Sound Waves:
Sound waves are a type of wave that travel through a medium, like air, as a series of compressions and rarefactions. They're called "longitudinal mechanical waves" because they move particles of the medium in the same direction as the wave is traveling.
Categories of Sound Waves:
Audible or Sound Waves: These are the sounds we can hear with our ears. They fall within the frequency range of 20 Hz to 20,000 Hz. Examples include the sound of voices, music, or a ringing phone.
Infrasonic Waves: These waves have frequencies lower than 20 Hz, making them too low for us to hear. They're produced by large-scale events like earthquakes, volcanic eruptions, or the movements of massive objects like ocean waves, elephants, or whales.
Ultrasonic Waves: These waves have frequencies higher than 20,000 Hz, making them beyond the range of human hearing. Animals like dogs, cats, bats, and mosquitoes can hear them. Some animals, like bats, even produce ultrasonic waves themselves.
How Ultrasonic Waves are Produced:
Galton’s Whistle or Hartman’s Generator: Devices that create ultrasonic waves through mechanical means.
Piezoelectric Effect: High-frequency vibrations of a quartz crystal under an electric field can generate ultrasonic waves.
Magnetostriction: Vibrations of a ferromagnetic rod under a magnetic field can also produce ultrasonic waves.
Applications of Ultrasonic Waves:
1. Sending Signals: Ultrasonic waves can be used to send signals, similar to how we use radio waves for communication.
2. Measuring Sea Depth: They're handy for measuring the depth of the sea or other bodies of water.
3. Cleaning: Ultrasonic waves are great for cleaning clothes, airplanes, and machinery parts in clocks. The waves can dislodge dirt and grime without causing damage.
4. Chimney Cleaning: They're also used to remove soot buildup from factory chimneys, keeping them clean and safe.
5. Sterilizing Liquids: Ultrasonic waves can sterilize liquids, making them free from harmful bacteria and germs.
6. Medical Imaging (Ultrasonography): In medicine, ultrasonic waves are used for imaging purposes, allowing doctors to see inside the body without the need for surgery. This technique is called ultrasonography.
Speed of Sound:
The speed of sound varies depending on the material it travels through. It's mainly influenced by how stretchy (elastic) and how tightly packed (dense) the material is.
- Speed in Different Mediums: Sound travels fastest through solids and slowest through gases.
- Changes When Entering a New Medium: When sound moves from one material to another, its speed and the distance between its waves (wavelength) change, but its pitch (frequency) stays the same.
- Independent of Frequency: In a material, the speed of sound doesn't depend on how high or low the pitch is. It stays constant.
- Effect of Pressure: Changes in pressure don't affect the speed of sound. Whether pressure goes up or down, the speed of sound remains the same.
| Medium | Speed of Sound (m/s) |
|---|---|
| Carbon dioxide | 260 |
| Air (0°C) | 332 |
| Air (20°C) | 343 |
| Steam (at 100°C) | 405 |
| Helium | 965 |
| Alcohol | 1213 |
| Hydrogen | 1269 |
| Mercury | 1450 |
| Water (20°C) | 1482 |
| Sea water | 1533 |
| Copper | 3560 |
| Iron | 5130 |
| Glass | 5640 |
| Granite | 6000 |
| Aluminium | 6420 |
Effect of Temperature on Speed of Sound:
- As the temperature of a medium increases, the speed of sound also increases. For example, in air, the speed of sound increases by 0.61 meters per second for every 1 degree Celsius increase in temperature.
Effect of Humidity on Speed of Sound:
- Sound travels faster in humid air compared to dry air because humid air is less dense than dry air.
Characteristics of Sound Waves:
- Intensity: Intensity of sound refers to the amount of energy passing through a unit area per unit time. It's measured in watts per square meter (W/m²).
- Intensity decreases as you move away from the sound source and is directly proportional to the square of the amplitude of vibration, the square of the frequency, and the density of the medium.
- Loudness: Loudness is the perception of sound in the ear, which depends on the intensity of the sound and the sensitivity of the ear.
- Loudness is often measured in decibels (dB), which is a practical unit derived from the bel. Another unit used is the phon.
Pitch:
- Pitch is how we describe whether a sound is high or low. It depends on the frequency of the sound wave. High frequency sounds have a higher pitch and are more shrill, while low frequency sounds have a lower pitch and are more dull or flat.
Quality:
- Quality is what helps us tell the difference between sounds that have the same loudness and pitch. It depends on factors like the number, frequency, and relative intensities of overtones, which are other sound waves mixed in with the main sound.
Echo:
- When sound waves bounce off a tall object, like a tower or mountains, and come back to us, it's called an echo. To hear an echo, the distance between us and the object must be at least 17 meters. Our ears can also remember sound for about 1/10th of a second.
Resonance:
- Resonance happens when a vibrating object is forced to vibrate at its natural frequency by another vibrating source. This can make the object vibrate a lot more. Think of how pushing someone on a swing can make them swing higher and higher.
Interference of Sound:
- When two or more sound waves of the same frequency meet, they can either make the sound louder (constructive interference) or quieter (destructive interference), depending on whether they're in sync or out of sync.
Diffraction of Sound:
- When a sound wave encounters an obstacle that's about the same size as its wavelength (which is usually around 1 meter for most sounds), the sound bends around the obstacle and continues to spread out. This bending of sound around obstacles is called diffraction.
Doppler Effect:
- If there's motion between the source of sound and the listener, the pitch of the sound heard by the listener changes. When the distance between the source and listener decreases, the pitch seems higher, and when it increases, the pitch seems lower. This is called the Doppler effect.
Mach Number:
- The Mach number compares the speed of an object to the speed of sound in the same conditions. If the Mach number is greater than 1, the object is traveling faster than sound (supersonic). If it's greater than 5, it's traveling at hypersonic speeds. If it's less than 1, it's traveling slower than sound (subsonic).
Shock Waves:
- When an object moves through the air faster than sound, it creates a disturbance that spreads out in a cone shape behind it. This disturbance is called a shock wave. Shock waves carry a lot of energy and can even cause damage to buildings or windows.
Bow Waves:
- When something like a motorboat moves faster than the speed of sound through water, it creates waves on the water's surface that resemble shock waves. These are called bow waves.
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