Investigation of Silo Honking: Slip-Stick Excitation and Wall Vibration

Silo honking is an acoustical emission with a fundamental frequency of several hundred Hertz and an intensity often greater than 100 dB. It occurs when a silo is discharging and is similar to the "honk" of a lorry horn. The high amplitude of the honk makes it a significant noise pollution issue for workers at the site and for neighboring businesses and residents. This paper considers some possible excitation mechanisms that may be responsible for honking and presents measurements obtained from a full scale honking silo detailing the acoustic emissions and the associated vibration of the silo walls. Experimental results are presented which are comprised of simultaneous measurements of the three components of the wall vibrations and the acoustic pressure. The wall vibrations have an initial impulse response with a high amplitude 0(100g) and subsequent oscillatory accelerations with amplitude O(10g). The frequency spectra of the acceleration and acoustic pressure measurements comprises a sharp peak at the fundamental acoustic frequency and a harmonic series of peaks at integer multiples of the fundamental frequency. It is shown that the honking is not generated by a resonance inside the silo, as in a flute or organ pipe; the sound is generated by the silo walls acting as large speakers. The interaction between the wall and the sliding pellets is considered as a possible excitation mechanism for the acoustic emissions. Laboratory friction measurements are presented using pellets from the honking silo and a wall sample. The results of these measurements show that the particles exhibit a slip-stick behavior when sheared against the wall material. This slip-stick behavior is characterized under different conditions for pellets that are known to produce honking. Particles that have not been observed to honk were also tested and did not produce slip-stick motion at the wall.