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# Acoustical applications

Subject is specified for students interested in acoustics. It assumes some basic knowledge of this subject that could be gained in Introduction to Acoustics.

Studets will measure 6 tasks, the first 5 of them should be resumed by written report.

1. Sound power measurements
• How to measure sound power? How we can use anechoic room and reverberation room for sound power measurements? Calculate sound power radiated by fan.
• File power2019.txt contains table of measured sound pressures for selected 1/3 oct. band bands in all measurement points (40); the first column contains central frequencies of 1/3oct. bands, the 22nd and last columns containe bacground noise levels, the rest of columns contains corresponding squares of sound pressure. Text containing guidelines for background noise correction and microphone positions for measurement of sound presure levels.
2. Vibration measurement
• Discuss the possible methods of vibration measurements. Which quantities can be measured? How the transducer influences the measured structure? Which method of analysis could be used for signal processsing (time x frequency domain)?
3. Measurement of transmission loss
• Using PULSE system measure transmission loss of the gate between the laboratory and corridor. What is the definition of the TL? What is the influence of transmitting and receiving rooms acoustics? Calculate weighted TL.
• rozdil.txt – frequency dependence of level differences
• dozvuk.txt– frequency dependence of reverberation time in receiving room
• Description of gauge line for single value transmission loss:
• 100 Hz – 36 dB
• 400 Hz – 54 dB (3 dB between adjacent bands)
• 1250 Hz – 59 dB (1 dB between adjacent bands)
• 3150 Hz – 59 dB
• Resulting single value (weighted) transmission loss corresponds to value on gauge line for frequency 500 Hz. Sum of negative deviations from the gauge line must be 32 dB. The gauge line should be shifted by 1 dB up or down.
4. Measurement in acoustic interferometer
• For selected samples measure absorption coefficients with respect to frequency using acoustic interferometer (Kundt's tube). Discuss limitations for using this method and influence of loading signal level.
• Some formulas for interferometer measurements
• Results are in the following form: 6 columns [1_frequency 2_mineral_wool_40mm 3_melamine foam 4_felt_20mm_2000g/m2 5_Helmholtz (diameter of the hole 6 mm, distance between centers of the holes 22 mm, depth of the volume 38 mm, thickness of the plate-length of the hole 1 mm) 6_Helmholtz_attenuated]
• Measurement parameters: sampling frequency fs=8000 Hz, pseudorandom sequence with length of 2^(11), FFT of 2048 points. Results are in 1024 rows corresponding to one half of sampling frequency.
5. Measurement of acoustic parameters of loudspeaker box
• Measured loudspeaker box B&W DM603 S2
• bwfft.txt - FFT analysis (0-25,6 kHz, 6400 lines) - three columns - the first is ordinal number of the line, the second is frequency and the third is level of the line in dB
• bwcpb.txt – 1/12 octave analysis - three columns - the first is ordinal number of the band, the second is the central frequency of the 1/12oct. band and the third is level in dB
• bwsin.txt – logarithmicly tuned sinus - tuning speed 0.05 decade/s (tuning time 60 s, tuning from 20kHz to 20 Hz, 480 values, levels in dB) - with respect to direction of tuning it is necessary to revert the order of the data and then to create frequency dependence!
• smerovost.txt – directional char., 1/3oct. analysis, pink noise, rotation of 360 degree in 80 s, sampling is two spectras per second, table has 31 rows and 192 columns,each row represents one 1/3 oct. band. Example: 1 2,0000000000e+001 4,27531e+001... ordinal number of the band, central frequency of the 1/3oct. band, followed by 190 values in dB (160 corresponds to 360 degrees and 30 are overlap). Plot the directional characteristics for at least five frequency bands in the form of polar diagram and calculate the directivity factor for all bands!
Last update: Thursday, 21 November 2019.