ISO 9611:1996 pdf download – Acoustics – – Characterization of sources of structure-borne sound with respect to sound radiation from connected structures一 Measurement of velocity at the contact points of machinery when resiliently mounted.
6.2 Instrumentation for addition and subtraction of electrical signals
The electrical signals of the two accelerometers of a pair shall be added to obtain the translation, and sub tracted to obtain th. rotation of the machinery foot.
Addition and subtraction of electrical signals can be done by special analog devices or by general digital (FFT) equipment Before addition and subtraction of the signals, possible differences in sens4tivities and phase shifts of accelerometers and analog devices shaM be corrected. This hok true also for FFT analysers due to non-negligible phase shifts of the antaliasing filters. When th. total phase shift is less than O.1. the correction may be omitted
Th. subtraction shall be replaced by addition, and addition replaced by subtraction, if one of the accelerometers of a pair is mounted in a reverse direction.
6.3 AmplIfier, fitters and evil recorder
The equipment used for this purpose shall satisfy the requirements specified for type 0 or type 1 instrument m IEC 651 or IEC 804 with the microphone replaced by an accelerometer. The filler characteristics shall corrily with IEC 1260.
Th. output signals of the electronic addition device shall be arrlitied. filtered in one-third-octave bands (octave bands if appropriate), indicated as r.m.s. values, and presented as translational velocity levels L., or angular velocity levels Lt
6.4 CalibratIon
Before commencing a series of measurements, the entire measuring system shaM be calibrated for amplitude and phase at one or more freQuencies with the aid of. known reference acceleration source (the two accelerometers are mounted simultaneously on the calibration exciter). In addition, the meesurrig system shall be tested electrically over the ente frequency range of riterest at least every half year, In cases where the signals are not directty analysed but recorded on magnetic tape, en amplitude and phase calibration shall be carried out at the beginning and at the end of eech measurement series. Each track of magnetic tape shall contain at least two electrical and one mechanical vibration calibration signals.
NOTE 10 In the case when only translational velocities we to be measured (see annes F), it may not be useful to calibrat. the accelerometers in phase
7 Test procedure
7.1 Test of the arrangement
Before commencing the measurements, determine the lower and upper limiting frequencies ft and h for defining the frequency range of validity in accordance with annexes A and B.
If it is possible to remove en isoletor without disturb- wig the mounting of the machine, measure the one- third-octave velocity spectrum (or octave velocrty spectrum, if appropriate) of the particular support with and without the isolator. The difference shell be less than 2 dB. Results for one-thicd-octav bands (or octave bends) in wtiich a larger difference occurs shall riot be given in th test report.
If it is not possible to remove the isolator, carry out the following test Stop the machine arid excite the foundation with a continuous random signal, for example with the aid of an electrodynamic shaker. The velocity levels (six components, see 7.2) are measured at both mounting sides of the isolator.. The difference between corresponding components at both sides shall be at least 10dB.
For a test on background noise, see 5.1
7.2 Measurement of v.Iocity I.v.I (translation or angular).