Illustration of a spectral analysis (multiple peak sounds within one tone) of a 55 Hz pitch tone.

<p>A regular sound vibration is a complex tone that consists of a fundamental frequency (F0 or harmonic number 1, h1 or A1 on the Figure, i.e., most left peak) + a series of harmonics or partials (h2–16, other peaks). Together, they blend, giving the impression of one single tone. The frequency of each partial is a multiple of the F0 (55–110–165 Hz and so forth…). These multiple relations are expressed in ratios on the Figure. For example h3 (165 Hz) is related to h2 (110 Hz) with a ratio of 3∶2. When considering the harmonics h2–16 in relation to the F0, you obtain the illustrated frequency ratios (see on the bottom of the Figure). Van Puyvelde et al. (2010) observed that, during tonal synchrony, mothers and infants share a tonal center (which corresponds with F0/h1 and its octaviations) to which their other uttered pitches are related in the same fashion as the partials of a harmonic series are related to the fundamental frequency or F0 (i.e., the mothers and infants use the same ratios as pictured on the Figure). The partials of the tonal center are indicated by rectangles and the arrows show the frequency ratios with regard to this tonal center. For example h6 is related to h4 (ratio 6∶4 = 3∶2) and not h5 because h5 is not part of the tonal center (no octaviation of F0). On the right, an overview of the distribution of the ratios during tonal synchrony and their approximated music interval is given, with a distinction between high consonance (2∶1, octave; 3∶2, perfect 5; 5∶4 major 3) medium consonance (7∶4, minor 7; 9∶8, major 2) and dissonance (11∶8, triton; 13∶8, minor 6; 15∶8 major 7). The same ratios and distribution were used in the tonal fragment, i.e., 70% high, 25% medium consonance and 5% dissonance).