Build Your Own Bat Detector
The term "bat detector" is usually used to describe a device used to detect the presence of bats by converting their echolocation ultrasound signals to audible frequencies as they are emitted by the bats. There are other types of detector which record bat calls so that they can be analysed afterwards, but these are more commonly referred to by their particular function. To use a bat detector it is necessary to have some understanding of frequencies in Hertz (Hz). 1 Hz is 1 complete cycle per second. 1 kilohertz (kHz) is 1000 Hz. Human ears are normally limited to the range from 20 Hz to 16 kHz, but in practice and with older ears, bat detector loudspeakers are limited to about 100 Hz to 5 kHz. Much of the terminology used with bat detectors is derived from radio transmission theory which can be confusing. Bats emit calls from about 12 kHz to 160 kHz, but the upper frequencies in this range are rapidly absorbed in air and many bat detectors are limited to around 15 kHz to 125 kHz at best. Bat detectors are available commercially and also can be self-built.
Common Pipistrelle call heard on a heterodyne bat detector - 68kb
Problems listening to the file? See media help. This is a typical echolocation call with "feeding buzzes" when it captures an insect. Using Bat DetectorsBat detectors are used to detect the presence of bats and also to come to conclusions about their species. Some bat calls are distinct and easy to recognise such as the Horseshoe bats; other calls are less distinct between similar species. While bats can vary their calls as they fly and hunt, the ear can be trained to recognise species according to the frequency ranges and repetition rates of the echolocation calls. Bats also emit social calls (non-echolocation calls) at ultrasound frequencies. A major limitation of acoustic bat detectors is their range which is limited by the absorption of ultrasound in air. At mid range frequencies around 50kHz, the maximum range is only about 25 to 30 metres in average atmospheric conditions when bats fly. This decreases with increasing frequency. Some bat calls have components around 20 kHz or even lower and sometimes these can be detected at 2 or 3 times the usual range. However, only the lower frequency components will be detected at a distance. The useable range of bat detectors decreases with humidity and in misty conditions the maximum range can be very low. It is important to recognise three types of bat echolocation call, Frequency Modulation (FM) Constant Frequency (CF) (sometimes called Amplitude Modulation) and composite calls with both FM and CF components. The following illustrates a bat making an FM type call followed by a bat which uses a CF type call: Frequency modulated call followed by a constant frequency call as heard on a heterodyne bat detector - 190 Kb
Problems listening to the file? See media help. The FM call is heard as rapid dry clicks and the CF call as peeps. These vary in frequency due to the Doppler effect as the bat flies past. A heterodyne bat detector exaggerates the doppler effect. As the bat making the CF calls swoops over the detector the pitch falls. Several species of bat use a composite FM and CF call starting with a rapid falling FM call which slows to become a CF call at the end, giving a "hockey stick" shape to the graph. This makes the call sound different on a bat detector: Soprano Pipistrelle call recorded on a heterodyne bat detector set to near the end frequency - 190 Kb
Problems listening to the file? See media help. This gives a much wetter sound that the pure FM call. Pipistrelles generally use the hockey stick call for general echolocation, but use only the FM part at times. The end frequencies for the Common Pipistrelle and the Soprano Pipistrelle are around 45 kHz and 55 kHz respectively, but these frequencies can vary widely. There are three types of "real time" audio bat detector in common use, the Heterodyne, Frequency Division and Time Expansion. Some bat detectors combine two or all three types. Bat detector typesHeterodyneHeterodyne detectors are the most commonly used, and most self-build detectors are of this type. A heterodyne function is often also built into the other types of detector. A heterodyne bat detector simply shifts all the ultrasound frequencies downwards by a fixed amount so we can hear them. A "heterodyne" is a beat frequency such as can be heard when two close musical notes are sounded together. A heterodyne bat detector combines the bat call with a constant internal frequency so that sum and difference frequencies are generated. For instance a bat call at 45 kHz and an internal frequency of 43 kHz produces output frequencies of 2 kHz and 88 kHz. The 88 kHz frequency is inaudible and is filtered out and the 2 kHz frequency is fed to a loudspeaker or headphones. The internal frequency is displayed on a dial or on a display. A better quality version of a heterodyne, or direct conversion, bat detector is the super-heterodyne detector. In this case the bat signal is mixed with a high frequency oscilator, typically around 450-600kHz. The difference frequency is then amplified and filtered in an 'intermediate frequency' or i.f. amplifier before being converted back to audible frequencies again. This design, which is based on standard radio design, gives improved frequency discrimination