Noise Band Measurement
PAMGuard contains two modules which can measure noise levels in multiple frequency bands. These are called the Noise Monitor and the Noise Band Monitor, both available from the File / Add Modules / Sound Processing menu.
Both modules should give the same results to within a fraction of a Decibel, so which you use is largely up to you. however, there are some subtle differences.
The Noise Monitor module works using FFT data as input. The resolution of the noise measurements in both time and frequency is therefore governed by how you’ve set up the FFF. For example, if you’ve data with a sample rate of 48kHz and are using a FFT length of 1024 samples, then the frequency resolution of the FFT will be 48000/1024 = 46.9Hz. Since the measurements are made by adding up multiple bins of data from the FFT’s, you can’t get a sensible measurement below a frequency bin size of about 200Hz. Further, the FFT time intervals may not align nicely with exact seconds, so the intervals between measurements may be a bit longer than you’ve asked for.
The Noise Band Monitor works differently, using a bank of filters to process the incoming raw data and these filters can go as low in frequency as you like, and take measurements from the data at the exat times you’ve asked for. On the other hand, the filter banks are slow to process, so this module will take more resources on your computer than the FFT based method.
The advantages and disadvantages of the two methods are laid out below
| Method | Advantages | Disadvantages |
|---|---|---|
| Noise Monitor (FFT method) | Faster (especially if you’re doing the FFT anyway); User can create non-standard bands | Poor resolution at low frequency; Time intervals between measurement may not be precise |
| Noise Band Monitor (Filter bank method) | Better resolution at low frequency; Accurate time intervals; | Slower |