Android SPL, deciBel and RT60 Meter, Spectrum Analyzer, Signal Generator and Polarity Checker Audio Tool
Recommended by "Sound and Vision" magazine.
Features 1/1, 1/3 & 1/6 octave RTA modes, RT60 (wideband, Octave), spectrogram, peak frequency, Fast, Medium & Slow filters, Flat & A/C Weighting, Averaging, SPL Chart Recorder, Noise Criteria, White/Pink Noise, Sine, Square, Triangle, Sweep, Ramp and Impulse signals & loudspeaker polarity checker. Frequency spectrum store & load, von Hann windowing, "Keep Screen On".
Now with ISO Octave Calibration feature - correct for your phone's microphone response! Move the calibration discs up or down so as to obtain a flat response, and match the overall SPL to that of an external meter.
AudioTools's Spectrum Analyzer component uses FFT on batches of samples recorded at the microphone. Hann windowing reduces aliasing. Recorded spectra may be stored as files on the SD card, and then loaded later and displayed together with the live spectrum.
(The "Store" button will store the current live spectrum with a filename of your choice. The "Load" button brings up a list of stored spectra. One of these can be chosen, or the "Cancel" item selected to return to the main display, or the "Clear" item selected to remove the currently loaded spectrum.)
Data collection can be paused and restarted.
To hide the buttons, tap the screen. To restore them, tap again.
The deciBel meter component uses IIR filters and exponential averaging to calculate Flat, A and C weighted response at speeds of Fast, Medium and Slow.
A pair of cursors can be shown on the RTA and Chart Recorder displays: each can be moved to any position, and will display the SPL & time/frequency values measured there. Initially, both cursors are positioned mid-screen - simply drag them to a new position with the fingertip. 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 (available from the Menu) 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 can be made by starting the RT60 component and using a loud clap (or similar) to trigger the measurement. RT60 times measure how much reverberation or echo there is in a room or auditorium.
The Signal Generator generates White and Pink Noise, Sine, Square, Triangle and Ramp waves, and Impulses. The generator uses multiple cycled buffers that are refreshed randomly so ensuring truly random White/Pink noise. The Pink Noise algorithm uses Kemmet's method to adjust White Noise into Pink.
Frequency is set via the arrow buttons or via the keyboard.
The Impulse signal is a delta issued approximately once per second. Impulses can be used to measure the frequency response of audio systems: they have a flat frequency distribution.
The accuracy in frequency of the generated periodic signals is quite good: on the Motorola Droid, errors were of the order of 1% throughout most of the range. Sine wave fidelity is good throughout most of the audio range. Other signals' leading and trailing edges show ringing and decreasing frequency response artefacts in the upper ranges.
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