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Damped Natural Frequency
The damped natural frequency is the frequency at which a damped system will oscillate in a free vibration situation.Damping
Any means of dissipating vibration energy within a vibrating system.
- 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.
- The action of frictional or dissipative forces on a dynamic system causing the system to lose energy and reduce the amplitude of movement.
- Removal of echoes and reverberation by the use of sound absorbing materials. See also Reverberation Time.
Damping Ratio
Ratio of actual damping to Critical Damping, at a resonant frequency. Also, the ratio of the decrease in signal amplitude as a function of time.DAT Recorder
Digital audio tape recorder.Data Source
The identity and location of the data that are used in an analysis.Day-Night Average Sound Level (Ldn)
A-weighted equivalent continuous sound exposure level for a 24-hour period with a 10 dB adjustment added to the sound levels occurring during nighttime hours (10 p.m. to 7 a.m.).dB
decibel. A bel (after Alexander Graham Bell) is defined as the logarithm to base ten of the ratio of two acoustical powers, or intensities. One tenth of a bel, the decibel, is the generally used unit. See Decibel.dBA, dB(A)
A sound-level meter reading with an A-weighting network simulating the human-ear response at a loudness level of 40 phons. The weighting is specified in ANSI Specifications for Sound Level Meter, S1.4-1983DC Coupling
The connection of a signal from one circuit to another in a manner that passes both AC and DC components. See also AC Coupling.DC Offset
The change in input voltage required to produce zero output voltage when no signal is applied to an amplifier.Decade
Ten times any quantity or frequency range. The range of the human ear is about 3 decades.Decay Rate
A measure of the decay of acoustical signals, expressed as a slope in dB/second. The rate at which a signal drops off.Decibel (dB)
The primary unit of sound measurement; used to quantify both sound pressure level and sound power level. Used for measuring the relative magnitude based on a logarithmic scale. See dB and Sound Pressure Level.
Quantity |
Symbol |
Formula |
Ref. Level* |
|
Sound Pressure Level |
Lp |
20 log(p/p0) dB |
20 µPa (in air)
1 µPa (in water) |
|
Acceleration Level |
La |
20 log(a/a0) dB |
1 µm/s2 |
|
Velocity Level |
Lv |
20 log(v/v0) dB |
1 nm/s |
|
Force Level |
LF |
20 log(F/F0) dB |
1 µN |
|
Power Level |
LW |
10 log(P/P0) dB |
1 pW |
|
Intensity Level |
LI |
10 log(I/I0) dB |
1 pW/m2 |
|
Energy Density Level |
Lw |
10 log(w/w0) dB |
1 pJ/m3 |
|
Energy Level |
LE |
10 log(E/E0) dB |
1 pJ |
|
* p0, a0, etc. |
Reference: ISO 1683-2008 - SI Units Decibel Scale
A linear numbering scale used to define a logarithmic amplitude scale, thereby compressing a wide range of amplitude values to a small set of numbers.Degree of Freedom
In a mechanical system, equals the minimum number of independent co-ordinates required to completely define the position of all parts of the system at any instant of time. Refers to a point and a direction in which a structure vibrates, or in which the vibration is measured.Delay
An entered time difference that has to elapse after (or before) triggering of a measurement and sampling of the time history data begins. There are situations where entering a trigger delay is useful.Delay Compensation
Compensation for the delay on the finite detector response time on a swept frequency analysis compared with the true result, which would be achieved with an infinitely slow rate of sweep.Delay Line
A digital, analog, or mechanical device employed to delay one audio signal with respect to another.Detector
An electronic circuit that determines the amplitude level of a signal in accordance with certain rules. The simplest type of detector consists of a resistor and a capacitor, and it measures the rectified average value of a fluctuating DC signal. A more complex, but much more useful type of detector is an RMS detector whose output is proportional to the power or energy present in the signal or a vibration.Deterministic
A type of signal whose spectrum consists of a collection of discrete components, as opposed to a random signal, whose spectrum is spread out or “smeared” in frequency. Some deterministic signals are periodic, and their spectra consist of harmonic series. Vibration signatures of machines are in general deterministic, containing one or more harmonic series, but they always have non-deterministic components, such as background noise. See also Stationary Signal. Transients can also be deterministic.DFCF
Diffuse field correction filter.DHCP
Dynamic Host Configuration Protocol.Diaphragm
Any surface that vibrates in response to sound or is vibrated to emit sound, such as in microphones and loudspeakers. Also applied to wall and floor surfaces vibrating in response to sound or in transmitting sound.Dielectric
An insulating material. The material between the plates of a capacitor.Differentiation
