Introduction to DSP - frequency: frequency spectra I

Using the Fourier transform, any signal can be analysed into its frequency components.

The diagram shows a recording of speech, and its analysis into frequency components.

With some signals it is easy to see that they are composed of different frequencies: for instance a chord played on the piano is obviously made up of the different pure tones generated by the keys pressed. For other signals the connection to frequency is less obvious: for example a hand clap has a frequency spectrum but it is less easy to see how the individual frequencies are generated.

You can use a piano as an acoustic spectrum analyser to show that a hand clap has a frequency spectrum:

• open the lid of the piano and hold down the 'loud' pedal
• clap your hands loudly over the piano
• you will hear (and see) the strings vibrate to echo the clap sound
• the strings that vibrate show the frequencies
• the amount of vibration shows the amplitude

Each string of the piano acts as a finely tuned resonator.