 Sounds are longitudinal mechanical waves.  They are created by a disturbance and must travel through a medium, usually air, but sound can travel through liquids and solids, too.

 Sound is created by a disturbance.  When a chair falls to the floor, the disturbance causes the particles of the floor to vibrate. That vibration transfers into the surrounding air, (the medium), creating a wave.  The wave will travel through the air as a longitudinal wave.

 The sound gets picked up by the pinna of the ear.  Once in the middle ear, the tympanic membrane begins to vibrate to the beat of the sound. This causes the fluid in the cochlea to vibrate as well, and nerve impulses are formed.  The nerve impulses then go on to the brain for processing.

 Sound can also propagate through liquids and solids.  In solids, sound can be perceived through vibrations. While the sound itself cannot necessarily be understood, one can determine how loud a sound is by its vibrations.  Certain animals, like whales and dolphins communicate with one another underwater by producing sounds.

 The speed of sound varies depending on the medium.  Does sound travel best through air, liquid or solid?

MediumSpeed (m/s) Air346 Water1490 Plastic1800 Wood4000 Steel5200 This explains why you can “hear” a train coming first by putting your ear to the tracks before you can hear it through the air.

 The intensity of sounds varies greatly.  The volume of sound depends on the amount of energy it transmits – ie, its amplitude, (remember, when looking at a sound wave, you need to look at it region of compression).  Sound amplitude is measured in a unit called decibels.  The decibel scale is measure by a factor of 10, so each increase by 10, sound is 10x louder.

 The human ear can withstand around 80 dB comfortably. Unfortunately, many events can produce much louder sounds.  Prolonged exposure to sounds at 100 dB can cause serious hearing damage. Sounds of 120 dB start to become painful and can cause immediate hearing damage.

 Sound also has different tones and pitches. This links to frequency.  Low-frequency sounds have lower tones.  High-frequency sounds have higher tones.

 In general, the human ear can detect frequencies ranging from 20 to 20,000 Hz.  This range is different for each animal species.

 Infrasounds and ultrasounds are sounds that are out of our frequency range.  Sounds lower than 20 Hz are infrasounds, and those higher than 20,000 Hz are ultrasounds.  Even though the human ear cannot detect these sounds, other animal species can.  Elephants use infrasound to communicate.  Dolphins use ultrasounds to communicate.  Dogs are sensitive to ultrasounds and can be trained with ultrasound whistles.

 The echo of ultrasounds hitting an object and bouncing back are used often. This is called echolocation.  Sonograms use ultrasound imaging to see a fetus.  Some animals emit ultrasounds to determine the location of objects, (bats, dolphins, whales).  Submarines and ships use this technique, as well.