and avoids problems with interference from the local oscillator. It is also possible to use a 'comb spectrum' generator as the local oscillator so that the detector is effective tuned to lots of frequencies, 10kHz apart, all at once. Some early bat detectors used ex Navy, low frequency radio sets, simply replacing the aerial with a microphone and pre-amplifier. It is also possible to modify a portable Long Wave radio to be a bat detector by adjusting the tuning frequencies and replacing the ferrite rod aerial with a microphone and pre-amplifier. How it is usedThe operator guesses the likely species to be present and tunes the frequency accordingly. Many users will start listening around 45 kHz. If a bat is seen or a bat-like call is heard, the frequency is tuned up and down until the clearest sound is heard. Species like Pipistrelles which end their call with a "hockey stick" CF component can be recognised according to the lowest frequency which gives the clearest "plop" sound. Horseshoe bats give a peeping sound at a frequency depending on their species. FM calls all tend to sound like clicks, but the start and end frequencies and the call repetition pattern can give clues as to the species. Pros and ConsThe advantages of a heterodyne bat detector is that it works in real time, exaggerates the frequency changes of a bat call, is easy to use, and is the least expensive. It is easy to recognise a doppler shift in CF calls of flying bats due to their speed of flight. Stereo recording is possible with one model. The disadvantages of a heterodyne bat detector are that it can only convert a narrow band of frequencies, typically 5 kHz, and has to be continually retuned, and can easily miss species out of its current tuned range. Frequency DivisionFrequency Division (FD) bat detectors synthesise a sound which is a fraction of the bat call frequencies, typically 1/10. This is done by converting the call into a square wave, otherwise called a zero crossing signal. This square wave is then divided using an electronic counter by 10 to provide another square wave. Square waves sound harsh and contain harmonics which can cause problems in analysis so these are filtered out where possible. Some FD detectors output this constant level signal which renders background noise and bat calls at the same high level. This causes problems with both listening and analysis. More sophisticated FD detectors such as the [Batbox Duet] measure the incoming volume level, limiting the noise threshold, and use this to restore the output level variations. This and other sophisticated FD detectors also include a heterodyne detector and provide a jack output so that independent outputs can be recorded for later analysis How it is usedWith dual output FD detectors, headphones can be used to monitor both outputs simultaneously, or the loudspeaker used with the heterodyne function and the FD output recorded and analysed later. Alternatively, listening to the FD output gives an audible rendering of the bat call at 1/10 frequency. Dual FD/heterodyne detectors are useful for cross country transects especially when there is a function provided for recording voice notes such as times, locations and recognised bat calls. The output or outputs are recorded on cassette tape, Minidisc or solid state recorders, downloaded to a computer, and analysed using custom software. Calls missed by the heterodyne function, if present, can be seen and measured on the analysis. Pros and ConsAdvantages, As with a heterodyne detector, an FD detector works in real time with or without a heterodyne function. Bat calls can be heard in their entirety over their whole range rather than over a limited frequency range. Retuning with an FD detector is not required although this is done with a dual type with heterodyne. By analysing the recording later, the entire call frequency range and the call pattern can be measured. A serious disadvantage with real time listening is that the speed of a bat call remains fast, often too fast for the species to be recognised. The frequency changes of CF calls are not exaggerated as with a heterodyne detector and so are less noticeable. Also with some species such as the Lesser Horseshoe bat with a call around 110 kHz, the resulting frequency is still quite high although it can be recorded. The synthesising of the call means that only one bat call can be reproduced at a time and a muddle is caused by simultaneous calls. Surprisingly, this is not a great disadvantage when analysing a recording later. Time ExpansionTime expansion (TE)detectors work by digitising the bat calls at a high sampling rate and replaying them at a lower sampling rate immediately afterwards. Typically the sampling rate ratios can be varied from 1:10 to 1:32. The output is audible on a loudspeaker or headphones and audio recordings can be made simultaneously for later analysis. TE detectors are "real time" devices in that they can be monitored at the time of recording, but there is an inevitable delay while the high speed sampled extract is slowed down and replayed. How it is usedIn real time mode, with or without an associated heterodyne or FD detector, the slowed down calls can be heard as a drawn-out bat call at audible frequencies. Therefore fast FM calls can be heard as a descending note instead of a click. Thus it is possible to hear the difference