Air is the most common medium for sound to propagate. The vibration produced by a sound source such as the voice or a horn, is transmitted in turn in all directions to the surrounding air molecules and these to their neighbors.
This disturbance causes pressure variations in the air, creating pressure waves. These variations are propagated and when they reach the eardrum, it begins to vibrate and the auditory signal is produced.
Waves carry energy at the same speed as the disturbance. In air, for example, sound travels at a rate of about 343.2 m / s under normal conditions of temperature and pressure, this speed being a characteristic of the medium, as we will see later.
How does the propagation of sound occur?
The propagation of sound basically occurs in two ways, the first is the sound that comes directly from the source that originates it. The second is through the sound waves that are reflected off obstacles such as the walls of the rooms, giving rise to a reverberating sound field.
These reflections from sound waves can occur many, many times and what is interpreted as sound is the acoustic pressure resulting from the direct sound field and the reverberant field.
In this process, the sound waves give up their energy to the medium and diminish with distance until they disappear.
Sound propagation speed
The speed with which sound propagates in different media depends on their properties. The most relevant are density , elasticity, humidity, salinity and temperature. When these change, so does the speed of sound.
The density of the medium is a measure of its inertia, which is a resistance to the passage of the pressure wave. A very dense material initially opposes the passage of sound.
For its part, elasticity indicates how easy it is for the medium to regain its properties once it is disturbed. In an elastic medium, sound waves travel more easily than in a rigid medium, because the molecules are more willing to vibrate over and over again.
There is a physical quantity called the modulus of compressibility to describe how elastic a medium is.
Equation for the speed of sound
In general, sound propagates in a medium with a speed given by:
Where the elastic property is the modulus of compressibility B and the property y is the density ρ:
Finally, temperature is another important factor when sound propagates through a gas such as air, which is the medium through which most of the sound waves propagate. When considering the ideal gas model, the quotient B / ρ depends only on its temperature T.
In this way, the speed of sound in air at 0ºC is 331 m / s, while at 20 ºC its value is 343 m / s. The difference is explained because when the temperature increases, the vibrational state of the air molecules also rises, facilitating the passage of the disturbance.
Means of propagation
Sound is a mechanical wave that needs a material medium to propagate. Therefore there is no way for sound to be transmitted in a vacuum, unlike electromagnetic waves that can do so without major problem.
Air is the most common environment for sound transmission, as well as other gases. Disturbances are transmitted by collisions between gaseous molecules, in such a way that the higher the density of the gas, the faster the sound travels.
As we have said before, temperature influences the propagation of sound in gases, since when it is higher, collisions between molecules are more frequent.
In air, the dependence of the speed of sound v on the temperature T in kelvin is given by:
Many times the temperature is not distributed evenly in a place, for example a concert hall. The warmer air is closer to the floor, while above the audience it can be up to 5ºC cooler, which affects the propagation of the sound in the room, since the sound moves faster in the areas more hot.
Liquids and solids
Sound travels faster in liquids than in gases, and even faster in solids. For example, in fresh water and salt water, both at a temperature of 25 ºC, the speed of sound is, respectively, 1493 m / s and 1533 m / s, about four times more than in air, approximately.
It is easy to check by putting your head in the water, so the noise of the engines of the boats is much better than in the air.
But in solid materials like steel and glass, the sound can reach up to 5920 m / s, therefore they conduct sound much better.
Simple sound propagation experiments
Sound waves interfere constructively or destructively, in other words, they overlap. You can easily experience this effect with a simple experiment:
-1 pair of speakers like the ones you use on desktop computers.
-Cell phone that has a wave generator application installed.
The experiment is carried out in a large, open room. The loudspeakers are placed side by side, 80 cm apart and in the same orientation.
Now the speakers are connected to the phone and both are turned on with equal volume . A specific frequency is selected in the generator, such as 1000 Hz.
Then you have to move along the line that joins the speakers, but maintaining a separation of about 3 m. It is immediately noticeable that at some points the intensity of the sound increases (constructive interference) at some points and decreases at others (destructive interference).
It is also observed that when standing at the equidistant point from the speakers, this is always a place of constructive interference.
This experience, which requires the participation of two people, serves to verify that the objects have characteristic frequencies.
2 identical empty bottles.
Participants must keep their bottles upright and vertical and be separated by a distance of approximately 2 m. One of the people blows through the mouth of the bottle, making the jet of air impinge obliquely, the other person holds his bottle vertically next to the ear.
The listener immediately notices that the sound appears to be coming from their own bottle, although the original sound is produced by the bottle being blown by the other person. This phenomenon is called resonance .
The experience can be repeated if the blowing person’s bottle is filled halfway with water. In this case the sound is also recorded, but higher.