Sound Pressure Level Variations and Response Due to Ambient Fluid Coupling Aperture Geometries


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Background Information





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Intro  Top

    For our research project we; Stephen, Tomas, and Ryan, decided to see how different port lengths affect the frequency response of a loudspeaker. Our background information starts off with a description of sound.  

Background Information  Top

    Sound is the vibration of air. The human ear can hear sound frequencies between about 16 Hz and 20 kHz, even though sound exists below and above these levels. When referring to the upper and lower sensitivity levels of things such as audio equipment, the term “frequency range” is used. Variations of sensitivity to specific frequencies in that range are referred to as “frequency response.” Sound quality, however, is all subjective as perceived by the human ear, therefore, sound theory is of practical value only if we seek to relate quantitative data in terms of our perception. The function of a loudspeaker is to use mechanical energy to create sound. An unmounted loudspeaker reproduces very little bass and it is at these lower frequencies that a mounting device or enclosure becomes essential. The reason that an unmounted speaker produces less than desirable results is because of the cancellation of waves from the front and back of the diaphragm. The most effective mounting device is a wall, however, mounting loudspeakers in a wall is sometimes not desired. The best alternative to a wall is a totally enclosed cabinet. The most prominent effect of an enclosure is the air inside, which acts as a pneumatic spring acting against diaphragm movement. A port in the enclosure will change the properties of this “pneumatic spring,” however, the effects of these changes are not known to this team of researchers. Our purpose of this investigation is to find the relationship, if any, between port length in a speaker enclosure, and frequency range. We believe that if we use five different port lengths in a speaker enclosure, then the speaker will reproduce lower frequencies at higher decibel levels with the shortest port length because it will have the least air resistance on the woofer cone, causing it to move more freely.

Procedure  Top

  Our method of experimentation was fairly simple. We used two Infinity 5" x 7" co-axial loudspeakers mounted in a polycarbonate enclosure (See Figure 1). The dimensions of the enclosure are approximately 20.5" x 8.5" x 10". The ports we used were located on the top of the enclosure and had an outside diameter of 2.25" and an inside diameter of 2". We used the electronics from a pair of computer speakers to power ours and the input is a standard .125" stereo jack. To take the data, we set the speaker enclosure on one chair and the decibel meter on another, directly in front of it. The decibel meter was exactly 24" from the front edge of the enclosure and was centered between the two speakers. (See Figure 2). We did the experimentation inside a small den to minimize the amount of ambient sound. We chose to set up our experiment like this because it was the simplest way to do it with our available resources. We gathered data by choosing 10 frequencies, then testing each port three times at each frequency. We played each frequency for 10 seconds and approximated the most prominent number on the decibel meter. This resulted in a total of 150 data points. 

Data  Top

Data File (Text)

Data File (Excel)

    The uncertainty for the frequency response is about plus or minus 5 decibels. We looked at the graphs and decided this uncertainty is appropriate because the 3 trial graphs are all pretty close and 5 decibels is a good approximation for the uncertainty. 

Bibliography/Links  Top

"A Guide to Loudspeaker Specifications." University of Salford page.
Collins, Martin. “High Performance Loudspeakers.” Fourth Edition, 1991. 
“Construction of the Loudspeakers.” Art Ludwig’s Sound Page.
Murphy, John. “Loudspeaker Diffraction Loss and Compensation.” True Audio
Roberts, Michael. “Sound Production.” Unesco, 1985.
Villchur, Edgar. “Reproduction of Sound.” Volume 2 1962. 

“A Guide to Loudspeakers Specifications.”-A college website guide to loudspeaker specifications

"Construction of the Loudspeakers."-How to build your own loudspeakers.

“Loudspeaker Diffraction Loss and Compensation.”-What causes diffraction loss and how to compensate.

"Loudspeaker Basics."-The basics of a general loudspeaker.

"Loudspeaker History."-The amazing history of the loudspeaker.

Diagrams  Top

Figure 1:

Figure 2:

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