between FM calls which just sound like clicks on the other types of detector. After downloading an audio recording to a computer, the original calls are analysed as if they were still at the original non-expanded rate. Pros and ConsThe output can be recorded with an audio recorder as with FD detectors. However the whole waveform is recorded with the full call range being preserved, rather than 1/10th of the waveform asn in a FD detector. While the recorded sample is being played back slowly, nothing is being recorded, so the bat calls are being sampled intermittantly. For intance, when a 1 second call is being played back at 1/32 rate, 32 seconds of bat calls are not being recorded. TE detectors are typically used for professional and research work, as they allow a complete analysis of the bats' calls at a later time. Other Bat Detector TypesZero Crossing AnalysisZCA is best known as an add-on device to the Anabat bat detector. The original bat calls are digitised and the zero crossing points used to produce a data stream which is recorded on a Compact Flash card. There are sophisticated timing controls and the device can be set to respond to bat calls, so that many hours of recording are available in unmanned situations. The solid state ZCA recording is analysed by custom software to produce a time/frequency plot of each call which can be examined for species recognition in a similar way to FD or TE recordings. How it is usedThe ZCA detector is usually placed in a bat roost or bat flight path and left for a number of days to collect data. Thus it is less labour intensive than using a manned bat detector in real time. Pros and ConsWhile the ZCA detector can also be used in real time, its value is for remote recording over long periods. The analysis is similar to that for FD recordings, but there is no amplitude data included. However it does accurately record each zero crossing point, rather than only one in ten. As with all recording devices triggered by an input, a ZCF detector recording automatically is prone to ultrasonic interference from insects such as crickets. Filters can be written to select a characteristic frequency of certain species and ignore others; some (CF species) are more easily filtered, others are nigh on impossible. High Frequency RecordingThis can be done by using a high speed digitiser peripheral on a computer such as a laptop. This is not a bat detector as such, but recordings of bat calls can be analysed similarly to TE recordings. This method produces large data files and produces no means of detecting bat calls without the simultaneous use of a bat detector. DSP detectorsThis type of bat detector is believed to be in pre-production or experimental and is not available commercially. DSP bat detectors utilise a digital signal processor to map bats' ultrasounds signals to audible sounds; different algorithms are being used to accomplish this, and there is active development and tuning of algorithms going on. Time Domain Signal CodingThis type of bat detector is believed to be in pre-production or experimental and is not available commercially. Research is in progress for analysing many types of ultrasound calls and sounds besides those of bats. A TDSC detector digitises the original calls and derives a two dimensional data string by analysing the parameters of each call with respect to time. This is analysed by a neural network to provide pattern recognition for each species. Non-acoustic detectionVisual observation is the obvious means of detecting bats, but of course this can only be done in daylight or crepuscular conditions. (dusk and dawn). Emergence counts are done visually at dusk, using a bat detector for confirmation of species. In lower light conditions a night vision device can be used but the more affordable generation 1 type has a lag time which fails to provide a suitable image of a flying bat. Infrared (IR) cameras and camcorders are used with an IR illuminator to observe bat emergences and bat behaviour inside and outside roosts. The problem with this method is that deriving a count from a recording is tedious and time consuming, but camcorders can be useful as a backup in roost emergence counts to observe bats re-entering the roost. Many Sony camcorders are sensitive to infrared. Infrared beam devices ususally consist of a dual array of invisible IR beams. The size of the roost entrance determines the number of beams necessary and thus the power required and potential for off-mains use. Single beam DIY systems are available for bat boxes but these do not log the direction of transit. Almost all the systems in use today are non-comercial or DIY. A system in use in some mines in Wisconsin uses two arrays of beams however they are spaced quite far apart and consequently only log approximately 50% of the bats although extrapolated figures are achieved through correlation of time stamped video and beam break data. The Countryside Council for Wales (CCW) uses two similar systems with beams spaced close enough together that every bat transiting the entrance is logged along with the temperature. These systems require either mains power or 12v deep cycle batteries. They can be used in conjunction with an Anabat Zcaim installed in a 6" soil pipe and pointed across the roost entrance to discriminate between species by correlating the time stamp data from