Tuesday, 2 August 2011

Echolocation is nothing to shout about.

The three previous bat postings have caused a fair bit of interest. I have had a few message exchanges with readers. In particular asking questions about the bat detector I use and how bats can see using sound. I am a bit more than an interested amateur, but I am certainly not an expert in the field of bat observations.

Science is only just beginning to understand some of the complexity of the echo location skills of bats. There is a certain amount of supposition in what bats can actually achieve using ultrasound. Supposition is then backed up by observational research to prove or disprove a point of supposition. We have learned a great deal about the life of bats from observations and yet at the same time we have come to know that we know so little.

So, first I will try and give a description of seeing using sound. There is a certain amount of descriptive licence being used to get the basic concept and understanding of seeing using ultrasound across.

First the basics.

Bats have eyes with which they can see reasonably well. Well enough to move from place to place by following landmarks. Bats have an array of additional senses as well as the familiar ones of vision, smell and normal hearing. These senses include the remarkable sense of ultrasound echolocation, used for finding their way about and hunting insects in the total darkness of a moonless night. A hard-bodied insect produces a different quality of echo from one with a soft body, so bats can distinguish between some different groups of insects in this way. They can also determine the size of the insect.

The four main functions of the ultrasound emitted by bats are:-

  • Navigating.
  • Foraging.
  • Targeting.
  • Contact.

Hearing range, is the range of different frequencies that can be heard by an animal or human. Usually stated as the lower and highest frequency. In humans the range is said to be 20Hz (cycles per second) to 20kHz (20,000 cycles per second) Although there is considerable variation between individuals, especially at the high frequency end, where a gradual decline with age is considered normal. Our hearing range is not flat throughout the range, we tend to be more sensitive at the centre point of our hearing frequency range.

So-called "silent" dog whistles at 25-30 kHz exploit this phenomenon by producing sounds at frequencies higher than those audible to humans but well within the range of a dog's hearing. The hearing range of a dog is 40 Hz to 60,000 Hz which is much greater than that of humans. As with humans, some dogs become more deaf with age.
Poppy our younger Wire Haired Fox Terrier can detect the sound of the Common Pipistrelle and follows the sound of them with her her head and ears. She soon gets bored with this and then starts to ignore them. Her preferred sense, is to use her nose and is much happier sniffing in the long grass. This scares up the insects in the grass and seems to attract the bats. Abbey our older Wire Haired Fox Terrier seems to ignore the bats all the time. She also has a severe case of selective deafness unless a chocolate drop is involved!

Our individual ears can also show differences in sensitivity and frequency range. We might for instance be slightly more deaf in one ear than the other. In my case my left ear is more sensitive to high frequency than my right ear. Never stand on my right at the bar when its my round!

Bats normal hearing range is 20 Hz to 120 kHz (twice that of a dog). However, they emit a specific set of ultrasound frequencies somewhere in the range between 20kHz to 120kHz. (starting just above our normal hearing range).

Different bats have a different set of ultrasound frequency ranges that they use. They also have a variety of sound types to their calls.
  1. Daubenton's bat ultrasound calls range from 35 - 85 kHz and peak at 45 - 50 kHz.
  2. Noctule bat ultrasound calls range from 20 - 45 kHz and peak around 20 kHz.
  3. Common Pipistrelle bat ultrasound calls range from 40 to 60kHz. The peak at 45kHz.
  4. Soprano Pipistrelle's ultrasound calls range from 40 to 60kHz. The peak at 55 kHz.
  5. Serotine bats range from 15 - 65 kHz and peak around 27 kHz.

  • Unlike light, sound travels at a low speed through the air.
Sound travels at around 340 metres/second or 765 mph. Because of the low speed compared to light (300,000,000 metres/second 186,000 miles/second) it takes time for the sound to travel to an object and for the echo to return back to the bat.

  • Bats make more than one type of sound.
Bats make changes in the way they make sound depending on what they are doing. The pulses of sound produced by the bat last only one or two thousandths of a second and silences between the calls give time to listen for the information coming back in the form of an echo.  
  • Ultrasound used by a bat is focused into a specific direction.
This is like the beam of light from a torch being swept from side to side and up and down. The bats do this by head movement when flying in a straight line or by moving their body position (when fluttering about chasing an insect) By keeping the head in a fixed position and the target insect dead ahead, the bat is always heading towards the insect no matter which way it is moving.
  • Bats also use silence when hunting.
Bats can also vector their flight to a position where an already detected and ranged (speed and direction) insect has been estimated to be. It is thought that they do this in silent mode so as not to alert other bats or the target insect. It is thought that they also listen to the sound of the insects wing beats during the silent vectoring period. Turning on the ultrasound again when within a couple of metres for any last moment flight adjustments.

