Sounds consist of pressure waves. The intensity of sound is known as the sound pressure level, or SPL.

The human ear can detect a wide range of sound pressure levels. Sounds can be very soft, such as the ticking of a wristwatch, or very loud, such as a top fuel dragster doing a burnout. The intensity of sound pressure can be measured, and is expressed as decibels, or dB.

Alexander Graham Bell founded the concept of decibels and formulated a logarithmic scale based on 10. "Deci" refers to the base 10 log scale, and "bel" refers to Alexander Bell.

Each 10-dB increase represents a tenfold increase in sound intensity. In addition, a 10-dB increase is perceived as roughly doubling loudness.
A few examples of decibel readings are:

	1 - 25 dB	Human ear begins to detect sounds at this baseline
	40 - 50 dB	Sound levels in the average home
	60 dB	Normal conversation
	70 dB	Negative responses begin in the body. The autonomic nervous system kicks in.

All sounds consist of waves of pressure moving through the air. As decibels increase, these waves of pressure get stronger and have more physical force. The human body reacts to this physical force through HEARING and FEELING the bombardment of sounds.

When sudden, strong sounds reach the ear, and are transmitted to the brain, the body reacts by triggering the autonomic nervous system. This automatic system is in place to protect us against danger. This system produces the "fight or flight" adrenaline response, which prepares the body to either fight a danger, or flee from danger.

In addition, when intense sound waves are combined with excessive low-frequency vibrations, the effect is tremendously damaging to the body, as well as to physical structures, such as buildings, etc.

	Noise over 70 dB	Increases the risk of heart attack by 20%
	Noise over 90 dB	As this intense sound bombards the body, the adrenaline reaction is so powerful that people become openly hostile and belligerent.
	120 dB	Standing behind a Boeing 707 while it is in full thrust, just before takeoff. Hearing loss can occur after just 7.5 minutes.
	120 - 130 dB	Sound threshold for pain
	120 - 140 dB	Inside the average street boom car. Boom cars with higher levels are usually seen in dB "drag racing" competitions.