What is a sound ?
A sound is an acoustic wave, and a mechanical wave which travels out from a definite source (like an instrument) through only a medium, such as air, water, glass or metal. However, a sound does not disperse into the vacuum.
The first person to discover that sound needs a medium was a brilliant British scientist, Robert Boyle (1627–1691). He carried out a classical experiment: he set a ringing alarm clock inside a large glass jar, and while the clock was still ringing, all the air was sucked out with a pump. As the air gradually disappeared, the sound died out because there was nothing left in the jar for it to travel through.
We did the same experiment, but with a bell:
Facts to know about sound waves
When we think of the sound of speed, we immediately say it is 340 meters per second. This is true, but only when it travels through air.
Sound travels at different speeds in solids, liquids, and gases and even its speed in one type of material can change. Very roughly speaking, how fast it travels varies according to the density of the medium. It's faster in solids than in liquids and faster in liquids than in gases. Sound travels at different speeds in different gases—and can go at different speeds even in the same gas. It travels much faster in warm air near the ground than in colder air higher up, for example. And it travels roughly three times faster in helium gas than in ordinary air because helium is much less dense. That's why people who breathe in helium talk in funny voices: the sound waves their voices make travel faster—with a higher frequency.
Balloons of helium, Google Images
Sound waves lose energy as they travel. That's why we can only hear things so at a certain distance and why sounds travel less well on blustery days (when the wind dissipates their energy) than on calm ones.
You can reflect a sound wave off an object. Stand some distance from a large flat wall and clap your hands repeatedly. Almost immediately you will hear a ghostly repeat of your clapping , slightly out of step with it. What you hear is, of course, sound reflection, better known as an echo: it's the sound energy of your clap traveling out to the wall, bouncing back, and eventually entering your ears. There's a delay between the sound and the echo because it takes time for the sound to race to the wall and back (the bigger the distance, the longer the delay).
A sound wave can also be refracted which means that it spreads out.
The principle of echo, Google Images
Sound and vibrations
If you bang a drum, you make the tight skin vibrate at very high speed (it's so fast that you can't usually see it), forcing the air all around it to vibrate as well. As the air moves, it carries energy out from the drum in all directions. Eventually, even the air inside your ears starts vibrating—and that's when you begin to perceive the vibrating drum as a sound. In short, there are two different aspects to sound: there's a physical process that produces sound energy to start with and sends it shooting through the air, and there's a separate psychological process that happens inside our ears and brains, which converts the incoming sound energy into sensations we interpret as noises, speech, and music.
We are going to look at the first one.
In this experiment we compare the drum skin with a speaker diaphragm.
The reason we made this video was to show that with bare eyes, at a certain point, we can not see the speaker's diaphragm vibrating. But the small paper balls show that it is still vibrating. It explains how wave sounds work.
How do sound waves travel ?
Sound is the energy things produce when they vibrate (move back and forth quickly). As a sound wave moves forward, it makes the air bunch together in some places and spread out in others. This creates an alternating pattern of squashed-together areas (known as compressions) and stretched-out areas (known as rarefactions). In other words, sound pushes and pulls the air back and forth. Sound waves thump energy through the body of the air. Sound waves are compression waves. They're also called longitudinal waves because the air vibrates along the same direction as the wave travels.
This image from http://w3.shorecrest.org/ represents the principle of compressions and rarefactions.
For a better understanding, this animation from www.web-sciences.com/ explains exactly how a sound wave travels.
The red line represents the thing vibrating (for exemple a string). All the grey points depict air particles which move back and forth quickly.
The main characteristics of a sound
A sound is characterized by three parameters:
One thing worth noting about sound waves is their pitch. Soprano singers make sound waves with a high pitch, while bass singers make waves with a much lower pitch. Scientifically, the pitch of a sound corresponds to its frequency which is simply the number of waves an object produces in one second. The unit is the hertz (Hz). A sound wave is high-pitched when its frequency is raised. Rather, it is deep when is frequency is low. So a soprano singer produces more energy waves in one second than a bass singer and a violin makes more energy than a double bass.
As this picture from www.web-sciences.com shows, humans can hear sounds whose frequency is between 20Hz and 20kHz. Below 20Hz, it is an infrasound. Over 20kHz, it is an ultrasound.
But there is a lot of diversity between humans' ability to hear.
As the picture shows, age plays a key role on our capacity to hear. Actually, young people have a better hearing than old people because they can hear high-pitched sounds.
Over time human's sense of hearing is reduced.
Obviously, some people can be affected by deficiencies which makes their hearing less developed.
picture by http://howgee.blogspot.fr/
We carried out an experiment in order to compare the ability to hear between young and older people, as explained above.
With a frequency generator, we varied the frequency from 0 Hz to 20 kHz in order to see when the two of us and our mathematics teacher hear various sounds. When our hands are raised we hear sound, when we put lower them, we can not. We are turning our backs to the camera because we wanted to focus on the sound test. We didn't see when the frequencies were rising and neither did we see when another person had his/her hand up or down.
/!\ We warn you that at one point, you will hear very high pitched sounds, so be careful if you have headphones on. /!\
Humans' audible spectrum ( from 20 Hz to 20 kHz ) is divided into octaves. An octave represents the interval which separates 2 musical notes whose the frequency of one is twice the other.
If the frequency is multiplied by 2, we thus switch up to an octave. Conversely, if the frequency is divided by 2, we thus switch down an octave.
For example, the La 440 ( situated in the middle of the piano keyboard ) means that the source vibrates 440 times a second. If we switch up its frequency, the sources vibrates 440*2=880 times a second. We let you discover by yourself thanks to this animation:
A music note with a pitch given is not equally perceived if it is played by a piano or by a diapason. His timbre is different. Its tone is linked to its spectral composition (characteristics of his harmonics) and its evolution over time. You will see more about it in the next part.
Actually the timbre of a sound is given by the number of harmonics and its intensity and, enables us to identify who is talking or which instrument is playing.
The energy something makes when it vibrates produces sound waves that have a definite pattern. Each wave can be big or small: big sound waves have what is called a high amplitude or intensity and we hear them as louder sounds. The intensity enables us to have an idea of the sound force: more the sound volume is raised, the more a sound is received is loud.
Sound intensity is noted with the letter l and expressed in watt per square meter (W.m-2) which is the power received per unit of area.
Humans' ears can on average, perceive sounds with a sound intensity up to a power of 10-12 W.m-2. This minimal sound volume is noted I0 and called the threshold of audibility. Otherwise, a sound with a very loud intensity can provoke earache and a partial or complete hearing loss: the threshold of pain corresponds to a value of about 10 W.m-2.
The perceptible sound intensity can take values included in an extremely big interval which goes from power 10-12 W.m-2 to 10 W.m-2, that is to say 10 000 thousand possibilities between the inferior limit and the superior one. To use an easier and more significant scale, we define the sound level of intensity.
This size is noted with the letter L and is expressed in decibel (dB). It is calculated thanks to the relation:
with
L: sound level (dB)
I: sound intensity (W/m²)
I0 = 10e-12 W/m²
The sound level is measured with a sonometer (picture by Google images).
The threshold of hearing corresponds to the sound intensity I0 = 10power-12.m-2
So
The threshold of pain corresponds to the sound intensity I0 = 10 W.m-2
So
Thus, the sound level of a perceptible sound (safe) by humans' ear is included between 0 and 130 dB.
This picture from http://www.hearingandaudiology.com.au/ shows some examples of sounds and their level in everyday life, from the safest to the most dangerous for humans' hearing.