02IntroToEcho REV

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Wildlife Acoustics: We Listen

Introduction to Bat Echolocation

Saturday, March 17, 12

Wildlife Acoustics has developed several models of acoustic detectorsspecifically designed to record the high-frequency sounds of echolocating bats.

This equipment forms the backbone of acoustic monitoring programs andhelps bat-workers throughout the world with a non-contact way of assessinghabitats for bats.

But, to learn how best to record bats using ultrasonic detectors, it is firstimportant to understand the amazing biological adaptation of echolocation andthe bats abilities to navigate and forage using sound.

Only then can we begin to meet the challenges inherent in identifying bats onthe basis of the sounds we record.


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Introduction to Echolocation

Bioacoustics: How Bats use Sound

Echolocation Call Types

Appreciating Species Repertoires


Before we delve deeper into identifying bats by analyzing their echolocation calls, we must firstunderstand a little bit more about bioacoustics, particularly how bats produce and process soundandhow the physics of sound functions in echolocation.

When thinking about how bats use sound, we must remember that the audio component of batecholocation calls is a waveform, just like all sound.

But unlike many ambient noises, bat echolocation employs discrete pulses of sound that often form aconsistent pattern.

Different bats will use different types of calls, based on their flight style and morphology, prey-selection, and changing information needs about their surroundings.

These call types range from frequency-modulated sounds that sweep through several frequencies overtime, to constant-frequency sounds, that stay at the same frequency throughout the entire duration ofthe call.

Identifying batson the basis of their echolocation calls is made more difficult due to the variability intheir call type repertoires.

Many different species can converge on similar call types when their information needs are similar.

Nevertheless, with understanding and practice it is possible to determine bat activity and presence in ahabitat, based on analysis of acoustic recordings alone.


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Bioacoustics: How Bats Use Sound

Sensory Perception: Human vs. Bat

Turning Sound into Pictures

The Physics of Sound

Operational Frequencies of Bats


We begin with an appreciation of animal bioacoustics, how bats use sound as acritical sensory input in their lives.

Physiologically and morphologically, bats have more of their brains devoted tointerpreting auditory stimuli than many other animals.

Echolocation is, at its most basic definition, a way to listen to and interpret

returning echoes of sound, and form sound-pictures in the brain, much likehuman brains turn reflected light into images about our surroundings.

Because bat echolocation sounds are almost always above the human range ofhearing, we are mostly deaf to just how noisy the night skies around us can be.

Using high-frequency soundsimparts several advantages to bats, not the leastof which is being able to operate in a soundscape with very little otherinterference from other noise sources.

High-frequency sounds also allow bats to discern and resolve very small itemsof interest in their immediate vicinities, like clutter from vegetation and thepresence and locations of target insect prey.

By fine-tuning their signal production capabilities, bats can resolve detailsabout their surroundingsat distances of up to 30-meters, in just fractions of asecond.


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Sensory Perception: Human vs. Bat

BAT HUMAN

ADAPTEDLOCOMOTION

Aerial Terrestrial

TEMPORAL

ACTIVITYNocturnal Diurnal

PRIMARY

SENSORYPATHWAY

Auditory Visual


Much of this is due to the adaptive differences between bats and humans, andthe ways in which each uses sensory perceptions to experience the worldaround them.

When we compare bats to humans, our respective resource partitioning couldnot be more different.

Bats can move higher, faster, farther, and with more maneuverability thru theirairspace than we can as we remain largely rooted to the ground.

Then, there are the bats adaptations for functioning in extremely low lightconditions or under the complete absence of illumination, as on a pitch blacknightor within the deep reaches of caves and underground mines.

Humans have evolved to be largely diurnal, and we must have supplementallight as soon as the sun goes down, otherwise we are in effect paralyzed inmovement and understanding of our surroundings.

Finally, bats have a highly developed sense of sound, relying upon auditoryinputs just as much we rely upon vision.


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Blind as a Bat? - or - Deaf as a Human?


This results in an interesting (human) perspective . . . we often feel justified inusing the phrase, blind as a bat.

But, were the tables turned, bats would likely endorse the moniker, deaf as ahuman.


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Turning Sound into Pictures


This is because a bats ability to use sound to echolocate is truly masterful.

When a bat echolocates, it produces an extremely loud, high-frequency pulse of sound in its larynx.

Sound vibrations create pressure waves on the air emanating outward from the bats mouth.

These waves propagate thru the air just like ocean waves propagate thru water.

But thru air the propagation velocity is extremely high . . . what we call the speed of sound.

When the sound hits an object, it bounces off and returns to the bat, at the same speed, but at afraction of the out-going intensity.

