Bat detectors
John Errington MSc(Physics) and
Jonathan Pounder BSc(Enviromental Studies)
John Errington
Lecturer in Electronics and IT at
University of Northumbria (retd)
30 years interest in bat detector
design
Webmaster for
Durham Bat Group
www.durhambats.org.uk
Jonathan Pounder
Lecturer in Environmental
Conservation at Houghall College
Environmental Consultant
Licensed Bat Worker with Durham
Bat Group
What does a bat sound like?
Bat call
is picked
up by
sensor
The bat detector takes
signals from a special
microphone, converts
them to match the
human hearing range,
and outputs them to a
speaker or headphones
Audible
sound out
from
speaker or
phones
Microphone or “sensor”
Detector electronics in
box – needs a battery
Loudspeaker,
headphones or earpiece
Types of bat detector
• Heterodyne
• Frequency division (FD)
• Time expansion (TE)
• Digital Signal Processing (DSP)
• Real time ultrasound recording
Detector types: heterodyne
Benefits:
Usually quite sensitive
o Pleasant sound.
o Tuning can be an aid to identification
Disadvantages:
o
o
o
o
o
Can be mis-tuned, leading to false identifications.
Can miss bats you're not tuned to.
Not suitable for call recording and analysis.
Heterodyning changes harmonic relationships in the
call.
Sound clip: common pip
Heterodyne detectors
Detector types : Frequency division
Benefits:
o Suitable (but limited) for recording calls for later analysis.
o No chance of missing a bat because they don’t need
tuning.
Disadvantages:
o Sound quality is not very pleasant unless a more
upmarket “AR” type is used.
o Creates artificial harmonics that can distort a sonogram.
o Can lack sensitivity
Sound clip: Belfry bat detector
Sound clip: Daubentons bat with BATON)
FD and dual-mode detectors
Detector types: Time expansion
Benefits:
o
o
o
Slow down call. This helps you hear changes during
the call.
Very pleasant to listen to.
Very good for recording and analysis.
Disadvantages :
o
o
Can miss bat calls while playing back sounds.
Rather expensive and can be fragile
Sound clip: TE compilation, daub, pip, noctule
TE detectors
Detector types: DSP
Benefits:
o
o
Better audio representation of bat call
More accurate for recording and analysis
Problems:
o
o
Not presently available in a portable detector
Need to use high quality components
(e.g. sensors) to justify added complexity
F Div AR
DSP FComp
Real time ultrasonic recording
Benefits:
Can record entire frequency range
o Can record continuously unattended
o Can play back original sound
o Ideal for bat call analysis and as an aid to
species identification.
Drawbacks:
o
o
o
o
Very expensive
Sometimes not very robust
Don’t provide audio output
Recording ultrasound bat calls
An ultrasound receiver picks up
bat calls and transfers them to a
computer or field recorder via
the USB interface
Software running on the computer receives
the data, supports analysis and storage of
data, and can produce sound output to
headphones (like a bat detector)
Ultrasound recording: Avisoft
Bat call recording on a budget
Recording bat calls
Data stored
usually
as .wav or
.mp3
USB
audio
Mic or
Line in
bat detector – to – sound recorder – to – computer
Bat call analysis
Bat calls are stored
on computer - usually
as .wav or .mp3 files
Sound recording and
editing software
e.g. AUDACITY (Freeware)
Call analysis software:
Freeware:
WaveSurfer, SoundRuler
Commercial software:
Avisoft “SASlab”, £2000
Batbox “Batscan” £30
Petterson “Batsound” £300
SonoBat (TE only) $320
The end result: a sonogram
Bat detector sensors
Piezo transducer
High voltage
Capacitance
Microphones
Small Electret
Miniature Electret –
hearing aid types
Micro-electricalmechanical system
(MEMS) transducers
Very sensitive, robust, cheap
very limited bandwidth
Wideband, flat response, good sensitivity,
expensive, not robust,
need high voltage supply
Sensitive, robust, cheap, best at lower
frequencies
Generally less sensitive than larger mic’s, and
more expensive, but better high frequency
response.
Similar to miniature electrets but with much
higher bandwidth and flatter frequency
response. Extremely robust.
Sensors in use
Type of sensor
Manufacturer / Model
Piezo Sensor
Belfry, Magenta 2 & 3
Capacitance Mic
Avisoft, BATech, Petterson D1000X
Electret
CSE, Ciel, BATBOX
Magenta 4 & 5
Mini Electret
Maplin kit (no longer available)
Used a tiny hearing aid microphone
MEMS Mic
BATON
Multiple sensors
Petterson D100 (2 piezo 1 electret)
MEMS microphones
A MEMS microphone combines a
capacitance microphone
micromachined on silicon as shown
here,
with all of the electronics
(drive circuit, amplifier, etc) needed
to make it work
Out in the field
This is where the bat detector is to be used, each design having its
own benefits and disadvantages. Areas to consider:
Weatherproof:
Rain
Cold Weather
Robust design:
Knocks bangs and
bumps
Dropping the detector
– impact resistant
These are now considered in most of the modern designs
Design and Use
The design in relation to fitting it into the hand and operating it.
This includes
o
o
o
o
Weight in hand
Supports & straps
Add-ons – particularly when recording for later analysis
Ease to change a battery quickly
Knobs and dials
Can sometimes be the deciding factor in identifying bats in the
field quickly. Factors to consider:
o
o
Low light levels
Ease of use with cold hands or gloves
Associated Equipment
Little extras come in very useful
o
o
o
o
o
Spare Batteries (ensure they are
the right type)
Earphones – allow you to focus
your attention on what you’re
hearing particularly with
background noise (road or river)
Head torch
MP3 player
Thermometer
Safety, theft and concealability
Particularly in urban
areas -
Keep them out of sight
If in doubt - Make sure
someone knows where
you are and when you
will be back.
Best buys
More information
For more information about bat
detectors, how they work, reviews etc.
visit our website
www.durhambats.org.uk