Bioacoustic Modeling

2D/3D Dolphin Bioacoustical Modeling.

Comments, suggestions, and/or questions are welcomed regarding any portion of the research described here. Email the author at .

Simple models of the dolphin echolocation emission system.

Click on the link below for an (unfinished) draft of an article on models of dolphin echolocation signal emission. I am hoping that readers will find this draft interesting enough to send their comments and suggestions. This article proposes a simple 3-component acoustical model of the dolphin forehead that appears to explain a great many features of dolphin echolocation signals. The acoustical behavior of each model component is illustrated using simulation movies and examples of model input-output signals are provided.

Note: This article contains an introduction to dolphin forehead anatomy and a brief description of dolphin echolocation signals. Readers who are not familiar with dolphin anatomy and/or echolocation signal characteristics may wish to read this introduction before downloading the next article on 2D computer modeling of dolphin echolocation beam formation.

I have not yet replaced the figures with thumbnails, so the HTML page with all figures may take several minutes to download (if you are using a modem).

HTML version of draft (~1 Mbyte including all figures): Click here to view.

The following figure is from the above article draft.

Figure 1. Diagram of selected common dolphin head tissues from the above article. [Figure adapted from Aroyan 1990.]

2D Modeling of acoustic beam formation in the common dolphin, Delphinus delphis.

The article below describes how 2D bioacoustic modeling was used to study sonar beam formation by the forehead tissues of the common dolphin. This article describes the methods and summarizes the results of my MS thesis research. Click on the link below for a PDF file.

Aroyan JL, Cranford TW, Kent J, Norris KS (1992) Computer modeling of acoustic beam formation in Delphinus delphis. J. Acoust. Soc. Am. 92:2539-2545.

It has been established that some dolphins possess well-developed acoustic orientation (echolocation) and information gathering abilities, though substantially less is known about the system of sound generation and beam formation. Dolphins use a narrowly focused sound beam which emanates from the forehead and rostrum during echolocation. The primary objectives of this study were to simulate the effects of anatomical structure on beam formation, and to test the viability of various hypothetical sound source locations. Outlines from parasagittal x-ray CT scans were used to construct a 2-D model of the head of the common dolphin, Delphinus delphis. Finite difference techniques were used to simulate sound propagation through tissues modeled as inhomogeneous fluids. Preliminary simulations confirm that beam formation results primarily from reflection off of the skull and the skull-supported air sac surfaces. For the frequencies tested, beam angles best approximate those measured by experimental methods for a source located in a region of the model referred to as the monkey lip/dorsal bursae (MLDB) complex. The results suggest that: 1) the skull and air sacs play the central role in beam formation; 2) the geometry of reflective tissue is more important than the exact acoustical properties assigned; 3) a melon velocity profile of the magnitude tested is capable of mild focusing effects; and 4) experimentally observed beam patterns are best approximated at all frequencies simulated when the sound source is placed in the vicinity of the MLDB complex. © 1992 Acoustical Society of America.

Article PDF file (file size 700 Kbyte): Click here to view.

The following figure illustrates a simulation result described in the above article.

Figure 1. A 2D computer simulation of sound production in the common dolphin. The model of the dolphin's head is indicated by a dotted outline. The yellow region represents the fatty melon tissues, the blue regions indicate skull bones, and the black regions are the nasal air sacs included in the head model. Sonar pulses from a spot (just beneath the uppermost air sac) below the dolphin's blowhole reflect and refract through these structures. The lines around the dolphin's head represent the direction and intensity of sound waves emitted from the model. Most of the acoustic energy is emitted in a forward and slightly upward-directed beam in this 100kHz simulation. The emitted field was collected at the ring of points surrounding the model and compared to the measured beam patterns of live dolphins for several different tissue and source models. Graphic © 1992 Acoustical Society of America.

3D Modeling of biosonar emission in Delphinus delphis.

The article below describes the use of 3D acoustic modeling to study sonar signal emission by the forehead tissues of the common dolphin. This research investigated several aspects of the signal emission process in this animal, including the location of the source tissues, focusing by the melon tissues, the focal properties of the skull, and the overall focal characteristics of the complete head of the dolphin. The methods described are more involved than the earlier 2D simulations, and include a novel approach to modeling the acoustic parameters of mammalian tissues based on x-ray CT data. This article appeared as part of a chapter contributed to Vol. 12 of the Springer Handbook of Auditory Research series. It summarizes the biosonar emission methods and results of my PhD research.

