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Manual TriggerA manual trigger is when the operator clicks on a button to initiate recording of data to disk or capture of time records for measurement.
Masked ThresholdThreshold of hearing measured in the presence of a masking sound (usually random noise of specified bandwidth).
MaskingThe process and amount by which the threshold of audibility of a signal at a particular frequency is raised by the presence of another signal at a frequency close by. The frequency range over which the masking effect extends is dependent on the SPL of the masking signal. Masking also occurs in the time domain when a short transient sound masks the audibility of other sounds both before and after the transient. The process by which the threshold of hearing of one sound is raised due to the presence of another sound (the “masker”).
Masking NoiseA noise that is intense enough to render inaudible or unintelligible another sound that is also present.
Mass LawRelationship of the doubling in mass or frequency to a 6 dB increase in threshold level for a uniform partition over a defined frequency range.
Mass MatrixIn modal testing, represents the inertial properties of the model.
Material TestingThis is the testing of the dynamic properties of materials, leading to quieter, stronger, more reliable manufactured products by minimising the effects of resonances. The main application areas are: the automotive and aerospace industries, consumer products and household goods, process machinery, machine tools and packaging machinery.
Mean Free PathFor sound waves in an enclosure, it is the average distance travelled between successive reflections.
MecatronicsThe intelligent integration of mechanical engineering, micro/electronics and computer control in product and process design. The main application areas are in consumer products (CD players, auto-focus cameras, disk drives, etc.) and process machinery (robotics, computer controlled lathes, textile machines, etc.).
Ratio of applied force to resulting velocity during simple harmonic excitation. Called driving point impedance if force and velocity are measured at the same point, otherwise called transfer impedance.
having units of
Dynamic stiffness: where all terms are phasors, having a magnitude and direction.
Mass = Force/Acceleration
MediumA substance carrying a sound wave.
MetricsA term used to describe objective psychoacoustic measurements such as those derived from Zwicker: Loudness, sharpness, roughness, and fluctuation strength.
MicrophoneAn acoustical-electrical transducer by which sound waves in air may be converted to electrical signals.
Middle earThe cavity between the eardrum and the cochlea housing the ossicles connecting the eardrum to the oval window of the cochlea.
MilliradianThis is one thousandth of a radian. A radian is an angle whose subtended arc is equal to the radius at which the arc is measured. It amounts to about 57.3 degrees. There are 2 radians in a circle. A unit (normally metric) used to describe the angle of one machine centreline to the other. It is the equivalent to 1 mils/inch. It can also be expressed as rise/run. (1 unit = 17.45 milliradians).
MillisecondOne thousandth of a second, abbreviated to ms.
MilsA unit of measure for displacement (thousandths of an inch). Usually measured in mils peak to peak, which represents total displacement.
Mils/InchA unit (normally English) used to describe the angle of one shaft centreline to the other. It is equivalent to milliradians. It can also be expressed as rise/run (1 unit = 17.45 mils/inch), as long as the rise is measured in mils and the run is measured in inches.
MixerA device, sometimes very elaborate, that is used for combining signals from many sources.
MobilityMechanical admittance. The frequency response function of velocity/force. Mobility is the inverse of mechanical impedance. It is a measure of the ease with which a structure is able to move in response to an applied force, and varies it with frequency. The vibration measured at a point on a machine is the result of vibratory forces acting somewhere in the machine. The magnitude of the vibration is equal to the magnitude of the force times the mobility of the structure. From this it follows that the amplitude of the destructive forces acting on a machine are not determined directly by measuring its vibration if the mobility of the machine is not known. For this reason, it is a good idea to measure the mobility at the bearings of a machine in order to find out the levels of the forces acting on the bearings due to imbalance or misalignment. See also Impedance, Mechanical.
Modal AnalysisThe process of determining a set of generalised co-ordinates for a system such that the equations of motion are both inertially and elastically uncoupled. More commonly, it is a process of determining the natural frequencies, damping factors, and mode shapes for a structure. This is usually done either experimentally through frequency response testing or mathematically using finite element analysis.
