| Type of Sound | Frequency Range | Examples | Details |
| Infrasonic Sound | Below 20 Hz |
Movement of hands or leaves. Whales, elephants, rhinoceroses, etc. |
1. Inaudible to humans 2. It is used by animals to communicate or detect danger 3. It can travel long distances |
| Audible Sound | 20 Hz to 20,000 Hz | Speech, music, environmental sounds, violin, flute, etc. |
1. Heard by humans 2. Common sounds in everyday life |
| Ultrasonic Sound | Above 20,000 Hz | Heard by dogs, bats, dolphins, etc. |
1. It is used in medical imaging (ultrasound) and instrument cleaning 2. Bats use it for echolocation and navigation |
Frequency of sound (Hz)
The Italian scientist Spallanzani discovered that bats rely on their ears, not their eyes, to navigate in the dark. Through experiments, he found that covering their ears caused bats to collide with obstacles, even with their eyes uncovered. Bats use ultrasonic sound waves that reflect off obstacles. By receiving these reflected sounds with their ears, they can locate objects and estimate distances, enabling them to fly safely even in complete darkness.
1. Aim: To understand how low-frequency oscillations produce sounds that are inaudible to humans.
2. Requirements: An oscillator, stopwatch or clock.
3. Procedure
4. Observation and Conclusion: Since the oscillator makes 3 to 4 oscillations per second, the frequency of the sound is 3 to 4 Hz. Humans can only hear sounds with a frequency greater than 20 Hz. This means the sound from the oscillator is an infrasonic sound, which is below the human hearing range. Animals like whales, elephants, and rhinoceroses can produce and detect infrasonic sounds.
1. Infrasonic Sound
2. Audible Sound
3. Ultrasonic Sound
A microphone is connected to the Y-input of a C.R.O Three different sounds are made in turn in front of the microphone. Their traces (a), (b) and (c) produces on the screen are shown in following figure.
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| (a) | (b) | (c) |
