What do you actually hear?

What we hear

The ringing cell phone, the murmur in the streetcar, the wind sweeping through the streets: there is hardly any place where there is real silence. But what is that, what our ears pick up there all the time?

  • What we hear are sound waves. Vibrating surfaces make air molecules vibrate. These collide with each other and propagate like waves.
  • The number of oscillations per second of a sound wave determines its frequency. Measured in hertz (Hz).
  • A young person can usually perceive frequencies between 16 hertz and 20 kilohertz (kHz).
  • The faster a sound wave vibrates, the higher you perceive a tone to be.
  • Tones consist of sound waves with constant frequencies. One speaks of a sound when harmonics are added. Its frequency must correspond to a multiple of the fundamental tone.

If an ambulance with a running siren moves very quickly past the listener, the sound apparently changes. The sound the siren makes, however, remains the same. So why does it sound different? This is due to a physical phenomenon – the Doppler effect. The Doppler effect occurs when the source of the sound or the observer is moving. When the ambulance approaches the receiver, the speed of the vehicle compresses the sound waves of the siren. As a result, they reach the ear at shorter intervals. So the frequency is higher and the tones sound higher than they are actually emitted. But if the car moves away, it pulls the sound waves apart, the frequency gets smaller and the sound gets darker. By the way, the term Doppler effect does not come from the fact that something is duplicated. Rather, the phenomenon was named after its discoverer, the Austrian physicist Christian Doppler.

Ear

The ear is not only the organ of hearing, but also of balance. A distinction is made between the outer ear with pinna and external auditory canal, the middle ear with eardrum and ossicles, and the actual organ of hearing and balance, the inner ear with cochlea and semicircular canals.

Life enters through the ears – the mother’s voice is the child’s first connection in the womb to the world outside. Later, thousands of sounds determine our day: the alarm clock that ends sleep, the warning horn of a car, the kind words of friends.

Yet the tones, sounds and noises that accompany everyday life are nothing more than waves invisible to the eye that push some air around and happen to hit the ears: sound waves. No chemical reactions are needed to generate them – physics is called for here. Sound is the wave-like movement of particles, such as air or water. Without particles, for example in the vacuum of space, there is no sound. The loud bang of exploding spaceships in science fiction movies can therefore not exist in reality.

Sound occurs when a body vibrates. Examples are the membrane of a drum under the beats of the musician or our vocal chords vibrating when breathing air flows through the larynx. How such a sound wave develops can be illustrated by the example of the ringing of a church bell. When the bell is struck, the clapper bangs against the metal wall and causes it to vibrate. If the metal bends outward, the molecules in the air are pushed away. They collide with neighboring particles, which collide again with their neighbors. If the metal vibrates inward again, there is more space between the particles, they are pulled apart. In this way, a wave-like motion is formed with dense and less dense parts. It spreads out in all directions, very similar to the circular waves after a stone has been dropped into the water.

Eye

Eyeball/bulbus oculi/eye bulb

The eye is the sensory organ for perceiving light stimuli – electromagnetic radiation of a certain frequency range. The light visible to humans is in the range between 380 and 780 nanometers.

Ten times as fast as a car on the highway

At 343 meters per second (at 20 degrees Celsius), sound travels at breakneck speed in the air – converted, that’s a good 1.200 kilometers per hour. So sound is ten times as fast as a car traveling 120 kilometers per hour on the highway. In water, sound propagates even much faster: With 1.480 meters per second. For the speed of sound, a submarine would have to travel a good 5.The sound can reach a speed of 300 km per hour, whereas an airplane can reach a speed of 1 km per hour.200 km/h.

How loud a sound is depends on the distance to the sound source. Directly in front of a loudspeaker at a concert, you can experience firsthand what the booming music does to the air. This works best with a song with a lot of drums or bass – if their sound waves hit the body very loudly, they can be perceived as a kind of fluttering in the chest.

Why open-air stages are very high

If the sound hits objects, the wave is weakened. To ensure that the music at an open-air festival is not swallowed up by the first rows of the audience, but reaches the audience further back in a direct way, the stage is usually particularly high. At concerts in halls, this is not quite so important, because there the walls reflect the sound waves.

