Why are languages louder in the tropics?

The ongoing evolution of human language is complex but now it’s been found that the natural environment affects the sounds of languages over time.

A new study has shown the physical properties of the very air we breathe – including its temperature – can influence how easy speech is to produce, and hear.

The study in the journal PNAS Nexus has found that languages that have a higher average sonority – the loudness of speech sounds – are concentrated around the equator and the Southern Hemisphere.

Sonority is influenced by the openness of the vocal tract – the structures that produce and shape sound for speech – during sound production. One of these factors that can influence this is the mean annual temperature.

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Global distribution of MSIs (mean sonority indexes) across 9,179 language varieties from the ASJP database. Color of dots represents the MSI of the language, with redder dots indicating higher and bluer dots indicating lower indices. The fill color of land areas represents the mean annual temperature. Credit: Wang et al 2023, PNAS Nexus

Researchers investigated the relationship between the average sonority of nearly 3/4 of the world’s languages and the average annual local temperature over the period of 1982-2022.

They found a positive relationship between the average sonority and average annual temperature across macroareas – North America, South America, Eurasia, Africa, Greater New Guinea, and Australia – and across language families. 

Voiced sounds are made by vibrating the vocal cords and are usually louder. If you put your fingers against the front of your throat you can feel these when you say “ahh”. Unvoiced sounds are made using free flowing air from the lungs to the mouth where the lips, teeth, modulate the sound. Try it by sounding out these consonants: T, Ch, F, or Sh.

Cold air is dryer and causes more water evaporation from the surface of the vocal cords. This makes it more difficult to produce louder voiced sounds in colder climates. But high temperatures boost the air’s ability to absorb high-frequency components of sounds. So, warm air tends to limit quieter unvoiced sounds.

Recording of the Colville-Okanagan word “t̓aq̓m̓kst” meaning “six.” Colville-Okanagan is a typical low-MSI language spoken around the western part of the US–Canada border. Credit: Salish School of Spokane

The authors explain some of the physics at work in their study. “In the tropics, the ‘lapse rate’ is generally greater, meaning that sound travels much faster in warmer air at lower altitudes, causing sound waves to bend upwards during transmission, and reducing the energy transmitted horizontally.”

Their conclusion is that such an environment leads to a preference for louder sounds, “which are more resilient to the upward bending effect.”

Recording of the Yoruba word “labalábá” meaning “butterfly.” Yoruba is a typical high-MSI language spoken in West Africa. Credit: African Studies Institute, University of Georgia

Languages in Oceania were found to have some of the highest average sonority.

According to the researchers, temperature effects on sonority evolve slowly on timescales of centuries, or even millennia, of linguistic evolution.

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