the IR array and filtered Anabat Zcaim data for horsehoe bats (relatively easy due to their easily identifiable CF echolocation which can be filtered automatically using Anabat software). Data from beam break systems must be carefully analysed to eliminate "light sampling behaviour" (environment sampling) where the bats repeatedly leave the roost and return immediately if the conditions are not suitable. CCW uses custom made event loggers which determine if the beams are broken by a bat sized animal and ignores all other transits. Data must be analysed using a methodology which takes light sampling behaviour into account. One method used is an analouge of the "human" method of counting emerging bats with a tally counter (clicker): count all the flights out and all the flights in, wait for the first ten or fifteen minute break in emergence after which it is considered that all the bats have emerged then subtract the "ins" from the "outs" to give the count for that day. Obviously this procedure must be automated due to the large amounts of data involved. Although many active infrared beam counters detect every transit, it appropriate to view the data as a series of evening emergence counts since data from after the main emergence has ceased cannot really be adequately analysed; the spike in numbers returning by dawn is almost always less than the total emergence the previous evening. Since the bats are continuously going in and out of the roost, the reason is most likely due to "early returnees". Emergence numbers do correlate closely with total returnees when "outs" are subtracted from "ins" in post emergence data. Thermal imagers[ which are of a high enough definition to register bats at over 30 metres range are expensive, but have been used to assess the dangers of wind turbines to birds and bats. "Affordable" thermal imagers have a bat detecting range about the same order of acoustic bat detectors due to the small size and the low heat emissions of bats. Passive infrared sensors are slow with a response speed of the order of a tenth of a second and will normally not detect a small fast mammal like a bat. Radar has been used to detect bats beyond the acoustic limit, but is very costly in equipment and man hours. Bird Aircraft Strike Hazard [(BASH)] installations are capable of detecting bats, but are usually situated where few bats fly. There are very few suitable mobile terrestrial radars available anywhere. Hand-held doppler radar modues have been used in the field to allow researchers to compensate for the doppler shift imposed on recordings of bat signals due to their flight speed. This allows the researchers to tell whether the bats are changing the pitch of their calls in flight. External linksBat Species Identification
Bat detectors are the most common way to identify the species of flying bats. There are distinct types of call which can indicate the genus, and variations in pattern and frequency which indicate the species. For readers not familiar with the different types of bat detector, there is further information below and elsewhere. Bats also make social calls, which are less useful for species identification. They sound different from the echolocation calls and do not have the same frequency patterns. Fuller details on the types of call and other clues to species identification follow below but Pipistrelles give good examples of what can be discovered with a bat detector and make a good start to learning how to identify bats. A whole world of ultrasound opens up when a bat detector is switched on. To distinguish bat and bat species it is important to recognise non-bat species. The following minimally edited recording was made in a small nature reserve in North Devon, UK, in September. None of these calls were audible normally. One comes from a bat see also Non-bat sounds. Crickets, unidentified mammal, and a bat social call recorded on a heterodyne detector set to 19 KHz - 379Kb
Problems listening to the file? See media help. Captured bats can be exactly identified in the hand but in many countries a licence is required before bats can be captured. [The Mammal Society] has published an excellent guide in the UK: Which Bat is it? by R.E. Stebbings, D.W. Yalden and J.S. Herman Types of callThere are four basic types of bat echolocation call. FM callsThe term "frequency modulation" (FM) refer to the "chirp" type of bat call. On a bat detector it sounds like a sharp click. Tuning a heterodyne detector does not change the sound much, but the level varies. This is a typical call from a Myotis species. It sounds like hard dry clicks. It was recorded at 40 KHz which was not critical, but this was chosen because it was above the crickets and below Pipistrelles. CF callsThese calls were recorded using a heterodyne bat detector tuned to an appropriate frequency. The constant frequency (CF) call is a series of peeping calls. The bat emits the calls at a constant frequency, but the heterodyne bat detector exaggerates the doppler effect as it flies. Hockey stick callsSome bats call with a composite call, starting with an FM component and ending in a constant frequency. This is often called a "hockey stick" call from its appearance on a spectrogram. A Typical hockey stick call with a ploppy sound as heard on a heterodyne bat detector - 68 Kb
Problems listening to the file? See media help. 
