On the Spectral Shape of the Sound Generated by Standard Tapping Machines

The standard tapping machine is the most widely used sound source to measure impact noise. The five hammers hitting at a frequency of 2 Hz each are usually modelled as one hammer acting at a frequency of 10 Hz. Starting with the experimental finding that measured velocity spectra exhibit a clear 2 Hz line spectrum, an analytical calculation model is derived to determine the sound power emitted by a five hammer configuration into infinitely large reception plates. The experimentally found 2 Hz line spectrum can be partly explained by runtime effects of the bending waves originating from an assembly of five point sources. Even more influential are the deviations between real tapping machines and the ideal tapping machine as described in ISO standard 10140. For the description of real tapping machines, measured impact velocities and time histories of the impacts could be used. These data are available at PTB from conformity tests with tapping machines. The conclusion is that real standard lapping machines emit a 2 Hz line spectrum which enables measurements down to the 50 Hz one-third octave band with an acceptable uncertainty.