Low frequency sound measurement in vehicles

Sound pressure level measurements in cars travelling at motorway speeds have shown that, in many cases, the overall level is very high in relation to the dB(A) and octave band levels, suggesting that much of the sound energy is in the low frequency and infrasonic regions. A technique has been developed to extend accurate octave band measurements down to the octave centred on 2 Hz. The system uses a calibrated sound level meter feeding a frequency modulation tape-recorder to record noise below 64 Hz, and an octave band analysis system to analyse the resultant tape recordings. Typical results are presented for a number of vehicles and it is found that sound pressure levels as high as 120 dB can be found in the octave bands between 2 and 16 Hz.