Sound Level Primer

This Sound Level Primer provides information to foster a better understanding of the physical aspects of sound, the subjective nature of noise, some of the issues involved in community noise management and the basic principles of noise mitigation. Noise is defined as unwanted or excessive sound. Sound becomes unwanted when it interferes with normal activities such as sleep, work or recreation. Under extreme conditions sound can cause physical harm such as hearing loss or adverse mental health effects. How people perceive any given sound depends on several measurable physical characteristics of the sound. These factors are:

• Intensity:  Sound intensity is often equated to loudness. The sound level magnitude (typically measured in decibels) is a measure of sound intensity. A 10 decibel increase in intensity is generally perceived as a doubling in loudness.

• Frequency Content:  Most common sounds are comprised of acoustic energy distributed over a variety of frequencies. Acoustic frequencies, commonly referred to as tone or pitch, are typically measured in Hertz (Hz). Pure tones (such as those generated by a tuning fork) have all their energy concentrated in a narrow frequency range. High-frequency (above 2,000 Hz) sound is typically considered more annoying than low-frequency (below 500 Hz) sound and may also be perceived as louder.

• Temporal Pattern:  The temporal nature of sound includes factors such as continuity, fluctuation, impulsiveness, and intermittence. Sound with increasing intensity is often perceived as louder than sound with decreasing intensity. Impulsive and intermittent sounds are usually perceived as louder than the actual sound level.

Individual human response to noise is subject to considerable variability.  There are many factors, both emotional and physical, which contribute to the variation in human reaction to noise. These factors are summarized in Table 1. The existence of numerous emotional and physical variables prohibits defining an exact individual or community response for any given noise level.  Community noise criteria are therefore based on statistical averages of human response to noise and applicable health criterion. Table 2 summarizes some generalized effects of noise on people in a residential environment. 

Sound levels are most often measured on a logarithmic scale of decibels (dB). The decibel scale compresses the audible acoustic pressure levels which can vary from 20 micropascals (µPa), the threshold of hearing and reference pressure (0 dB), to 20 million µPa, the threshold of pain (120 dB). Because sound levels are measured in dB, the addition of two sound levels is not linear. To add sound levels in dB, the levels (dB) must be converted into “energy” terms, which are added and then converted back to dB. Adding two equal sound levels creates a 3 dB increase in overall level. An example is 50 dB (100,000 in “energy”) added to 50 dB (100,000 in “energy”) equals 53 dB (200,000 in “energy”). If more than a 10 dB difference exists between two sound levels, there is no significant additive effect. An example is 50 dB (100,000 in “energy”) added to 60 dB (1,000,000 in “energy”) equals 60.4 dB (1,100,000 in “energy”). Figure 1 further illustrates the range of acoustic pressures (µPa) and their relationship to the acoustic decibel scale.

Because the human ear does not hear sound energy linearly (on a one-to-one basis), humans do not perceive changes in sound level as equally loud. Research indicates the following general relationships between sound level and human perception:

• A 3 dB increase is a doubling of acoustic energy and is the threshold of perceptibility. The average person will not be able to distinguish a 3 dB difference in sound level in a laboratory condition.
• A 10 dB increase is a tenfold increase in acoustic energy but is perceived as a doubling in loudness to the average person. The average person will judge a 10 dB change in sound level to be twice or half as loud.

The human ear does not perceive sound levels from every frequency as equally loud. As part of the hearing process, the human ear will attenuate low and high-frequency sounds. To compensate for these phenomena in perception, the A-weighted decibel scale, referred to as dBA, is used to measure and evaluate environmental noise levels. The A-weighted scale adjusts sound pressure levels by frequency, reducing low and high-frequency sound, similar to the way people hear sound.

TABLE 1 - Factors Involved In the Human Reaction to Noise

EMOTIONAL VARIABLES

• Feelings about the Necessity or Possibility of Prevention of the Noise:  If people feel that their needs and concerns are being ignored, they are more likely to feel hostile towards the noise.
• Judgment of the Importance and Value of the Activity which is Producing the Noise:  If the noise is produced by an activity which people feel is vital, they are not as bothered by it as they would be if the noise-producing activity was considered superfluous.
• Activity at the Time an Individual Hears the Noise:  A person's sleep, rest and relaxation have been found to be more easily disrupted by noise than by communication and entertainment activities.
• Attitudes about Environment:  The existence of undesirable features in a person's environment will influence the way in which the person reacts to a particular intrusion.
• General Sensitivity to Noise:  People vary in their ability to hear sound, their physiological predisposition to noise and their emotional experience of annoyance to a given noise.
• Belief about the Effect of Noise on Health:  The extent to which people believe that the noise exposure will damage their health, affects their response to the noise.
• Feeling of Fear Associated with the Noise:  The extent to which people fear physical harm from the sources of the noise, affects their attitude towards the noise.

PHYSICAL VARIABLES

• Type of Neighborhood:  The type of neighborhood sometimes associated with one's expectations regarding noise there.  Instances of annoyance, disturbance and complaints associated with a particular noise exposure will be greater in areas which are typically noted for their quiet and less active nature.
• Time of Day:  Noise intrusions are typically considered more annoying in the early evening and at night than during the day.
• Season:  Noise is considered more disturbing during the summer than in winter, especially in climates where, during the summer, windows are likely to be open and recreational activities occur outside more frequently.
• Predictability of the Noise:  Research has revealed that individuals exposed to unpredictable noise have a lower noise tolerance than those exposed to predictable noise.
• Control over the Noise Source:  Someone who has no control over the noise source will be more annoyed than one who is able to exercise some control.
• Length of Time an Individual is Exposed to a Noise:  There is little evidence supporting the argument that annoyance resulting from noise will decrease with continued exposures, and, under some circumstances, annoyance increase with longer exposures.

     Source:  Aviation Noise Effects, USDOT-FAA, March 1985.