  • Depending on the size and distance of an object, the strength of the returning echo will vary.
The larger an object, the stronger the echo. The nearer and object the stronger the echo. The texture of an object being detected will also vary the strength of an echo.

  • The closer a bat gets to an object the sooner the echo will return.
Imagine shouting and the echo comes back whilst you are in mid shout, you would be deafening yourself to the echo. Bats can alter the length of a sound pulse to stop calling before the echo arrives. The closer they get to an object, the shorter the sound pulse is, giving more time between sound pulses for the echo to arrive.

  • Bats use a wide range of ultrasound frequencies to see better.
Bats can alter the frequency as well as the length of a sound pulse, it is thought that this gives the bat a better more detailed view of the detected object.

  • Doppler shift.
There is also evidence to suggest that bats use the change in pitch of the returning echo (the Doppler effect) to assess their flight speed in relation to objects around them.

The information regarding size, shape and texture is built up to form a picture of their surroundings and the location of their prey. Using these factors a bat can successfully track change in movements and therefore hunt down their prey.

  • Bats can shout quietly or more loudly if needed.
A bit like the Memsahib in this case! However, the bat can at the same time use different levels of loudness (intensity of sound) at different frequencies (Sound pitch). This is again thought to give more information on the detected objects texture.

  • There are two types of calls used by bats. Constant frequency (CF) and frequency modulated (FM) calls.

Constant Frequency is an ultrasonic call pulse sent at a fixed ultrasound frequency. Frequency Modulation is an ultrasonic call that descends in ultrasound frequency during the pulse. Each type reveals different information for the bat. CF is used to detect an object and FM is used to provide information regarding the nature of the object and its distance.

  • Close range ultrasound disables and or confuses flying insects.
It is thought that the ultra sound frequency and the number of rapid pulses can disable or confuse an insect long enough for the bat to press home its hunting attack. Some bats produce sounds louder than 110 decibels. That's the same volume as a jet plane flying low overhead.

OK here comes the bit where you have to use your imagination and become a bat.

A bat when it is navigating is sending individual Constant Frequency (CF) sound pulses which allow enough time for the sound to travel say a maximum of 10 metres to an object and for the echo to return back 10 metres to the bat. This is a bit like being out in a thick fog with a torch that turns itself on an off at a varying speed. However, you can still only see a maximum of 10 metres in any direction because of the fog. High ultrasound frequencies have a shorter range of about 6 m (20 ft), and perhaps less than 3 m (10 ft) when a small insect is first detected.

Remember when navigating from place to place bats also use their normal eyesight to pick out landmarks and large objects on their flight path.

Frequency Modulated (FM) sound pulses are like different colours of light, Red light is at the low end of the frequency range. Blue light is at the high end of the frequency range. Different colours of light will when mixed together travel different distances through the fog. (Remember yellow fog lights) By mixing the coloured lights together (mixing different frequencies) we get a better view of the objects ahead.

When as a bat we detect (or see with ultrasound) an insect, we can shorten the length of the pulse to increase the actual number of pulses we can send in the same time period and still (see with ultrasound) hear the echo. We would then hear (see with ultrasound) a faster flickering picture where we can detect even tiny changes in the position of the insect. As we get closer to the target insect, we can emit even more short pulses (up to 100 a second) of sound (light) because the echos (flickering pictures) are returning back much sooner. When we are almost in capture range the ultrasound pulse rate can be as high as 200 pulses a second and the range to the insect be less than one metre. Its a bit like a flashing strobe light being used to illuminate people. it appears to make people move in a jerky rather than a smooth way.

Bat Detectors.

Bat Detectors come in three types, hetrodyne, frequency division and time expansion. They also come in three price ranges, cheap, expensive and Oh My God! As a rule of thumb, £40-£100, £150-£400 and £500-£1000's.

For general use a hetrodyne bat detector is all you will need and most fall into the £40-£200 range.

We know that bat calls are of a frequency too high for us to hear. However the electronics of a bat detector can convert the ultrasounds to "normal sound" at a level that we can comfortably hear. The calls made by bats will vary in repetition rate, their intensity as well as frequency and length of sound pulse. It takes some practice to be able to distinguish the species, from their hetrodyne detector sounds. However hetrodyne bat detectors are now one of the main tools of bat researchers.


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