Sound occurs from vibrations in the air or in fluids, but it can also travel through such solid matter as bone or steel.
For instance, the thump that is heard when the head is bumped is a result of vibrations traveling through the skull and not the air.
As sound travels, vibrations travel toward the outer ear and on into the eardrum. This triggers a chain reaction in the middle ear, a chemical reaction in the cochlea, and an electrical impulse in the auditory nerve (cranial nerve VIII).
From here, it is sent to the brain to be transcribed into a sound. Some animals are able to rotate the outer ear to face the source of the sound, but humans have to rely on turning the whole head until the sound can be distinguished.
One sound can be vastly different from another but there are three main qualities that can help identify sound: frequency, intensity, and timbre. The first two can be measured while the third is subjective.
- Frequency, also known as pitch, happens when a disturbed pressure wave fluctuates within a given period of time. Frequency is usually measured in cycles per second, or hertz (Hz). In other words, the more fluctuations, the higher the frequency. On average, sound frequencies audible to humans range from a very low pitch of around 20 Hz to a very high pitch of 20,000 Hz.
- Intensity of a sound is measured in decibels (db). Decibels measure loudness, or amplitude, associated with the level of disturbance in the pressure wave. For example, a whisper might be measured at 30 decibels sound pressure level (db SPL), whereas a gunshot might register between 140 and 170 db SPL. Such high level sounds can cause permanent damage to the ears if they are not protected.
- Timbre is the subjective quality that allows the distinction between sounds of the same frequency and intensity. Examples would be the same note being played by two different musical instruments or enunciations of a vowel by two different voices.
In reality, when a sound is heard, it is perceived by all three qualities at once. Sounds are seldom composed of only a single frequency but rather as a combination of many.
For instance, sound from a modern orchestra will show energy concentrated from about 25 Hz to 5,000 Hz, and the noise in a large computer room will range from 30 Hz to 10,000 Hz.
Human listeners perceive the spectrum of a sound as its quality or timbre. A violin playing middle C and a trumpet playing middle C have different qualities because their spectra are different.
Hertz (Hz) is the standard measurement of frequency. One hertz is equal to one vibration cycle per second. Other expressions include 1000 Hz equals one kilohertz (kHz) and 1000 kHz equals one megahertz (MHz)
The name Hertz comes from the German physicist, Heinrich Rudolf Hertz (1847-1894), who proved that electricity can be transmitted in electromagnetic waves. These waves travel at the speed of light and possess many other properties of light. His experiments led to the development of the wireless telegraph and the radio.
The range that humans can typically hear is from about 20 Hz (bass) to 20,000 Hz (treble), but can vary substantially with the individual. Frequencies lower than 20 Hz are usually felt as vibrations.
As we age, however, the frequency range narrows. In addition, the hearing range for men worsens more quickly than the hearing range for women. This means that women will have the ability to hear notes of higher pitch than men of the same age.
Decibels are units of measure used to define the intensity of sound. Usually, a measure of intensity is expressed in terms of db (decibels) while hearing level measurements will include the HL (hearing level) after the db.
Measuring the force of a sound wave in the environment or the amount of pressure it exerts on the eardrum is referred to as sound pressure level (SPL). The intensity of normal speech is generally about 60 db SPL.
It also establishes just how well one person hears compared with the average of a large group of people of comparable age.
A person with a hearing threshold (the faintest point at which a sound is perceived) measured between 0 and 25 db HL is considered to have normal or near normal hearing. Someone who has trouble understanding conversational speech may barely hear sounds at 40 db HL but no lower.
This is considered to be a moderate loss of hearing. A person who can hear only a relatively loud voice close by may have a hearing threshold of 70 db HL and would have a severe hearing loss.
Common Noise Sound Levels
- 30 db – Whisper
- 60 db – Normal conversation
- 80 db – Ringing telephone
- 90 db – Hair dryer, power lawn mower
- 98 db – Hand drill
- 105 db – Bulldozer
- 110 db – Chain saw
- 120 db – Ambulance siren
- 140 db – Jet engine take-off
- 165 db – 12-gauge shotgun blast
(From the National Institute for Occupational Safety and Health – 2001)