Acoustics & Vibration Fundamentals

FUNDAMENTAL Concepts and Terminology

Our acoustics and vibration consulting encompasses a lot of complex technical analyses and designs. Bearing this in mind, we deliver our projects in plain English so our clients can understand the results we provide.

This section presents basic concepts to aid understanding of fundamental acoustics. Also check below the chart for a glossary of acoustics and vibration terms.

Common Outdoor and Indoor Sound Levels

Changes in the instantaneous noise level on the order of 3 decibels are barely noticeable to most people. However, this concept does not apply to the Ldn (Day Night Average Noise Level). The Ldn is an average of noise levels over 24 hours with a 10 dBA factor added to nighttime sounds to account for people's increased sensitivity to noise during the night. A doubling of the number of noise events that occur within a 24 hour period results in a 3 dBA increase (as long as the hourly distribution of events does not change), and people certainly notice a doubling of noise events.

GLOSSARY OF ACOUSTICS & VIBRATION TERMS

A-Weighted Sound Level (dBA): A number representing the sound level which is frequency weighted according to a prescribed frequency response established by the American National Standards Institute (ANSI S1.4-1971) and accounts for the response of the human ear.

Accelerometer: A vibration sensitive transducer that responds to the vibration acceleration of a surface to which it is attached. The electronic signal generated by an accelerometer is directly proportional to the surface acceleration.

Acceleration Level: Also referred to as "vibration acceleration level." Vibration acceleration is the rate of change of speed and direction of a vibration. An accelerometer generates an electronic signal that is proportional to the vibration acceleration of the surface to which it is attached. The acceleration level is 20 times the logarithm to the base 10 of the ratio of the RMS value of the acceleration to a reference acceleration. The generally accepted reference vibration acceleration is 10^-6g(10^-5 m/sec²).

Acoustics - The science of sound including the generation, transmission, and effects of sound waves, both audible and inaudible.

Ambient Noise Level - That sound level that exists at any instant regardless of source.

Background Noise - The total of all noise in a system or situation, independent of the presence of the desired sound.

Barrier Insertion Loss - The difference in the sound level at a particular location with and without a noise barrier.

Community Noise Equivalent Level (CNEL) - The A-weighted acoustical energy during 24 hours with weightings of 5 dB for the evening hours (7 p.m. to 10 p.m.) and 10 dB for nighttime hours (10 p.m. to 7 a.m.).

Decibel (dB) - A unit of measurement on a logarithmic scale which describes the magnitude of a particular quantity of sound pressure or power with respect to a standard reference value.

Flanking Path - The transmission of sound from a source to an adjacent receiving room by paths other than the common partition.

Frequency - The time rate (number of times per second) that the wave of sound repeats itself, or that a vibrating object repeats itself--now expressed in Hertz (Hz), formerly in cycles per second (cps).

Inverse Square Law - That acoustical situation where the mean square sound pressure decreases in inverse proportion to the square of the distance from the source. This amounts to a decrease of 6 decibels with each doubling of distance from a point source.

Level - The value of an acoustical quantity in decibels.

L₁, L₁₀, L₅₀, L₉₀, L₉₉: The noise (or vibration) levels that are exceeded for 1%, 10%, 50%, 90%, and 99% of a specified time period, respectively.

L₉₉ and L₉₀, represent the general background or residual noise level observed during a measurement period. The background or residual noise is the cumulative effect of distant sound that can be observed in periods of relative quiet and absence of nearby noise sources–for example, during lulls in local traffic. The most prevalent source of this residual noise is distant street and highway traffic and is not strongly influenced by occasional local motor vehicle passbys. However, L₉₉ and L₉₀ can be influenced by nearby stationary sources such as air conditioning equipment. L₅₀ represents a long term statistical average or median sound level over the measurement period and reveals the long-term influence of local traffic. L₁₀ describes the average peak or maximum sound level occurring, for example, during nearby passbys of trucks, buses, automobiles, trains, or airplanes. L₁ represents the occasional maximum or peak sound level that occurs in an area.

L_dn - Sound level, day, night. The A-weighted acoustical energy during 24 hours with the nighttime level from 2200 to 0700 hours weighted by 10 dB.

L_eq - The equivalent steady state sound level which in a stated period of time would contain the same acoustical energy as time-varying sound level during the same period.

Loudness - The judgment of intensity of a sound by a human being.

Masking - The action of bringing one sound (audible when heard alone) to inaudibility or to unintelligibility by the introduction of another sound.

Microphone - An electro-acoustic transducer that responds to sound waves and delivers essentially equivalent electrical waves.

Noise - Any sound that is undesirable because it interferes with speech and hearing, or is intense enough to damage hearing, or is otherwise annoying (unwanted sound).

Noise Attenuation - The reduction of a noise level from a source by such means as distance, ground effects, or shielding.

Noise Barrier - Any solid obstruction blocking the line of sight from a noise source to a listener.

Noise Reduction Coefficient (NRC) - Noise reduction coefficient is a measure of the acoustical absorption performance of a material calculated by averaging its sound absorption coefficients at 250 Hz, 500 Hz, 1000 Hz and 2000 Hz.

Octave and 1/3 Octave Bands - One octave is an interval between to sound frequencies that have a ratio of two. For example, the frequency range of 200 Hz to 400 Hz is one octave, as is the frequency range of 2000 Hz to 4000 Hz. An octave band is a frequency range that is one octave wide. A standard series of octaves is used in acoustics, and they are specified by their center frequencies. In acoustics, to increase resolution, the frequency content of a sound or vibration is often analyzed in terms of 1/3 octave bands, where each octave is divided into three 1/3 octave bands.

Receptor - A location where either a noise level measurement was made, or noise levels were calculated, or both.

Reverberant Field - The region in a room where the reflected sound dominates, as opposed to the region close to the noise source, where the direct sound dominates.

Reverberation - The persistence of sound in an enclosure or partially enclosed space after the source of sound has stopped; the persistence is a result of repeated reflection and/or scattering.

Shielding - The attenuation of a sound by placing walls, buildings, or other barriers between a sound source and the receiver.

Sound - The auditory sensation evoked by the compression and rarefaction of the air or other transmitting medium.

Sound Absorption Coefficient (a) - The absorption coefficient of a material is the ratio of the sound absorbed by the material to that absorbed by an equivalent area of open window. The absorption coefficient of a perfectly absorbing surface would be 1.0 while that for concrete or marble slate is approximately 0.01 (a perfect reflector would have an absorption of 0.00).

Sound Level (Noise Level) - The weighted sound pressure level obtained by use of a sound level meter having a standard frequency-filter.

Sound Level Meter - An instrument, comprising a microphone, an amplifier, an output display, and frequency weighting networks, that is used for the measurement of noise and sound levels in a specified manner.

Sound Pressure - (1) The minute fluctuations in atmospheric pressure that accompany the passage of a sound wave; the pressure fluctuation on the tympanic membrane are transmitted to the inner ear and give rise to the sensation of audible sound. (2) For a steady sound, the value of the sound pressure averaged over a period of time.

Sound Pressure Level - The root-mean-square value of the pressure fluctuations above and below atmospheric pressure due to a sound wave; expressed in decibels based on a reference pressure of 20 micropascals (2 x 10-5 newtons per square meter).

Sound Shadow - The acoustical equivalent of a light shadow. A receiver that cannot see the sound source is in a "shadow zone." A sound shadow is often partial because of diffraction effects.

Velocity Level - Also referred to as the "vibration velocity level." Vibration velocity is the rate of change of displacement of a vibration. The velocity level is 20 times the logarithm to the base 10 of the ratio of the RMS value of the velocity to the reference velocity.