You can see the swell, a mechanical wave that propagates on the surface of the water; you can also see an earthquake; but can we see a sound wave? And what if it were possible? We will see that this seemingly simple question opens up exciting educational scenarios for using the inquiry method with students.
1. Can we see a sound ?
Experience shows that we cannot see a sound: whether the music is loud or not, whether a note is high or low, the visual appearance of the world around us does not change according to the sounds that are emitted around us. To detect a sound we can only rely on two senses, that of hearing, and, for very low or very loud sounds, that of touch.
A sound is created by an object vibrating in a medium such as air or water. The movements of the object compress then relax the immediate medium around the object, for example the air, and these pressure variations propagate in the rest of the medium, this is the sound wave.
Like other mechanical waves, the sound wave does not generate a displacement of matter, but a disturbance of it. Leonardo da Vinci, the first to have glimpsed the mechanism of the swell, made this analogy: "We see in the month of May running through the countryside the undulations that the wind makes in the wheat, and yet the wheat has not not moved".
Why can't we see a sound? We hear often the following explanation : we cannot see a sound because the air is transparent. This explanation is inadequate. It is not because the air is transparent that one cannot see the sound, but because the characteristics of light transmission in the air vary very little with the pressure (approximately 0.025% for a doubling of the atmospheric pressure).
One cannot see the effect of the sound on the medium since the optical characteristics of the medium are not appreciably affected by its passage. It would be different if, for example, the index of refraction of air varied with the pressure of the medium, then we could see distortions of light rays as we see the breaking of a knife in a glass of water due to the difference in refractive index between air and water.
In such a world, each new noise would cause visual distortions in the form of circular waves whose source, frequency and intensity would depend on the emitting objects, and which would interfere with each other... In addition to the refractive index, we can imagine that other optical characteristics could also be modified by the pressure variations of the medium, leading for example to variations in color or luminosity. Crossing a busy street would then become a real psychedelic experience!
If that the transmission characteristics of light in air were sensitive enough to pressure, could we then see a sound wave? For a sound wave to qualify as a sound wave, it must have a frequency high enough to be detected by for our sense of hearing. In other terms, the sound wave must vibrate at least 20 oscillations per second, or 20 hertz, which is much faster than the frequency of a sea wave (about one oscillation every 10 seconds) or an earthquake (several oscillations per second). The human voice "vibrates" rather at 250-2000 hertz.
Could we see variations of the medium at such frequency? It is often mentioned that the rate of acquisition of images of the eye is approximately 20 images per second. In fact, MIT researchers have shown that when a subject could anticipate an event, he was able to recognize an image in just 13 milliseconds, which corresponds to an acquisition frequency of 76 images per second. This rate is much lower than the usual frequency of a sound and what we could best detect by vision is a blurry movement, much like watching the strings of a guitar vibrate.
Even if the characteristics of the medium in which the sound wave evolves could in theory make it visible, it would nevertheless not be possible to "see" it because of the rapidity of the phenomenon. May be the only thing we could see would be a blur around the source.
2. How to see a sound
Faced with these difficulties, scientists have developed several types of approaches to see a sound and thus be able to conduct their research on acoustic phenomena.
A first solution comes from Galileo who first noticed patterns on a vibrating plate. The physicist Ernst Chladni formalized the phenomenon at the end of the 18th century by using metal plates, on which he made sand vibrate with a bow. The mathematician Sophie Germain a few years later will give the first mathematical modeling of the Chladni figures, work for which she obtains the grand prize of mathematical sciences in 1815. The Chladni figures make it possible to visualize the location of the vibration nodes of the excited plate by a sound wave and which depend on the plate and the frequencies used.
We can extend the concept of Galileo and Chladni by studying many effects of sound, for example on water or on paint, a field of research called "cymatics". Although the effect is visually very impressive, these experiments do not, however, strictly speaking allow the visualization of sound waves.