Pipistrelle spectrogram
The Pipistrelle call is an example of a "hockey stick" composite type FM and CF call with a fast falling frequency FM part ending with a constant frequency, CF section. A spectrogram is a graphic representation of frequencies against time. The colour represents the loudness of each frequency. This spectrogram shows a falling call which becomes a steady note. The yellow and green blotches are noise. A single Pipistrelle call slowed down 256 times or 8 octaves - 22Kb
Problems listening to the file? See media help. On a heterodyne detector this sounds like a ploppy click, but when it is slowed down by eight octaves, you can hear how a sharply falling call slows to a single note. A heterodyne bat detector will only handle a small range of bat frequencies, so it is necessary to keep retuning the heterodyne frequency to find the point of maximum loudness or, in the case of bats with a hockey stick call, the frequency which gives the lowest sound. This gives the lowest plop sound from the CF end of the calls. Using a heterodyne bat detectorA heterodyne bat detector simply downshifts the bat frequencies by the amount shown on the dial or the display on the more posh detectors. For instance with a hockey stick call ending at 45 KHz, this will produce a near zero end frequency when tuned to 45 KHz, If it is tuned too low, or too high, the difference frequency rises as illustrated in Tuning the heterodyne in the Pipistrelle section below. It will only reproduce a limited range of frequencies, typically only 10 KHz out of a bat spectrum of over 90 KHz. A heterodyne bat detector does not give a very accurate measurement of the frequency of a bat call, One reason is that the call frequencies can easily vary by 1KHz or more due to the doppler shift. To track these changes and to get a more precise frequency, a frequency division bat detector or a time expansion bat detector is used using a computer with sound analysis software. Time expansion detectors are beyond the scope of this article but are described in bat detector Using a frequency division bat detectorA frequency division bat detector analyses each bat call and re-synthesises it at typically a tenth of its frequency to make then audible as explained in the bat detector article. It can produce anomalies with sounds with a random structure and can only process periodic or tonal calls with a measurable frequency, but a recording can be used to measure the frequency of all parts of the bat call using a spectrogram display as illustrated below. You don't tune a FD detector as it works on a full range of bat frequencies, but some, like the Duet, also have a heterodyne detector built in. The entire call is preserved and can be recorded on an audio recorder and studied later on a computer. A spectrogram and other analysis software can also show the repetition rates and patterns of the calls which is also useful for species identification. This is what a Pipistrelle sounds like on a frequency division bat detector: 
Pipistrelle spectrogram with feeding buzz
A Pipistrelle flying and feeding over a pond on a Devon farm - 225 Kb
Problems listening to the file? See media help. The sound is different from a heterodyne recording and is a squeaky "teek" a bit more like a heterodyne tuned off frequency. With a FD detector you cannot easily tell the difference between 45's and 55's by ear. And this is what that recording looks like on a spectrogram up to the feeding buzz. We can measure the end frequency (selected) which comes out as 44.6 KHz after multiplying by 10. The call rate speeds up as a food target is approached ending in a very rapid "feeding buzz". not the call becomes FM only during the feeding buzz. Social callsThis is a wide subject and there is still a lot to be discovered about bat social communication and how they use social calls in roosts and when flying. Generally a bat social call is not tonal, in other words it does not consist of a musical type note. Some bat detectors do not produce an accurate recording of a bat social call. Typically bat social calls use a lower frequency range than echolocation calls, and can thus be heard further away. Sometimes a bat will make a social call while echolocating which can cause confusion. 
