High-Frequency Noise Monitor

"High-Frequency Noise Monitor" is a noise analysis tool that specializes in high-frequency bands above 8 kHz.

You may be exposed to artificial high-frequency noise on a daily basis that is inaudible to the human ear. This app allows you to monitor these noises and examine their occurrence.

Generally, noise contains relatively high-frequency components depending on its source, which is classified as high-frequency noise.

For instance, harmonic noise radiates from electronic devices' main bodies or power lines of power conversion devices, such as AC-DC power adapters. Depending on its power level, this noise may affect peripheral devices.

Some devices generate high-frequency noise to drive away vermin, such as rats and weasels. The frequency of the sound varies depending on the type of vermin. Some devices incorporate sensors that change the volume and frequency over time. When such devices are installed nearby, they can sometimes cause noise problems that have adverse effects on human health.

High-frequency noise does not often occur at a constant volume or frequency, but rather in cycles or randomly. Such noise is not easy to detect.

This app collects randomly fluctuating high-frequency noise from the iPhone's built-in microphone and performs a frequency analysis. It also retains PCM data from the 20 seconds just before measurement stop, enabling the display of spectrograms for FFT and octave band analysis. A spectrogram is a graph showing the results of a frequency spectrum analysis, with time on the vertical axis, frequency on the horizontal axis, and signal strength represented by colors. With this app, you can visually monitor the occurrence of high-frequency noise.

Features:
# Main: Real-time frequency analysis function.
- Real-time measurement: Automatically stops after a maximum of approximately 180 seconds.
- Maximum frequency detection and power level display (dB unit in 20 μPa standard)
- Time trace display: approx. 10 seconds
- Display gain adjustment (Range): 3 levels (0dB - 90dB / -10dB - 80dB / -20dB - 70dB)

# Analysis: FFT/octave band spectrogram analysis function (PCM data for 20 seconds immediately before the measurement is stopped is saved, and spectrogram analysis is performed.)
- FFT spectrogram analysis (FFT)
- Octave spectrogram analysis (OCT)
- Display gain switching (Range): 3 levels
- Playback function: approx. 20 seconds.
- Playback sound filter (Filter): 8kHz high-pass filter to cut off low-mid range sound.

# Snapshot (Snap): Save the analysis result as an image file.

Operation Procedure:
1) Sound measurement and real-time frequency analysis (Main)
Tap Start button in Main mode to start measurement. It displays the real-time instantaneous frequency analysis result and the trace of frequency time variation. The frequency and signal level at which the maximum level is detected can be displayed numerically.

2) Spectrogram analysis (Analysis)
After measurement in Main mode, tap the Analysis button to perform spe