Every day we hear sounds coming from different types of sources, from humans to vehicles, from musical instruments to televisions. Just as with other forms of energy (mechanical, thermal, light, etc.), sound energy can neither be created nor destroyed. This statement refers to the principle of conservation of energy formulated by American physicist Feynman. As Feynman himself states, we can only change its form, but energy will always be present.
What is sound propagation?
Sound propagation is the process through which sound waves spread through a medium, such as air or water. This phenomenon is of fundamental importance for understanding how we perceive and analyze sounds in the environment around us.
Sound is produced by the vibrations of objects. The object or substance through which it propagates is defined as the medium and can be solid, liquid, or gaseous.
Sound waves travel from their point of origin (source) toward the listener.
Specifically, when an object vibrates, it transmits these vibrations to the particles of the medium adjacent to it; the vibrations themselves do not travel from the source to the ear. Each particle of the medium located near the source is “displaced” from its equilibrium position and set in motion. In turn, this will transmit energy to the particles near it, thus allowing the diffusion of sound. The particles of the medium do not move forward on their own: sound propagation, in fact, can be described as a wave motion produced by the acoustic wave.
Air is certainly the most common propagation medium; when an object vibrates, it pushes and compresses the air adjacent to it. The compressed zone begins to move away from the source; when the source “relaxes” and returns to its equilibrium state, an area of low pressure called rarefaction is created. This alternating movement repeats many times, generating several areas of compression and rarefaction in the air. It is precisely the alternation of these areas that allows the sound wave to propagate through the medium. The zones of high pressure or low pressure present in the medium refer to the density of particles present in a given volume. The more there are, the higher the pressure will be. It can therefore be stated that sound propagation occurs through pressure variation within the medium itself.
A medium is necessary for sound propagation
As a mechanical wave, sound requires a physical medium (water, air, metal, etc.) to be transmitted. It cannot travel in a vacuum, as can be demonstrated with a simple example.
If a common bell is placed inside a hermetically sealed glass bell jar and connected to a vacuum pump, it can be observed that, as the amount of air inside decreases, the sound of the bell will become increasingly attenuated, until it is no longer audible once all the air has been removed from the container.
Speed of Sound and Sound Propagation
Sound propagates through a given medium at a constant speed. As happens during a thunderstorm, thunder is heard a few moments after seeing the lightning: sound travels more slowly than light, and its speed depends strictly on the physical properties of the medium through which it propagates. In particular, there are two main aspects:
The speed of sound is closely related to the temperature of the medium. The higher the temperature, the faster the sound will be. For example, its speed in air at 0°C is 331 m/s, while if the air is at 22°C it is 344 m/s.
The speed of sound decreases when passing from a solid medium to a gaseous one.
Both assumptions can be easily demonstrated in the table below, which shows the different speeds that sound assumes in relation to the medium and its temperature.
Speed of Sound in Different Media at 25°C
State
Substance
Speed in m/s
Solids
Aluminum 6420
6420
Nickel
6040
Steel
5960
Iron
5950
Brass
4700
Glass (Flint)
3980
Liquids
Water (Sea)
1530
Water (Distilled)
1498
Ethanol
1207
Methanol
1103
Gases
Hydrogen
1284
Helium
965
Air
346
Oxygen
316
Sulfur Dioxide
213
Measuring Sound Propagation
This effect can be measured through various parameters, including the intensity and speed of sound waves. Sound intensity refers to the power per unit area through which the wave propagates. It is generally expressed in decibels (dB), which is a logarithmic scale. The speed of sound depends on the medium through which it travels; for example, in a vacuum, sound cannot propagate since it lacks a transmission medium.
The Sonic Boom
When the speed of any object exceeds the speed of sound, it is said to be traveling at supersonic speed. What does this mean exactly?
Objects such as bullets and jet aircraft often travel at supersonic speeds; when a source moves at a speed greater than that of the sound it generates, it produces shock waves in the air that carry a large amount of energy. The variation in air pressure associated with this type of shock wave produces a very loud and noisy sound called a “sonic boom.” This energy is so powerful that, for example, the energy generated by a supersonic aircraft has sufficient power to shatter glass and even damage buildings.
How to Reduce Sound Propagation with Sound-Absorbing Solutions
Reducing sound propagation is essential for creating comfortable environments free from acoustic disturbances. Sound-absorbing solutions represent an effective approach to minimize echo and reverberation effects within interior spaces. Let us explore together how these solutions can improve the acoustic quality of environments.
In summary:
Sound is produced by vibrating objects.
Sound travels longitudinally through a physical medium.
Sound spreads through the presence of compressions and rarefactions in the medium.
Sound propagation consists of the transmission of acoustic energy from one particle to another (it is not, in fact, a displacement of particles, but a transmission of energy from one to another).
Sound cannot travel in a vacuum.
The speed of sound depends primarily on the nature, temperature, and state of the medium through which it propagates.
In conclusion, this is a complex process that plays a crucial role in our daily sensory experience. Understanding the mechanisms of how sound spreads can help optimize the acoustic design of environments and avoid unwanted phenomena such as sonic booms.
