Ultrasonic Sound

*sound science series # 4 

Ultrasonic Sound

The term "ultrasonic" applied to sound refers to anything above the frequencies of audible sound, and nominally includes anything over 20,000 Hz. Frequencies used for medical diagnostic ultrasound scans extend to 10 MHz and beyond. 

Approximate frequency ranges corresponding to ultrasound, with rough guide of some applications

Uses of Ultrasonic sound:  Sounds in the range 20-100kHz are commonly used for communication and navigation by bats, dolphins, and some other species. Much higher frequencies, in the range 1-20 MHz, are used for medical ultrasound. Such sounds are produced by ultrasonic transducers. A wide variety of medical diagnostic applications use both the echo time and the Doppler shift of the reflected sounds to measure the distance to internal organs and structures and the speed of movement of those structures. Typical is the echocardiogram, in which a moving image of the heart's action is produced in video form with false colors to indicate the speed and direction of blood flow and heart valve movements. Ultrasound imaging near the surface of the body is capable of resolutions less than a millimeter. The resolution decreases with the depth of penetration since lower frequencies must be used (the attenuation of the waves in tissue goes up with increasing frequency.) The use of longer wavelengths implies lower resolution since the maximum resolution of any imaging process is proportional to the wavelength of the imaging wave.

Radar and ultrasonic sound waves made by dolphins Even though dolphins live in the darkness of the sea, they can still search for food in the form of fish.

They do this by emitting sound that cannot be detected by the human ear (ultrasonic sound waves) and catching the sound that comes back like an echo. Even though they cannot see their food, they can tell their direction and distance by using ultrasonic sound waves.
The way radar works and the way that dolphins use ultrasonic sound waves are much the same.
Radars used at airports emit radio waves from an antenna and catch the radio waves that are reflected off aircraft.
Through this, the direction and distance of aircraft can be detected.

Principle of an active sonar

Ultrasonic range finding: A common use of ultrasound is in range finding; this use is also called SONAR, (sound navigation and ranging). This works similarly to RADAR (radio detection and ranging): An ultrasonic pulse is generated in a particular direction. If there is an object in the path of this pulse, part or all of the pulse will be reflected back to the transmitter as an echo and can be detected through the receiver path. By measuring the difference in time between the pulse being transmitted and the echo being received, it is possible to determine how far away the object is.
Bats and ultrasound: Bats use ultrasonic sound for navigation. Their ability to catch flying insects while flying full speed in pitch darkness is astounding. Their sophisticated echolocation permits them to distinguish between a moth (supper) and a falling leaf.
About 800 species of bats grouped into 17 families. The ultrasonic signals utilized by these bats fall into three main categories. 1. short clicks, 2. Frequency-swept pulses, and 3. constant frequency pulses. There are two suborders, Megachiroptera and Microchiroptera. Megas use short clicks, Micros use the other two. Tongue clicks produce click pairs separated by about 30ms, with 140-430 ms between pairs. (Sales and Pye, Ultrasonic Communication by Animals). 10-60 kHz in frequency swept clicks. One kind of bat, the verspertilionidae, have frequency swept pulses 78 kHz to 39 kHz in 2.3 ms. Emits pulses 8 to 15 times a second, but can increase to 150-200/s when there is a tricky maneuver to be made. 

Bats use ultrasounds to navigate in the darkness.

Some more with ultrasound ability: The upper frequency limit in humans (approximately 20 kHz) is due to limitations of the middle ear, which acts as a low-pass filter. Ultrasonic hearing can occur if ultrasound is fed directly into the skull bone and reaches the cochlea through bone conduction without passing through the middle ear.

       It is a fact in psychoacoustics that children can hear some high-pitched sounds that older adults cannot hear, because in humans the upper limit pitch of hearing tends to become lower with age , which may be due to the considerable variation of age-related deterioration in the upper hearing threshold.

                  Many animals—such as dogs, cats, dolphins, bats, and mice—have an upper frequency limit that is higher than that of the human ear and thus can hear ultrasound. This is why a dog whistle can be heard by a dog.

Ultrasonic technology in wide range of applications:

http://www.angelfire.com/nj3/soundweapon/ultrales.htm
  
*Note: all pictures thankfully shared from various sources..