Chapter Reference:
Aroyan JL, McDonald MA, Webb SC, Hildebrand JA, Clark D, Laitman JT, Reidenberg JS (2000) "Acoustic Models of Sound Production and Propagation." In: Au WWL, Popper AN, Fay RR (eds), Hearing by Whales and Dolphins. New York: Springer-Verlag, pp. 409-469.

Article Overview:
Measurements of the acoustic field of echolocating dolphins have demonstrated that dolphins emit a rapid series of pulses in a narrowly focused beam that emanates from the forehead and rostrum. Despite application of a variety of experimental techniques, the exact mechanisms involved in the generation, emission, and reception of delphinid biosonar signals have remained conjectural. Advances in the methodology of bioacoustic simulations have led to powerful combinations of techniques capable of addressing questions that have proven difficult to resolve experimentally. Aroyan (1996) combined methods for 3D acoustic simulation and extrapolation with a novel approach to the mapping of acoustic tissue parameters from x-ray CT data. These techniques, applied to models of the forehead and lower jaw tissues of the common dolphin, Delphinus delphis, allowed sound propagation within the modeled tissues to be studied in detail. The first part of the current chapter discusses the methods of investigation and presents several results concerning the location of the biosonar signal source tissues, the acoustical consequences of forehead asymmetry in D. delphis, and the roles of the skull, air sacs, and soft tissues (including the melon) in beam formation.

3D Modeling of hearing in Delphinus delphis.

The article below appeared in the December 2001 issue of JASA. This article summarizes the hearing simulation portion my PhD research, describing how 3D bioacoustic modeling was used to investigate pathways of hearing and directional sound reception in the common dolphin. Similar approaches could be used to investigate hearing mechanisms in many other marine mammals. The article notes in conclusion a number of potential refinements and/or extensions of the techniques that may be useful in future applications.

Please email me (contact link below) for additional information. I would be delighted to work with sincere collaborators in applying these techniques to other marine mammals.

Aroyan JL (2001) Three-dimensional modeling of hearing in Delphinus delphis. J. Acoust. Soc. Am. 110(6), 3305-3318.

Physical modeling is a fertile approach to investigating sound emission and reception (hearing) in marine mammals. A method for simulation of hearing was developed combining three-dimensional acoustic propagation and extrapolation techniques with a novel approach to modeling the acoustic parameters of mammalian tissues. Models of the forehead and lower jaw tissues of the common dolphin, Delphinus delphis, were created in order to simulate the biosonar emission and hearing processes. This paper outlines the methods used in the hearing simulations and offers observations concerning the mechanisms of acoustic reception in this dolphin based on model results. These results include: 1) The left and right mandibular fat bodies were found to channel sound incident from forward directions to the left and right tympanic bulla and to create sharp maxima against the lateral surfaces of each respective bulla; 2) The soft tissues of the lower jaw improved the forward directivity of the simulated receptivity patterns; 3) A focal property of the lower jaw pan bones appeared to contribute to the creation of distinct forward receptivity peaks for each ear; 4) The reception patterns contained features that may correspond to lateral hearing pathways. A “fast” lens mechanism is proposed to explain the focal contribution of the pan bones in this dolphin. Similar techniques may be used to study hearing in other marine mammals. © 2001 Acoustical Society of America.

Article draft PDF file (approx. 1.4 Mbyte): Click here to view.

The PDF file is actually an author proof of the above article. The color figure resolution has been reduced to minimize file size, but the content is otherwise the same as a reprint.

Incidentally, several "puzzling" results of the hearing simulations decribed in the above article have turned out to be supported by more detailed experimental measurements with live dolphins. For example, the simulations indicate that as frequency decreases, the directions of peak hearing sensitivity drop down from the forward horizon to point (at 12.5 kHz) in lateral-ventral directions for each ear. It now appears that this frequency dependence is actually supported in principle by recent jaw-phone sensitivity data for a bottlenose dolphin [see Brill RL, Moore PWB, Helweg DA, Dankiewicz LA (2001) "Investigating the Dolphin's Peripheral Hearing System: Acoustic Sensitivity About the Head and Lower Jaw," SPAWAR Technical Report No. 1865]. A pdf file of Brill et al.'s report is available at the web site:

The hearing simulations also showed the same left-right side asymmetry as Brill et al.'s experimental data. Unfortunately, Brill et al.'s report was released (to the general public) after I submitted my final draft, and I did not have an opportunity to discuss these correlations in the article.

3D Modeling of hearing in Delphinus delphis -- Additional Results

Full model (D. delphis) simulated hearing receptivity plots for frequencies
12.5 kHz, 25 kHz, and 75 kHz.

(PDF file – approx. 2.0 Mb).

This section under construction – please check back soon.

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