One of the many challenges in the development of hearing aid technology has been creating accurate directionality or the ability to understand where a sound is coming from in relation to the person. Normally, humans use something known as the head shadow effect, where a sound in one ear will be louder and slightly faster than the sound in the other ear. While this isn’t something we consciously calculate our brains analyze and interpret this information and allow us to identify where sounds are coming from. Unfortunately, when someone has a hearing loss, this reduces their ability to localize and the hearing aids must talk to each other and calculate the difference between sounds to compensate. The hearing aids are able to analyze the sound using multiple microphones placed as far apart as the hearing aid allows. However, as hearing aids get smaller and the components get closer together it can create meaningless noise, which is why researchers are always looking for new and novel ways to overcome this problem.
So this leads to the question, how do tiny insects that don’t have heads large enough to disrupt sound waves know where the sound is coming from? This is what researchers have been looking into as we may be able to apply the same principles to cellphone and hearing aid microphones. Below are a couple of examples of how these amazing insects locate sounds.
The Orimia ochracea, is a tiny parasitic fly that lays its eggs on crickets. In order to find its hosts in the dark, it must be able to accurately identify where the male cricket’s mating call is coming from. It does this using its two tympanic membranes (ear drums) that are located at the base of the fly’s legs and coupled together by a strut. This creates a tiny see-saw-like structure that is highly sensitive to any differences in timing or intensity (loudness) of the sound waves and helps indicate which side of the fly the sound is coming from. While researchers have tried to model directional microphones based on the hearing structure of the fly it is a concept that hasn’t been widely applied in commercial microphones, at this point.
Another insect with an ingenious hearing system is the Lesser Wax Moth or Achroia grisella. For this small creature hearing and locating sounds is critical to keeping them off the dinner menus of predators like bats. Unlike our hearing which requires two ears and brain processing to localize sounds, these moths are able to pinpoint sounds with only one membrane and three neurons. The membrane or tympanum is similar to our eardrums but theirs is divided into two sections that have different thicknesses and vibrate differently when stimulated by sound. The thicker portion of the membrane is located towards the head of the moth, while the thinner portion is located towards the moth’s back. This thinner part is where the neurons attach and it vibrates with larger movements and more complicated patterns than the thicker portion. This thinner portion is also stimulated more intensely when a sound is presented from the front and may be how the moths are able to determine the direction of the sound with only one ear. At this point, researchers are still working to build a functional model of the moth's ear but this knowledge might be helpful in designing directional microphones in the future.
It’s pretty amazing to think that these tiny insects might hold the answers to the future development of cellphone microphones and hearing aids!
If you are interested in reading further about this topic we have included some links to articles we used in our research.
https://medium.com/@ResearchFeatures/from-ears-to-engineering-5fa5417b2f5d
https://insidescience.org/news/these-moths-can-track-sounds-one-ridiculously-simple-ear
https://www.karger.com/Article/Fulltext/380741
https://www.nature.com/articles/nature.2013.12941.pdf
https://www.pnas.org/doi/epdf/10.1073/pnas.1615691113