In vibration analysis, differentiation is a mathematical operation that converts a displacement signature to a velocity signature, or a velocity signature to an acceleration signature. It is performed electronically on an analog signal or can be performed digitally on a spectrum. Differentiation is an inherently noisy operation, if performed on an analog signal, adding a significant amount of high-frequency noise to the signal, and is generally not used very much in machinery vibration analysis. It is not inherently noisy if it is done digitally on the FFT spectrum. See also Integration, which is the inverse of differentiation.Diffraction
The distortion of a wave front caused by the presence of an obstacle in the sound field. The scattering of radiation at an object smaller than one wavelength and the subsequent interference of the scattered wave fronts.Diffuse Field
A sound field in which the sound pressure level is the same everywhere and the flow of energy is equally probable in all directions.Diffuse Sound
Sound that is completely random in phase; sound that appears to have no single source.Diffusion
The act of sound waves spreading out over a wide area, reflecting off a convex or other uneven surface.Diffusor
(Diffuser) A proprietary device for the diffusion of sound through reflection-phase-grating means.Digital
Digital instrumentation consists of devices that convert analog signals into a series of numbers through a sampling process and an analog to digital converter. They then perform operations on the numbers to achieve such effects as equalisation, data storage, data compression, frequency analysis, etc. This process in general is called digital signal processing. It is characterised by several advantages and disadvantages. One advantage is that the converted signals can be manipulated, transformed and copied without introducing any added noise or distortion. The disadvantage is that the signal representation may not be truly representative of the original signal. Digital signal processing requires analog to digital (A/D) conversion of the input signal. The first step in A/D conversion is sampling of the instantaneous amplitudes of the signal at specific times determined by the sampling rate. If the signal contains any information at frequencies above one-half the sampling frequency, the signal will not be sampled correctly, and the sampled version of the signal will contain spurious components. This is called aliasing. The theoretical maximum frequency that can be correctly sampled is equal to one-half the sampling rate, and is called the Nyquist frequency. In all digital signal processing systems, including FFT analyzers, the sampling rate is made to be significantly greater than twice the highest frequency present in the signal in order to be certain the aliasing will not occur.Digital Filter
A digital processor that receives a sequence of input data values, executes an operation on them, and outputs a corresponding sequence of values that have been filtered with respect to the input.Digital Filter Analyzer
Uses constant percentage (or relative) bandwidth resolution. This is often preferred for acoustic measurements because it best simulates the way in which the human ear perceives sound.Digital Signal Processing (DSP)
DSP is the analysis of digital signal data. The original analog signal is sampled at regular time intervals, and an Anolog to Digital converter converts the sampled amplitudes into a number series.Dimensionless Frequency Domain Functions
see Coherence.Direct Field
The region in which the sound measured can be attributed to the source alone without the effects of obstructions, walls, etc.Directivity Factor
The ratio of the mean-square pressure (or intensity) on the axis of a transducer at a certain distance to the mean-square pressure (or intensity) that a spherical source radiating the same power would produce at that point.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.Directivity Pattern
A graphical description, usually in polar co-ordinates, of the response of the transducer as a function of the direction of the transmitted or incident sound waves in a specified plane and at a specified frequency.Discrete
With reference to a spectrum, discrete means consisting of separate distinct points, rather than continuous. An example of a discrete spectrum is a harmonic series. An FFT spectrum, which consists of information only at specific frequencies (the FFT lines), is actually discrete regardless of the input signal. For instance, the true spectrum of a transient is continuous, and the FFT of a transient appears continuous on the screen, but still only contains information at the frequencies of the FFT lines. The input signal to an FFT analyzer is continuous, but the sampling process necessary to implement the FFT algorithm converts it into a discrete form, with information only at the specific sampled times.The mathematical calculation that converts or “transforms” a sampled and digitised waveform into a sampled spectrum. The fast Fourier transform, or FFT, is an algorithm that allows a computer to calculate the discrete Fourier transform very quickly. See also Fast Fourier Transform.
