Sound Spectrum Pro for Windows

Measurements

Is the normal mode for looking at the spectrum of the environment sounds (the internal signal generators are not used).
The app gathers continuously audio data and submits it periodically to the calculation pipeline to extract frequency information.
The FFT size can be quite large (64k of 44.1kHz samples takes approx 1.5 seconds, 1M takes 25 seconds). In order to have more frequent display updates, the calculations are done before a full new set of samples is available. Some previous samples are used together with the new ones, and a part of the old samples may be masked out. The actual numbers are auto-adjusting and depend on several settings. The minimum number of new samples used in each cycle is 4k. In accurate mode (see FFT), the window size is always the full size of the FFT.
The FFT window is important for this mode. You can best see the window effect by starting a sine-wave and enabling loopback. Some hints on usage:
- Rectangular - when the input signal is noise
- Hamming or Blackman - generally good compromise
- Nutall, Blackman-Nutall or Blackman-Harris - good dynamic range (small signals close to large signals)
- Flat-top - good amplitude accuracy

Is the best choice for transfer characteristics (what sound output is produced for certain inputs applied).
It is used together with the internal signal generators. Except "Tone", all signal types can be used in synchronous mode.
In this mode the calculations are run only when the full number of samples (FFT size) are ready. For a 64k FFT size the updates come more than a second apart.
To start this mode:
- Start a signal generator.
- Press the SYNC button to start synchronous mode. This will also auto-adjust some settings:
- The generated signal is set to be periodic with the same number of samples (and time period) as the FFT size (mandatory, internally adjusted).
- The FFT window type is set to Rectangular. Other windows may distort the results (especially true for sweeps).
- Smoothing is turned off for faster response.
To exit this mode: stop the signal generator, or press ASYNC.
As always, you can see the "ideal" response by enabling loopback.

The accuracy of all measurements improves with the FFT size. Larger FFT size means smaller FFT bin (calculation interval) from 3Hz at size=16k to 0.05Hz at size=1M. This brings:
- Frequency precision.
- SPL accuracy due to more data points to work with (especially visible at low frequencies).
In accurate mode (filter times >= 25 seconds), the window covers the full number of samples and the frequency resolution is the maximum achievable. Filter times from 50 seconds up use under-sampling to further increase the frequency resolution without increasing the computation time. The trade-off is the decrease of the maximum frequency. With a setting of 100 seconds, the accuracy (FFT bin size) is 0.01Hz. Please note that this may be affected by the device's quartz precision and jitter.
The correct choice for the FFT window can affect the measurement a lot.