10.6 - Audible and Inaudible Sounds

In this section, we will delve into the concept of audible and inaudible sounds, which are critical to understanding how sound behaves and how humans perceive it. Sound is a mechanical wave that requires a medium (like air, water, or solids) to travel through. The frequency of a sound wave determines its pitch, and this frequency range is essential in classifying sounds as audible or inaudible.

Frequency and Pitch: The frequency of a sound wave is the number of vibrations per second, measured in Hertz (Hz). The pitch refers to how high or low a sound is perceived, and this perception depends on the frequency. Higher frequencies result in higher-pitched sounds, while lower frequencies produce lower-pitched sounds.

Audible Sound: Humans can hear sounds within a specific frequency range, generally from 20 Hz to 20,000 Hz (20 kHz). This range is called the audible range. Sounds within this range can be perceived by the human ear. The threshold of hearing is at 20 Hz, and the threshold of pain is at around 120 dB, which is the loudest sound humans can endure without pain.

Inaudible Sound: Sounds with frequencies below 20 Hz are called infrasonic sounds, and those with frequencies above 20 kHz are called ultrasonic sounds. Both infrasonic and ultrasonic sounds are inaudible to the human ear, but they can still have significant effects in various applications.

Infrasonic Sounds: Infrasonic waves have frequencies lower than 20 Hz. These sounds are produced by natural phenomena like earthquakes, volcanic eruptions, and even large animals like elephants. Although humans cannot hear infrasonic waves, they can sometimes feel them, such as in the case of low-frequency vibrations.

Ultrasonic Sounds: Ultrasonic waves have frequencies higher than 20,000 Hz. These sounds are used in various technologies, such as medical imaging (ultrasound), cleaning delicate items, and even in some pest repellent devices. While humans cannot hear these sounds, animals such as dogs, bats, and dolphins can detect them.

Applications of Ultrasound: Ultrasound has several practical applications:

• Medical Imaging: Ultrasonography is used to view internal organs, monitor pregnancies, and detect abnormalities.
• Cleaning: Ultrasonic cleaners are used to clean items such as jewelry, instruments, and electronic components.
• Distance Measurement: Ultrasonic sensors are used in devices for measuring distance, such as parking sensors in vehicles.

Sound Intensity and Decibels: The intensity of a sound is a measure of the energy it carries, and it is related to the amplitude of the sound wave. The unit of sound intensity is the decibel (dB). The formula to calculate sound intensity is given by:

$ I = \frac{P}{A} $, where I is the intensity, P is the power, and A is the area.

The decibel scale is logarithmic, which means a small increase in dB represents a large increase in intensity. The relationship between intensity (I) and decibel level (dB) is given by:

$ L_{dB} = 10 \cdot \log_{10} \left( \frac{I}{I_0} \right) $, where $I_0$ is the reference intensity (the threshold of hearing).

Conclusion of Frequency Ranges: The human ear can perceive sounds within the range of 20 Hz to 20 kHz. Sounds outside this range, either infrasonic or ultrasonic, are inaudible to humans but have numerous applications in different fields. The study of these frequencies helps us understand not just sound, but also how it can be used in technology and nature.