Displacement
A vector quantity that specifies the change of position of a body. Usually measured from the rest position.Displacement, Velocity, Acceleration Relationships
(for sinusoidal motion only)
- do = peak displacement
- D = pk-pk displacement
- f = frequency in Hz
- t = 1/f seconds = period in seconds
- g = 9.80665 m/s2 = 386.09 in/s2 = 32.174 ft/s2
- G = acceleration in g units
Displacement: 
Velocity: 
Acceleration: 
G = a/g
vo = 6.28 f do = 3.14 f D
vo = 61.42 
in/s pk = 1.560 m/s pk
do = 9.780 
inches pk; or G = 0.0511 f2 D (for D in inches pk-pk)
=0.2484 metres pk; or G = 2.013 f2 D (for D in metres pk-pk)Distance Double Law
In pure spherical divergence of sound from a point source in free space, the sound pressure level decreases 6 dB for each doubling of the distance. This condition is rarely encountered in practice, but it is a handy rule to remember in estimating sound changes with distance.Distortion
Any change in the waveform or harmonic content of an original signal as it passes through a device. The result of non-linearity within the device.Distortion, Harmonic
Changing the harmonic content of a signal by passing it through a non-linear device.Dodd Bars
A secondary alignment method. Consists of two bars that are similar in configuration to reverse dial indicator bars. However, these bars are not mounted on the shaft, they are mounted to the machine. Each bar is fitted with a proximity probe and it corresponds to a block on the other bar. As the machines move to their on-line condition the gap between the proximity probes and the metal blocks changes, which changes the voltages. The analyzer converts the voltages to distances from which the alignment corrections can be calculated.Domain
A domain is a set of co-ordinates in which a mathematical function resides. A waveform, for instance, has dimensions of amplitude and time, and it is said to exist in the time domain, while a spectrum has dimensions of amplitude and frequency, and is said to exist in the frequency domain.Doppler Effect (Doppler Shift)
The apparent upward shift in frequency of a sound as a noise source approaches the listener or the apparent downward shift when the noise source recedes. The classic example is the decrease in pitch of a railroad whistle as a locomotive passes by.Dosimeter
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.Driving Point Measurement
A frequency response measurement where the excitation point and direction are the same as the response point and direction.Dynamic Capability (for Sound Intensity)
The dynamic capability of an Intensity measurement system is determined by adding normally 7 or 10 dB (for a measuring error less than 1 or 0.5 dB, respectively) to the pressure-Residual Intensity Index.Dynamic Mass
Ratio of applied force to resulting acceleration during simple harmonic motion.Dynamic Modulus
Ratio of stress to strain under vibratory conditions.Dynamic Range
All audio systems are limited by inherent noise at low levels and by overload distortion at high levels. The usable region between these two extremes is the dynamic range of the system. Expressed in dB.Dynamic Stiffness
The frequency response function of force/displacement.Dyne
The force that will accelerate a 1 gram mass at the rate of 1 cm/s. The old standard reference level for sound pressure was 0.0002 dyne/cm2. The same level today is expressed as 20 micro pascals, or 20 µPa.Brüel & Kjær Sound & Vibration Dictionary End User Agreement
