In the previous article I presented general information and technical details concerning the accordion and sound generation process. In this post I present a results of spectral analysis of the accordion music signal, including a different configurations of the registers.

Fast Fourier transform

For the frequency analysis of the digital data, FFT algorithm can be used. This algorithm based on a vector of N samples (256, 512 …, 16384), converts signal of time-domain to the frequency. The precision of computing depends on the size of the input vector and can be expressed by the formula: fp / N [Hz], where fp means sampling frequency.

Registers

The register is responsible for activating specified voices in the accordion. When the specified register is switched on, a combination of active voices is changed. The air flows through the corresponding reeds in all active voices. In this section we will see, how the register affects the music signal.

Melody line

A3 musical note with F0 – 220 Hz. We will analyze it using a three different registers:

Unison register

The unison register has active only one voice, so the generated signal contain one harmonic series with the first harmonic uquals 220 Hz.

A3 - unison, spectrum of the treble side

 

A3 spectral view, unison register

Musette register

The full tremolous register has active 3 voices: an unison, a voice tuned slightly below unison and a voice tuned slightly above. This register is often used to play the French waltzes. The tremulous register causes that produced sound is a vibrant ‑ fluctuations above and below unison can be noticed.

A3 - musette, spectrum of the treble side

 

A3 spectral view, musette register

 

In the spectral view, we see additional pulses near the original frequency (220 Hz, 440 Hz…).

Tutti register

The register which active all voices in the accordion is called the tutti. In comparision to the Musette register, Tutti register also activates voice one octave lower. And as a result, distance between harmonics equals 110 Hz.

 

A3 - tutti, spectrum of the treble side

 

A3 spectral view, tutti register

 

Bass line

Similarly to the section of the melody line, I will present spectral analysis of the root note A coming from the bass side of the accordion.

Unison register

 

A - unison, spectrum of the bass side

 

Root note A, unison register

 

One octave lower register

 

A - lower octave, spectrum of the bass side

 

Root note A, voice one octave lower

 

Tutti register

 

A -tutti, spectrum of the bass side

 

Root note A, tutti register

 

Chords

The chord is a composition of at least three different sounds. In this section I will present spectral views of chords coming from the bass and melody line.

A-major - unison, spectrum of the melody line

 

A-major chord, melody line, unison register
F-major - lower octave, spectrum of the bass line

 

F-Major chord, bass line, unison register
F-major - tutti, spectrum of the bass line

 

F-Major chord, bass line, tutti register

Summary

The register mechanism allows to easy change the timbre of sounds, affecting the degree of polyphony produced by the accordion.


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