Voyage au centre de l'audition : spécialistes

> What do I hear ?

Drawings and animations: S. Blatrix; scientific contributors: A. Lorenzi, M. Camilleri; editing: B.Chaix, S. Irving

We hear sounds: vibrations that spread in air or water and stimulate our ears.
In a natural environment, everything that vibrates creates a sound: leaves rustling in the wind, running water, violin or guitar strings, vocal cords…

What is sound ?
	A sound wave is the vibration of air molecules around their resting state following the disruption of the surrounding environment, which can be solid, liquid or gaseous. When the membrane of the speaker vibrates, the closer air molecules vibrate around their resting state and this vibration spreads to neighbouring molecules, and so on. After each environmental disruption, the air molecules return to their initial position: there is therefore no molecular displacement. This is what sound waves are. Once captured by our ear, the vibrations cause the movement of the eardrum, the starting point for the ear’s stimulation and the perception of sound.

What is sound ?

A sound wave is the vibration of air molecules around their resting state following the disruption of the surrounding environment, which can be solid, liquid or gaseous.
When the membrane of the speaker vibrates, the closer air molecules vibrate around their resting state and this vibration spreads to neighbouring molecules, and so on. After each environmental disruption, the air molecules return to their initial position: there is therefore no molecular displacement. This is what sound waves are.
Once captured by our ear, the vibrations cause the movement of the eardrum, the starting point for the ear’s stimulation and the perception of sound.

Sound characteristics: frequency, intensity and duration
Frequency refers to the number of vibrations that occur per second: few vibrations produce a low sound, whereas more vibrations produce a higher sound. Frequency is measured in Hertz (Hz). Intensity depends upon the amplitude of the vibrations: the bigger the amplitude, the louder the sound; and conversely, the smaller the amplitude, the quieter the sound. Intensity is commonly measured in decibels (dB). Duration depends upon the length of time that the environment is disrupted. It is measured in seconds (s).
High pitch sound (click on thumbnail)	Low pitch sound (click on thumbnail)

Sound characteristics: frequency, intensity and duration

Frequency refers to the number of vibrations that occur per second: few vibrations produce a low sound, whereas more vibrations produce a higher sound. Frequency is measured in Hertz (Hz).
Intensity depends upon the amplitude of the vibrations: the bigger the amplitude, the louder the sound; and conversely, the smaller the amplitude, the quieter the sound. Intensity is commonly measured in decibels (dB).
Duration depends upon the length of time that the environment is disrupted. It is measured in seconds (s).

High pitch sound (click on thumbnail)

Low pitch sound (click on thumbnail)

Environmental influences on sound
The speed of the soundwave Soundwaves move at 340 m/s in air, 1500 m/s in water and at even higher speeds in denser materials (3500 m/s in bone and up to 6000 m/s in steel!). In a vacuum, with an absence of matter, sound does not propagate. Example: if you put a sound source underneath a jar, you will hear the sound. However, if you create a vacuum underneath the jar, the sound disappears as there are no remaining air molecules to vibrate. Other factors, such as the humidity levels or temperature, also influence the propagation speed of the soundwave. Impedance A soundwave can experience difficulties passing from one environment to another, as each environment imposes a resistance, which can vary in strength, called impedance. Therefore, a soundwave that propagates easily through air may be difficult to perceive in water, as the latter has a high impedance. Example: at the beach, if you have your head under water, you won’t be able to hear the person that is speaking whilst walking along next to you. However, you’ll hear sounds produced under water perfectly well, such as the outboard motor of a boat that’s 100 m away!
The human auditory range
Frequencies perceived by man and some common animals (click on thumbnail)	Intensities perceived by man (click on thumbnail)
A few common sounds
	Hear four common sounds of our environment
Other properties linked to sound propagation
When a soundwave is produced, it will be modified by parameters such as distance, or potential obstacles. Attenuation: in free-field, a space where there are no obstacles to alter the soundwave, the acoustic intensity will decrease the further it gets from the sound source. Reflection: When a soundwave meets an obstacle, such as the ceiling of a room, a certain amount of the energy is bounced back into the room: this is reflection. Successive reflections make up reverberation. Absorption: Another amount of energy is partially absorbed by the ceiling itself: this occurs more readily for higher frequencies than for lower ones. Transmission: A part of the acoustic energy is transmitted into the next-door room through the ceiling, which acts like a secondary sound source.
	Sound in a close environment The sound of the drumkit bounces off of the walls and room dividers. One of the dividers is used as an example to show that some of the sound's energy is absorbed and the rest is transmitted. In the room next door, the remaining sound intensity depends upon the ceiling's absorbing ability. Theoretically, the sound will be quieter, otherwise the ceiling is inefficient.

A few sound particularities
Sonic boom	The Doppler effect

The infinite gamme	The Larsen effect: whistle

(click on thumbnails)

> What do I hear ?

What is sound ?

Sound characteristics: frequency, intensity and duration

Environmental influences on sound

The human auditory range

A few common sounds

Other properties linked to sound propagation

A few sound particularities