Building a Simple

Bat Detector
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Sound Frequencies
Types of Detectors
Finding Bats in NJ

Last updated Sun Oct 15 12:09:00 EDT 2000

Sound Frequencies

  Frequency Range of Hearing for Humans and Selected Animals
  animal                           frequency (hertz) 
                                    low       high 
  Humans                             20      20,000 
  Cats                              100      32,000 
  Dogs                               40      46,000 
  Horses                             31      40,000 
  Elephants                          16      12,000 
  Cattle                             16      40,000 
  Bats                            1,000     150,000 
  Grasshoppers and locusts          100      50,000 
  Rodents                         1,000     100,000 
  Whales and dolphins                70     150,000 
  Seals and sea lions               200      55,000 

  Reference: Encyc. Britannica.

Humans can hear sounds with frequencies between 20 hz and 20 khz, but in practice most of us hear only up to 16 khz. In contrast, bats commonly emit echolocation ultrasounds at frequencies between 12 khz and 100 khz. Some bat species can hear up to 160 khz. Other animal species can hear a full octave (double the frequency) above humans, for example cats and dogs can hear up to 40 khz. Some animal species can hear several octaves below humans. Moles, for example, can hear frequencies of a only a few hertz.

Bats are not the only species that use ultrasounds for echolocation. Dolphins, for example. emit echolocation sounds between 1 khz and 100 khz. Nor do all echolocating species use ultrasounds. Certain cave dwelling swiftlets, birds from South East Asia, use sound pulses in the human audible range between 1,000 and 5,500 hertz.

The speed of sound in air is 343 m/s at a temperature of 20 C, so a 1 khz tone has a wavelength of 34.3 cm, 10 khz 3.4 cm and 100 khz only 3.4 mm.

Types Of Detectors

A bat detector is an instrument that will detect the presence of bats based on their echolocation ultrasounds.

frequency division detectors:

These are the easiest to build. They will map, for example, 16 cycles of the original bat signal to 1 cycle of a new, frequency divided, signal. This frequency division by 16 means the new signal will have a frequency in the audible range. The best feature of this method is that division simply and effectively maps a wide range of ultrasounds (16 to 122 khz) to audible sounds (1 to 8 khz). In its simplest implementation the technique doesn't preserve the amplitude (sound level) of the original signal, but plays it at a constant level. This makes the bats appear to jump out of nowhere and disappear suddenly. This is a good first detector.

heterodyne detectors:

The original, inaudible, bat frequency is modulated (multiplied) by a tunable frequency to produce an audible signal at the difference frequency. (Modulation occurs electronically after the bat sound has been converted to an analog, volt based, signal by the microphone. The tuning frequency is entirely analog and doesn't leave the detector as ultrasound.) This method works by frequency subtraction and can therefore only bring 8 khz chunks of the ultrasound spectrum into the audible range at any given time. The frequency variations within this chunk will be easier to hear than with a division detector, but a narrower part of the spectrum is scanned at any given time, so some information is lost. Heterodyning also preserve more of the original signal amplitude so the feeling of a bat coming and going is better preserved. This detector makes a nice complement to a frequency divider.

time expansion detectors:

The ultrasound pulses are recorded at high speed and replayed at lower speed later. The recording is often already digitized and may be saved to tape or solid state memory. This is a professional method that allows a complete analysis of all features of the bat signal. The main drawback is that this can be quite expensive. Another drawback is that the analysis is delayed, so this approach is unsuitable for simple bat detection in real time.

DSP detectors:

Some newer detectors, based on the ever increasing processing power of microcontrollers, are starting to appear. With the help of digital signal processing methods, ultrasounds can be mapped to audible sounds using algorithms that go beyond division and subtraction. And they can do this in real time! The new mapping methods are still the object of active research, the goal being to produce an audible signal that preserves the subtleties of the ultrasound spectrum.

Reference: Comparing Mini Bat Detectors, informative newsletter from the UK's National Bat Monitoring Programme.


The CJ-SAS project from Popular Electronics:

Popular Electronics Heterodyne, summer 2000 project. The detector we originally proposed to build.

Fall 2000 lineup:

by Vincent Goffin.
Complete reports on the belfry detector, the sk207 detector, and my own batty ultrasound generator, a simple ultrasound chirp generator useful for testing and calibrating detectors. Here are some of the better recordings I made.

Your experiment:

by You!
If you have have built a bat detector, or if you some original bat recordings consider writing it up for inclusion here. Contact the webmaster@njsas.org for questions or comments.

Finding Bats in NJ

Eastern pipistrel Pipistrellus subflavus,
little brown bat Myotis lucifugus,
Indiana bat Myotis sodalis+,
northern long-eared bat Myotis septentrionalis,
small-footed myotis Myotis leibii,
big brown bat Eptesicus fuscus,
hoary bat Lasiurus cinereus,
red bat Lasiurus borealis and
silver-haired bat Lasionycteris noctivagans.
+ state and federally endangered
Reference: New Jersey Division of Fish, Game and Wildlife, a and b.
Nine bat species are considered regular residents of New Jersey.

During the summer bats can be found in many rural and suburban settings. Since they feed on insects, woods and lakes can be good places for a search. Bat activity starts soon after sundown.

During the winter, bats that do not migrate will hibernate. New Jersey's largest known bat hibernaculum is the Hibernia Mine in Rockaway Township, Morris County. This little brown bat colony is one of the spectacles described in Nature Spectacles in New Jersey, J. Burger and M. Gochfeld, Rutgers University Press, 2000. ISBN 0-8135-2766-X. Call (908)735-8975 for information on guided tours. Guided tours this year (2000) will be organized in late August.



Google search


Bat Conservation International.
Bat Conservation In NJ, 9 species of bats are known to live in New Jersey, 44 in North America.
The UK Bat Conservation Trust, 16 species of bats in the United Kingdom.


Google search,
Pettersson Elektronic AB , professional bat detectors from $200 to $4K+, including analysis software. This time vs. frequency spectrum from their site is quite impressive.
BVL Von Laar's Bat Detectors and Bioacoustic Systems, heterodyne and time expansion units from $130 to $2,000+. They also have bat sound recordings.
The Simple Bat Detector, Tony Messina's frequency division detector. He also sells parts, kits ($35) and assembled units ($50).
Transtronics' SK-207 Ultrasonic Translator, a heterodyne ultrasound kit for $20. It can double as a bat detector.
Belfry Bat Detector, Convergence Technologies' bat detector ($45.95 kit, $79 assembled). This is a frequency division detector similar to Tony Messina's.
Bertrik's bat detector page very informative pages with a discussion of how bat detectors work, design tradeoffs and bat call analysis. Also many good links.

bat detector projects:

University of Northumbria at Newcastle, Bats and bat detectors.
DSP Based Bat Detector, an ambitious university project for designing the best DSP based bat detector. Challenging!

analytical software:

Sound Aquistion/Analysis Database, from the Dolphin Study Group in Singapore.
Spectrogram 5.1 for Windows 95/98/NT. See this bat echolocation call, with mp3 audio!

call libraries:

Leeds University (UK), recorded on a Pettersson D980 bat recorder and time expanded by a factor of 10.
Ashley Walker's Sonar Gallery, unfortunately many of the links lead nowhere.

bat research:

Identification of Bat Echolocation Calls, an attempt to identify bats from their ultrasonic calls.
Micro-bat, Caltech is building a mechanical bat.

rat research:

Rat detector, different... and interesting.