Reflection and absorption of sound – echo and reverberation

Understanding sound and its behavior is an essential part of physics. In this explanation, we'll take a deeper look at how sound waves interact with surfaces through processes such as reflection and absorption, and how these interactions lead to phenomena such as resonance and echo.

Sound waves: The basics

Sound travels in waves. These waves are created when an object vibrates, causing the particles in the surrounding medium (usually air) to vibrate as well. As sound waves travel, they carry energy, which helps us hear sounds.

How does sound travel?

Sound needs a medium to travel, which means it cannot travel in a vacuum. On Earth, sound travels most often through air, but it can also travel through water, solid objects, and other gases. The speed of sound varies depending on the medium. For example, sound travels faster in water than in air.

Reflection of sound

When a sound wave strikes a surface, some part of the wave bounces back. This phenomenon is called reflection. The angle at which a sound wave strikes a surface (angle of incidence) is equal to the angle at which it reflects (angle of reflection). This principle is similar to the way light reflects from surfaces.

Visual representation of sound reflection

Examples of sound reflection

An everyday example of sound reflection is when you hear an echo. If you shout in a mountainous area or across a large empty room, you may hear your voice come back to you. This happens because sound waves reflect off surfaces and travel back to your ears.

Echo

Echo is a reflected sound that can be heard clearly by the original source. To hear the echo clearly, the reflecting surface must be at least 17 meters away from the sound source. This is because the human ear cannot distinguish between direct and reflected sounds that are heard within a time interval of less than 0.1 seconds.

Mathematical explanation of resonance

The time taken for the echo to return can be calculated using the following formula:

t = 2d / v

Where:

• t is the time in seconds,
• d is the distance in meters from the reflecting surface,
• v is the speed of sound in that medium in meters per second.

Example calculation

Suppose you shout across the valley, and the reflecting surface is 68 m away. If the speed of sound is about 343 m/s, you can calculate the time it takes for the echo to return as follows:

t = 2 * 68 / 343 ≈ 0.4 seconds

Sound absorption

When sound waves strike a surface, not all of them are reflected. Some of the sound energy is absorbed by the surface. This means that sound energy is converted into other forms of energy, usually heat. The amount of energy absorbed depends on the material of the surface: soft and porous materials absorb more sound than hard, reflective surfaces.

Materials and sound absorption

Different materials have different sound absorption coefficients. Here are some examples:

• Curtains, carpets and cushions: High absorption, good for reducing noise.
• Concrete walls: Low absorption, reflects sound effectively.
• Wood panels: Used in acoustic settings to balance medium absorption, absorption and reflection.

Resonance

Reverberation or reverberation occurs when sound waves reflect off surfaces and persist for some time. Unlike resonance, reverberation involves multiple reflections that blend into each other, creating a prolonged sound effect. This effect is common in enclosed spaces such as concert halls.

Visualization of echo

The above diagram shows sound waves reflecting in a closed area, resulting in an echo. Multiple reflecting paths combine to produce the echo effect.

Reverberation time

Reverberation time is the time it takes for sound to decay by 60 decibels after the sound source is turned off. This time varies depending on the dimensions and materials of the room. It is important in building design, especially in spaces like auditoriums and theatres where acoustics matter a lot.

The formula used to calculate the reverberation time (RT) is:

RT = 0.161 * V / A

Where:

• V is the volume of the room in cubic meters,
• A is the total absorption (the sum of the products of the surface area and absorption coefficient of all materials).

Example calculation

Consider a room of volume 200 cubic meters. The absorption sum for all surfaces is 30. The reverberation time will be calculated as:

RT = 0.161 * 200 / 30 ≈ 1.073 seconds

Practical applications of resonance and resonance

Resonance in technology and nature

• Sonar systems: Ships and submarines use echoes to determine the distance, speed, and direction of underwater objects.
• Bats and dolphins: These animals use echolocation to locate and hunt prey by emitting sound waves and interpreting the returning echo.

Resonance in architecture and music

• Concert hall: The design takes resonance into account to ensure sound clarity and richness, optimising the listener’s experience.
• Recording studio: Use customized resonance for audio effects, enhancing the quality of music and sound recordings.

Conclusion

Sound is an interesting element of physics that affects our daily experiences. The reflection and absorption of sound leads to phenomena such as resonance and echo, which have practical applications in both nature and technology. Understanding these processes provides insight into how sound interacts with the environment and how we can harness these interactions for various uses.

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