“Dol­phins are phe­nom­e­nally good at us­ing echolo­ca­tion, much bet­ter than man-made de­vices”

Dol­phins echolo­cate with two-part acous­tic beams. &T ,QUGƂP 5VCTMJCOOCT of Lund Univer­sity ex­plains how this could help us im­prove ul­tra­sound tech­nol­ogy

Focus-Science and Technology - - DISCOVERIES -

Why do dol­phins need echolo­ca­tion?

They use it for nav­i­ga­tion, hunt­ing for prey and pos­si­bly in so­cial con­texts. Dol­phins al­ways use acous­tics as their pri­mary sense. They gen­er­ate short sound pulses, which bounce off sur­faces and come back as echoes – the time it takes to re­turn is a mea­sure of how far away an ob­ject is. The beam is quite fo­cused so the dol­phins turn their heads to scan their en­vi­ron­ment.

How do they pro­duce acous­tic beams?

They have a struc­ture below the blow­hole called the phonic lips. Sounds come out through the melon, the rounded fore­head – it’s one of the tis­sues re­spon­si­ble for the shape and for­ma­tion of the beam. It’s ba­si­cally an acous­tic lens: the speed of sound is faster along the edges com­pared to the core of the melon, so the beam ends up cone-shaped. Dol­phins have ex­tremely short sig­nals, usu­ally much shorter than bats.

How did you study dol­phin sig­nals?

You need one dol­phin echolo­cat­ing in a spe­cific di­rec­tion and hy­drophones – mi­cro­phones for un­der­wa­ter use. I built my mea­sure­ment sys­tem as a PhD stu­dent with 47 hy­drophones. If we use all of them and record the cross-sec­tion of the beam, we can see finer de­tails. I wanted to learn more about how they use th­ese small de­tails to solve tasks, be­cause dol­phins are phe­nom­e­nally good at us­ing echolo­ca­tion, much bet­ter than man­made de­vices. I was look­ing at the sig­nals and re­alised that reg­u­lar meth­ods couldn’t give me the in­for­ma­tion, so I talked with col­leagues work­ing with math­e­mat­i­cal sta­tis­tics and we de­vel­oped a sig­nal-pro­cess­ing al­go­rithm that helps us look at sig­nals in a much more de­tailed way.

What did you dis­cover about the beam?

Even though the sig­nal from the dol­phins is very short, about 70 mi­crosec­onds long, my pre­vi­ous re­search found that it ac­tu­ally con­sists of two in­ter­twined beam com­po­nents. The al­go­rithm helped us de­ci­pher this, and we’ve dis­cov­ered that parts of the beam con­sist of over­lap­ping pulses. You get two slightly time-sep­a­rated echoes from the up­per part of the beam first as a low-pitched note and then a high-pitched one. From the lower part of the beam, you only hear a low-pitched echo. Dol­phins have a fre­quency gra­di­ent across the beam, so in the­ory they could use this

in­for­ma­tion to lo­cate ob­jects more pre­cisely: if prey is mov­ing up­wards in the beam, the pitch will get higher and higher in fre­quency.

What are the prac­ti­cal ap­pli­ca­tions?

With the al­go­rithm, we hope it can be ap­pli­ca­ble to non-de­struc­tive test­ing meth­ods for di­ag­nos­tics, such as if we want to mea­sure the thick­ness of a very thin layer in the body. So im­proved im­age res­o­lu­tion. The other as­pect is what we can learn from dol­phins. For in­stance, the con­cept of us­ing two in­ter­twined sound beams with a fre­quency gra­di­ent cross-sec­tion might im­prove ul­tra­sound ma­chine per­for­mance. We could also make boat sonars more ef­fec­tive by em­ploy­ing th­ese prin­ci­ples. After all, mil­lions of years of evo­lu­tion have formed dol­phin echolo­ca­tion to per­fec­tion, so there are def­i­nitely things we can learn from them.

ABOVE: Dol­phins use echolo­ca­tion to de­ter­mine where they, the rest of their pod and any po­ten­tial prey are in the wa­ter

BELOW: Dol­phins pro­duce acous­tic waves and fo­cus the sound with their melons (yel­low). Sound waves re­flected by ob­jects are chan­nelled to the au­di­tory bulla (orange), which trans­mits the nerve sig­nals to the brain

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