Aims
Studying the vibrational modes of organ pipes
Introduction
Sound is a vibration that arises from a mechanical wave of pressure and displacement, through a medium like air or water (Berg and Stock, 23). The reception and perception of these waves by the brain is referred to as sound. On the other hand, acoustics deals with the study of these mechanical waves in matter (pg. 40). In this experiment, the organ pipe is the main target of discussion. This is a sound producing wind instrument that resonates at a specific pitch when pressurized air is driven through it. A complete set usually has numerous pipes, where each is tuned to a specific note. Organ pipes mainly find use in cathedrals. The pipe organ is a keyboard instrument played using one or more manuals and a pedal board (Backus, 59). It is classified into the aerophones group, as it produces sound by vibrating columns of air. The wind is moving through metal or wooden pipes remain constant while a key is depressed (Backus, 60).
Workability
Vibrations in wind instruments are brought by a disturbance produced at or close to the end of an enclosed air column (Rossing, 33). The sound in the pipe comes about when air enters a sharp edge of a hole that oscillates back and forth. The frequency is altered by manipulating the flow rate of air as well as changing the orifice to wedge distance (pg. 45). Generally, the frequencies depend on the speed of air, length of the pipe and the state of the pipe (open or closed).
Apparatus
Organ pipes (2 ranks)
Microphone/microphone amplifier
Frequency spectrum analyzer
Thermometer
Oscilloscope
Hoses and Valves
Air compressor
Cables
Connectors
Rubber gloves
Printer
Ruler
Safety
The following safety procedure was observed before the experiment was carried out.
The students were instructed not to touch the lead tab with bare hands for health reasons.
Rubber gloves were provided for doing the above, alongside handling other lab equipment
A metal lifter was used to lift one side of the cover
The cover was lifted enough and pulled completely out of the slot
The cover was placed back into position by applying pressure directly above the slot each time the pipe was sounded in the open position.
The students were instructed not to take data while the compressor is running
Procedure
Familiarizing with the equipment
The compressor gauges, valves and hoses were checked, and all students made sure that they understood the connections and operations.
The oscilloscope, frequency spectrum analyzer, microphone and its amplifier were checked in order to understand the connections, settings, controls and operation.
The cables and connections of the setup were counterchecked once more to identify faults and wrong connections.
Pipe measurements and setup
4 pipes from both ranks with different sizes were selected and noted
The length of the pipes was measured from open end to the air outlet edge
The spectrum analyzer and oscilloscope were setup
Vibration modes and spectrum analysis of organ pipes
A measurement of the room temperature was taken
The compressor hose was connected and blown into. The pipes were the sounded with the end open and closed.
The pipes were blown into and sounded using the air compressor to produce a steady airflow. A microphone was placed close to the outlet of each pipe. Using a T-connector and cables, the microphone was connected to both the frequency spectrum analyzer and the oscilloscope.
The waveform of the sound was observed on the oscilloscope screen while the frequency spectrum was observed on the spectrum analyzer screen.
The oscilloscope was connected to a printer, and a sample of each pipe’s waveform was printed
The spectrum analyzer was used to compare the frequency spectrum of the selected pipes for both open and stopped cases by recording the frequency of the main peaks and their relative amplitudes.
Results
The table below shows the results of each of the 4 different pipes while open and closed. The results include the frequency as well as the power ratios of the pipes. This helps to relate to the size and the frequency. Different pressure values were applied, where their corresponding frequency values were read and recorded. Same case applied for the power ratios.
The room temperature was found to be 250C.
Data Analysis
The above results represent the frequencies of the respective power ratios of the pipes. The frequency, in this case, was measured in Hertz (Hz) while the power ratio was measured in decibel milliwatts (dBm) (White and White, 57).
Fractional uncertainty = least count / actual value.
Percentage uncertainty = fractional uncertainty x 100.
Frequency least count = 0.005Hz
Power ratio least count = 0.005dBm
The discovered uncertainties may have resulted from different factors, such as the presence of solid impurities in the air inside the pipes or the unexpected change in air temperature inside the pipes. This affects the pressure thus causing the changes in the frequency and pitch of the sound (White and White, 15).
Pipe b had a higher pitch when open compared to when it was closed. This was the highest among all pipes. Pipe b was followed by open pipe c, which exhibited similar features when open and closed. Closed pipe f followed closely in front of open pipe d while closed pipe d had the least pitch. Generally, pipes had a higher pitch while open than when closed. Nevertheless, closed pipes were louder. The results of this experiment were successful since they were close to the actual and expected readings. This is because the theory has it that open pipes increase the flow rate, thus leading to a wave frequency. This results into a higher sound pitch.
Conclusion
In conclusion, the pitch in organ pipes is higher when the pipes are open compared to when they are closed. Furthermore, the wind pressure inside the pipe as well as the air flow rate also affects the nature of the sound output of the organ pipe.
References
Berg and Stock, The Physics of Sound, Addison-Wesley; 2nd edition, 1994.
Backus, The Acoustical Foundations of Music, W. W. Norton & Co Inc,; 2nd Revised edition, 1977.
White and White, Physics and Music: The Science of Musical Sound, Dover Publications; Reprint edition, 2014.
Rossing, T. D. The Science of Sound, Addison-Wesley; 1982.
