3.4.1 Input and radiated sound power measurement
Experimental setup to measure input and radiated sound power is shown in Fig. 3.3. A speaker is attached to a cylindrical tube (with dimension of 0.1m diameter) to generate plane wave excitation as an input source to the duct. The specifications of the speaker are: 0.102m diameter, 20W power, 8Ω impedance with the working frequency range of 20-8000 Hz. A rectangular duct made up of galvanized iron (GI) sheet (with dimensions 0.3 m x 0.4 m x 1.2 m and 20 gauge (1.01 mm) thickness) is considered in the present study. Duct is fabricated by bending a rectangular sheet at three corners and edges joined with Pittsburgh locking mechanism. Simply supported (SS) boundary condition is applied at both ends of the duct and is established by making a point contact with the steel balls in a rectangular frame as shown in the Fig. 3.3(b).
The cut-off frequencies for plane wave propagation in the chosen cylindrical and rectangular ducts are 1991.4 Hz and 425 Hz, respectively. Since, breakout noise is predominant at lower frequencies, up to 400 Hz is considered for analysis in the present study.
Figure 3.4 shows the schematic diagram to measure the input sound power provided to a rectangular duct with speaker excitation. Progressive and reflecting pressure waves in the cylindrical tube are represented as “A and B”. Two different microphones (of IEPE type) are used to measure the sound pressure at two positions (1 and 2 as shown in Fig. 3.4) which are separated by a distance of 0.3 m.
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(a)
(b)
Figure 3.3: (a) Experimental setup for measuring input and radiated sound power measurement, (b) Simply supported boundary condition.
Figure 3.4: Schematic diagram of input sound power measurement setup.
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Radiated sound power from all four flexible surfaces of a duct is measured by two different methods such as P-P (pressure–pressure) and P-U (pressure-velocity). The experimental arrangements for both methods are shown in Figs. 3.5 and 3.6. In both the cases, measurement was done with different scanning pattern and it is verified that the repeatability is good.
The intensity probe method (P-P) measures radiated sound pressure with two matched microphones separated by a spacer, over a virtual surface as shown in Fig. 3.5. In the present study spacer with 50mm length and ½ inch diameter is used to measure up to 1.25 kHz frequency. Intensity probe (Brüel & Kjær (B&K) of type 2270) is calibrated using sound intensity calibrator type B&K 4297 for amplitude of the sound pressure and phase difference between the microphones. Error in sound power, due to phase mismatch, is expressed in terms of pressure-residual intensity (p-RI) index. According to international standard IEC 1043, the p-RI index should be as high as possible. It should be at least 17 dB above 125 Hz for the microphone separation distance of 50mm [109]. So in present case p-RI index is 23.5 dB above 125 Hz after calibration of the intensity probe.
Figure 3.5: Experimental setup for sound power measurement by intensity probe method (P-P method).
Microflown technique (P-U) measures both sound pressure and particle velocity near the duct surface as shown in Fig. 3.6. A high sensitivity Microflown probe containing both pressure and velocity sensors is used for scanning the duct surface. The distance from the duct surface for scanning is chosen based on reactivity (ratio of reactive intensity to active intensity), which should be less than 7 dB to obtain good measurements [110]. Based on different measurements, a distance of 0.08m from the duct surface is used in the present study. The radiated sound power from all four flexible surfaces of a duct is obtained by using sound intensity over the measured surface area.
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Figure 3.6: Experimental setup for sound power measurement by Microflown technique (P-U method).
3.4.2 Directivity measurement
Plane-wave excitation given to the duct is verified for two different cases using measurements and numerical simulations. In the first case, directivity measured in a plane parallel to the cross-section of cylindrical tube without test duct attachment. Sound pressure distribution inside the test configuration as shown in Fig. 3.3(a) is simulated using numerical models as a second case.
The schematic diagram of an experimental setup for directivity measurement is shown in Fig.
3.7. The measurement plane has a dimension of 0.5 m radius at a distance of 0.6 m from the cylindrical tube end. Sound pressure levels were measured with a sound level meter at 12 locations in the plane at an equal angle interval of 300. Two tonal sounds at 141 Hz, 220 Hz, and one random signal are used as input sound signals for the directivity test.
Figure 3.7: Schematic diagram of input source directivity measurement setup without test duct attachment.
54 3.4.3 Vibrations measurement
Vibrations on the duct walls surface are measured with an accelerometer and using measured acceleration data, vibration velocity and displacement are calculated. High sensitivity and low weight sensors are used for acquiring the vibration data. Measurements are taken at the mid- section of a duct (0.5L2). Five measuring points are considered on each duct surface. The schematic diagram of test setup used for vibration measurement is shown in Fig. 3.8. A speaker attached to the cylindrical tube is used as a source for excitation of the duct and simultaneously vibration generated on flexible walls of the duct is measured from 10-400 Hz (frequency range) by accelerometers mounted on the duct wall.
Figure 3.8: Schematic diagram of vibrations measurement with accelerometers at duct mid- section along the perimeter.