Why Bat Detectors?

The frequency of the echolocation sounds that bats produce, is out of the range of human hearing. In order for humans to hear the calls of bats,  bat detectors have been developed.

While human hearing can only detect sounds within the 0 to 16kHz range, the call frequency of bats ranges from 20 to 210kHz with each bat species having a unique call.  Bat detectors convert the ultrasonic calls into audio which we can hear, enabling us to identify different species and to note various behavioural patterns. Much of this can be done without ever seeing the bats. 

Bat detectors range from simple devices which merely allow one to ‘hear’ the bat to highly sophisticated detectors which can, once coupled to a computer, allow one to ‘hear’ and ‘see’ the call through sonographs.

Is a Bat detector only for Scientists?

NO!! Anyone fascinated by bats will be richly rewarded by discovering more secrets of bats, their hiding places and their identity with the aid of a bat detector.

A bat detector is as useful to the bat enthusiast as binoculars for the bird watcher!

There are various types of detectors available on the market and may be purchased for about R1000!

Types of Bat Detectors

Bat detectors can be divided up into three basic types although some can combine the Heterodyne and time expansion modes.
 
A.    Heterodyne bat detectors

The receiver converts the noise made by bats into sound that we can hear.  They can be tuned into a number of different frequencies which allow one to listed to a wide range of species.  Depending on the species of bat being listened to, the calls sound like a series of clicks which are either dry and crackly or wet and gurgly.  Until a bat locks onto an item of food, it emits calls at fairly regular intervals.  Once it finds a suitable insect, the rate of the calls emitted speeds up and this sounds like a buzz on the bat detector.  This is known as a ‘feeding-buzz’.  One can thus not only identify the species but also determine whether or not the bat is feeding.  This type of detector is fairly inexpensive.

B.   Frequency division bat detectors

These record one pulse out of every 8 to 10 incoming pulses thus reducing a 56kHz call to a 7 or 5.6kHz call which is audible to humans.  Without computer software, these detectors merely tell you that a bat is present.  However, once the sound is analysed on computer, the species can be identified.  These detectors are more suited to researchers than the bat enthusiast.

C.   Time expansion bat detectors

These are very expensive detectors and work in much the same way a high-speed tape-recorder does.  They record the sound and then play it back at 10, 32 or 64 times slower to make the sound audible to humans.  These systems not only enable one to identify the species but also the amplitude, frequency and harmonic detail of the call.

Click To View: Call Parameters of Some Southern African Bats

Terminology required in order to successfully use Bat Detectors:

• Wavelength and frequency – A sound wave consists of a complete up and down movement and the number of these waves in a given time is known as the frequency.  1,000 complete waves per second is equal to 1 kilohertz (kHz).  When we speak of a bat echolocating at 40kHz, we mean it emits 40,000 complete waves per second.
• Frequency Modulation and Bandwidth – single notes that don’t vary in pitch are referred to as constant frequency (CF).  Conversely, sounds that do vary in pitch are referred to as frequency modulated (FM).  Bats use either CF or FM or a combination of both.
   
  The difference between the minimum and maximum frequency is known as the bandwidth.  A CF call has a zero bandwidth as it does not vary in pitch.  Bandwidth can be measured on a FM call as the minimum and maximum frequency or pitch changes.
• Harmonics – harmonics are what allows humans to differentiate between sounds.  For example, we can recognize the voices of different people based on the harmonics of their voice.  Bats use precise patterns of harmonics.  The lowest frequency is known as the fundamental or first harmonic.  Subsequent higher harmonics are exact copies of the first but at higher frequencies and are always multiples of the first.  These subsequent harmonics are always weaker than the fundamental and assist in bat identification.

Sonograms –these allow us to ‘see’ a call.  They are produced on computer when sophisticated software is used in conjunction with a bat detector.  The horizontal axis gives the duration of the call and the vertical axis gives the call’s frequency range.

• Intensity – The loudness of bats calls vary with species.  In order to echolocate in a open field, a bat has to be able to emit a loud call which can travel over a greater distance.  Bats that forage in forests can emit a softer call as it doesn’t have to travel over such a large distance.  Calls may also be louder in one part than in another – this is referred to as a dominant frequency.
• Call duration – the time over which a sound is emitted is referred to as call duration and varies from 0.2 milliseconds to 100 milliseconds.  The gaps in-between calls allow the bat to hear the returning echo and home in on their prey.

Click To View: Call Parameters of Some Southern African Bats