Acoustic features Animal echolocation

1 acoustic features

1.1 frequency
1.2 frequency modulation , constant frequency
1.3 intensity
1.4 harmonic composition
1.5 call duration
1.6 pulse interval
1.7 fm signal advantages
1.8 cf signal advantages
1.9 acoustic environments of fm , cf signals

acoustic features

describing diversity of bat echolocation calls requires examination of frequency , temporal features of calls. variations in these aspects produce echolocation calls suited different acoustic environments , hunting behaviors.

frequency

bat call frequencies range low 11 khz high 212 khz. insectivorous aerial-hawking bats have call frequency between 20 khz , 60 khz because frequency gives best range , image acuity , makes them less conspicuous insects. however, low frequencies adaptive species different prey , environments. euderma maculatum, species feeds on moths, uses particularly low frequency of 12.7 khz cannot heard moths.

frequency modulation , constant frequency

echolocation calls can composed of 2 different types of frequency structures: frequency modulated (fm) sweeps, , constant frequency (cf) tones. particular call can consist of one, other, or both structures. fm sweep broadband signal – is, contains downward sweep through range of frequencies. cf tone narrowband signal: sound stays constant @ 1 frequency throughout duration.

intensity

echolocation calls have been measured @ intensities anywhere between 60 , 140 decibels. microbat species can modify call intensity mid-call, lowering intensity approach objects reflect sound strongly. prevents returning echo deafening bat. high-intensity calls such aerial-hawking bats (133 db) adaptive hunting in open skies. high intensity calls necessary have moderate detection of surroundings because air has high absorption of ultrasound , because insects’ size provide small target sound reflection. additionally, so-called whispering bats have adapted low-amplitude echolocation prey, moths, able hear echolocation calls, less able detect , avoid oncoming bat.

harmonic composition

calls can composed of 1 frequency or multiple frequencies comprising harmonic series. in latter case, call dominated harmonic ( dominant frequencies present @ higher intensities other harmonics present in call).

call duration

a single echolocation call (a call being single continuous trace on sound spectrogram, , series of calls comprising sequence or pass) can last anywhere 0.2 100 milliseconds in duration, depending on stage of prey-catching behavior bat engaged in. example, duration of call decreases when bat in final stages of prey capture – enables bat call more rapidly without overlap of call , echo. reducing duration comes @ cost of having less total sound available reflecting off objects , being heard bat.

pulse interval

the time interval between subsequent echolocation calls (or pulses) determines 2 aspects of bat s perception. first, establishes how bat s auditory scene information updated. example, bats increase repetition rate of calls (that is, decrease pulse interval) home in on target. allows bat new information regarding target s location @ faster rate when needs most. secondly, pulse interval determines maximum range bats can detect objects. because bats can keep track of echoes 1 call @ time; make call stop listening echoes made call. example, pulse interval of 100 ms (typical of bat searching insects) allows sound travel in air 34 meters bat can detect objects far away 17 meters (the sound has travel out , back). pulse interval of 5 ms (typical of bat in final moments of capture attempt), bat can detect objects 85 cm away. therefore, bat has make choice between getting new information updated , detecting objects far away.

fm signal advantages


echolocation call produced pipistrellus pipistrellus, fm bat. ultrasonic call has been heterodyned - multiplied constant frequency produce frequency subtraction, , audible sound - bat detector. key feature of recording increase in repetition rate of call bat nears target - called terminal buzz .

the major advantage conferred fm signal extremely precise range discrimination, or localization, of target. j.a. simmons demonstrated effect series of elegant experiments showed how bats using fm signals distinguish between 2 separate targets when targets less half millimeter apart. ability due broadband sweep of signal, allows better resolution of time delay between call , returning echo, thereby improving cross correlation of two. additionally, if harmonic frequencies added fm signal, localization becomes more precise.

one possible disadvantage of fm signal decreased operational range of call. because energy of call spread out among many frequencies, distance @ fm-bat can detect targets limited. in part because echo returning @ particular frequency can evaluated brief fraction of millisecond, fast downward sweep of call not remain @ 1 frequency long.

cf signal advantages

the structure of cf signal adaptive in allows cf-bat detect both velocity of target, , fluttering of target s wings doppler shifted frequencies. doppler shift alteration in sound wave frequency, , produced in 2 relevant situations: when bat , target moving relative each other, , when target s wings oscillating , forth. cf-bats must compensate doppler shifts, lowering frequency of call in response echoes of elevated frequency - ensures returning echo remains @ frequency ears of bat finely tuned. oscillation of target s wings produces amplitude shifts, gives cf-bat additional in distinguishing flying target stationary one.

additionally, because signal energy of cf call concentrated narrow frequency band, operational range of call greater of fm signal. relies on fact echoes returning within narrow frequency band can summed on entire length of call, maintains constant frequency 100 milliseconds.

acoustic environments of fm , cf signals

a frequency modulated (fm) component excellent hunting prey while flying in close, cluttered environments. 2 aspects of fm signal account fact: precise target localization conferred broadband signal, , short duration of call. first of these essential because in cluttered environment, bats must able resolve prey large amounts of background noise. 3d localization abilities of broadband signal enable bat that, providing simmons , stein (1980) call clutter rejection strategy. strategy further improved use of harmonics, which, stated, enhance localization properties of call. short duration of fm call best in close, cluttered environments because enables bat emit many calls extremely rapidly without overlap. means bat can continuous stream of information – essential when objects close, because pass – without confusing echo corresponds call.

a constant frequency (cf) component used bats hunting prey while flying in open, clutter-free environments, or bats wait on perches prey appear. success of former strategy due 2 aspects of cf call, both of confer excellent prey-detection abilities. first, greater working range of call allows bats detect targets present @ great distances – common situation in open environments. second, length of call suited targets @ great distances: in case, there decreased chance long call overlap returning echo. latter strategy made possible fact long, narrowband call allows bat detect doppler shifts, produced insect moving either towards or away perched bat.