14 DAY TRIAL // 1.Movement or chocking of the objects produce sounds. 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).
2.Human ear has three parts: external, median and internal. The external ear is made of pinna (the only visible part) and auditory canal. The pinna acts like a parabolic antennae, directing the sounds towards the canal. The sounds move the tympanic membrane (eardrum) located at the entrance to the middle ear.
The middle ear is carved on the temporal bone, communicating with the nasal cavity through the Eustachian tube. That's why the pressure in the middle ear is the same with the external pressure and changes in the atmosphere pressure cause "crackles" in the middle ear till the pressure adapts. This is also the reason why nose infections can cause ear infections as well.
The sounds from the eardrum to the elliptical window of the inner ear are transmitted by three minute bones (the smallest of the human body): malleus, incus and stapes. They are moved by the smallest muscles of the body.
3.In the inner ear, vibrations are transformed into nervous signals transmitted to the brain. The semicircular canals of the inner ear are involved in balance. The hearing sensors are located in the cochlea ("snail"). Vibrations move the basal and tectorial membranes which act on the four rows of cilia of the auditory sensory cells which transmit signals to the brain.
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.
4.Bats, dolphins, rodents and dogs perceive sounds over 20 kHz, called ultrasounds. Bats and dolphins also emit ultrasounds using their sonar to detect their prey (insects and fishes, respectively) from tens of meters away. 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.
5.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.
6.There are acoustic microscopes that employ scanning sounds to investigate, measure, or image an object. These microscopes can detect fabrication errors (like impurities at micronic scale).
7.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 polyurethane, 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.
8.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, decreasing auditory sensitivity.
Continuous and uniform noise is less harmful than intermittent one. The sharp sounds are more irritating than the grave ones. Powerful noises, like those emitted by planes, even if uniform, produce physical and intellectual tiresome and sleepiness.
9.The sound's intensity (energy carried by the sound) is measured in decibels (one tenth of a bel). Human ear perceives only up to 1140 decibels. After that humans only feel an intense pain in their ears; in fact, this can affect the eardrum.
Even sounds over 100 decibels (dB) are unbearable. Lifting off jet planes produce 130 dB, while only their started engine produces 120 dB. A rock concert produces 110 dB, a pneumatic hammer 100 dB.
A whisper has 15 dB, the gurgle of a classroom or crowded office 50 dB. A vacuum cleaner produces 70 dB, a kitchen mixer 75, a medium radio 61, clock ticking 30, normal speech 40, loud speech 60, a tramway 70, a motorbike 70, a train 80, a crowded street 90 and leaf sloughing ...10!
The average noise level in the big cities is of about 80 dB while in a library 20 dB. Nature has a noise level of 10 dB.
10.With the aging, hearing decreases, especially because of the lowered mobility of the ear bones, which can no longer transmit properly vibrations to the inner ear.
Ear infections can affect the eardrum or the ear bones. Impairment of the auditory nerve can weaken the hearing. This can be produced by load noises or long term noises (like those produced by the engines of a fabric). Load walkmen can have the same effect.