The effect of sound reflection is particularly clear in the mountains. If you call in the direction of a distant mountain wall, you hear an echo. If you count the seconds until the echo comes back, you can even calculate the distance of the wall. At 343 meters per second, the sound takes 1 kilometer, or 1.000 meters, just under three seconds. The sound wave must first reach the wall and then come back again, so it makes the way twice. That’s why you have to divide the counted seconds by six to get the distance in kilometers. A similar calculation can be made with thunderstorms: When you see lightning, you can count the seconds until thunder. Unlike echoes, sound only has to travel once – from the point of impact to the ear. That’s why you can directly use the speed of sound for calculation: If you divide the counted seconds by three, you get the distance in kilometers. The time it takes for the light of the flash to reach the eye can be neglected, because light is about a million times faster than sound.

Ear

The ear is not only the organ of hearing, but also of balance. A distinction is made between the outer ear with pinna and external auditory canal, the middle ear with eardrum and ossicles, and the actual organ of hearing and balance, the inner ear with cochlea and semicircular canals.

Eye

Eyeball/bulbus oculi/eye bulb

The eye is the sensory organ for perceiving light stimuli – electromagnetic radiation of a certain frequency range. The light visible to humans is in the range between 380 and 780 nanometers.

When the clapper of a bell hits the metal wall, it vibrates and moves the air particles. We humans perceive these vibrations as noise. © Steve Evans /flickr

Frequency determines pitch

Sound is not equal to sound. Whether we perceive a sound as melodious music or as loud noise depends on the nature of the vibrations. If they move in slow regular waves, we perceive them as low tones. We interpret fast air vibrations as high-pitched tones. To determine the speed of a wave, one determines its frequency, i.e. the number of oscillations per second. It is measured in Hertz (Hz). The larger the hertz number, the higher the sound appears to be.

Sounds that consist exclusively of sound waves with identical frequencies are called sinusoidal tones. They usually sound shrill, as you can hear from the ringing of old cell phones. We perceive the sounds of instruments as not so glaring because, in addition to the fundamental tone that determines pitch, they have other sound waves in higher frequencies. These are called overtones. So that they do not distort the pure sound, they must fulfill a condition: Their frequency must be a multiple of the fundamental tone. The first overtone is twice as fast, the second three times, the third four times and so on. With each overtone the timbre changes. This is why an "a" sounds different on different instruments, even though the frequency of the fundamental is the same.

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Inaudible field mouse whine

When all overtones match the fundamental tone, the acoustician calls it a "sound". With the term "noise is the term he uses when different frequencies mix without context. A sound can therefore not be assigned a clear pitch, but only an intensity and a frequency spectrum.

Sounds and noises occur almost everywhere, but we cannot perceive them all. A young, healthy person generally hears frequencies between 16 and 20.000 hertz. The bat’s whine is usually so high that we humans can’t hear it – its sounds are in the range of 15.000 to 150.000 hertz and are thus largely located in the ultrasound range. Animals use their calls to orient themselves and locate prey via the echo reflected from their bodies. Only people with very good hearing can sometimes hear the lowest tones of the bat, as a very high pitched beep.

The sound pressure determines the volume

Not only the pitch but also the volume determines what can be understood and which sounds remain unheard. Once again, the shape of the sound wave is decisive here: we perceive particularly large wave crests as particularly loud. To determine the height of a wave, technicians measure its amplitude, i.e. the distance from the highest point of the wave crest to the original value. Sound consists of pressure differences, therefore the amplitude is measured in the unit of pressure Pascal. The healthy human ear can process a huge range: From about 0.00002 pascals at the hearing threshold to 20 pascals at the pain threshold, it barely misses a thing.

More common in everyday life than the pascal is another unit of volume: the decibel (dB). Thus, the enormous spectrum of audible sound levels can be represented in a simple logarithmic scale: Thus, zero decibels corresponds to the weakest audible sound. Whispering is at 30 decibels, city traffic roars at 90 decibels and the jet engines of an airplane roar at 140 decibels during takeoff. Every increase of ten decibels is roughly equivalent to a doubling of the perceived loudness.

Whether we perceive something as loud depends not only on the decibels – the frequency is also important. When we listen to the music of our neighbor across the street, we usually only hear the low bass notes of the song, but not the high-pitched singing voice. The higher a sound is, the quieter it seems to us.

But regardless of whether it’s loud or quiet, pleasant or annoying: sound is almost everywhere. In order to understand the tones and sounds, to distinguish between them and to assign meaning to them, they must first be converted into a language that the brain can understand. This is what happens when the sound waves find their way through the passages, convolutions and cavities of the ears and trigger nerve impulses there. Then a little physics becomes music.

Ear

The ear is not only the organ of hearing, but also of balance. A distinction is made between the outer ear with pinna and external auditory canal, the middle ear with eardrum and ossicles, and the actual organ of hearing and balance, the inner ear with cochlea and semicircular canals.

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