Sampling Rate and Frequency Resolution

Higher sampling rates are not automatically better, because a higher sampling rate means lower frequency resolution for the same FFT size. To understand what that means, open the Analyzer Settings dialog and experiment with changing the sampling rate from 11025Hz to 96000Hz. Notice how the frequency resolution decreases as the sampling rate increases.

If the sampling rate is doubled, the FFT Size also needs to be doubled to get the same frequency resolution. Therefore, set the sampling rate as high as necessary for the highest frequency that you want to measure, but not higher. If you want to measure very low frequencies (100Hz or lower), lowering the sampling rate will increase the accuracy of the measurement without requiring a larger FFT Size.

 

The table below shows the spectrum of a tuning fork that is tuned to 105Hz at various FFT Sizes to compare the analyzer accuracy at sampling rates of 11025Hz and of 44100Hz. You can download the recordings here and load them into Overtone Analyzer yourself:

Tuning Fork tuned to 105Hz sampled at 11025Hz

Tuning Fork tuned to 105Hz sampled at 44100Hz

 

Sampling Rate: 11025HzSampling Rate: 44100Hz

FFT Size: 512 Points
512 Points per transform, 21.5Hz Resolution

Frequency Resolution: 21.5Hz

512 Points, 86.1Hz

Resolution: 86.1Hz

FFT Size: 1024 Points
1024Points, 10.8Hz

Resolution: 10.8Hz

1024Points, 43.1Hz

Resolution: 43.1Hz

FFT Size: 2048 Points
2048Points, 5.4Hz

Resolution: 5.4Hz

2048Points, 21.5Hz

Resolution: 21.5Hz

FFT Size: 4096 Points
4096 Points, 2.7Hz

Resolution: 2.7Hz

4096 Points, 10.8Hz

Resolution: 10.8Hz

FFT Size: 8192 Points
8192 Points, 1.3Hz

Resolution: 1.3Hz

8192 Points, 5.4Hz

Resolution: 5.4Hz

FFT Size: 16384 Points
16384 Points, 0.7Hz

Resolution: 0.7Hz

16384 Points, 2.7Hz

Resolution: 2.7Hz

Sampling Rate: 11025HzSampling Rate: 44100Hz

 

In this example, the frequency scale range is 75Hz to 135Hz, and the dynamic range is 66dB, starting at -8dB. The time resolution was set to 1 update per second.