ACOUSTICAL & VIBRATION TERMINOLOGY
For more information please contact the executive.
ACOUSTICAL TERMINOLOGY
ACOUSTICAL
Pertaining to the science of sound, including the generation, propagation, effects and control of both noise and vibration.
AMBIENT NOISE
The ambient noise level at a particular location is the overall environmental noise level caused by all noise sources in the area, both near and far, including all forms of traffic, factories, wind in the trees, birds, insects, animals, etc..
AUDIBLE
means that a sound can be heard. However, there are a wide range of audibility grades, varying from “barely audible” to “just audible”, “clearly audible” and “prominent”.
BACKGROUND NOISE LEVEL
Silence does not exist in the natural or built-environments, only varying degrees of noise. The Background Noise Level is the minimum repeatable dBA level of noise measured in the absence of the noise under investigation and any other short-term noises such as those caused by cicadas, lawnmowers, etc. It is quantified by the LA90 or the dBA noise level that is exceeded for 90 % of the measurement period. Background Noise Levels are determined for the day, evening and night time periods by statistically analysing (typically 15 minute) LA90 measurements over multiple (typically 7) days.
DECIBEL
The human ear has a vast sound-sensitivity range of over a thousand billion to one. The decibel is a logarithmic unit that allows this same range to be compressed into a somewhat more comprehensible range of 0 to 120 dB. The decibel is ten times the logarithm of the ratio of a sound level to a reference sound level. See also Sound Pressure Level and Sound Power Level.
Decibel noise levels cannot be added arithmetically since they are logarithmic numbers. If one machine is generating a noise level of 50 dBA, and another similar machine is placed beside it, the level will increase to 53 dBA, not 100 dBA. Ten similar machines placed side by side increase the sound level by 10 dBA, and one hundred machines increase the sound level by 20 dBA.
dBC
The dBC scale of a sound level meter is similar to the dBA scale defined above, except that at high sound intensity levels, the human ear frequency response is more linear. The dBC scale approximates the 100 phon equal loudness contour.
dBA
The human ear is less sensitive to low frequency sound than high frequency sound. We are most sensitive to high frequency sounds, such as a child’s scream. Sound level meters have an inbuilt weighting network, termed the dBA scale, that approximates the human loudness response at quiet sound levels (roughly approximates the 40 phon equal loudness contour).
EQUIVALENT CONTINUOUS NOISE LEVEL, LAeq
Many noises, such as road traffic or construction noise, vary continually in level over a period of time. More sophisticated sound level meters have an integrating electronic device inbuilt, which average the A weighted sound pressure levels over a period of time and then display the energy average or LAeq sound level. Because the decibel scale is a logarithmic ratio, the higher noise levels have far more sound energy, and therefore the LAeq level tends to indicate an average which is strongly influenced by short term, high level noise events. Many studies show that human reaction to level-varying sounds tends to relate closely to the LAeq noise level.
FREE FIELD
This is a sound field not subject to significant reflection of acoustical energy. A free field measurement is typically not closer than 4 metres to any large flat object such as a fence or wall; or inside an anechoic chamber.
FREQUENCY
The number of oscillations or cycles of a wave motion per unit time, the SI unit being the Hertz, or one cycle per second.
IMPACT ISOLATION CLASS (IIC)
The American Society for Testing and Materials (ASTM) has specified that the IIC of a floor/ceiling system shall be determined by operating an ISO 140 Standard Tapping Machine on the floor and measuring the noise generated in the room below. The IIC is a number found by fitting a reference curve to the measured octave band levels and then deducting the sound pressure level at 500 Hz from 110 decibels. Thus the higher the IIC, the better the impact sound isolation.
IMPACT SOUND INSULATION (LnT,w )
Australian Standard AS ISO 717.2 – 2004 has specified that the Impact Sound Insulation of a floor/ceiling system be quantified by operating an ISO 140 Standard Tapping Machine on the floor and measuring the noise generated in the room below. The Weighted Standardised Impact Sound Pressure Level (LnT,w ) is the sound pressure level at 500 Hz for a reference curve fitted to the measured octave band levels. Thus the lower LnT,w the better the impact sound insulation.
IMPULSE NOISE
An impulse noise is typified by a sudden rise time and a rapid sound decay, such as a hammer blow, rifle shot or balloon burst.
LOUDNESS
The degree to which a sound is audible to a listener is termed the loudness. The human ear perceives a 10 dBA noise level increase as a doubling of loudness and a 20 dBA noise increase as a quadrupling of the loudness.
VIBRATION TERMINOLOGY
AMPLITUDE
The measurement of energy or movement in a vibrating object. Amplitude is measured and expressed in three ways: Displacement (commonly in mm Pk-Pk); Velocity (commonly in mm/s Pk); and Acceleration (commonly in m/s2 RMS). Amplitude is also the y-axis of the vibration time waveform and spectrum, it helps define the severity of the vibration.
DAMPING
Energy dissipation in an oscillating structure. For free vibration, that results in a decay in the amplitude of motion over time.
DYNAMIC STIFFNESS
The frequency response function of force/displacement.
FAST FOURIER TRANSFORM (FFT)
The FFT is an algorithm, or digital calculation routine, that efficiently calculates the discrete Fourier transform from the sampled time waveform. In other words it converts, or “transforms” a signal from the time domain into the frequency domain.
FINITE ELEMENT ANALYSIS or MODELLING
A computer-aided design technique for mathematically modelling a structure. Finite element modelling is used for structure analysis and modal analysis.
FREQUENCY
The repetition rate of a periodic vibration, per unit of time, determined by taking the reciprocal of the period (T). Frequency is expressed in three ways: Hz (how many cycles per second); Frequency is also the x-axis of the vibration spectrum.
FREQUENCY RESPONSE
The is a characteristic of a system which has a measured response resulting from a known applied input. In a mechanical structure, the frequency response function, also called the FRF, is the spectrum of the vibration of the structure divided by the spectrum of the input force to the system. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response.
HARMONIC
A frequency that is an integer multiple of a given frequency.
HERTZ (Hz)
Vibration can occur over a range of frequencies extending from the very low, such as the rumble of thunder, up to the very high such as the crash of cymbals. The frequency of vibration and sound is measured in hertz (Hz). Once hertz is one cycle per second. Structural Vibration is generally measured over the frequency range from 1Hz to 500Hz (0.5kHz).
IMPACT TESTING
A method of measuring the frequency response function of a structure by hitting it with a calibrated hammer and measuring the system’s response. The impact hammer is instrumented with a load cell to measure the input force pulse while the response is typically measured using an accelerometer. The impact imparts a force pulse to the structure which excites it over a broad frequency range.
NATURAL FREQUENCY
The frequency of oscillation of the free vibration of a system.
PEAK TO PEAK (Pk-Pk)
This is the measure of the vibration amplitude, maximum to minimum, equal to twice of Ö2 times the RMS value of a sine wave.
PEAK PARTICLE VELOCITY (PPV)
Vibration velocity can be measured in a number of ways. For some projects vibration levels can be given in terms of Peak Particle Velocity (PPV).
PEAK VECTOR SUM
The resultant particle velocity magnitude or vector sum of the transverse, vertical and longitudinal particle velocity components.
RESONANCE
When a forcing frequency is the same as a resonant frequency of the structure, the structure is said to be in resonance.
RMS VELOCITY
For most applications where there is continuous vibration, vibration is measured in terms of root mean square RMS velocity (mm/sec).
SPECTRUM
The spectrum is the result of transforming a time domain signal to the frequency domain. Spectrum analysis is the procedure of doing the transformation, and it is most commonly done with an FFT analyzer.
|