All living organisms have a range of sound frequencies that they can tolerate. We can only hear the sound frequencies that lie within our range. Humans can hear the sounds whose frequency lies between 20-and 20,000 Hz. That means we can not hear the sounds with a frequency less than 20 Hz like our heartbeat (we need a stethoscope to hear it) and greater than 20,000 Hz like the sound of dolphins.
Infrasonic and ultrasonic waves
Infrasonic waves: The sound waves with a frequency less than 20 Hz are called infrasonic waves. Those are beyond the audible range of human ears. Some animals like whales and elephants produce infrasounds, so those sounds are not audible to us.
Ultrasonic waves: The sound waves with a frequency greater than 20,000 Hz are called ultrasonic waves. Ultrasonic waves or ultrasounds have a frequency too high to be heard by humans. For example, a sound frequency of 100,000 Hz is an ultrasound.
Properties of ultrasonic waves:
- The ultrasound is reflected just like ordinary sound waves, producing echoes. But the echoes produced by ultrasound are not audio to our ears. They can only be perceived by special equipment.
- Because ultrasound waves have a high frequency, the wavelength is shorter. Wavelength is the complete length of a wave. It is denoted by lambda.
- Because it has a very high frequency, ultrasound has a much greater penetration power than the ordinary sound. This property of ultrasound is very useful to humans.
- Ultrasounds can navigate well-defined paths even if obstacles cover them.
- Ultrasounds are used enormously in the medical field and industries.
- Ultrasonic waves navigate very easily through solids like glass and polystyrene.
- Ultrasonic waves can travel enormous distances.
Applications of ultrasound:
- Ultrasound waves can be used to visualise the internal organs of the human body. So, they are used for the diagnosis of medical conditions. They are used to diagnose the presence of stones inside organs like the urinary bladder. The stones reflect the ultrasound waves, and this activity is recorded in an ultrasound film and helps the doctors diagnose. It can also be used to investigate other internal organs of the human body, such as the liver, gallbladder, pancreas, kidney, etc. It helps healthcare professionals to detect abnormalities such as stones in the gallbladder and the presence of cancerous cells in different organs. I
In a diagnostic ultrasound technique, the ultrasonic waves navigate through the body’s structure and get reflected from a site where there is a defect or even a slight change of tissue density. These waves are then converted into electrical energy to produce a pictorial representation of the organ. This pictorial representation is then exhibited on a screen or printed on a film.
- Diagnostic procedures such as colour doppler and echocardiography all use the principle of ultrasound. Doppler is a procedure where ultrasound scans are used to monitor the development of the foetus (unborn baby) inside the pregnant mother’s uterus. This technique can sometimes be misused and lead to infanticide as it can detect the sex of the fetus. Ultrasound is also used as a diagnostic procedure to detect congenital disabilities and anomalies in the growing foetus.
This is very important as this helps the healthcare team stay vigilant and provide extra care for such mothers. Echocardiography is a technique in which ultrasonic waves are made to reflect from various structures of the heart so that an image of the internal structure of the heart is formed. It helps doctors to know about any congenital disabilities or anomalies
- Ultrasounds are used to break stones in the kidney into smaller particles or grains, which then get excreted through the urine.
- Ultrasound is used in sonar apparatus to measure the depth of a sea or an ocean and locate under-sea objects like shoals of fish, shipwrecks, submarines, sea rocks, and hidden icebergs in the sea. The sonar apparatus is attached to the undersurface of a ship. It has a transmitter and a detector. The transmitter produces ultrasonic waves, which are transmitted through the water.
These waves hit any object or resistance that comes in their path. The object reflects the waves and sends those to the detector. The detector changes the ultrasonic waves into electrical energy that provides us with information regarding the object’s depth under the sea and the depth of the water body.
- The ultrasound waves are used in industries to detect defects (cracks, etc.) in metal blocks without damaging them. This is based on the fact that an internal crack (or hole, etc.) does not allow ultrasound waves to enter it, rather it reflects ultrasound waves. Metals are mostly used to establish huge structures like buildings, roads, bridges, machines, and scientific equipment. The defects and imperfections inside the metal blocks, which we can not detect from outside, make the structure weak and susceptible to collapse.
This situation, if ignored, can prove catastrophic. Ultrasonic waves are allowed to cross the metal block, and detectors are used to detect the wave that gets transmitted. In case there is even a small anomaly in the metal, the ultrasonic waves get reflected, indicating the presence of the flaw or defect. Ordinary sound waves of longer wavelengths can not be used to carry out this function because those will bend around the corners of the defective location and enter the detector; they do not have much penetration power.
- Ultrasounds are used to clean parts of the objects that are hard to reach manually, for example, spiral tubes, odd-shaped parts, electronic components, etc. Objects to be cleaned are immersed in a cleaning solution, and ultrasonic waves are then passed through the solution. The dust and dirt particles get separated from the object and drop out. Therefore, the object gets completely cleaned.
- Animals like bats use ultrasonic waves to locate their prey. Bats produce high—pitched ultrasonic waves, which are reflected by the resistance that comes in their path, such as prey and return to the bats’ ears. This is how the bat gets to know how far the prey is. Similarly, dolphins use ultrasounds to find fish to feed on and to protect themselves from predators such as sharks.
Preparing for CBSE Class 9? Here are some pages that will be helpful for you.
Important Concepts for Physics
1. What are the properties of sound waves?
1. Sound waves are longitudinal; their oscillation is parallel to the direction in which they move.
2. Sound waves travel through all types of matter. The velocity of sound waves is the highest in solids like glass and the least in gases.
3. Sounds which human beings can hear are called audible sounds. The sound waves between the frequency range of 20-20,000 Hz are audible to the human ear. The sound waves with a frequency of more than 20,000 Hz are called ultrasonic waves, and those of a frequency less than 20 Hz are called infrasonic waves. Ultrasonic waves can be heard and produced by bats, while infrasonic waves can be produced by dolphins and snakes.
4. Sound waves have the property of reflection and refraction. Reflection means that whenever a sound wave hits an object, it is sent back by the object. The speed, frequency and wavelength of the sound wave do not change during this process of reflection. Refraction is the process where the direction of the sound wave changes while travelling from one medium to the other. During refraction, the frequency of the sound wave does not change but the velocity and wavelength change.
Sound waves can not be polarised as their oscillation is not perpendicular to the direction in which they move
5. A vacuum can not transmit sound waves. Except for vacuum, sound waves travel with the speed of sound in any medium
Sound waves can be represented graphically.
2. What is infrasound?
A sound with a frequency below 20 Hz is called infrasound or infrasonic sound. It is produced due to very mild vibration. For example, a simple pendulum produces infrasonic sounds while oscillating. Some animals such as snakes, whales, elephants and dolphins also produce infrasound. Rhinoceroses communicate with each other by producing infrasonic waves with a frequency of as low as 5 Hz. Humans can’t hear their sounds because their frequency is beyond our audible range. Earthquakes also produce infrasonic waves. Animals perceive the onset of earthquakes before us. This is because some animals can hear infrasonic waves.
3. What is the sound?
Sound is a form of energy. When we talk, potential energy is converted into sound energy. When a stimulus is perceived by our ears, the sensation is produced in the form of sound. We hear different types of sounds daily. Humans produce sounds. The vehicles on the road produce sound. Animals produce sound. Even when we pour water into a glass, the sound is produced. All living and nonliving beings produce sound. Vibrations produce sound . To and fro motion of an object or a body is called vibration. When we vibrate the strings of a guitar, the sound is produced.
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