Upwards spread of masking for fun and profit… or at least to get away from bats

Marshall Chasin
November 11, 2014

Here is a scenario that may occur to you once or twice a day, as it does to me: Pretend you are a moth flying around, minding your own business, and a bat decides that you would be a tasty meal. You quickly have to review your audiology training and knowledge of a bat’s echolocation signal (anywhere from 11 kHz to 212 kHz depending on the species) and come up with a survival strategy.

One obvious approach is to match and emit an identical signal that is 180 degrees out of phase with the bat’s echo-locating signal, just as modern digital hearing aids do with feedback cancellation. This innovation, first appearing in the literature in 1933, would work long enough for the moth to take cover. It is questionable, however, if this approach would have any evolutionary advantage. Since the echo location signal can range from 11 kHz to over 200 kHz, the selection of one frequency would provide protection from one species of bat, but none of the others. Not a very effective approach.

Another solution would be to “jam” the bat’s signal.

Aaron Corcoran, a post-doc fellow in biology at the University of Maryland, discovered that some insects may use a jamming signal to block a bat’s echo-location of its prey. This is the first indirect evidence that insects study audiology.

According to Dr. Corcoran’s research, some moths (Grote’s tiger moth, to name one) emit a high-pitched clicking sound that masks the signal of the bat’s echo-location signal. Rather than trying to match the exact signal of the bat and then flipping its phase 180 degrees, the moth seeks to create a clicking that has a broadband high-frequency-biased spectrum sufficient to mask some (or all) of the bat’s signal. In many cases, the moth’s signal has its energy at a lower frequency than the echo-location signal, but thanks to upwards spread of masking in the bat’s ear, this would be sufficient to mask, or otherwise create confusion about the moth’s location.

Dr. Corcoran also found that Mexican free-tailed bats emitted similar sounds to the tiger moth, but these sounds were designed to distract other hungry Mexican free-tailed bats from their meal. When the scientist aimed recorded sounds of a Mexican free-tail bat directly at another bat about to have a tasty moth, the bat was up to 85.9% less likely to catch its prey.

Remember when the definition of a human being was a “tool user”? We can no longer even claim that humans are the only ones who know how to use upwards spread of masking!



Upwards spread of masking is related to the asymmetry of the travelling wave in the mammalian cochlea. Even though the cochlear tuning curves of individual hair cells are most sensitive at their characteristic or best frequency, they also respond to lower frequency sounds if they are sufficiently loud. This is true of all mammals (as well as some other non-mammals).

This is a necessary aspect of normal hearing function; without a sufficiently asymmetric cochlear hair cell tuning curve, speech and all other acoustic stimuli would sound quite odd.

A salient feature of speech is that it is sequential; lower frequency vowels and nasals are followed in time by other low-frequency sounds or other high-frequency consonant sounds. Rarely do two or more speech sounds occur at the same time. This has allowed upwards spread of masking to remain a relatively minor phenomenon when it comes to speech perception, at least in quiet.

In contrast, music is co-incident, meaning that two or more notes tend to occur simultaneously and also closer together than speech sounds; the creation of musical notes is not limited by the neurology of the vocal mechanism. There can be 16th or even 32nd notes where temporal masking from one moment in time can occur next to another, one a small fraction of a second later.

Music would sound very odd indeed without upwards spread of masking rearing its important head. Each note, and its harmonics, would create a significant upwards spread of masking, and this is why music sounds so good. Too much masking can be as deleterious as too little.

And in the case of moths, there are more of them because of their knowledge of upwards spread of masking.

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