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Dr Fernando Montealegre Zapata

Dr Fernando Montealegre Zapata

Dr Fernando Montealegre Zapata
BSc(Valle), PhD(Tor)

HFSP Research Fellow

Area of research

Biophysics of unconventional ears with acoustic wave guides

University of Bristol,
 Woodland Road, Bristol BS8 1UG
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+44 (0) 117 331 7322

Summary

  • The transfer of mechanical energy via hollow structures is of great interest in many areas of engineering. Tubes as waveguides are matters of investigation since they are subject to many constraints, one being external acoustic radiation via the tube wall. In human hearing aids for example, acoustic radiation from tube walls causes feedback. Acoustic insects like bush-crickets detect sound with a system of paired ear drums on each foreleg, just below the knee. An air-filled tube, the acoustic trachea, delivers sound from the acoustic spiracle on the side of the body down the foreleg to the internal face of the eardrums. The wall structure of this tracheal tube is flexible and elastic, allowing sharp curvatures (produced by leg articulation) along its length. This "air tube" is thus confined inside a fluid-filled chitin cylinder. The acoustic trachea of insects and human hearing aids tubing have remarkably analogous functions; the former is however effective at much smaller length scales, opening up the opportunity of further miniaturisation of acoustic wave guides. This project aims at elucidating the mechanisms by which ears endowed with wave guides function to generate high sensitivity and directional accuracy. The role of accessory structures backing a conventional eardrum has been largely neglected thus far. Using mathematical modelling, I am exploiting experimental and theoretical data to understand the modalities of sound propagation these auditory systems.
  • In katydids and crickets the ear consists of two adjacent tympana  located on the forelegs (one pair per leg), which vibrate in response to both external and internal sound pressure because they are backed by a pipe-like structure derived from the insect’s tracheal (breathing) system, called the acoustic trachea. The acoustic trachea also connects the ears with the insect’s thorax through a semicircular opening, the acoustic spiracle. In the ear of these creatures unusually the mechanoreceptors arew not in direct contact with the tympana, as it is in many other acoustic insects (e.g. locusts and moths). It is unknown how vibrations caused by sound are transmitted from the tympana to the mechanoreceptors. My current research focuses on the study of these 'unconventional' ears.
  • The principles of sound propagation in biological wave guides may find direct applications for the development of novel detection technologies, notably in hearing aids and smart miniature pressure sensors. This project is sponsored by a Human Frontier Science Program (HFSP) - Cross Disciplinary Fellowship.

Activities / Findings

My current HFSP-fellowship research aims at elucidating the mechanisms by which ears endowed with waveguides function to generate high sensitivity and directional accuracy. The role and structure of three of the four major components of the katydid ear (waveguide, tympanum, and mechanoreceptor) have been studied independently in a few species. So far no comprehensive approach to integrate the functionality of these organs has been undertaken. Using Laser Doppler Vibrometry, and Micro-CT x-ray scans of the ears I have build high-resolution (µm) large-scale computer models (in AMIRA) for finite element acoustic modelling and further applications in COMSOL.

I discovered a novel structure that constitutes the fourth ear component, not considered in previous work. This organ (a fluid vesicle) seems to be the missing piece in the understanding the process of energy transformation in these ears. The tympanal vibrations are transmitted to this fluid and then to the receptors. This cavity might play a role in hard tissue sound conduction, acting as impedance matching transformer. Enticingly, this possibly constitutes a mechanism analogous to the mammalian cochlea.

Summary of outcomes:

a) The tuning of the eardrums of katydids and crickets in some species is close to the frequency of the call, and in these cases the tympanum acts as a mechanical filter (e.g. the New Zealand weta Lomas et al. 2010 coauthored). In other species, depending on different factors (e.g., ecological conditions), the mechanical response of the tympanic membrane may be flat to a broad range of frequencies. Therefore, discriminating tuning and sensitivity should occur at the nerve level (e.g. Indian tree crickets, Mhatre et al., 2009, coauthored).

b) A new hearing organ (a fluid vesicle) seems to mediate the transferring of mechanical energy into electrochemical signals in katydids (Montealegre-Z et al. in review).

c) The operational function of this vesicle might be analogous to the melon organ of dolphins, and might play a similar role as the mammalian cochlea in the process of frequency selection acoustic energy transformation (Montealegre-Z et al. in review).

Memberships

Organisations

School of Biological Sciences

Recent publications

View complete publications list in the University of Bristol publications system

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