Audacity for recording sound in range frequency 20-100hz

Hy All,
Let me introduce my self, I’m Richard, lecturer from Singapore.
I’m new user audacity. I have try some feature in Audacity and its really helpful me in school.
I have little problem when used audacity in low frequency. The problem is when I want to record audio from microphone in range frequency 20-100hz.
can anyone in this forum help me to teach how set up the configuration to recording that frequency? and next question, we assumed I have an audio file (WAV Format), how to analyze spectrum and check spectrogram audio only in range 20-100hz?

all answer really appreciated.

The first problem is that you need equipment (hardware) appropriate for the job.
Many microphones and microphone pre-amps are optimised for recording the vocal range and purposefully reduce the level of frequencies below 100 Hz so as to reduce “popping” and “rumble”. Good quality studio equipment will often have a “low cut” switch so that you can choose whether to reduce the level below 100 Hz or not. There are also “measurement microphones” that are designed to be equally sensitive across the full 20 Hz to 20,000 Hz range.

When analyzing a spectrum, Audacity (and most other frequency analysis tools) use a technique called “FFT analysis”. This is a mathematical process of splitting the audio into narrow frequency bands. The frequency bands are defined at equal intervals over the overall band-width. Typically the number of bands will be a power of 2 (for example, 1024, 2048 or 4096 bands). Because they split at equal intervals, this gives lots of bands per octave at high frequencies, but only a few bands per octave at low frequencies.

An “octave” is a doubling of frequency. 100 Hz to 200 Hz is one octave. 10,000 Hz to 20,000 Hz is one octave. If the frequency bands are at intervals of 50 Hz, then there will be 200 bands in the 10,000 to 20,000 Hz octave, but only 2 bands in the 100 to 200 Hz octave. This is a problem for analyzing low frequencies, and you will notice that in Audacity’s “spectrogram view” and “Plot Spectrum” that there are very few bands at low frequencies.

The solution to this problem is to make the overall bandwidth much narrower by reducing the sample rate. The upper frequency limit for digital audio is half the sample rate. Thus to represent frequencies of 100 Hz, the sample rate must be (theoretically) at least 200 Hz. Due to technical limitations, the actual sample rate must be somewhat higher. For your purposes a sample rate of 800 Hz should work well. To change the sample rate of a track, use “Tracks menu > Resample”.

In Plot Spectrum, you can increase the number of frequency band by increasing the “Size” parameter. You can also make the lower frequencies more visible by changing the
“Axis” setting from “Linear” to “Logarithmic”. Note that increasing the frequency resolution reduces the time resolution - the FFT algorithm analyzes the audio in bigger chunks.

In “Spectrogram view” you can increase the number of bands in “Edit menu > Preferences > Spectrograms → Window Size”. A larger window size gives higher frequency resolution (more frequency bands) and lower time resolution. You can make lower frequencies more visible by using “Spectrogram (log f)” rather than the normal (linear scale) “Spectrogram” view.

Thanks for your answer,
I saw in options audacity, in there hace options min and max frequency. Can I used the options to filter frequency from microphone?

They only set the default upper and lower frequencies for the vertical scale. They have no affect on the actual analysis, so by setting those to 20 / 100 all you will achieve is a close-up view of a very blurred image. Try it and see.

If you ever need to check what the “factory default” setting are for any part of Audacity, the defaults are shown in the manual. For example: