Former U.S. President Bill Clinton playing in a band during his second inauguration in 1997. Scientists are closer to understanding what makes some notes sound pleasant - and therefore music - and others not.

Credit: Clinton Presidential Library

SYDNEY: We know a beautiful sound when we hear it, but the underlying reasons for why some musical chords sound better than others have baffled scholars for centuries. Until now.

In a new study published in Current Biology, psychologists may be a step closer to unravelling the secrets of what makes certain sounds pleasing to human ears.

A team at the University of Minnesota asked more than 250 students to rate how pleasing they perceived a variety of sounds. The most appealing, they found, were combinations of notes that had harmonically related frequencies.

Harmonic spectra

"I think understanding why music is pleasurable is likely to be key to understanding why there is music to begin with," said Josh McDermott, now of New York University, who led the study.

Harmonic spectra are prevalent in natural sounds and vocal signals, and animals are well attuned to listening for them, say psychologists. According to the new study, these frequency relationships may also play a key role in our perception of consonance.

The ancient Greeks attributed pleasing chords to simple ratios in instrument string lengths. Others have since come to believe that a listener's cultural context and familiarity with certain types of music exerts a strong influence on their musical perception.

Enjoyment of sound

This new study examines specific acoustical properties and their influence on our enjoyment of sound.

The researchers compared students' perceptions of sounds containing harmonically related frequencies with those that contained beating. Harmonic sounds contain frequencies in multiples: 200, 300 and 400 Hertz, for example.

Beating occurs when close, but not identical, frequencies shift in and out of phase with one another. It gives sound a rough, wobbling quality.

For decades, the predominant theory stated that beating caused us to recognise sounds as dissonant. McDermott and his colleagues, however, suggest otherwise - that harmonically related frequencies had more effect on sound's perceived pleasantness than the presence of beating.

Western music

Further, they found that trained musicians had an even stronger preference for harmonic frequencies. Instrumentalists learn to listen for certain notes within chords, and McDermott believes their preference for consonance stems from the importance of these pleasing intervals in Western music.

"It is evidence that there is a significant learned component to our aesthetic response to sound, and in particular to the response to consonance," he said.

The study predominantly included students accustomed to Western music. "I suspect if we examined subjects with qualitatively different musical experience, we might be able to find some preference for dissonance, or at least a much smaller preference for consonance," said McDermott.

Is a preference for consonance malleable?

The preferences of infants or non-Western adults may be subject of future study, but based on results so far, he believes that the preference for consonance is malleable.

Psychologist Neil McLachlan of the University of Melbourne supported the shift in thinking that beating does not cause dissonance, and said the study confirmed some of his work in the area.

Based on his own experiments, "roughness cannot account for the perception of dissonance," McLachlan said.

However, McLachlan added that the effects of harmonicity and beating that the American team see may be difficult to correlate. Separating beating from hamonicity is complex, and "there is no coherent neurobiological basis for dissonance."

New York University's McDermott recognised the issue is far from settled. "This will be part of a larger story that has yet to fully emerge," he added. "I … hope that it will ultimately tell us something about the big unresolved mysteries surrounding music."