damaged pile. Listening to the quality or tone of the acoustic response 

 is a subjective process. NCEL conducted an engineering analysis of this 

 acoustic inspection technique to quantify the hammer sounding method. 



It is known that if the equilibrium condition of an object is dis- 

 turbed by an impact, it is the nature of the object to transfer the force 

 and the motion: to deflect, vibrate, resonate, and conduct sound 

 (Ref 13). Upon impact, a timber pile will vibrate laterally, axially 

 and radially. The overall resonant frequency response evaluated by SWRI 

 showed no appreciable change with damage for the frequency response of 

 lateral vibrations (Ref 8). The goal of the NCEL impact tests was to 

 analyze the localized sound waves transmitted axially through the pile. 



The hammer impact excites resonant modes of vibration affecting the 

 quality of tone heard. Loudness, pitch, and timbre are the three funda- 

 mental quantities which characterize a tone. To the human ear, pitch is 

 one attribute of auditory sensation for which sounds may be ranked and 

 compared. Pitch is primarily a function of frequency, although intensity 

 and waveform are also influential. A curve giving the relationship be- 

 tween subjective pitch and frequency at a fixed loudness is shown in 

 Reference 14: pitch increasing with higher frequencies. 



An objective approach to inspection of timber piles was devised 

 that utilized a hydrophone or an accelerometer to record the acoustic 

 response of the pile to the hammer impact. An instrumented hammer ex- 

 cited the timber pile with a nearly constant force over a broad frequency 

 range. The time domain signal was used to generate the Fast Fourier 

 Transform (FFT) of the signal for later analysis. 



When disturbed by an impulse forcing function (hammer tap), the 

 pile resonates at its damped natural frequency. It was anticipated that 

 an analysis of the frequency spectrum would show a higher frequency re- 

 sponse (higher pitch) for solid timber piles and lower frequency response 

 for damaged piles. Correlation between the axial frequency response of 

 the timber pile and the amount of cross-sectional wood loss was desired. 

 Consequently, the following variations in the signals from solid and 

 damaged wood were investigated: (1) changes in bandwidth, (2) number of 

 discrete frequency bands (peaks) , and (3) amplitude and frequency of the 

 highest energy peaks. Laboratory and field tests were conducted. 



Types of Impact Testing 



NCEL impact testing concentrated on two basic applications. Both 

 approaches deal with impulse impact and the analysis of the frequency 

 response of the timber pile. The primary difference is the specific 

 equipment used to measure the acoustic timber response after impact; 

 either a hydrophone or an accelerometer was used to record the acoustic 

 response. 



Ideally, both the stimulus (impulse force) and the frequency re- 

 sponse (axial vibration of the pile) could be objectively measured. The 

 overall testing procedure for the impact tests was divided into two 

 separate series: (1) hydrophone impact tests with an instrumented hammer 

 and a hydrophone and (2) accelerometer impact tests with an instrumented 

 hammer and an accelerometer. A detailed description for each series 

 follows. 



36 



