ISO 9611 pdf download – Acoustics — Characterization of sources ofstructure-borne sound with respect to soundradiation from connected structures 一 Measurement of velocity at the contactpoints of machinery when resiliently mounted.
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 sensitivities and phase shifts of accelerometers and analog devices shall be corrected. This holds true also for FFT ana lysers due to non-negligible phase shifts of the antialiasing filters. When the total phase shift is less than 0,1, the correction may be omitted.
The 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, filters and lev•I r.cord•r
The equipment used for this purpose shall satisfy the requirements specified for type 0 or type ‘I instrument in IEC 651 or IEC 804 with the microphone repLaced by an accelerometer. The filter characteristics shall comply with IEC 1260.
The output signals of the electronic addition device shall be amplified, 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 L
6.4 Calibration
Before commencing a series of measurements, the entire measuring system shall be calibrated for amplitude and phase at one or more frequencies with the aid of a known reference acceleration source (the two accelerometers are mounted simultaneously on the calibration exciter). In addition, the measuring system shall be tested electrically over the entire frequency range of interest at least every half year. In cases where the signals are not directly analysed but recorded on magnetic tape, an amplitude and phase calibration shall be carried Out at the beginning and at the end of each 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 are to be measured (see annex F), it may not be useful to calibrate the accelerometers m phase.
7 Test procedure
7.1 Test of the arrangement
Before commencing the measurements, determine the lower and upper limiting frequencies ft end 12 for defining the frequency range of validity in accordance with annexes A and B.
During the tests, any possible influences of the test surroundings on the vibration levels of the machine supports should be considered. There are two causes of undesirable influences:
— structural connecUons other than the supports and the isolators, and
— directly radiated or reflected airborne sound.
Examples of other structural connections are an exhaust pipe, an intake p4pe, a cooling water pipe, a shaft, secondary supports and electric cables. With the exception of the electric cables, such connections shell be provided with suitable flexible elements. Elec tric cables shall not have a stiff covering and shall not be rigidly fixed to the surrounding structure over a distance of lOOdEc. where dEC is the diameter of the electric cable. Another helpful measure is a free bend over at least 90.
In principle, the tests should be performed in an acoustical free field over a reflecting plane. In most cases, however, it is not necessary to require such surroundings because the directly radiated and reflected Sound does not significantly affect the vibration levels of the supports. Exceptions may occur in the following cases.
ISO 9611 pdf download – Acoustics — Characterization of sources ofstructure-borne sound with respect to soundradiation from connected structures 一 Measurement of velocity at the contactpoints of machinery when resiliently mounted
