Acoustical 101

Acoustics is defined as the scientific study of sound especially of its generation, transmission, reception and reduction. Due to differing legislation, no one standard fits all geographical regions. Therefore, the focus in this report is mainly an overview of acoustics in the area of noise, noise pollution and noise reduction

Environmental noise is a worldwide problem. Common sources for noise are automotive and truck traffic, airplanes, trains, sirens, generators, air compressors and air conditioning units. Noise can have adverse health effects such as diminish hearing, disturb sleep, disrupt conversation, and diminish one’s quality of life. Immediate safety factors should also be considered. The fact that you can’t see, taste or smell noise may help explain why it has not received as much attention as other types of pollution, such as air pollution or water pollution.

Noise is typically defined as “unwantedsound” or the human sensation of pressure fluctuations in the air. For noise to exist, there must be three components: Source, Path and Receiver.

The frequency of sound is the number of times in a period of one second that the pressure changes from zero to maximum to minimum to zero, thus completing a cycle. Amplitude refers to the loudness level of the noise and is oen measured in decibels (dB). Decibels (dB) are logarithmic units, where 5 dB is nominally the threshold of hearing” and 130dB is nominally the “threshold of pain”.

Since decibels are logarithmic, multiple dB cannot be added by ordinary arithmetic means. For example, if one automobile generates 70 dB when it passes an individual, two cars passing simultaneously would not produce 140 dB. In fact, they would combine to produce 73 dB. The following shows how to add two decibel values:


When Two Decibel Values Differ By:

Add this Amount of the Higher Value:


0 or 1 dB

3 dB

70 + 69 =73

2 or 3 dB

2 dB

74 + 71 = 76

4 to 9 dB

1 dB

66 + 60 = 67

10 dB or more

0 dB

65 + 55 = 65

Both the frequency and amplitude of sound are significantly altered by the physical variables in the path to the receivers. For example, walls, structures, ground absorption, atmospheric conditions such as temperature, humidity, wind and rain all contribute to changes in source noise levels before it reaches the receiver

Our main concern with sound is the effect it has on the receiver…..when a sound becomes an unwanted annoyance. Variations between sound power levels from the source and the characteristics of the path between the source and the receiver determine the sound levels upon the receiver. The most critical element, however, in determining the sound levels upon the receiver is the individual sensitivity of the human receiver.

Using Sound Walls to Control Unwanted Noise

Noise levels can be lowered by eliminating or reducing the noise at the source with the use of noise-control walls and enclosures. Sound walls are most effective when built close to the source or close to the receiver. The height of the wall should interrupt line-of-sight between the source and the receiver.

Sound walls are classified as reflective or absorptive. Hard surfaces such as masonry or concrete are reflective. This means most of the noise is reflected back towards the noise source and beyond.

A barrier wall with a surface material that is porous with many voids is said to be absorptive. This means little or no noise is reflected back towards the source. Sound-absorptive walls installed between the noise source and the receivers are effective in reducing reflective noise.

In evaluating the effectiveness of a barrier or enclosure, criteria such as local, state, and federal requirements must be considered. These codes cover a large range of situations from vehicle, industrial and residential boundary noise. In order to be in compliance with these criteria, octave band frequency analysis is typically required to investigate the noise source in order to calculate the amount of reduction that will be required in order to merit the criteria that is trying to be achieved. Individual octave measurements is also critical in determining the amount of ‘aer the fact’ reduction is required in order to meet the desired results in the most cost effective manner. Individual octave band measurements are a detail procedure that requires precautions from false measurements such as wind and increases in ambient noise levels. The procedure itself is more in-depth than this introduction allows.

Sound walls are performance rated in two categories Sound Transmission Class (STC) and Noise Reduction Coefficient (NRC). The STC is a single number rating of a material’s or assembly’s barrier effect. A higher STC rating blocks more noise from transmitting through a wall. The NRC is a single number index for rating how absorptive a material is. A material with an NRC of .8 will absorb 80% of the sound that comes into contact with it and will reflect 20% of the sound back into space. A good sound wall is a sound-absorbing wall with a STC rating of 30 or more and a minimum NRC rating of .85.


ABSORPTION: A property of materials that allows a reduction in the amount of sound energy reflected. The introduction of an absorbent into the surfaces of a room will reduce the sound pressure level in that room by not reflecting all of the sound energy striking the room’s surfaces. The effect of absorption merely reduces the resultant sound level in the room produced by energy that has already entered the room.

A measure of the sound-absorbing ability of a surface. It is defined as the fraction of incident sound energy absorbed or otherwise not reflected by a surface. Unless otherwise specified, a diffuse sound field is assumed. The values at the sound-absorption coefficient usually range from about 0.01 for marble slate to almost 1.0 for long absorbing wedges oen used in anechoic rooms.

(1) The science of sound, including the generation, transmission, and effects of sound waves, both audible and inaudible. (2) The physical qualities of a room or other enclosure (such as size, shape, amount of noise) that determine the audibility and perception of speech and music within the room.

Damage to the hearing mechanism caused by a sudden burst of intense noise, or by a blast. The term usually implies a single traumatic event.

Sound that reaches the point of interest by propagation through air.

AMBIENT NOISE: The total of all noise in the environment, other than the noise from the source of interest. This term is used interchangeably with background noise.

A room in which the boundaries absorb nearly all the incident sound, thereby, effectively creating free field conditions.

ANSI: The American National Standards Institute.

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

AUDIO FREQUENCY: The frequency of oscillation of an audible sound wave. Any frequency between 20 and 20,000 Hz.

AUDIOGRAM: A graph showing individual hearing acuity as a function of frequency.

An instrument for measuring individual hearing acuity.

A-WEIGHTED SOUND LEVEL: A measure of sound pressure level designed to reflect the acuity of the human ear, which does not respond equally to all frequencies. The ear is less efficient at low and high frequencies than at medium or speech-range frequencies. Therefore, to describe a sound containing a wide range of frequencies in a manner representative of the ear’s response, it is necessary to reduce the effects of the low and high frequencies with respect to the medium frequencies. The resultant sound level is said to be A-weighted, and the units are dBA. The A-weighted sound level is also called the noise level. Sound level meters have an A-weighting network for measuring A-weighted sound level.

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 oen used to mean the noise in the environment, other than the noise from the source of interest.

. Any segment of the frequency spectrum.

CALIBRATOR (ACOUSTICAL). A device which produces a known sound pressure on the microphone of a sound level measurement system, and is used to adjust the system to Standard specifications.

COCHLEA. A spirally coiled organ located within the inner ear which contains the receptor organs essential to hearing.

A wave in which the surfaces of constant phase are coaxial cylinders. A line of closely-spaced sound sources radiating into an open space produces a free sound field of cylindrical waves.

DAMPING. The dissipation of energy with time or distance. The term is generally applied to the attenuation of sound in a structure owing to the internal sound-dissipative properties of the structure or to the addition of sound-dissipative materials.

dBA. Unit of sound level. The weighted sound pressure level by the use of the A metering characteristic and weighting specified in ANSI Specifications for Sound Level Metere,

DECIBEL. A unit of sound pressure level, abbreviated dB.

. A modification which soundwaves undergo in passing by the edges of solid bodies.

DIRECTIVITY INDEX. In a given direction from a sound source, the difference in decibels between (a) the sound pressure level produced by the source in that direction, and (b) the space-average sound pressure level of that source, measured at the same distance.

DOPPLER EFFECT (DOPPLER SHI). The apparent upward shi in frequency of a sound as a noise source approaches the listener or the apparent downward shi when the noise source recedes. The classic example is the change in pitch of a railroad whistle as the locomotive approaches and passes by.

. A device worn by a worker for determining the worker’s accumulated noise exposure with regard to level and time according to a pre-determined integration formula.

ECHO. A wave that has been reflected or otherwise returned with sufficient magnitude and delay, so as to be detected as a wave distinct from that directly transmitted.

* Effective Percieved Noise levels This is the result of applying tone and duration corections to the PERCIEVED Noise Level based on one third octive band data

The constant sound level that, in a given time period, would convey the same sound energy as the actual time-varying A-weighted sound level.

FREQUENCY: The number of times per second that the sine wave of sound repeats itself, or that the sine wave of a vibrating object repeats itself. Now expressed in hertz (Hz), formerly in cycles per second (cps).

Sensory cells in the cochlea which transform the mechanical energy of sound into nerve impulses.

HEARING: The subjective human response to sound.

A measured threshold of hearing at a specified frequency, expressed in decibels relative to a specified standard of normal hearing. The deviation in decibels of an individual’s threshold from the zero reference of the audiometer.