Bats then listen to these relatively quiet echoes, discerning minute differences in their returns to eachear.

Through this process, bats can calculate discrete differences in distance to an object, all while in flight,in just fractions of a second.

Moreover, bats are challenged by the need to produce a very intense (loud) sound, emitting it throughtheir mouth, closing their ears so as not to deafen themselves, then letting their ears becomesensitive to the faint returning echo, and processing that echo into a picture of their surroundings.

And, they do this several, to several hundred, times a second; coordinating the outgoing signal withtheir wing-beats, remaining sensitive to the in-coming echo, interpreting it in their brains, all beforeproducing the next outgoing signal.

Anyone who has ever watched a bat twist and turn thru the sky at dusk, can begin to appreciate justhow much signal processing must be going on in a bats brain, at very short time intervals, and withastonishing precision.


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The Physics of Sound

V= f

V

300 m/s


Using high-frequency sounds is key to how bats have been able to capitalize ontheir echolocation abilities.

Basically, its governed by the following physical constant: Velocity is equal tothe wavelength of the sound times its frequency.

And, because velocity is essentially constant, it is equal to the speed of sound,

which is approximately 343.6 meters per second in air, there is an inverserelationship between wavelength and frequency.

For simplicity of the math we are about to do, lets round-off the speed ofsound to 300-meters per second.

Regardless, this means that as frequency increases, wavelength must decrease.

This influences how a bat can use different frequencies of sound to get differentinformation about a target.

To generate an echo, a target must be larger in size than at least the half thewavelength, otherwise the sound waves will pass right past or around the target,without producing an echo.


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Operational Frequencies of Bats

FREQUENCY APPROXIMATE TARGET SIZE

100 kHz 1.5 mm (gnat)

50 kHz 3 mm (fly)

20 kHz 7.5 mm (beetle/small moth)

10 kHz 15 mm (large moth)


So, for bats this means: If they need to get echoes off very small insects, thewavelength needs to be very small so they need to use a very high frequency.

A bat with a 100kHz frequency, can generate echoes from gnat-sized insects.To wrap your head around this . . .Consider the speed of sound to be 300m/s; or 300,000 mm/s.Divide that by 100kHz; or 100,000 cycles/second

And we get a wavelength of 3mm; and an object half that size, or 1.5mm,will be sufficient to generate a usable echo for a bat.

By extension then, a bat echolocating at 50kHz, can receive echoes from fly-sized insects.

For a bat echolocating 30kHz, the smallest insect detectable would be a beetleor small moth.

And, at 10 kHz, bats can identify large moths.

Therefore, bats that use lower frequencies are constrained to identifying largertargets.

This also means, that if bats only need to identify large objects, such as trees,brick walls, and beehive hairdos, they can switch to lower frequency calls.


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Frequency-modulated: FM

Constant-frequency: CF

Quasi-constant-frequency: QCF


Of course, in addition to needing different information about different sized objects, bats havedifferent wing morphologies leading to different flight styles and speeds, so they need this informationat different rates.

As a result, there are many different echolocation call types that bats use to form their sound-pictures.

Most temperate bat species produce pulses of sound that are frequency-modulated or FMin nature.

FM calls change in frequency over time.

Sometimes this change is very rapid, especially for bats using broad band calls that sweep throughmany frequencies over a short duration.

Other bats use constant frequency or CFcalls which remain at nearly the same frequency over theirentire duration.

CF calls are narrow band sweeping through very few frequencies, and they are often longer induration.

Each type of call is better suited to returning different types of information and many bats will use acombination of broad-band and narrow-band calls, depending on their information needs at the time.

Sometimes these intermediate calls are referred to as quasi-constant frequency or QCF.

To make echolocation even more complex, or interesting, depending on your perspective, bats caninclude CF, FM, and QCF components, all in a single pulse of sound.


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Call Shapes: Structure and CharacteristicsCharacteristic Frequency

Duration

Slope and Call Shape

Bat Repertoires


This gives echolocation calls interesting shapes due to these different structures.

Some of the structural characteristics we look at when we categorize echolocation calls are:

The characteristic frequencyis the frequency at which the call reaches its lowest slope or flattestpart.

For some bats, it is also often the part of the call with the most energy (the loudest, or most intenseportion of the call).

Call duration is also an important factor and gives an echolocation pulse its shape.

Most bat calls are very short in duration; 2-6ms.

But some can be upwards of 10ms or even 100ms. Usually longer duration calls have lower slopes, orare almost flat.

The relative amount of time that bats spend at any one frequency can give calls very interestingshapes and slopesthat change throughout the call.

And, bats will vary their power output along the duration of the call, which also can lead to interestingcall shapes.

