In the novel “The Secret of Two Oceans” and in the adventure film of the same name, the heroes did unimaginable things with ultrasonic weapons: they destroyed a rock, killed a huge whale, destroyed the ship of their enemies. The work was published back in the 30s of the 20th century, and then it was believed that in the near future the existence of powerful ultrasonic weapons would become possible - all that matters was the availability of technology. Today, science claims that ultrasonic waves as weapons are fantastic.
Another thing is the use of ultrasound for peaceful purposes (ultrasonic cleaning, drilling holes, crushing kidney stones, etc.). Further we will understand how acoustic waves with large amplitudes and sound intensities behave.
Powerful Sound Feature
There is a concept of nonlinear effects. These are effects characteristic only of sufficiently strong waves and depending on their amplitude. In physics, there is even a special section that studies powerful waves - nonlinear acoustics. A few examples of what she explores: thunder, underwater explosions, seismic waves from earthquakes. Two questions arise.
- First: what is the power of sound?
- Second: what are the nonlinear effects, what is unusual in them, where are they used?
What is an acoustic wave?
The sound wave is the areas of compression-rarefaction that diverge in the medium. In any place, pressure changes. This is due to a change in the compression ratio. The changes superimposed on the initial pressure that was in the medium are called sound pressure.
Flow of sound energy
A wave has energy that deforms the medium (if sound propagates in the atmosphere, this is the energy of elastic deformation of air). In addition, the wave has the kinetic energy of the molecules. The direction of the flow of energy coincides with that in which the sound diverges. The flow of energy passing per unit time through a unit area characterizes the intensity. And this refers to a region perpendicular to the movement of the wave.
Intensity
Both the intensity I and the acoustic pressure p depend on the properties of the medium. We will not dwell on these dependences, we give only the formula of sound intensity connecting p, I and the characteristics of the medium - density (ρ) and sound velocity in the medium (s):
I = p 0 2 / 2ρc.
Here p 0 is the amplitude of the acoustic pressure.
What is strong and weak noise? The force (N) is usually determined by the level of sound pressure - a value that is associated with the amplitude of the wave. The unit of sound intensity is decibel (dB).
N = 20 × log (p / p p ), dB.
Here p p is the threshold pressure, conventionally assumed equal to 2 × 10 -5 Pa. The pressure p p approximately corresponds to the intensity I p = 10 -12 W / m 2 of a very weak sound, which the human ear can still perceive in air at a frequency of 1000 Hz. The sound is stronger, the higher the level of acoustic pressure.
Volume
Subjective ideas about the strength of sound are associated with the concept of loudness, that is, they are tied to the range of frequencies perceived by the ear (see table).
But what about when the frequency lies outside this range - in the field of ultrasound? It is in this situation (in experiments with ultrasound at frequencies of the order of 1 megahertz) that it is easier to observe non-linear effects in laboratory conditions. We conclude: it makes sense to call powerful such acoustic waves for which nonlinear effects become noticeable.
Nonlinear effects
It is known that an ordinary (linear) wave, the sound intensity of which is small, propagates in the medium without changing its shape. In this case, both rarefaction and compression regions move in space at the same speed - this is the speed of sound in the medium. If the source generates a wave, then its profile remains in the form of a sinusoid at any distance from it.
In an intense sound wave, the picture is different: the compression regions (sound pressure is positive) move at a speed exceeding the speed of sound, and the rarefaction regions move at a speed lower than the speed of sound in a given medium. As a result, the profile changes a lot. The front surfaces become very steep, and the backs of the waves become more gentle. Such strong changes in shape - this is the non-linear effect. The stronger the wave, the greater its amplitude, the faster the profile is distorted.
For a long time, it was considered possible to transmit high energy densities over long distances using an acoustic beam. An inspirational example was a laser capable of destroying structures, punching holes at a great distance. It seems that replacing the light with sound is possible. However, there are difficulties due to which it is unrealistic to create an ultrasonic weapon.
It turns out that for any distance there is a boundary value of the intensity of the sound that gets to the target. The greater the distance, the lower the intensity. And the usual attenuation of acoustic waves when passing through a medium has nothing to do with it. Attenuation noticeably increases with increasing frequency. However, it can be chosen so that the usual (linear) attenuation at the desired distances can be neglected. For a 1 MHz signal in water, this is 50 m; for ultrasound, a sufficiently large amplitude can be only 10 cm.
Imagine that a wave is generated at a certain place in space, the sound intensity of which is such that non-linear effects significantly affect its behavior. The amplitude of the oscillations will decrease with distance from the source. This will happen the sooner the larger the initial amplitude p 0 . At very high values, the rate of decrease of the wave does not depend on the value of the initial signal p 0 . Such a process occurs until the wave decays and the nonlinear effects cease. After that, it will diverge in nonlinear mode. Further attenuation occurs according to the laws of linear acoustics, i.e., it is much weaker and does not depend on the magnitude of the initial perturbation.
How then, ultrasound is successfully used in many industrial sectors: they are drilled, cleaned, etc. With these manipulations, the distance from the emitter is small, so the nonlinear attenuation does not yet have time to gain momentum.
Why are shock waves acting so hard on obstacles? It is known that explosions can destroy structures located quite far away. But the shock wave is nonlinear, so the decay rate should be higher than that of weaker waves.
The bottom line is this: a single signal does not act like a periodic signal. Its peak value decreases with distance from the source. By increasing the wave amplitude (for example, the force of the explosion), it is possible to achieve large pressures on the obstacle at a given (even small) distance and thereby destroy it.