A term denoting an impairment of auditory acuity. The amount of hearing impairment, in decibels, measured as a set of hearing threshold levels at specified frequencies. Types of hearing loss are: 1. Conductive: A loss originating in the conductive mechanism of the ear; 2. Sensor-neural: A loss originating in the cochlea or the fibers of the auditory nerve; 3. Noise induced: A sensor-neural loss attributed to the effects of noise.

HEARING THRESHOLD LEVEL (HTL). Amount (in decibels) by which an individual’s threshold of audibility differs from a standard audiometric threshold.

HERTZ (Hz): Unit of measurement of frequency, numerically equal to cycles per second

* IMPULSE NOISE pilr drivers punch presses transient noise vehical and aircra pure tones blowers electrical generators, power transformers

INFRASONIC: Sounds of a frequency lower than 20 hertz.

INTENSITY: The sound energy flow through a unit area in a unit time.

A description of the acoustic wave behavior in which the mean-square pressure varies inversely with the square of the distance from the source. This behavior occurs in free field situations, where the sound pressure level decreases 6 dB with each doubling of distance from the source.
ISO: The International Organization for Standardization.

LEVEL: The logarithm of the ratio of a quantity to a reference quantity of the same kind. The base of the logarithm, the reference quantity, and the kind of level must be specified.

LOGARITHM: The exponent that indicates the power to which a number must be raised to produce a given number. For example, for the base 10 logarithm, used in acoustics, 2 is the logarithm of 100.

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.
Measured in phons it is numerically equal to the median sound pressure level (dB) of a free progressive 1000 Hz wave presented to listeners facing the source, which in a number of trials is judged by the listeners to be equally loud.

MASKING: 1. The process by which the threshold of audibility for a sound is raised by the presence of another (masking) sound. 2. The amount by which the threshold of audibility of a sound is raised by the presence of another (masking) sound.

A noise that is intense enough to render inaudible or unintelligible another sound that is also present.

MEDIUM: A substance carrying a sound wave.

NEAR FIELD: The sound field very near to a source, where the sound pressure does not obey the inverse-square law and the particle velocity is not in phase with the sound pressure.

NOISE: 1. Unwanted sound. 2. Any sound not occurring in the natural environment, such as sounds emanating from aircra, highways, industrial, commercial and residential sources. 3. An erratic, intermittent, or statistically random oscillation.

NOISE ISOLATION CLASS. (NIC): A single number rating derived in a prescribed manner from the measured values of noise reduction between two areas or rooms. It provides an evaluation of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.

NOISE LEVEL: For airborne sound , unless specified to the contrary, it is the A-weighted sound level.

NOISE REDUCTION (NR): The numerical difference, in decibels, of the average sound pressure levels in two areas or rooms. A measurement of “noise reduction” combines the effect of the sound transmission loss performance of structures separating the two areas or rooms, plus the effect of acoustic absorption present in the receiving room.

NOISE REDUCTION COEFFICIENT (NRC): A measure of the acoustical absorption performance of a material, calculated by averaging its sound absorption coefficients at 250, 500, 1000 and 2000 Hz, expressed to the nearest multiple of 0.05.

NON-IMPULSIVE NOISE: all noise not included in the definition of impulsive noise.

OSCILLATION: The variation with time, alternately increasing and decreasing, of (a) some feature of an audible sound, such as the sound pressure; or (b) some feature of a vibrating solid object, such as the displacement of its surface.

OSHA: The Occupational Safety and Health Administration.

: The maximum absolute value of the instantaneous sound pressure in a specific time interval. Note: in the case of a periodic wave, if the time interval considered is a complete period, the peak sound pressure becomes identical with the maximum sound pressure.

PERIOD. The duration of time it takes for a periodic wave form (like a sine wave) to repeat itself.

PERMANENT THRESHOLD SHI (PTS). A permanent decrease of the acuity of the ear at a specified frequency as compared to a previously established reference level. The amount of permanent threshold shi is customarily expressed in decibels.

PHON. The unit of measurement for loudnes level.

PITCH. The attribute of auditory sensation that orders sounds on a scale extending from low to high. Pitch depends primarily upon the frequency of the sound stimulus, but it also depends upon the sound pressure and wave form of the stimulus.

