For frequencies below 300 Hz there is a noticeable range of difference between the trends of the absorption coefficients of MDF and fiber cement. Above this frequency, similarities of these trends can be observed not including the region where that of the MDF peaks. It is also evident from the graph that the absorption coefficient values for both materials rage only from 0 to 0. 05 within the investigated range of frequencies. These values are quit small to say that the two samples are good absorbing at the chosen frequency region. The accumulated data are also superficial to predict the materials’ absorbing behavior beyond this range.
Figure 3 Graph of absorption coefficients of MDF (black) and Fiber Cement (red) as functions of frequency. In our experiment, we cannot expect for an absolute accuracy of the measured values as we consider the uncertainties in measuring the speed of sound and length of the tube for the computation of the fundamental resonant frequency to produce a standing wave inside the tube. Furthermore, through the frequency generator, we were not able to produce a signal whose frequency is exactly the same as that of our computation (see Appendix).
These differences introduce an initial phase shift of the reflected sound added to that of what is caused by the absorbing material and yields a different value for the standing wave ratio. The rapid fluctuation of the sound pressure amplitude inside the tube as read by the LabView program is also a great contributor to the inaccuracy since we are not able to read all the fluctuating values in order to get a more reliable average. The uncertainties in finding the maxima and minima because of the former also have to be taken into account.
In the sample, a circular opening with minimal radius is made as channel for the microphone probe introduces a leak that can change the pressure inside the tube, thus; also have a contribution to the discrepancy. 4. Conclusion Using the improvised standing wave apparatus, we were able to demonstrate the principles in measuring the absorption coefficient of a material through standing wave ratio. On the other hand, we failed in making a reliable evaluation for the absorbing properties of the sample material in response to varying frequencies since we only have chosen a very short frequency rage not sufficient for the evaluation.
The inaccuracy of the measured values is brought about by inadequate calibration of the experimental set-up as a whole. Nevertheless, we can still conclude from our data that fiber cement is more absorbing than MDF at the chosen frequency rage (100 Hz to 700 Hz). In experiments like ours, a highly calibrated standing wave apparatus that can provide a wide range of frequencies for measurement is well recommended so that accurate results and responsible evaluation can be realized. References 1. D.