Modal Damping (or decay rate)Describes the rate at which a structure dissipates vibrational energy.
Modal DensityDescribes how closely the modal frequencies are spaced in a structure.
Modal FrequencyThe natural frequency at which a structure vibrates in a free response with no external forces.
Modal MassThe scaling constant that relates Mode Shapes (which are intrinsically unscaled) and Residues.
Modal MatrixA collection of all Mode Shapes gathered together as columns.
Modal ParametersA set of parameters necessary to quantify the modal properties of a structure, for example, Modal Frequency, Modal Damping, etc.
Modal Property (or Modal Behaviour)The rather surprising observation that all structures, regardless of their geometrical continuity (properties) behave in a discrete sense, that is, they only seem to absorb energy at particular frequencies, the frequencies at which they vibrate and, in turn, dissipate energy.
Modal TestingThe term is synonymous with Experimental Modal Analysis. This requires a physical structure, typically a prototype or scale model. For example, using “shaker excitation”, the structure is excited by a measurable force and the associated response is observed over a representative number of Degrees of Freedom (DOFs). Frequency response functions are estimated between excitation and response DOFs, and form the data set from which the modal parameters are estimated. The parameter estimation is made using the curve fitting technique, where the modal parameters of the underlying theoretical model are estimated using a least squares technique. See also Modal Analysis.
ModeA room resonance. Axial modes in rectangular rooms are associated with pairs of parallel walls. Tangential modes involve four room surfaces and oblique modes all six surfaces. Their effect is greatest at low frequencies and for small rooms.
Mode ShapeThe relative position of all points on a structure at a given natural frequency.
Model CorrelationThis structural testing technique involves testing an analytical model by comparing it directly with an experimental model.
Model ReductionIn this structural analysis technique, the number of degrees of freedom is reduced in the analytical model to fit the number of degrees of freedom used for experimental models.
Model UpdateWhere an analytical model is adjusted (improved) by applying the data from an experimental model.
Model VerificationFor experimental models, reciprocity measurements are used where synthesised non-measured frequency response functions are compared to measurements in order to verify the validity of a model.
Monaural Sound RecordingA sound recording in which only one channel is used. If there are facilities for recording two channels (stereo or binaural sound) but only one channel is chosen, then the left channel is usually chosen as the default.
MountStructure that isolates/separates active source (engine, etc.) from passive structure.
Mount Stiffness MethodA method of analysing structure-borne contributions, wherein the operational forces are estimated using displacement and mount stiffness. Also referred to as displacement method, stiffness method, complex stiffness method, force vector method, or connection stiffness method.
Multi-degree-of-freedomA system where more than one input can vary with time.
Multi-orderWhere there are several predominant frequency orders present.
Multi-pass AnalysisAn analysis based on using a number of passes to record the measurement data.
Multi-spectrumA one- or two-dimensional array of spectra. A multispectrum consists of two or more spectra that were recorded during the same measurement.
Multi-trackA system of recording multiple tracks on magnetic tape or other media. The signals recorded on the various tracks are then “mixed down” to obtain the final recording.
Multi-value GraphA type of graph that allows the overall levels for a number of different functions to be displayed side by side.
Multiple-degree-of-freedom SystemAn N-degree-of-freedom system is a system whose position in space can be completely described by N co-ordinates or independent variables. This is the generalised structural model.
Multiple ExcitationIn structural dynamic testing, for multiple input techniques, two or more exciters act simultaneously. The setup must be such that the forces remain uncorrelated. The measurement configuration must be designed so that the mechanical coupling between the exciters is minimal.
MultisineA periodic signal where the lines in the spectrum have the same magnitude. The phase increases with the square of the frequency. This minimises the crest factor.Brüel & Kjær Sound & Vibration Dictionary End User Agreement