FD Spectrogram of social and echolocation calls
Pipistrelle Social and Echolocation calls on a FD Detector Problems listening to the file? See media help. The spectrogram shows combined slower social calls with faster echolocation calls at a higher frequency. We can see and hear how the lower frequency social calls are heard at a greater distance than the higher echolocation calls as the bat approaches and departs. 
FD Spectrogram detail of social and echolocation calls
Zooming in on the spectrogram, we can see that the social calls are atonal and repeated rapidly about five times in each call. The social calls are interleaved between the echolocation calls. They show a ragged frequency distribution around 20 KHz Note the FD detector divides frequencies by 10. The echolocation calls are single hockey stick calls at a higher repetition rate. At this scale the hockey stick shape is not very clear, but the end frequency can be measured as 45.2 KHz. Note a doppler shift as the bat approaches. The frequency was measured as it passes. 
FD Spectrogram of echo on calls
This is further zoomed in on two echolocation calls. They appear double due to an echo. The selected portion is 10.8 ms, giving a path length difference of 10.8 times 340 M/sec or about 3.7 metres. Note the timings are not altered by the frequency division. Flying speedA technicality is creeping in here. A spectrogram can easily reveal a doppler shift in a passing bat. You can hear some shifts in the heterodyne calls above, but doppler shifts are most readily heard in CF calls such as the Horseshoe's. You can make a rough estimate the speed of a bat from the doppler shift as it flies past. The rule of thumb is: At around 50 KHz a shift is 1KHz indicates about 6.8 m/sec or 15 MPH. A passing bat will produce a total shift of about double this. The Pips below showed an estimated shift of around 1.5KHz indicating a speed just over 5 m/sec or a bit under 14 MPH. UK SpeciesThis section is also a practical introduction to the recognition of bat species, using Pipistrelles as an example. More detailed and technical information is given below. Seventeen species of bat are regarded as resident in the UK. The species most often seen and heard are the Common Pipistrelle and the Soprano Pipistrelle, and are a good reference point for comparison with other bat species. In fact it is worth taking time to get familiar with the various calls of the two common species. Common Pipistrelle Pipistrellus pipistrellusSoprano Pipistrelle Pipistrellus pygmaeusThese two species are considered together here. This section also acts as a tutorial for analysing bat calls. The only technical knowledge needed is that KHz is the number dialled into a bat detector. These two pips are distinct species but the frequencies of their calls are very variable and are not an exact indication of the species. They are frequently referred to as "45 Pips" and "55 Pips" from the calls as heard on a heterodyne detector.
Common Pipistrelle call heard on a heterodyne bat detector - 68Kb
Problems listening to the file? See media help. Note the "ploppy" sound of the call and the "feeding buzzes" as it
homed in on insects. The Soprano Pipistrelle's call sounds very
similar, but at a higher frequency setting on the heterodyne bat
detector. Tuning the heterodyneAs heterodyne bat detector only shifts a limited range of bat call frequencies, it needs to be constantly retuned so as not to miss some species and to identify those heard. One solution sometimes used in bat surveys is to use a second heterodyne detector tuned to a different frequency to detect other species such as Horseshoe bats if these are likely to be present. With Pipistrelles, if it is tuned too low or too high, the difference frequency rises as illustrated in the following example. Tuning to a Pipistrelle hockey stick call Four typical hockey stick calls: 1) tuned at CF frequency; 2) tuned low; 3) tuned high; 4) back to CF frequency - 213 Kb
Problems listening to the file? See media help. The first and last sections of this edited recording at about the same frequency as the last part of the hockey stick call, This produces a deeper and wetter ploppy sound. The second section is with the detector tuned too low - this brings the end frequency up and gives a squeaky click sound. The third section is with the detector tuned too high and also gives a squeaky sound but a bit harder. By tuning up and down, the deepest sound as in the fourth section is again produced, and this indicates the approximate frequency of the end of the bat's call. This is important for species identification. 45 and 55 PipsHow do you distinguish between P. pipistrellus and P. pygmaeus? In principle, if a call is around 45 KHz it is a Common Pipistrelle and around 55 KHz it is a Soprano. The rare P. nathusii calls at around 39 KHz and so is easier to distinguish. The problem is that there seems to be an almost continuous spectrum of Pip frequencies from 43 KHz to 59 KHz. More studies need to be done on the call frequency ranges of each species. Another small problem with differing frequencies is the doppler shift and a Pip passing by at 3.4 m/sec (8 MPH) will show a doppler shift of about 1 KHz. For bat workers with a suitable licence, an examination in the hand or close up, shows distinct characteristics between the 45 and 55 Pips:
Resolving callsThe following recording was made on a Duet combined heterodyne and FD detector. The heterodyne frequency was 53 KHz and the corresponding track sounds like this: What can be heard is a lot of background noise from crickets and a rather muddled pass by what is probably a pipistrelle. 
