Lab Report: Acoustic Effectiveness of Sound Barriers

Categories: Physics

Report, Pages 5 (1077 words)

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Introduction

The purpose of this laboratory experiment and the subsequent report is to investigate and demonstrate the influence of various laboratory variables on acoustic outcomes. Specifically, we aim to evaluate the effectiveness of sound barriers in reducing sound levels at different distances compared to no sound barrier. This research has practical relevance for Building Service Engineers who need to design systems while complying with regulations, such as NR ratings, which require knowledge of sound levels and their calculations.

Principles and Theoretical Background

In this section, we will discuss the fundamental principles and theories related to acoustic behavior and the effectiveness of sound barriers.

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We will explore concepts like the Inverse Square Law, which states that the magnitude of a physical quantity varies inversely with the square of the distance from its source. This law will help us understand the expected trends in our experimental results.

Laboratory Apparatus and Use

Disclaimer: Due to time constraints, the Sound Level Meter (SLM) used in this experiment was calibrated externally, and our results are based on this calibration.

Equipment

Calibrated Sound Level Meter (SLM) with connected tripod (See Appendix item (1))
Measuring Tape
Sound Source (See Appendix Item (2))
Sound Barrier – Acoustic resistant foam board

Equipment Use

The calibrated SLM was employed to measure the LZeq and LAeq at different distances (2m, 4m, and 8m) away from the source, both with and without the sound barrier.
A tripod was used to ensure the SLM was positioned accurately at the correct height to record the LZeq and LAeq frequencies at different distances.

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Measuring tape was used to precisely measure the distances from the sound source at different intervals.
The sound source emitted a pre-calibrated frequency for collecting readings of LZeq and LAeq at the SLM.
An acoustic sound barrier was placed to evaluate its effect on LAeq and LZeq at different distances.

Experiment Methodology

We collected the necessary equipment as listed above in the "Equipment" section.
We set up the sound source, which had already been pre-calibrated, and marked its position.
After marking the source's position, we measured distances ranging from 2m to 8m from the source and marked these intervals.
The SLM was then set up at the correct height (approximately 1.3m high), with a wind/noise cancellation hood to prevent background sound interference.
We took a baseline measurement of the decibel (dB) level in the laboratory at the 2m mark we had measured earlier. This was followed by measuring the LZeq(10sec) and LAeq(10sec) in 1/1 spectrum in octaves. (Note: The guidance suggested LZeq/LAeq to be measured at 20sec, but due to time constraints, we used 10sec intervals.)
After recording the baseline measurement (see baseline chart in the Results Section), we switched on the sound source and obtained the LZeq at 2m, which was recorded at 90dB. We marked this as the reference point for the experiment.
Following the baseline assessment, we measured LZeq(10sec) and LAeq(10sec) at 2m, 4m, and 8m without the sound barrier, and the results can be found in the "Results Section - 2m No Barrier" and subsequent sections, each octave band was plotted.
After completing the tests without a barrier at the specified distances, we switched off the source and introduced the acoustic foam barrier at 2m, 4m, and 8m. We recorded the results at each distance.
Finally, we measured the background noise again to confirm our baseline measurements.
Photos of the source and sound meter were taken and included in the appendix along with the results.

Analysis and Discussion

The results presented in the appendix below demonstrate a clear trend consistent with our expectations. As the distance between the SLM and the source increased, there was a corresponding decrease in the emitted sound levels (dB). This phenomenon can be partially explained by the Inverse Square Law, which states that the magnitude of a physical quantity varies inversely with the square of the distance from its source. Consequently, we observed a decrease in LAeq and LZeq values with increasing distance.

However, when the sound barrier was introduced, the results indicated that sound waves exhibited a fluid-like motion, arcing over the barrier. This effect can be attributed to the increased distance the sound waves had to travel due to the barrier's presence. Additionally, the barrier absorbed some of the energy from the source, resulting in reduced potential energy of the waves reaching the SLM. Consequently, the dB reaching the SLM was significantly reduced with the introduction of the barrier.

The effectiveness of a noise barrier depends on its ability to disrupt the direct path of sound from the source to the receiver, causing the sound to diffract around the barrier, thereby attenuating the noise.

Conclusion

Based on the collected results from this experiment, it is evident that as the distance between the SLM and the source increases, both LAeq and LZeq decrease proportionally. When a sound barrier is introduced, LAeq and LZeq experience even greater reductions in dB, particularly when the SLM is positioned at a lower height directly behind the barrier.

If we were to repeat this experiment, several changes could improve its accuracy. Firstly, allocating more time for data collection would lead to more precise results. Secondly, conducting the experiment in a near-silent room without other students performing experiments nearby would ensure a more accurate assessment since background noise would be minimized and consistent.