Victor V. Krylov
Ctr. for Res. into the Built Environment, Nottingham Trent Univ., Burton St., Nottingham NG1 4BU, UK
The problem of disturbing low-frequency noise, also called low-frequency hum, has been known for at least two decades. However, in many aspects the nature of low-frequency hum still remains a mystery (see, e.g., ``The Independent'' of 22 June 1994). It may be possible that in some cases the sources of this hum are underground gas or petrol pipes where turbulent flows of gas or liquid generate sound waves of high amplitude propagating in a pipeline as in a waveguide. The velocities of sound C[sub 0] inside the pipes (450 m/s for methane) sometimes may be higher than the velocities of Rayleigh surface waves C[sub R] in the ground at the frequencies of interest (5--50 Hz). Typical values of C[sub R] are 300--600 m/s. If so, i.e., if C[sub 0]>C[sub R], then ground Rayleigh waves might be effectively generated by sound wave propagating inside the pipes, the mechanism of generation being similar to that of a sonic boom from supersonic jets. Preliminary calculations show that central frequencies of generated Rayleigh wave ground vibration spectra depend on depth of the pipe and are in the low-frequency range. The amplitudes of generated ground vibration velocity due to sound waves propagating in gas pipes buried at a depth of 2 m can be around 70 dB (relative to 10[sup -9] m/s). This may be enough to annoy some people both because of direct impact of vibrations and due to generated structure-borne noise.