Chapter 8

Mechanical waves

Energy study of sound waves

FondamentalEquation of local conservation of sound energy

Summing the equation : (see the lesson "Acoustic waves")

multiplied by the velocity and the equation :

multiplied by the pressure .

It comes :

Let :

  • Volume density of sound energy :

    The first term refers to the density of kinetic energy of the fluid and the second the potential energy density associated with the pressure forces.

  • The vector surface density of sound power :

So :

One recognizes an equation of type "local energy conservation", already seen for example for the energy of an electromagnetic field or for the conservation equation of energy in the case of thermal conduction phenomena.

AttentionEquation of local conservation of sound energy

With :

ComplémentCase of a progressive plane wave

  • Energy densities :

    Consider a HPPW which propagates in the direction of . So .

    So :

    But :

    Whence :

    Thus, for a HPPW, densities of kinetic and potential energies are equal.

    The density of total energy is then :

  • Vector surface density of sound power :

  • Speed of Energy :

    Consider a sound pipe of cross section and we will calculate two different ways the energy flowing through the cross section during the time interval .

Speed of Energy

Energy can be calculated from the flux of the vector  :

It can be calculated saying it was contained in the volume :

Equating these two expressions that are obtained :

We find the same result as the electromagnetic waves in vacuum, for which the speed of energy is equal to the speed of wave propagation in a vacuum, is .

DéfinitionSound Intensity

The sound auditory sensation (or sound intensity) depends on the frequency spectrum, duration and especially the intensity of sound.

In general, the intensity of an elastic wave is proportional to both the square of the frequency and the square of the amplitude.

In terms of sound pressure, the sound intensity is proportional to the square of the amplitude of the pressure wave.

The intensity of natural sounds varies enormously and the human ear is sensitive to a wide range of variation in intensity.

One can support intensities up about , which is such a strong sound that begins to hear no more ; it is the pain threshold and corresponds to a sound pressure of only about .

At the other extreme, the ear detects just the sounds of intensity as low as .

This is the threshold of audibility, which is associated with a sound pressure of the order of .

Rayleigh showed that at , this corresponds to a sound pressure vibration of the air molecules on the order of only.

The human ear operates over a wide range corresponding to a factor of a million (from to ) for pressure and a factor equal to the square of this quantity for sound intensity, is .

This is a very broad operating range for a detector.

It is equivalent to that of a device capable of measuring both the diameter of an atom and the length of a football field.

The figure shows the limits of the sensitivity of the ear in function of frequency.

Limits of the sensitivity of the ear in function of frequency

When listening to two sounds, to judge their relative volume, not by the difference of the intensities but by the ratio of the intensities.

An increase in the intensity of sound by the factor of is generally seen as a multiplication of the volume by .

And an intensity sound is perceived as twice stronger than its intensity and  is perceived as twice louder than .

This is equivalent to saying that the ear is sensitive to the logarithm of the intensity.

We define the sound intensity (or acoustic) in decibels ( ) :

corresponds to the hearing threshold for a reference frequency of .

The pain threshold corresponds approximately to a sound level of .

Some orders of magnitude :

  • silent room :

  • quieter dishwasher :

  • Busy street :

  • School canteen :

  • Plane :

  • Nightclub :

  • Rocket :

Additional lessons
Additional exercises