Audible and Inaudible Sounds

Sound is a form of mechanical energy produced by vibrating objects and transmitted through a medium (such as air, water, or solids) in the form of longitudinal waves. These waves consist of compressions and rarefactions that travel through the medium and reach our ears. However, the human ear is incapable of detecting all sound waves present in nature. Based on their frequency, sounds are classified into audible and inaudible sounds.

Frequency and earing

The frequency of a sound wave determines whether it is audible or inaudible. It represents the number of vibrations or oscillations per second.

• Unit: Hertz (Hz)
• Formula: f = \frac{1}{T}

where T is the time period of one vibration.

Human Hearing Range

A normal human ear can detect sounds in the range of 20 Hz to 20,000 Hz (20 kHz). This range is called the audible range. Sounds outside this range are classified as inaudible.

It is important to note that this range is not fixed:

• Young individuals can hear closer to 20 kHz.
• With age, the upper limit decreases (often to ~15–17 kHz).

Audible Sound

Audible sound refers to sound waves whose frequencies lie within the hearing range of the human ear, i.e., 20 Hz to 20 kHz.

Characteristics of Audible Sound

1. Perceived by the human ear.
2. Produced by everyday sources.
3. Includes a wide range of pitches (low to high).
4. Sensitivity varies with frequency (ears are most sensitive to 1–5 kHz).
5. Requires a medium to travel.

Factors Affecting Audibility

1. Frequency – Must lie within the audible range.
2. Amplitude (Loudness)—Sound must be strong enough to be heard.
3. Distance from Source – Sound weakens with distance.
4. Medium – Sound travels differently in air, water, and solids.
5. Health of the Ear – Hearing ability varies among individuals.

Examples of Audible Sound

• Human speech (approximately 300 Hz to 3 kHz).
• Musical instruments (varied frequencies).
• Sounds of vehicles and machinery.
• Natural sounds like wind, rain, and animal calls.

Inaudible Sound

Inaudible sounds are those sounds whose frequencies lie outside the human hearing range, i.e., Below 20 Hz or Above 20 kHz. Even though humans cannot hear them, these sounds are very important in nature and technology.

Types of Inaudible Sounds

1. Infrasonic Sound (Infrasound)

Infrasonic sounds are sound waves with frequencies less than 20 Hz.

Characteristics

1. Very low frequency.
2. Long wavelength.
3. Can travel very long distances with little energy loss.
4. Can pass through obstacles easily.
5. Not detectable by human ears.

Applications of Infrasound

• Disaster detection: Earthquakes and volcanic eruptions.
• Atmospheric studies.
• Wildlife communication research.

2. Ultrasonic Sound (Ultrasound)

Ultrasonic sounds are sound waves with frequencies greater than 20,000 Hz (20 kHz).

Characteristics

1. Very high frequency.
2. Short wavelength.
3. Travel in straight lines.
4. Easily reflected and focused.
5. Can penetrate materials.

Applications of Ultrasound

• Medical Field: Ultrasonography: Imaging internal organs like Monitoring pregnancy (safe imaging technique) and Detecting tumors and stones.
• SONAR (Sound Navigation and Ranging): Used in ships and submarines to measures depth of the sea and also to detects underwater objects.
• Industrial Applications: Detecting cracks in metals and Non-destructive testing.

Comparison Between Audible and Inaudible Sounds

Points to Remember

• Frequency determines pitch, not loudness
• Amplitude determines loudness

Solved Problems

Question 1: An instrument produces a sound of frequency 250 Hz. Will be able to perceive its sound?

Solution: Yes, we will be able to perceive its sound as it falls under our audible range that is between 20 Hz – 20,000 Hz.

20 Hz < 250 Hz < 20,000 Hz

Question 2: A bagpiper produces a sound of frequency 360 Hz. Can he be able to gather any audience with his music?

Solution: Yes, he will be able to gather audience as its musical sound falls under the audible range that is between 20 Hz – 20,000 Hz.

20 Hz < 360 Hz < 20,000 Hz

Question 3: A machine produces a sound of frequency 25,000 Hz. Will the people near the manufacturing hub be able to perceive its sound?

Solution: No, people will not be able to hear the sound because its frequency (25,000 Hz) is greater than the audible range of humans (20 Hz – 20,000 Hz). Hence, it is ultrasonic sound.

Question 4: Calculate the frequency of waves if the time period given is 25 sec.

Solution: As we know,

f=\frac{1}{T}

Where f=? And T= 25 sec

\therefore f=\frac{1}{25}

f= 0.04 Hz.

Therefore, the frequency is 0.04 Hz.

Unsolved Problems

Question 1: A sound wave has a frequency of 250 Hz and travels with a velocity of 500 m/s. Calculate its wavelength.

Question 2: The time period of a sound wave is 0.01 s. Determine its frequency and state whether it is audible or inaudible.

Question 3: A sound wave travels in air with a velocity of 340 m/s and has a wavelength of 0.85 m. Find its frequency.

Question 4: A sound wave of frequency 30 kHz travels through a medium with a speed of 1500 m/s. Calculate its wavelength and identify the type of sound.

Question 5: Two sound waves travel in the same medium. The first has a frequency of 200 Hz and the second has twice the wavelength of the first. Find the frequency of the second wave.