14 DAY TRIAL // A sound is the vibration of elastic waves through different environments (solid, liquid, gaseous), with a frequency between 16 Hz to 20,000 Hz (20 kHz) (which is detected by the human ear).
In humans, the main phonic organ is the larynx, which produces sounds and words.
There is a complex neuro-muscular mechanism involving the vocal cords and controlled by brain centers that induces sound production. Inside the inner ear, there are the hairy cells, located on a membrane which vibrates under the influence of acoustic signals. The cilia on the top of the hairy cells are inserted on other membrane. When the membrane below vibrates, the hairy cells are deformed and produce electric signals transmitted to the brain.
The bats and dolphins perceive sounds over 100 kHz, called ultrasounds. They detect this way their prey (insects and fishes, respectively) from tens of meters away.
In fact, these animals emit intermittent ultrasounds (90 to 200 kHz) and pick up their echo during the pauses. The echo can be 1 million times weaker than the emitted signal, but their ears detect it. This way, bats can hunt during the night and dolphins in murky waters.
Elephants emit infrasounds (less than 16 Hz).
These sounds travel enormous distances and are not stopped by physical barriers. This way, herds located at great distances can communicate and lost individuals are easily located.
There are even acoustic microscopes that employ scanning sounds to investigate, measure, or image an object. These microscopes can detect fabrication errors (like impurities at micronic scale).
To analyze a pure sound, researchers employ "deaf rooms". The walls of such rooms, with 80 cubic meter volumes, are wrapped in 4,000 pieces of polyurethan, which adsorb 99 % of the waves that touch them, turning them into heat. "Deaf rooms" are employed for measuring sonic power of the most varied machines and vehicles.
The noise is formed by a succession of irregular, without periodicity, sounds. It is present everywhere in nature and human environments. Noise can harm the auditory system, but also the brain, which lowers its ability of understanding and speaking.
Continuous and uniform noise is less harmful than intermittent one. Sharp sounds are more irritating than grave ones. Powerful noises, like those emitted by planes, even if uniform, produce physical and intellectual tiresome and sleepiness.
The sound's intensity is measured in decibels (one tenth of a bel). Human ear perceives only 130 decibels; over this value, the sound is not heard and humans only feel an intense pain in their ears.
This intensity is reached by lifting off jet planes. A vacuum cleaner produces 68 decibels, a kitchen mixer 75, a medium radio 61, whispering 20, clock ticking 30, normal speech 40, loud speech 60, a tramway 70, a motorbike 70 and leaf sloughing ...10!
The average noise level in the big cities is of about 80 decibels.