- Noise Basics
- Basics of Sound
- What models and metrics are used to measure Community Noise Exposure?
- What can I learn about Sound Perception and Hearing?
- What has been done to reduce Aircraft Noise?
- What are the Federal Aviation Regulations that deal with Aircraft Noise?
- How does Weather affect Aviation Operations?
Am I Hearing What I Think I'm Hearing?
Hearing is a complicated process and the ear is a complicated organ. The ear, through its complexity, does an extraordinary job of picking up tiny pressure fluctuations in the air. These pressure fluctuations, when they occur in cycles or waves, are what we call sound.
The ear is broken into three sections:
- Outer ear
- Middle ear
- Inner ear
Each section serves a purpose that is vital to our hearing:
The Outer Ear
The outer ear consists of the pinna, the ear canal and the eardrum.
The pinna is the outer cup-like part of the ear. It serves to catch sound and funnel it into the ear canal. The pinna also helps us detect where a sound is coming from — sound coming from different directions will bounce off the curves of the pinna in different ways that the brain can distinguish and interpret.
The ear canal is a tube through which sound travels before reaching the eardrum.
The eardrum is a thin membrane that vibrates with the slightest movement of air. It will vibrate more quickly for higher frequency sounds and more slowly for low frequency sounds. We hear high frequency sounds as higher in pitch and low frequency sounds as lower in pitch.
In addition to transmitting sound to the middle ear, the eardrum can be tightened to help protect the inner ear from very loud, and potentially damaging, sounds. This tightening is called the acoustic reflex. If the brain perceives a loud sound, it tells the muscles near the ear canal to contract. This causes the eardrum to be less flexible and to transmit less vibration to the middle and inner ear, protecting the ear from damage.
The Middle Ear
The middle ear consists of the ossicles, the three smallest bones in our bodies, and the oval window (the separation between the middle and inner ear).
The three ossicles are connected in a chain between the eardrum and the much smaller oval window. This chain of bones includes the malleus, the incus, and the stapes. These are more commonly called the hammer, anvil and stirrup.
Sound is passed from the larger eardrum membrane through the three bones. The footplate of the stirrup fits in the smaller oval window membrane, amplifying (or strengthening) the vibrations that enter the fluid-filled inner ear. This occurs because smaller vibrations over a larger area, the eardrum, are leveraged through the bones to create bigger vibrations over a smaller area, the oval window.
Perception of the loudness of a sound - Sounds carry a certain amount of energy. The ratio of energy between the sounds we can just barely hear and the sounds that are so loud that they cause us pain is about one to ten trillion. However, we don't hear the painful sound as being ten trillion times louder than the sound we can just barely hear. An increase of 10 decibel (dB) in a 1 kilohertz (1 thousand vibrations cycles per second) sound is perceived as twice as loud. This is why we use the decibel, a logarithmic scale, to measure loudness. A logarithmic scale more closely mirrors the way we hear.
Most measurements of airplane noise, including the Equivalent Noise Level (LEQ) (See Noise Basics), use an A-weighted decibel scale. The LEQ is essentially the average sound level as measured by the A-weighted decibel scale over a period of time. It is used over a 24 hour period with 10 dB added to the sounds occurring between 10 pm and 7 am to calculate the Day-Night Level (DNL). The DNL is used in evaluating the noise levels in a community. Therefore, our hearing process and perception of sound are an important part of noise assessment.
Luce, R. Duncan, Sound & Hearing, Lawrence Erlbaum. 1993. pp.65-90. ISBN: 0-8058-1389-6. (Properties of the ear).
For definitions of words used in this section go to the NoiseQuest Glossary of Terms.