FD detector whole track
A spectrogram of the FD track reveals what happened: 
FD detector Lesser Horseshoe pass
Another bat was completely missed by the heterodyne detector which was set to the wrong frequency for that bat. It was a Lesser Horseshoe emerging from its roost. 
FD detector two Pips pass
The muddle following the LHS resolves into two Pipistrelles flying together with frequencies of 47.7 KHz and 54.5 KHz, in other words, a Common and a Soprano Pipistrelle. Three bats have thus been positively identified using a FD recording These three bats would not have been identified by a heterodyne bat detector. From other recordings taken at the time, the insistent "chuck" sound was associated with the Soprano Pipistrelle at around 20 KHz, which habitually made this social call while flying. This is where a FD detector falls down as it regenerated the social call at around 40 KHz. FD detectors can only process tonal calls with a measurable frequency and not calls which have a random frequency pattern. Space for other bat speciesIn preparation Non-bat soundsA whole world of ultrasound opens up when a bat detector is switched on. To distinguish bat and bat species it is important to recognise non-bat species. The following minimally edited recording was made in a small nature reserve in North Devon, UK, in September. None of these calls were audible normally. However, the cheat here is that the last sound is from a bat which is making a social call as it flies. Crickets, unidentified mammal, and a bat social call recorded on a heterodyne detector set to 19 KHz - 379Kb
Problems listening to the file? See media help. Rodents and insectivoresMice, voles and shrews emit ultrasound calls which can sometimes be mistaken for bat social calls. Sometimes other clues must be used to be certain, such as a sound coming and going as a bat flies past. CricketsCrickets make a distinctive sound both audibly and in ultrasound. Some species cannot be heard by the human ear. In the height of summer, they can mask out bat calls and interfere with spectrogram analysis. They can trigger "voice activated" recorders which can be very annoying when listening back later. This is the other relationship between cricket and bat. Several species of ultrasound crickets recorded on a heterodyne bat detector set to 19 KHz - 169Kb
Problems listening to the file? See media help. Note the reaction of one cricket when approached by a bat detector. Background noisesThere is an irreducible hiss in the background of every bat detector recording. This is "system noise" from the microphone and electronics. This can vary widely between bat detectors and various types of bat detector and in practice sets the distance limit for detecting calls. Footsteps and contents of pockets like keys and small change can be very noisy at ultrasound frequencies. Wind in trees is often less of a problem as this noise is absorbed by air at a distance. More detailed notesDistance range of bat callsHigher sound frequencies are absorbed by the air over a distance. The amount of absorption depends on the frequency and the humidity of the air. This is why close thunder crackles and distant thunder rumbles; the air has absorbed the higher frequencies. At bat echolocation frequencies, air absorption limits the range both for the bat and for bat detectors. Typically at around 50 KHz the sound level halves every six metres, or put more technically, it is absorbed at around 1 dB per metre. In practice this puts the maximum range of a bat detector at around 30 metres and a bat's ability to locate any object at roughly 20 metres. These are very approximate figures and bats which call at lower frequencies can hear and be heard at much greater distances. Conversely a bat like the Lesser Horseshoe which calls mainly at about 110 KHz is more difficult to detect over 10 metres. With a small target like an insect, the level of the reflection coming back falls with the inverse fourth power of the distance. In other words, at twice the distance, the level falls 16 times. This puts the maximum range at which a bat can detect an insect in metres. A large object such as a tree, a building or the ground can be detected by the bat at much greater distances. Types of bat callThe three basic types of bat call described an illustrated above provide different information to the bat. We cannot know what the bat actually hears, but research is continuing on what a bat can hear and discriminate. A bat does not receive a detailed image like a visual image although it has good eyesight as well, but essentially any ultrasound image it detects will be defocusssed due to the comparatively long wavelengths of the sound frequencies used. At 50 