Page 22 - Spring 2007

Fig. 13. Impact hammer measurements on cylindrical shell structure submerged in ARL/Penn State Reverberant Water Tank. Top–shell submerged in tank with diver; bottom, shell suspended over tank.
cies! It does not include the coincidence dip, or the high-fre- quency stiffness effects. To consider these effects, and those of finite panel boundaries, other techniques are used, like SEA. In fact, one of the early SEA applications was for single and double panel transmission loss calculations. Price and Crocker’s famous papers12, 13 clearly show the coincidence dips in transmission loss, and the importance of varying panel thicknesses in double panel systems.
Measurements of sound—structure interaction
The sound radiated by vibrating structures, and conversely, the vibrations induced in structures by sound fields, are com- monly measured in two types of chambers: reverberation rooms, and anechoic rooms. Interconnected rooms are used to measure the transmission loss of barriers, like windows and doors.
Accelerometers mounted to structures can provide a spa- tially averaged normal velocity. Also, non-contact velocity measurements are often made using laser vibrometry, elimi- nating the mass-loading effects of the accelerometers on the test structure, and reducing test times.
Measuring sound power is more difficult than measur- ing vibration. In air, arrays of microphones may be used to measure the spatial variability of pressure. In water, hydrophones are used. Sound pressures are measured quite differently in reverberant and anechoic rooms, though.
In a reverberant room, the acoustic modes of the air in the room are excited by a vibrating structure. When the acoustic cavity modal density is high, the room’s pressure field becomes nearly diffuse, such that pressures measured at just a few locations randomly spaced throughout the room may be used to estimate the total sound power radiated by a vibrating object. The sound power is computed using the measured pressures, along with a few of the reverberation room’s parameters, such as the volume and the reverberation time. The reverberation time is inversely proportional to the room’s loss factor—the longer a pressure impulse takes to decay, the lower the room’s loss factor.
The accuracy of a reverberation chamber sound power measurement depends on the room’s modal density and rever- beration time (among other things). The sound power is actual- ly a statistical mean, which is bracketed by a standard deviation at each measurement frequency. In general, the higher a room’s modal density and the wider the frequency bandwidth, the smaller the standard deviation. Most reverberation room sound power measurements are therefore made over wide frequency bands that contain several acoustic modes. One-third octave fre- quency bands are commonly used.
There are standards14, 15 available to help an experimen- talist quantify a reverberation chamber’s characteristics, and conduct a sound power measurement. The standards, how- ever, are specific to rooms filled with air. To make measure- ments in a reverberant water tank, some modifications to the approaches in the standards are necessary. Conlon describes these modifications, and applies them to ARL/Penn State’s
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sures measured at five hydrophone locations are used to esti- mate the sound power transfer function (Pραδ/F2).
The sound power transfer functions for several drive loca- tions on an elbowed pipe (the same pipe described in Part 1 of this article) are shown in Fig. 14, adapted from another NoiseCon article,17 this one from 2005. The measurements were made in one-third octave bands, and show that the sound power transfer functions vary with drive location (just as mobility functions do). Annotations on the graph show where various shell modes cut on (again, see Part 1 of this article to refresh your memory on what shell modes are).
It is often useful to compare sound power transfer func- tions to that of an ideal dipole source:
, (26)
where |F|2 represents the square of the r.m.s. force amplitude. This would be the sound made by an oscillating point drive in space. At some frequencies, most notably those of struc- tural resonance, the sound power transfer function for the elbowed pipe is higher than that of a point dipole, showing that the structure amplifies the force drive at those frequen-
reverberant water tank in his NoiseCon 2004 article. Photographs of a metal pressure vessel being tested in ARL/Penn State’s tank are shown in Fig. 13. The vessel is struck by an impact hammer at several points, and the pres-
20 Acoustics Today, April 2007