Acoustics — Important Questions

20 Hz to 20,000 Hz

Step 1: The human ear can detect sound only within a specific range of frequencies. Step 2: This range is from 20 Hz (lowest pitch we can hear) to 20,000 Hz or 20 kHz (highest pitch we can hear). Step 3: Sounds below 20 Hz are called infrasonic waves — earthquakes and whale sounds fall here. Step 4: Sounds above 20,000 Hz are called ultrasonic waves — bats and dogs can detect these. Therefore, the audible range for humans is 20 Hz to 20,000 Hz.

Infrasonic waves

Step 1: Sound waves are categorized based on their frequency. Step 2: Waves with frequency below 20 Hz are called infrasonic waves — the prefix 'infra' means 'below'. Step 3: Examples include waves produced during earthquakes, ocean waves, and sounds produced by whales. Step 4: These cannot be heard by humans but can be detected by some animals. Ultrasonic waves are above 20,000 Hz, and audible waves are in between. So, infrasonic waves have frequency below 20 Hz.

Second law of reflection

Step 1: Sound waves, like light waves, obey the laws of reflection when they bounce off a surface. Step 2: The first law states that the incident wave, normal, and reflected wave all lie in the same plane. Step 3: The second law of reflection states that the angle of incidence (i) equals the angle of reflection (r), i.e., ∠i = ∠r. Step 4: Both angles are measured from the normal (a line perpendicular to the reflecting surface). Refraction and diffraction are different phenomena altogether.

Repetition of sound due to reflection from a rigid surface

Step 1: When you shout near a mountain or a large building, you sometimes hear your voice again after a short time. Step 2: This happens because the sound waves travel to the reflecting surface (like a wall or mountain), bounce back, and reach your ears again. Step 3: This repeated sound is called an echo. Step 4: For an echo to be heard clearly, the time gap between the original sound and the echo must be at least 0.1 seconds. Absorption is the opposite of reflection, and change in frequency is the Doppler effect.