1. Basic Sound terms and definitions

Sound: Any pressure variation that can be detected by the human ear. (↑)

Noise: Noise is most commonly referred to as an unwanted sound. (↑)

Decibel: This is the unit used to measure the sound pressure level, or loudness of noise. Abbreviated dB. The following chart details common sounds and their average dB levels. (↑)

Because the decibel scale is logarithmic, a decrease of 10 dB is a change of approximately half of the perceived noise level. So a large truck is about twice as loud as a small passenger car, even thought the change in decibels is only 10. (↑)

Generally acceptable noise levels are in the range of 60-65 dB. Prolonged exposure to noise levels 85 dB or above can result in hearing damage. The human threshold of pain is about 130 dB. (↑)

Decibel Weighting: Decibels are commonly measured in two different 'weights', A and C. A weighting is used to measure sound perceived by the human ear and is therefore the weighting most commonly used when talking about noise. It is abbreviated dBA. (↑)

Frequency: Sound occurs at different frequencies, measured in hertz. A hertz is simply defined as a cycle per second. If you imagine that a sound wave moving through the air looks like a sine wave, then the number of cycles this sign wave completes per second is the measured number of hertz. (↑)

Frequency Characteristics: The human ear can detect sound in the range of 20-20,000 hertz. Low frequency sounds are deep noises like those from a bass speaker, or a truck engine. High frequency sounds are those from a tea kettle or a whistle. All of these frequencies are commonly heard everyday. (↑)

2. Noise Characteristics

Environmental Noise: Noise that comes from any external source. It is usually an unwanted or annoying noise, and is often heard in the background during everyday living. Environmental noise can come from freeways, air traffic, constructions sites, air-conditioners, pool equipment, and even household appliances such as a refrigerator or a fan. Many times, environmental noise goes relatively unnoticed, as we become accustomed to having it in the background at all times. (↑)

Noise Source: The noise source is the cause of noise in your environment. It is important to understand the characteristics of the noise source when talking about how to control noise. For example, where is the noise source coming from? Is it above or below you? Is it constant or intermittent? Is the noise of a high or low frequency? Or both? (↑)

Line of Sight: In order to effectively block unwanted noise, it is necessary to break the line of sight with the noise source. This will not completely block unwanted noise, as noise has certain characteristics that allow it travel around sound barriers. These characteristics are detailed in the following definitions. (↑)

Noise Reflection: One characteristic of noise is that when it comes into contact with dense surfaces, it reflects in other directions. For example, noise from a nearby freeway bounces off the sides of homes and can be heard even when the freeway is not visually in sight. (↑)

Noise Diffraction: A characteristic of noise that allows sound waves to bend around objects they come into contact with. For example, noise can diffract (bend) over the top of a wall or fence that has been put in place to reduce unwanted noise. (↑)

3. Noise Reduction

Noise Absorption: Some materials can actually absorb sound, rather than reflect it. In general, these materials are porous in nature, allowing noise to filter through the material and be absorbed. Some common types of porous material are fiber glass, insulation, thick futon and blanket. (↑)

Noise Cancellation: Noise cancellation occurs when sound waves of an equivalent frequency are projected back towards the noise source. The result is a cancellation of sound waves and no noise at all. (↑)

Noise Barrier: Noise barriers are walls constructed to diminish the unwanted noise created by a noise source. Common noise barriers are constructed of masonry block or cement and are line the edges of a noisy freeway. These noise barriers commonly reflect noise rather than absorb it. (↑)

Transmission Loss: Noise barriers can be constructed of a variety of materials and are not necessarily limited to walls or fences. Transmission loss refers to how effectively the material used to construct a noise barrier cuts down on noise through such materials. For example, cement cuts down noise levels much more effectively than cardboard. (↑)

Noise Barrier Effectiveness by Frequency: Noise barriers have greater diminishing effects on some frequencies more than others. In general, noise barriers are most effective on higher frequency sounds than lower frequency sounds, as higher frequency sound do not bend much over the noise barriers. The following graph depicts the effectiveness of a sound barrier by frequency. (↑)

4. Sound Masking

The use of sound to make a noise unintelligible or less distracting. For more details please see sound masking tips.