Traffic Noise & Noise Relationships

Traffic noise is a complex phenomenon. It includes tire, exhaust, transmission, and engine noises that are all distinguished by their "pitch" character. These noise components are forever changing in intensity, fluctuating up and down as vehicles come and go or as gears shift up or down. To understand traffic noise requires an understanding of the physics of sound, and a familiarity with the terminology and definitions of noise descriptors. Some of the pertinent definitions are:

• Ambient Noise Level – the sound level of background noise characteristics. The level of a specific sound must be above the ambient noise level in order to be perceived. As an example, the ambient noise level in the residential area west of Circle Drive and south of 8th Street East was recorded at 54 dBA Ldn.
• A-Weighted Sound Level – the sound level as measured on a sound level meter, using a setting that emphasizes the middle frequency components similar to the response of the human ear. The A-weighted sound level is found to correlate well with subjective assessments of the annoying or disturbing effects of sound.
• Decibel (dB) – sound is measured in decibels. Zero on the decibel scale is based on the lowest sound level that a healthy, unimpaired human ear can detect. Decibels are not linear units, but representative points on a sharply rising (exponential) curve. Thus, 100 decibels represent 10 billion times as much acoustic energy as one decibel.
• Equivalent Level (Leq) – the Leq is an average of the moment-to-moment fluctuations in noise and is equivalent in the sense that it represents an equivalent noise "dose" or amount of energy. The Leq is also called the "energy-equivalent level". Leq1h condenses an hours worth of noise fluctuations into a single number, with units of dBA. Leq24 is the average noise level over a 24-hour period.
• Day-Night Average Sound Level (Ldn) – day-night sound level in dBA is derived by averaging time varying sound energy over the daytime (daytime Leq) with the varying sound energy over the nighttime (nighttime Leq) to which an additional 10 decibel weighting is applied to the nighttime hours of 2200 to 0700. Weighting the nighttime noise is done to reflect increased sensitivity to nighttime noise.

How loud is too loud?

Experts agree that continued exposure to noise above 85 dBA will cause hearing loss. To know if a sound is loud enough to damage your ears, it is important to know both the loudness level (dBA) and the length of exposure to the sound. In general, the louder the noise, the less time required before hearing loss will occur. According to the National Institute for Occupational Safety and Health (1998), the maximum exposure time at 85 dBA is 8 hours. At 100 dBA, the maximum exposure time is one minute and 29 seconds.

The following are decibel levels of common noise sources around us. These are typical levels – actual noise levels may vary depending on the particular item. Remember, noise levels above 85 dBA will harm hearing over time, anything above 140 dBA can cause damage to hearing after just one exposure.