What We Hear
In theory, the human ear can hear from 20hz to 20,000hz. This is definitely the case when we are babies but by the age of 5, it usually has dropped to 19khz. Then as we get older, the top end of the spectrum slowly diminishes and the highest frequency an adult can hear becomes 14khz. This highest frequency is usually dependant upon surroundings which means that if a person has spent most of his or her teenage years in loud nightclubs, then their hearing will have become more affected. It is worth noting that despite being able to hear 20hz frequencies, we tend to feel sound below 40hz or so. These can can a big affect depending on the bodies' reaction, for example, bowel movements (otherwise known as 'brown noise'), epileptic fits, and even kill people! However, some of these frequencies are so low and precise that the chances of death are very slim. On the other end of the scale, anything above 20khz is known as ultrasonic frequencies. As humans cannot hear ultrasonic frequencies, they can be used for other things such as dog whistles (dogs can hear from 40hz to 60khz), as well as remote controls for car alarms.