depending on the cellphone being used.

The above description is a very terse version of the Online Manual, which may be accessed here: 
https://sites.google.com/site/bofinit/audiotool

The friendly AudioTool Discussion Group can be used to request features, find/request Calibration files, or report problems. Feedback is always welcome:
http://groups.google.com/group/audiotool-discussion-group

Disclaimer: AudioTool's performance depends on your Android hardware, and is not guaranteed to meet any audio measurement or safety standards.

AudioTool: Fully Featured SPL, dB and Sone Meter, plus Audio Spectrum Analyzer