Humans as well as most other land-living vertebrates use the time delay between the arrival of a sound wave at each ear to discern the direction of the source. In frogs, lizards and birds the distance between the ears is too small to do this. However, they have an air-filled cavity that serves as an “inner ear” connecting the two eardrums, in which internal and external sound waves are processed.

Devising a universal mathematical model, the research team at TUM, in collaboration with colleagues abroad, have shown for the first time how new signals are created in this “inner ear” used by animals for localizing sounds.

Almost all mammals, including humans, localize sound sources horizontally via the the delay in time at which sound signals arrive at each ear. Frogs, many reptiles and birds do not have this option since the distance between their ears often measures just a few centimeters. The time difference is thus so small that it cannot be processed by the brain.

To make up for this disadvantage, these animals have developed an effective system using the cavity between their two eardrums. This cavity, which runs right through the head, couples the eardrums. The scientists refer to this as “internally coupled ears” or ICE. This “tunnel in the head” is clearly visible when light falls into one ear of a gecko, for example, because the light then shines out of the other ear.

Unlike humans, animals with ICE perceive not only external signals, but also a superposition of external sound waves with those that are created internally through the coupling of the two sides. Scientists have determined in previous experiments that animals use the resulting signals for pinpointing sound sources. But what exactly happens within the coupled ears had remained a mystery. Now, scientists working in a team led by Leo van Hemmen, PhD, professor of theoretical biophysics at the Technical University of Munich (TUM) have developed a universal mathematical model that describes how sound waves propagate through the internally coupled ears and how clues for localizing sound sources are created in the process.

Over 15,000 species have internally coupled ears, reports van Hemmen, which is more than half of all land-dwelling vertebrate animals. Using their model, van Hemmen and his team discovered that the animals have even developed two different methods of hearing with internally coupled ears, which occur in different frequency domains and augment each other.

In sounds below the fundamental frequency of the eardrum, the time difference in the superposition of the internal and external signals is amplified up to five-fold, which is sufficient to facilitate sound localization. In higher frequencies, the time difference can no longer be evaluated. Here, another property of the signal becomes relevant: The difference in the amplitude (the loudness of the sound perceived by the ears).

This new insight on the mechanisms and advantages of hearing with internally coupled ears is also relevant for industrial applications. It is conceivable that robots in the future will be equipped with this kind of hearing system.

Source: Technical University of Munich (TUM)

Image credits: TUM; © Volodymyr Byrdyak