KHz, the wavelength is about 7 mm. It is remarkable how well a bat can echolocate. Frequency modulationThis is a rapidly falling whistle note and is used by most bat species. An FM call gives a very precise time measurement and enables a bat to detect distances to the accuracy of tens of centimetres. This technique is also used in radars where it has advantages over a very short pulse. As with radar itself, it was discovered that bats had been using these techniques or millions of years before man "invented" it. Constant frequencyThe Horseshoe bats Rhinolophus spp. use a mainly CF call. This is heard as a characteristic peeping sound on a bat detector. The frequency of the emitted call depends on the species and gives an immediate identification. Their call is not completely CF as it starts and ends with a "grunt" as can be seen on a spectrogram. A CF call does not give a precise distance measurement. It however gives a precise relative speed measurement due to the doppler effect. A "beep" lasting 60 milliseconds gives a linear length of the call of about 20 metres, thus if a target is less than 10 metres distance, the echo will start to return while the bat is still emitting the call. The doppler effect of the CF call from a moving bat is greatly exaggerated by a heterodyne bat detector. This can be used to estimate the speed of a flying bat or to identify bats which are echolocating while roosting. A bat call from a bat approaching or departing at 6.8 m/s (15 MPH) calling at 50 KHz will typically show a doppler shift of +- 1 KHz and pro rats. This can cause uncertainty with some species such as Pipistrelles. Composite FM and CFSome species such as the Pinstripes start their call with a FM component but the rate of change of frequency slows to an almost CF end part. On a spectrogram, his appears as a "hockey stick" shape. This in effect gives the best of both worlds, enabling precise distance and speed measurement. Some species alternate calls with and without a CF component, and Pipistrelles usually use a FM only call at short ranges. Detecting batsAcoustic bat detectorsSee bat detector for details of the different types of acoustic bat detection. There is a range limitation with this method because of the absorption of ultrasound by air as discussed above. For many bat species the range limit is around 30 metres, but it can be much less with Horseshoe bats and Long-eared bats. Visual identificationBats fly mostly at night but some indication of the species by sight at dusk or dawn can be given by size, flight patterns and proximity to known roosts. An example is when doing a bat roost emergence count at dusk when the likely species is further confirmed using an acoustic bat detector. The range limit depends on the light, surroundings and the night vision of the observer. InfraredInfrared imaging enables bats to be observed without causing them disturbance. This requires an IR illuminator such as a spotlight with a suitable filter or an IR LED emitter. Observation is done by IR binoculars or by IR CCTV. Some home camcorders made by Sony are sensitive to IR enabling an IR video recording to be made at low cost. The range limit is set by the illuminator and the definition of the imager. Species recognition is only by estimating size and by flight patterns. The power of an IR floodlight drops rapidly with distance, and a spotlight will miss bats not in the beam. Thermal ImagingA bat has to be warm to fly and emits heat of the order of 1 watt when flying, depending on size. The range is limited as with IR detection by the definition of the thermal imager. Affordable imagers will not detect distant bats and this method is unlikely to be better than IR illumination except with the most expensive high definition equipment. Ground radarFixed ground radar is used to detect birds flying near aircraft. Bird/Aircraft Strike Hazard (BASH) systems are deployed at airfields. Mobile BASH installations are still in the development stage and await further research with bats. They are very expensive to deploy and the licensing arrangements for mobile zoological ground radars is unknown. Anecdotal reports suggest that the most sophisticated radar systems with detection software can identify the presence of bats up to around 1 KM. The near limit with radars is more distant than the maximum range of the above methods and there may be a substantial distance gap between these systems and radar in which bats cannot be observed. Links[The Mammal Society] Publishers of: Which Bat is it? by R.E. Stebbings, D.W. Yalden and J.S. Herman This article is licensed under the GNU Free Documentation License. It uses material from Wikipedia Encyclopedia article "Bat Detector" |