. An oscillation whose instantaneous magnitude is not specified for any given instant of time. It can be described statistically by probability distribution functions giving the traction of the total time that the magnitude of the noise lies within a specified range.

REFLECTION. The return of a sound wave from a  surface.

REFRACTION. The bending of a sound wave from its original path, either because it is passing from one medium to another or by changes in the physical properties of the medium, e.g., a temperature or wind gradient in the air.

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.

. A device that resounds or vibrates in sympathy with a source of sound or vibration.

REVERBERATION. The persistence of sound in an enclosed space, as a result of multiple reflections, aer the sound source has stopped.

The square root of the time averaged square of the sound pressure.

SABIN. A measure of the sound absorption of a surface; it is the equivalent of one square foot of a perfectly absorptive surface.

SHIELDING. The attenuation of a sound, achieved by placing barriers between a sound source and the receiver

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.

SOCIOCUSIS. Loss of hearing caused by noise exposures that are part of the social environment, exclusive of occupational-noise exposure, physiological changes with age, and disease.

1. An oscillation in pressure, stress, particle displacement, particle velocity, etc., in an elastic or partially elastic medium, or the superposition of such propagated alterations. 2. An auditory sensation evoked by the oscillation described above. Not all sound waves can evoke an auditory sensation: e.g. ultrasound.

. The weighted sound pressure level obtained by the use of a sound level meter and frequency weighting network, such as A, B, or C as specified in ANSI specifications for sound level meters (ANSI Sl.4-1971, or the latest approved revision). If the frequency weighting employed is not indicated, the A-weighting is implied.

An instrument comprised of a microphone, amplifier, output meter, and frequency-weighting networks which is used for the measurement of noise and sound levels.

SOUND POWER. The total sound energy radiated by a source per unit time. The unit of measurement is the watt.

SOUND PRESSURE. The instantaneous difference between the actual pressure produced by a sound wave and the average or barometric pressure at a given point in space.

SOUND PRESSURE LEVEL (SPL). 20 times the logarithm, to the base 10, of the ratio of the pressure of the sound measured to the reference pressure, which is 20 micronewtons per square meter. In equation form, sound pressure level in units of decibels is expressed as SPL (dB) = 20 log p/pr.

The preferred single figure rating system designed to give an estimate of the sound insulation properties of a structure or a rank ordering of a series of structures.

SOUND TRANSMISSION LOSS (STL). A measure of sound insulation provided by a structural configuration. Expressed in decibels, it is 10 times the logarithm to the base 10 of the reciprocal of the sound transmission coefficient of the configuration.

SPECTRUM. The description of a sound wave’s resolution into its components of frequency and amplitude.

SPEED (VELOCITY) OF SOUND IN AIR. 344 m/sec (l128 /sec) at 70 degrees F in air at sea level.

. Sounds whose average characteristics remain relatively constant in time. A practical example of a steady-state sound source is an air conditioning unit.

. A temporary impairment of hearing acuity as indicated by a change in the threshold of audibility.

The minimum sound pressure level at which a person can hear a specified frequency of sound over a specified number of trials.

THRESHOLD OF PAIN. The minimum sound pressure level of a sound outside the ear that will produce a transition from discomfort to definite pain.

THRESHOLD SHI. A change in the threshold of audibility at a specified frequency from a threshold previously established. The amount of threshold shi is customarily expressed in decibels.

TINNITUS. Ringing in the ear or noise sensed in the head. Onset may be due to an acoustic trauma and persist in the absence of acoustical stimulation (in which case it may indicate a lesion of the auditory system).

TONE. A sound of definite pitch. A pure tone has a sinusoidal wave form.

ULTRASONIC. Sounds or a frequency higher than 20,000 hertz.

VIBRATION. An oscillatory motion of solid bodies described by displacement, velocity, or acceleration with respect to a given reference point.

VIBRATION ISOLATOR. A resilient support for vibrating equipment designed to reduce the amount of vibration transmitted to the other structures.

WAVE. A disturbance that travels through a medium by virtue of the elastic properties of that medium.

WAVELENGTH. For a periodic wave (such as sound in air), the distance between analogous points on any two successive waves. The wavelength of sound in air or in water is inversely proportional to the frequency of the sound. Thus, the lower the frequency, the longer the wavelength.

WEIGHTING.: Prescribed frequency filtering provided in a sound level meter.