Glossary

"A Weighting" = [Expressed as dB(A)] You can measure the sound pressure level with various "weighting filters" whose characteristics approximate the sensitivity of the ear at various frequencies and levels based on the Fletcher-Munson curves. Most commonly used is the "A Weighting," which is most sensitive in the mid-frequencies. Other, less common weightings are B, C, and D. (See "Sound Pressure Level" and "Frequency Weighting").

The basic instrument for objectively measuring sound is the sound level meter. In its simplest form it is calibrated to read sound level over a short period of time with a similar response to all frequencies, that is a 'linear' weighting. Since the human ear is not uniformly sensitive to all frequencies, several weighting scales have been developed to simulate the various sensitivities. These weightings are known as A, B, C or D weightings (see figure 1).

The A, B and C weightings mainly differ in the degree of sensitivity at lower frequencies, relative to 1000Hz. The least sensitivity to lower frequencies is provided by the A-scale, the most by the C-scale. The D-scale gives an indication of perceived noisiness and is used in aircraft noise measurements (IEC 537).

Measurements of sound pressure levels with a weighted response are usually referred to as 'sound levels with the appropriate suffix' (i.e. dB(A), dB(B), dB(C) or dB(D)). The A-weighted sound level has been shown to correlate with subjective responses and two sounds judged to be of similar loudness would produce similar dB(A) values, although their unweighted dB values would vary considerably. The A-weighting compares well with other noise sources. It is, therefore, the most widely used. All four weightings are internationally standardized.

"Ambient noise level" = the all encompassing noise level associated with a given environment, being a composite of sounds from all sources, excluding the alleged offensive noise, at the location and approximate time at which a comparison with the alleged offensive noise is to be made. Also see "Background Noise" below.

A factor to remember when measuring sound involves the contribution to the overall level of all the other noise sources present at the time a noise violator is cited. This extraneous noise is called the ambient noise level. The violator might ask, "There were a number of other loud noise sources present when you cited me, so how do you know that they didn’t cause the readings to be too high?" The rule that applies here is: a violator should not be cited unless the level measured when the violation occurs is at least 10dB above the ambient noise level immediately before the violation. If this condition is met, then the additional noise caused by all the other sources producing noise will add less than 0.4dB to the level produced by the violator.

Attenuation = The reduction of sound intensity by various means (e.g., air, humidity, porous materials...).

Background noise = The total of all noise in a system or situation, independent of the presence of the desired signal. In acoustical measurements, strictly speaking, the term "background noise" means electrical noise in the measurement system. However, in popular usage the term "background noise" is often used to mean the noise in the environment, other than the noise from the source of interest.

British Standard 4142 defines the background noise level as: L_{A90, T} This is the “A weighted” noise level exceeded for 90 per cent of the specified measurement period (T).

One factor that may substantially affect the accuracy of environmental measurements is the level of background noise in relation to the noise being measured. In practice, no correction is necessary if the difference between the measured noise and the background noise is greater than 10dB. If the difference is less than 3dB the background noise is too high for accurate measurement. For differences between 3dB and 10dB a correction is necessary, as illustrated in figure 2.

If the spectra of the source and background noise are very different, it may still be possible to provide useful information concerning the source, despite comparable overall noise levels. (Also see Combining Sound Pressure Levels)

Figure 2: Correction for background noise (LS+N is the total noise level and LN is the background noise level)

Bel = Unit of level when the base of the logarithm is ten, and the quantities concerned are proportional to power; unit symbol B (ANSI 1994) See Decibel below.

CNEL = Community Noise Equivalent Level. A noise measurement system introduced in the early 1970's by the State of California as a simplified alternative to the Noise Exposure Forecast (NEF) system for community noise exposure, with particular emphasis on airport noise. The major difference is that CNEL can be measured using ordinary dB(A) readings, as opposed to the computer calculation of Effective Perceived Noise Level used in the NEF. The CNEL represents the daily energy noise exposure averaged on an annual basis. It is not measured, but computed.

The CNEL is a calculated noise average over a 24 hour period. Because it is a calculation, based on a mixture of noises, it is not possible to hear what 65 CNEL sounds like. A deafening explosion, on an otherwise quiet day, will yield a low CNEL calculation due to the averaging. In order to account for increased human sensitivity at night, the CNEL level includes a 5-decibel penalty on noise during the 7:00 p.m. to 10:00 p.m. time period and a 10-decibel penalty on noise during the 10:00 p.m. to 7:00 a.m. time period.

