Actual tone frequency

I’m generating a 250 hz tone with audacity 2.1.0 for windows, but when I view on spectogram it seems that the tone’s frequency is less than 200 hz. Anyone can explain this?

I attach the screen shot below. My assumption is what I mark with that green line indicating the height of tone frequency.

I don’t know, but I trust the tone generation algorithm because it’s mathematically simple… There’s almost no way the frequency can be off.*

A spectrogram requires FFT and is about 100 times as complicated. And, there are compromises, trade-offs, and and choices (as you can see from the Spectrogram preference options in the menu).


  • When you actually play the file, the frequency depends on the accuracy of your soundcard’s clock (oscillator). No real-world clock is perfect, but your soundcard won’t be off by 25% unless it’s badly-badly defective. And the spectrogram analysis is not going through your soundcard.

The tone generation in the digital domain is essentially perfect and as close as you an get with integers. (And the sample rate is just an integer number, so it’s value is “perfect” in the digital-data domain.)

The frequency is the “brightest” part of the spectrum. With your current settings it only shows that frequency is “somewhere around” 200 to 700 Hz.
Increase the “Window size” (“Edit > Preferences > Spectrograms”) to about 2048.

Ah, got it! Thanks Steve :smiley: