Washington, Dec 3 (IANS) Neuroscientists have unearthed powerful new evidence of a deep biological link between human music and speech.

A pair of new studies by Duke University researchers found that the musical scales most commonly used over the centuries are those that come closest to mimicking the physics of the human voice.

The Duke team explains why we understand emotions expressed through music because it mimics the way emotions are expressed in speech.

Composers have long exploited the perception of minor chord music as sad and major chord music as happy.

The Duke researchers, led by Dale Purves, professor of neurobiology, found that sad or happy speech can be categorised in major and minor intervals, just as music can. So your mother was right: It's not only the words you say, but how you say them.

In a separate study, Kamraan Gill, another member of the team, found the most commonly used musical scales are also based on the physics of the vocal tones humans produce.

"There is a strong biological basis to the aesthetics of sound," Purves said. "Humans prefer tone combinations that are similar to those found in speech."

This evidence suggests the main biological reason we appreciate music is because it mimics speech, which has been critical to our evolutionary success, said Purves.

To delve into the emotional content of music, the Duke team collected a database of major and minor melodies from about 1,000 classical music compositions and more than 6,000 folk songs and then analysed their tonal qualities.

They also had people speak a series of single words with 10 different vowel sounds in either excited or subdued voices, as well as short monologues.

The team then compared the tones that distinguished the major and minor melodies with the tones of speech uttered in the different emotional states.

They found the sound spectra of the speech tones could be sorted the same way as the music, with excited speech exhibiting more major musical intervals and subdued speech more minor ones, said a Duke release.

The tones in speech are a series of harmonic frequencies, whose relative power distinguishes the different vowels. Vowels are produced by the physics of air moving through the vocal cords; consonants are produced by other parts of the vocal tract.

These two studies were published in the Thursday online issue of PLOS and in the Journal of the Acoustical Society of America (JASA).