The CNEL and L descriptors have been found to provide good correlation to the potential for dn annoyance from transportation-related noise sources (ie: roadways, airports, railroad operations). However, they do not provide a good correlation to the potential for annoyance from non-transportation or stationary noise sources such as industrial and commercial operations. This is due to the fact that many times stationary noise sources may operate between 8 and 10 hours per day, or will have noise sources such as loading docks, pressure relief valves or alarms which tend to be short duration noise events. When applying an L or CNEL dn criterion, the noise levels associated with these types of short term operations will be averaged over a 24-hour period, thus underscoring the potential for annoyance.

Combining Sound Pressure Levels = Suppose we have two identical sound sources, each alone producing the same dB level. So what is the SPL of the combined sources? It is not the sum of the two. We cannot simply add decibels directly to get the overall effect. The correct answer is obtained by using the following rule: Each time the number of identical noise sources is doubled, the SPL increases by 3dB; each time the number is halved, the SPL is decreased by 3dB. This rule is called 3dB doubling or 3dB exchange rate.

How does the 3dB rule help you? Suppose your noise ordinance has an 80dB noise limit. You cite a violator for causing an 89dB noise level and the case comes to court. The judge asks you how loud 89 dB is. Knowing this rule of thumb, you are able to tell the judge that 89 dB is the same noise level that would be generated by 8 identical vehicles, each producing the maximum allowable sound level of 80 dB. Case closed!! Be sure to apply the doubling rule contained in your particular ordinance.

Day-Night Average Level (Ldn) = The average equivalent A-weighted sound level during a 24-hour day, obtained after addition of ten decibels to sound levels during the night time from 10 p.m. to 7 a.m. The 10 decibel penalty is applied to account for increased noise sensitivity during the nighttime hours. The Ldn represents the daily energy noise exposure averaged on an annual basis. Where evening sound levels are not substantial, an Ldn value is generally within 2 dB of a CNEL value.

Decibel, [Expressed as dB] one-tenth of a Bel. Unit of level when the base of the logarithm is the tenth root of ten, and the quantities concerned are proportional to power; unit symbol dB (ANSI 1994) The rigorous definition of decibel (since deci means tenth) is 10 times the logarithm of a ratio.

Harmonic = A sinusoidal (pure-tone) component whose frequency is a whole-number multiple of the fundamental frequency of the wave. If a component has a frequency twice that of the fundamental it is called the second harmonic, etc...

Health = (Mental Health) The absence of identifiable psychiatric disorder according to current norms (Freeman 1984). In noise research, mental health covers a variety of symptoms, ranging from anxiety, emotional stress, nervous complaints, nausea, headaches, instability, argumentativeness, sexual impotency, changes in general mood and anxiety, and social conflicts, to more general psychiatric categories like neurosis, psychosis and hysteria (Berglund and Lindvall 1995). Also see: Long Term Health Effects of Noise Exposure.

Hertz = Unit of frequency, the number of times a phenomenon repeats itself in a unit of time. The frequency of a vibration is expressed as the number of vibrations per second (i.e. inversely proportional to the period) in Hertz (Hz). Sound may consist of a single frequency, or pure tone, but mostly contains tones of varying frequencies and intensities. The disturbance caused by a sound is dependent on both the frequency and level of the sound. A pure tone causes more disturbance than a complex sound at the same level.

"Impulsive noise" = a noise of short duration usually less than one second and of high intensity with an abrupt onset and rapid decay. See Time Weighing.

L_10, L₅₀, and L₉₀ = "A" weighted noise level that was statistically exceeded during 10%, 50%, and 90% of a stated time period.

LAeq = Equivalent Continuous Noise Level. The noise level in dB(A) which if present for the entire measurement period would produce the same sound energy to be received as was actually received as a result of a signal which varied with time. Normally abbreviated to "Leq" or "LAeq", often followed by a specification of the time period (such as 1 hour or 8 hours) indicating the period of time to which the measured value has been normalized; for example, "LAeq(8hr)".

LAeq,T is the average energy equivalent level of the "A Weighted" sound over a period "T". Expressed as LAeq(8hr), LAeq(12hr), LAeq(6-24hr), etc.

Leq = Equivalent Sound Level is a single value of sound level for any desired duration, which includes all of the time-varying sound energy in the measurement period. Used to identify the average sound level over a given period of time. A method of quantifying the noise environment is to determine the value of a steady-state sound which has the same A-weighted sound energy as that contained in the time-varying sound.

In general, Leq represents short-term measurements (15- or 30-minute), while CNEL or Ldn represent long-term measurements (24-hour).

The major virtue of the Equivalent Sound Level is that it correlates reasonably well with the effects of noise on people, even for wide variations in environmental sound levels and time patterns. It is used when only the durations and levels of sound, and not their times of occurrence (day or night), are relevant. It is easily measurable by available equipment.

Lmax = Maximum noise level the maximum A-weighted sound level for a given event.

