Bat detectors

John Errington MSc(Physics) and

Jonathan Pounder BSc(Enviromental Studies)

John ErringtonLecturer 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 PounderLecturer in Environmental Conservation at Houghall College

Environmental Consultant

Licensed Bat Worker with Durham Bat Group

What does a bat sound like?

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

Bat call is picked

up by sensor

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: heterodyneBenefits:

o Usually quite sensitiveo Pleasant sound.o Tuning can be an aid to identification

Disadvantages: o Can be mis-tuned, leading to false identifications. o Can miss bats you're not tuned to.o Not suitable for call recording and analysis.o 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 expansionBenefits:

o Slow down call. This helps you hear changes during the call.

o Very pleasant to listen to.o Very good for recording and analysis.

Disadvantages :o Can miss bat calls while playing back sounds.o Rather expensive and can be fragile

Sound clip: TE compilation, daub, pip, noctule

TE detectors

Detector types: DSPBenefits:

o Better audio representation of bat callo More accurate for recording and analysis

Problems:o Not presently available in a portable detectoro Need to use high quality components

(e.g. sensors) to justify added complexity

F Div AR DSP FComp

Real time ultrasonic recordingBenefits:

o Can record entire frequency rangeo Can record continuously unattendedo Can play back original soundo Ideal for bat call analysis and as an aid to

species identification.

Drawbacks: o Very expensiveo Sometimes not very robusto 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

bat detector – to – sound recorder – to – computer

audio

Data storedusuallyas .wav or.mp3

USB

Mic orLine in

Bat call analysisSound 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

Bat calls are stored on computer - usually as .wav or .mp3 files

The end result: a sonogram

Bat detector sensors

Piezo transducer

High voltage Capacitance Microphones

Small Electret

Miniature Electret – hearing aid types

Micro-electrical-mechanical system(MEMS) transducers

Very sensitive, robust, cheapvery 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 useType 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 fieldThis is where the bat detector is to be used, each design having its own benefits and disadvantages. Areas to consider:

Weatherproof: RainCold Weather

Robust design: Knocks bangs and bumpsDropping 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 includeso Weight in hando Supports & strapso Add-ons – particularly when recording for later analysiso 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 Low light levelso Ease of use with cold hands or gloves

Associated Equipment

Little extras come in very useful

o Spare Batteries (ensure they are the right type)

o Earphones – allow you to focus your attention on what you’re hearing particularly with background noise (road or river)

o Head torch o MP3 playero Thermometer

Safety, theft and concealabilityParticularly 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