A second solution is to use was is called the Schlieren effect. We have seen that the refractive index of air showed very small variations when the pressure was modified. How to amplify these variations to visualize them? The Schlieren effect makes it possible to optically isolate the minute deviations of the light rays which cross a medium due to changes in its refractive index. This method was invented by Léon Foucault in the 19th century (known for the demonstration of the pendulum at the Panthéon) and perfected by Auguste Toepler. By coupling an optical device using the Schlieren effect to a high-frequency camera, and emitting high-frequency, high-intensity sounds, we can visualize the variations in air pressure and therefore see the sound.
The analysis using the Schlieren effect provides an global of the disturbances created by the sound wave. A last solution provides an analysis of the local variations of the air pressure with a microphone, invented by Emile Berliner in 1876. Under the effect of pressure, a membrane with an attached magnet moves in a coil which generates voltage variations in an electrical circuit. By analyzing these variations with an oscilloscope, we can then see the evolution over time of the pressure variations created by the sound wave, at the location of the microphone.
Until recently, it was necessary to go to a laboratory to make such measurements with an oscilloscope, but with advances in technology, a mobile phone or a tablet is enough to see the sound.
3. See a sound with FizziQ
FizziQ is a free app for scientific experimentation with a smartphone. You can download it on iOS or Android store. We will use it to generate and analyse various sounds and study their sapes.
Let's first generate a regular sound wave with a frequency generator or more simply with the sound synthesizer of the FizziQ app. Press on the Tools tab, in the bottom banner, then Synthesizer, and select a frequency, for example 600 Hz. The Play key is used to play the sound. Adjust the volume so it's loud enough, but not so loud that it bothers you.
If you have an Android device, you will be able to analyze this sound directly with the same smartphone in the FizziQ application. If you have an iOS device, you will need a second smartphone on which FizziQ will be installed because Apple devices cannot both emit sound and analyze it.
On the device that will analyze the sound, press the Measurements tab, then in the central circle to select the Microphone instrument, then Amplitude. This measurement displays an oscillogram of the sound captured at the frequency of 44,000 Hertz, in other words it captures the movements of the microphone membrane every 22 microseconds! With the oscillogram we will therefore have a very precise image of the air pressure over time at the location of the microphone.
Let's visualize this sound wave and study it by pressing the red measurement capture button, then in the notebook we can study the curve. We see that we obtain a beautiful sinusoidal cut, typical of a pure sound which makes our eardrum resonate harmoniously.
4. Mapping the sound waves
We often talk about the shape of a sound wave but what we really measure is the curve describing the pressure as a function of time, as we would see the swell in a vertical section. Each sound wave has different shapes and characteristics that can be studied using the FizziQ oscilloscope. Some sounds present beautiful sinusoidal curves, and others non-sinusoidal curves, but periodic, other curves finally are totally irregular.
With sounds from the sound library, it's easy to visualize all kinds of different sounds. For example, let's compare the curves produced by three sounds:
The sound of a tuning fork
The note A of a flute
The noise of a busy street
Like the synthesizer sound, the tuning fork sound also produces a sine curve. It is a pure sound, that is to say composed of a single frequency.
The sound of the flute is also periodic, that is, the pattern repeats, but it is no longer a sinusoid. In fact this sound is composed of several frequencies which add up and create a more complex curve. This sound is called a complex harmonic sound. Harmonic because its pattern repeats itself. We can study harmonics in detail with spectrum analysis, another FizziQ tool.
Finally, let's study the noise of the busy street. This sound is not periodic, the curve does not repeat itself and the peaks seem to happen randomly. This sound is called noise to indicate that it is not periodic and that the frequencies that compose it are random.
5. Conclusion
It is not possible to see the sound, but this is precisely what makes it an exciting field of study for students. Invisible, very fast, the phenomenon can only be studied with the sense of hearing, a very sophisticated natural instrument, or a scientific instrument. It is therefore always a particular study for the students which has great pedagogical value for learning the scientific method. Until now, these phenomena could only be studied in the lab, but thanks to applications like FizziQ, you can conduct investigative procedures very simply and quickly on any smartphone or tablet, in class or outside of class. Another reason to carry out activities on sound!