Loudness = The subjective judgment of intensity of a sound by humans. Loudness depends upon the sound pressure and frequency of the stimulus. Over much of the frequency range it takes about a threefold increase in sound pressure (a tenfold increase in acoustical energy, or, 10 dB) to produce a doubling of loudness.

Noise: The word NOISE is believed to be derived from the Latin word "nausea," meaning seasickness. Etymologically the word can be traced back to Old French (noyse) and to 11th century Provençal (noysa, nosa, nausa), but its origin is uncertain. Physically, there is no distinction between sound and noise. Sound is a sensory perception and the complex pattern of sound waves is labeled noise, music, speech etc. Noise is thus defined as unwanted sound. Sound Pollution is an imbalance in a soundscape caused by intruding or disrupting sound of any kind. (See Annoyance.)

Noise 2:. Any sound which annoys or disturbs human beings or which causes or tends to cause an adverse psychological or physiological effect on human beings. Our ear receives sound as information. Often continuous sounds are not perceived consciously. Only changes in sound or sound level cause us to notice. The faster the sound level changes, the more obtrusive is the sound is perceived as noise.

Octave = The interval between two sounds having a frequency ratio of two.- There are 8 octaves on the keyboard of a standard piano.

Octave band filter = means a filter that attenuates all noise except that falling between two frequencies an octave apart. Octave band filters are used to measure which frequencies are present in a given noise.

Receiving Land Use = A geographical area or Zone such as a Residential Zone, that receives noise generated by a source in the same or another area.

Reflection = The significant sound measurement factor to remember is the effect of large objects on sound reflection. Think of the stone in the water. If the water waves encounter an obstacle as they move away from where the stone entered the water, you will see part of the wave reflected back in the direction it came from, modifying the height of the waves, which is equivalent to the sound pressure in air. The same phenomenon occurs in air when measuring sound.

Resonance = The relatively large amplitude of vibration produced when the frequency of some source of sound or vibration "matches" the natural frequency of vibration of some object, component, or system.

SEL = Sound exposure Level the measure of the physical energy of the noise event which takes into account both intensity and duration. SEL is typically used to compare noise events of varying durations and intensities.

SIL = SPEECH-INTERFERENCE LEVEL. A calculated quantity providing a guide to the interference of a noise with the reception of speech. The speech-interference level is the arithmetic average of the octave band levels of the interfering noise in the most important part of the speech frequency range. The levels in octave bands centered at 500, 1000, and 2000 Hz are commonly averaged to determine the speech-interference level.

"Simple tone noise" = any sound which is distinctly audible as a single pitch or a set of single pitches as judged by any person empowered to enforce this chapter.

Single Frequency Sound = There are a number of common sources of sound that act much like the spring because they cause a single frequency sound to be produced. The keys of a piano are a good example. Pressing the middle C key causes its string to vibrate about 260 times per second. The vibrating string and soundboard cause the air adjacent to it to compress and expand with the same frequency. Just like the balloon, the changing pressure moves outward as a sound wave. Other examples of tones are the hum of a motor (60 Hz) and the sound of a police whistle (3500 Hz).

SONE = The unit of measurement for loudness. One sone is the loudness of a sound whose loudness level is 40 phons. Loudness is proportional to the sound's loudness rating, e.g., two sones are twice as loud as one sone.

Sound = Vibrations transmitted through an elastic material or a solid, liquid, or gas, capable of being detected by human organs of hearing. The response of the human ear to audible sound depends both on the sound frequency (measured in Hertz, Hz) and the sound pressure, measured in decibels (dB). Audible sound for fit, healthy young people is from 20Hz-20,000Hz, with maximum sensitivity at around 3,000Hz. As we age it is normal to detect sounds in the middle or speech frequencies. Frequencies below 20Hz are called 'infrasound' and those above 20,000Hz 'ultrasound'. The numerical magnitude of a sound is normally expressed as the sound pressure level in logarithmic decibel units (dB).

"Sound Level Meter" (SLM) = an instrument used for measurement of sound levels, which meets the American National Standard Institute's Standard S14. Does all this complicated noise measurement jargon mean that an enforcement officer will have to have a degree in mathematics? NO! All enforcement equipment is calibrated directly in decibels, so no calculations are involved. Click on picture for a larger image:

The SLM performs three basic operations. It uses a microphone to convert the energy in the sound into an electrical signal. An electronic circuit then conditions the signal to provide meaningful results. Finally, the SLM communicates the results to the operator in a display, digital readout, or printed record.

Sound Pollution = An imbalance in a soundscape caused by intruding or disrupting sound of any kind. Such an intrusion need not necessarily be excessively loud (as recorded on a sound level meter for instance), but rather it needs only to have characteristics which disturb the perceived balance of a soundscape.