Finally because bats will change their call types as their needs for information changes, there is usuallyno single call type that can be associated with a single bat species.

Instead, bats have repertoires of calls which they use.

Becoming familiar with a species repertoire is one of the most challenging aspects of acousticidentification.


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This schematic illustrates some of the variability of echolocation call shape. Frequency is depicted onthe Y-axis and time along the X-axis.

At one extreme we have the FM calls, which can be both variable in frequency and duration, also Inbandwidth, giving them different shapes.

FM calls provide bats with many advantages for navigation and foraging.

The major advantage of an FM signal is extremely precise range discrimination, or localization of the

target.

In laboratory studies, bats using FM signals could distinguish between two separate targets even whenthe targets were less than half a millimeter apart.

This amazing ability is due to the broadband sweep of the signal, which allows for better resolution ofthe time delay between the call and the returning echo, thereby improving the cross correlation of thetwo.

The steepest FM calls provide key information about distance to an object, its size and shape, and

something about texture.

One disadvantage of the FM signal is a decreased operational range of the call.

Because the energy of the call is spread out among many frequencies, the distance at which the FM-batcan detect a target is limited.

This is in part because any echo returning at a particular frequency can only be evaluated for a brieffraction of a millisecond, as the fast downward sweep of the call does not remain at any one frequencyfor long.


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At the other extreme, we have bats that produce CF or QCF calls.

Because the signal energy of a CF call is concentrated into a narrow frequency band, the operationalrange of the call is much greater than that of an FM signal.

This is because echoes returning within the narrow frequency band can be summed over the entirelength of the call, which can maintain a near constant frequency for up to 100 milliseconds.

Most bats produce calls that are long, low, and shallow in slope, either CF or QCF, when in a search

phase or out in the open.

The structure of a CF signal is adaptive in that it allows the bat to detect both the velocity of a target,and the fluttering of a target's wings as Doppler shifted frequencies.

A Doppler Shift is an alteration in sound wave frequency, and can be produced in two situations:

1. when the bat and its target are moving relative to each other, and2. when the target's wings are oscillating back and forth.

The oscillation of a target's wings also produces amplitude shifts, which gives a CF-bat additional helpin distinguishing a flying target from a stationary one.


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Between these two extremes, are calls that combine FM and CF or QCFcomponents in a single pulse.

This is helpful because, as we have already discussed, each call type impartsspecific information to the bat, and this information can become increasinglyuseful as bats move between open-air and cluttered environments, or as theirinterest in a target becomes greater or the target-size of interest changes.


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We can think of bat echolocation calls as being a lot like an adjustable beamheadlight or flashlight.

When the beam is very focused and bright, a lot of detail can be determinedabout a very small area, but when the beam widens to detect a larger area, theillumination over any one spot is not as intense, and detail can be lost.

So, just like us, walking around with our headlamps at night, bats will use aconstant, low-level call with limited bandwidth but a long range whennavigating through open areas with few obstacles to worry about.

Then they will switch to more intense, focused call types with shorteroperational ranges when they need a lot of information about a very small,close-in areas, over a short amount of time.


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Bat Vocal Repertoires


Unfortunately, we must remember, bats dont produce just a single call type.

It is tempting to think that different species of bats will produce different types of echolocation calls sowe can easily determine the species based on their sounds. We can do this for most species of birds,but sadly this is not easily achieved with bats.

This is because, a bird call is very social and it is meant to identify the bird to other members of thesame species, to advertise its presence to a mate . . .

. . . a bat call is very functional.

Bats use echolocation to see their immediate environments with sound. Depending on what is goingon in its environment, a bat will vary its echolocation call considerably.

This illustration shows how canopy height (on the Y-axis) and operational frequency (on the X-axis)compare for different species as they move in and out of clutter, or as they change their behavior whileforaging.

The take home message is that bat echolocation calls are very variable, and more so for species that

are

generalists

operating successfully in many different habitats, foraging on many different types ofinsects, and contending with varying degrees of clutter.


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This means a bat that is a generalist can produce many different looking echolocation calls.

Bats will often vary the frequency of their calls depending on their operational need for target distanceand size determination.

Look at these call sequences. They are all from the same species, a big brown bat, doing the samething: navigating thru its habitat.

But, the habitat varies (it becomes more or less cluttered) as the bat moves thru different habitat types.

Sonograph A illustrates how a big brown bat needs more information, at a finer resolution (a smallertarget size) when in a highly cluttered habitat.

Sonograph Bshows that as the bat flies through a less cluttered habitat, it needs information at aslightly coarser resolution (because target size has increased) so it can lower the frequency of thesounds it produces.

Sonograph Cdepicts how as the clutter disappears completely, the bat can change its call shape andlower its frequency even more.

Finally sonograph Ddepicts a complex habitat where the bat travels from an open habitat to acluttered one and then back into the open again, using more complex calls to obtain information as theclutter level changes.

So, as you can see recognizing all these different looking call sequences and call shapes as comingfrom the same species of bat takes a certain degree of experience and finesse.


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Almost every species is capable of changing their call shape and duration.

Call pulses can range from short duration calls with greater bandwidth that batsproduce in a more cluttered environment, one that has multiple objects andtargets . . .

. . . to longer duration, lower bandwidth calls like bats often produce when in

open air.

This sonogram depicts a diversity of calls from the eastern red bat, showing itsrepertoire from short to long call variants.

The call pulses in this compilation were stitched from different individuals flyingin different habitat types, with clutter quotients decreasing from high-clutter,understory foraging behavior on the left; to low clutter, open-air flight on theright.


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search phaseapproach phase acquisitionphase


Bats are also capable of drastically varying their call types within a single sequence.

This most often happens when bats are intercepting prey (or even when they investigating themicrophone of a bat detector sticking up into their airspace).

This sequence shows a Mexican free-tailed bat approaching a target, represented by the 4 call pulseson the extreme left,

acquiring the target, represented by the 10 increasingly steep call pulses in the center,

then returning to a typical search-phase call type, represented by the 5 call pulses on the extremeright.

If we were just looking at any one of those sections of the sequence in isolation, we might think thatwe were looking at three distinctly different bat species.

This is why knowledge of a bats entire vocal repertoire is important for making species identificationdeterminations.

As is being able to record the longest possible, highest quality call sequences, so important behavioralchanges in the call can be incorporated into our analysis.


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It should be clear by now, that the more we understand about echolocation, themore we realize:

1. Bats are not birds . . . they cannot always be reliably expected to producespecies-specific calls,

2. Plus, the range of characteristics we currently use for bat identification can

overlap among one or more species,

3. Bats of different species will converge on similar call characteristics if theyhave similar needs for information,


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4. And any individual bat may emit calls outside its typical known ranges andcall characteristics, and will thus mimic those of another species.

So, for many species and recording conditions, confident species classificationcan only be achieved on a subset of call types within a repertoire that fallsoutside of data space shared with another species.

Therefore, the more bioacoustics monitoring is used to record bat echolocationcalls, the more likely you are to overcome some of these challenges by having alarger repertoire of bat calls to study.


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Equipment for Recording Calls

Wildlife Acoustics Song Meter SM2BAT+

Wildlife Acoustics Echo Meter EM3

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In summary the more we understand about echolocation, the more we realizejust how important the proper equipment is for capturing calls for potentialspecies ID.

Wildlife Acousticshas developed state of the art detectors for monitoring thehigh-frequency sounds of bats, both in passive and active modes of recording.

The current Wildlife Acoustics SM2BAT+is a robust, field-ready detector withfeatures specifically designed for capturing high-quality, passively collectedecholocation calls

And the new Echo Meter EM3unit was specifically designed for the demands ofactive monitoring, voucher call collection, and is an excellent tool aiding speciesidentification of bats on the wing.

Both detectors have options for multiple recording modes and can store files inany format used by popular signal analysis software packages.

As such, the Wildlife Acoustics line of bat detectors are important foundationsto any acoustic inventory attempt with bats.


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For More Information . . .

Please visit http://www.wildlifeacoustics.comclick on Products then click on Ultrasonic

Monitoring.

Other Training Modules in this Series

Un-boxing the SM2BAT+/EM3

Overview of the SM2BAT+/EM3 HardwareField Use and Deployment Tips for Ultrasonic Recording


To learn more about the SM2BAT+ and EM3 models, browse the WildlifeAcoustics website.

And, detailed, guided information about the hardware, settings, and recordingtips for the SM2BAT+ and EM3 units as they are designed to monitor and recordbats, are addressed in additional Information and Training Modules on thissite.


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Wildlife Acoustics: We Listen


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Wildlife Acoustics, Song Scope, Song Meter, Echo Meter,SM2, SM2BAT+, SM2M and EM3 are trademarks of Wildlife

Acoustics, Inc. 2011/2012.Product names, logos, brands, and other trademarks

featured or referred to are the property of their respective

trademark holders.


Wildlife Acoustics, Song Scope, Song Meter, Echo Meter, SM2, SM2BAT+, SM2Mand EM3 are trademarks of Wildlife Acoustics, Inc. 2011/2012.Product names, logos, brands, and other trademarks featured or referred to arethe property of their respective trademark holders.

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02IntroToEcho REV