SPL and Spectrum Analyser

SPLMeter monitors sound picked up by the mic on your Android device and converts it to an SPL in deciBels, as well as showing its frequency composition as a spectrum. The meter has three speeds: Fast, Medium and Slow. Peak spectrum can be shown as well as the detected frequency of maximum strength, in Hz.

Operation Details
Input audio data from the microphone is read out as 16 bit PCM samples at a sample rate of 22,050 samples per second. This sets the maximum detectable SPL level to 90dB (equivalent to 20log(65536)), and the maximum detectable frequency to around 10kHz.

A calibration adjustment is made internally that matches the dB SPL shown by SPLMeter to that measured by the Ivie Technologies IE35 professional grade meter. Metering uses an exponential filter whose speed can be adjusted to Slow, Medium or Fast using a button on screen.

The PCM samples are sent to a FFT in batches of 2048 ... this fixes the lowest detectable frequency to around 10Hz. The frequuency spectrum is plotted on a Hz log scale in the standard way, with dB levels shown on the vertical axis for each point.

The frequency at which the maximum value of SPL is detected is shown on the display, and its position indicated with a small downwards pointing arrow.

The application also shows the real time audio PCM signals as they are collected from the Mic via the Android device's audio subsystem. These are plotted on the lower display from left to right, as a function of time. This display auto scales as a function of the maximum detected signal level.

Buttons are provided that allow the display to be paused and to show or hide the peak measured spectrum.

If the mic signals clip due to overload (very loud noise) then the Mic sample display background changes to red.

Processing Speed
The number shown immediately after the "Mic" legend indicates the frame rate of the application, in milliseconds - in other words the time for data collection and processing ... ideally, at the chosen sample rate, this should be around 200ms, but will vary depending on the speed of the Android device's processor and whether other applications are taking shares of the CPU. On the Motorola Droid it runs at around 250ms, on the HTC G1 it is slower, around 800ms.