In this sense, the nature of sound pollution depends largely on how an observer perceives the original balance and the nature of the sound intrusion. For example, a transistor radio in a quiet rural or park area is often perceived as disrupting the natural soundscape, even if it is not excessively loud; it is perceived as simply being out of place, since it overpowers or masks quieter sounds, and cannot be easily ignored by the listener. Even less consciously perceived sounds, such as low frequency ambient sound may affect the balance of a soundscape and human behavior within it over the long term.

Soundscape = An environment of sound (or sonic environment) with emphasis on the way it is perceived and understood by the individual, or by a society. It thus depends on the relationship between the individual and any such environment. The term may refer to actual environments, or to abstract constructions such as musical compositions and tape montages, particularly when considered as an artificial environment.

Since a soundscape is shaped by both the conscious and subliminal perceptions of the listener, soundscape analysis is based on perceptual and cognitive attributes such as foreground, background, contour, rhythm, silence, density, space and volume,

"Sound pressure level" (SPL) = The level, in decibels, of acoustic pressure waves. Sound pressure is the variation of the pressure of air and is measured in force per unit area. Sound pressure is normally represented on a logarithmic amplitude scale, which gives a better relationship to the human perception of hearing. Typical values on this scale for example are a sound level of 0 dB, which is the average threshold of human hearing, and 60 to 70 dB for normal conversation, 110 dB at an extremely loud concert, and 150 dB for the noise of a rocket takeoff or a jet engine at close range.

You can measure the sound pressure level with various "weighting filters" whose characteristics approximate the sensitivity of the ear at various frequencies and levels. Most commonly used is the "A Weighting," which is most sensitive in the mid-frequencies. Other, less common weightings are B, C, and D.

Statistical Parameters Ln (i.e., the sound level which is exceeded an n% of time). For example a noise ordinance establishes a basic noise standard for a land use Zone, such as Residential at 45 dB. That noise standard sound Level may not to be exceeded for a cumulative period of more than thirty (30) minutes in any hour or:

a. The noise standard plus five dB for a cumulative period of more than fifteen (15) minutes in any hour; or
b. The noise standard plus ten (10) dB for a cumulative period of more than five minutes in any hour; or
c. The noise standard plus fifteen (15) dB for a cumulative period of more than one minute in any hour; or
d. The noise standard plus twenty (20) dB or the maximum measured ambient for any period of time.

Some call this type of noise standard "Tiered Levels".

Time weighting = Different, internationally recognized, meter damping characteristics are available on sound level measuring instruments: 'Slow' (S), 'Fast' (F) and 'Impulse' (I) (IEC 651, 1979; BS 5969, 1983). The 'Slow' characteristic gives an effective averaging time of approximately 1 second, the 'Fast' characteristic, approximately 0.125s.

The 'Peak' characteristic, on the other hand, measures the actual peak sound pressure level of a short duration sound, as short as 50 microseconds, and is therefore used to determine hearing damage risk. There is little standardization of criteria for assessing hearing damage risk due to impulsive noise. Some national standards suggest impulse noise limits in terms of dB(A) 'Impulse' level; ISO suggests the addition of 10dB(A) to the measured 'Slow' dB(A) value for a series of impulse sounds but does not cover the single impulse situation. The Occupational Safety and Health Administration (OSHA), which is the accepted standard in parts of North America and Europe, classes repetitive events as 'steady noise' if the interval between events is less than 0.5s, and as 'impulses' if the interval is greater than 0.5s. They also set a maximum limit of 140dB peak sound pressure level for up to 100 impulses a day, with a 10dB decrease in allowable level for a 10-fold increase in the number of impulses.

Weighting Filters = A family of standardized broad-band filters that attempt to simulate human response to sound by the analyzer. Filters such as A-, B-, C-, D- and E-weighting have been defined over the years. At present, A- and C-weighting filters are the most commonly used. A-weighting is essentially a high-pass filter, defined in ANSI S1.4, which was originally intended to simulate perception of the loudness of low-level tones. A-weighting progressively de-emphasizes frequencies below 500 Hz. C-weighting essentially filters out sounds outside the audio range (20 Hz to 20 kHz), and was originally intended to simulate perception of the loudness of high-level tones.

The ear has different sensitivities to different frequencies, being least sensitive to extremely high and extremely low frequencies. Because of this varied sensitivity, the term "A weighting" is used: all the different frequencies, that make up the sound, are assessed to give a sound pressure level. The sound pressure level measured in dB is referred to as "A-weighted" and expressed as dB(A).

Zoning Ordinance = A zoning ordinance is adopted to protect and promote the public health, safety and general welfare of the community and is intended to provide a precise guide to the physical development of the City in accord with the goals and policies expressed in the General Plan. The zoning ordinance consists of a separate Zoning Map designating certain zoning districts on real property and a set of regulations known as the Zoning Ordinance. Two of the objectives of the Healdsburg noise regulation are: