Lily M. Wang

The effects of simple coupled volume geometry on the objective and subjective results from nonexponential decay

This project focuses on the individual and interactive acoustic effects of three architectural parameters on the double slope profile from a simple coupled volume system created in the computer modeling program ODEON. The three variables studied are the volume ratio between the main and secondary spaces, the absorption ratio between the two spaces, and aperture size. The resulting energy decay profiles are analyzed using T30∕T15 coupling coefficient ratios and Bayesian analysis. Coupling coefficient results show general trends in the effects of the three architectural parameters that match previous research results and the predominant interactive effect between the three variables involving a large coupled volume. Similar results are suggested by the Bayesian analysis based on the newly developed quantifiers, decay ratio and ΔdB, although a more complex relationship among the variables may exist at larger volume ratios. A subset of the simulated sound fields have been auralized and used to conduct psychoa...

Acoustic radiation from bowed violins

Near-field acoustic holography (NAH) is applied to visualize the acoustic radiation from bowed violins across a frequency range from 294 Hz to 3 kHz. These visualizations are employed to localize regions of acoustic radiation from surfaces of violins. Three violins were tested: a common student instrument by Scherl and Roth; Hutchins violin SUS295, which has been the subject of many previous investigations; and a Hutchins mezzo violin from the Violin Octet set of instruments, which is longer, broader, and thinner than a standard instrument. The violins were bowed continuously with an open-frame mechanical bowing machine, while NAH measurements were made on four planes surrounding the instrument. Mappings of the acoustic intensity are presented that show locations of maximum radiation at low and high frequencies with a spatial resolution smaller than the acoustic wavelength. Comparisons are made of the radiation patterns between the two conventional instruments and the mezzo violin. Radiation patterns from...

Implications of human performance and perception under tonal noise conditions on indoor noise criteria

This research investigated differences in task performance and perception under six non-time-varying ventilation-type background noise spectra with differing tonality. The results were related to five indoor noise criteria systems: noise criteria, balanced noise criteria, room criteria, room criteria mark II, and the A-weighted equivalent sound pressure level (L(Aeq)). These criteria systems are commonly used in the U.S. building industry, but concerns exist over whether they are appropriate for all noise situations. Thirty test subjects completed three types of performance tasks (typing, reasoning, and math) and answered questions about their perception of the indoor environment under each noise condition. Results showed that performance scores did not change significantly across the six noise conditions, but there were differences in subjective perception. For example, perception trends for tonality, annoyance, and distraction changed based on the frequency and prominence of discrete tones in noise. However, these perceptual changes were not fully reflected in the criteria level or spectral quality ratings. Additionally, task performance was related to subjective perception but not to criteria level predictions. As a result, the authors suggest that the current criteria should be modified to account for the frequency and prominence of tones in background noise.

Optimum absorption and aperture parameters for realistic coupled volume spaces determined from computational analysis and subjective testing results

This project utilizes computational modeling to study the effects of varying two architectural parameters, absorption ratio and aperture size, in a realistic coupled volume concert hall. Coupled volumes have been shown to exhibit non-exponential sound energy decay profiles, referred to as double slope effect. A number of objective metrics (T30/T15, LDT/T10, decay ratio, and DeltaL) have been used to quantify the double slope effect of the profiles generated in the virtual hall. T30/T15 and LDT/T10 showed similar trends across all hall configurations, indicating decreasing double slope effect with increasing coupled volume absorption ratio for each aperture size, and producing highest values at a specific aperture size for each absorption ratio. Generally, LDT/T10 provides finer resolution than T30/T15 when analyzing the decay profiles in this study. Results from the two metrics derived from Bayesian analysis, decay ratio and DeltaL, seem less consistent. Subjective testing has also been conducted to determine the effect of varying the two architectural parameters in the hall, and multidimensional scaling analysis shows that, in general, listener preference is inversely proportional to the level of double slope effect, with the highest levels of preference occurring at low and medium levels of double slope effect. Recommended design guidelines for coupled volume halls are provided based on these computational and subjective results.

Comparison of Measured and Computer-Modeled Objective Parameters for an Existing Coupled Volume Concert Hall

This paper details the comparison of a built coupled volume concert hall and a computer-generated model of the hall. Coupled volume spaces typically have an auxiliary volume connected to the main volume through an acoustically transparent aperture. In this type of space, architectural parameters such as volume size and surface absorption can be designed to produce a non-linear reverberant decay, known as double-slope decay. Objective reverberation and clarity parameters were calculated for the sound fields measured in the built hall and predicted in the computer modeled hall. These objective parameters were compared between the two modalities for several configurations of the hall to determine the accuracy of the computer modeling prediction scheme for coupled volume spaces. The results showed a high level of computer model accuracy in the high frequency range, with differences in reverberation parameters being less than 1 just noticeable difference (jnd). Lower frequencies proved computer modeling to be less accurate, particularly for the clarity parameter, which had differences up to 8 jnd.

Relationships between unoccupied classroom acoustical conditions and elementary student achievement measured in eastern Nebraska

Building standards recommend maximum background noise levels (BNL) and reverberation times (RT) for unoccupied classrooms. However, existing research does not show a consistent correlation between these parameters and student achievement. Through in situ testing, this research seeks to determine what acoustical conditions should be attained in elementary schools for students to meet educational goals. Acoustical measurements were conducted in a Nebraska public school system and correlated to achievement scores from students in the surveyed classrooms. Unoccupied BNLs and RTs were gathered in 34 third and 33 fifth-grade classrooms. Additionally, binaural room impulse response measurements were gathered in a subset of the classrooms. The results suggest that student reading and language subject areas may be negatively impacted by higher unoccupied BNLs; to meet the upper half of NE state targets, these levels should be less than 45 dBA. However, the percentage of students receiving free or reduced price lunches is more strongly correlated to achievement than BNLs, and the negative correlations between noise and achievement are not significant when controlling for this demographic variable. One statistically significant relationship that remained when controlling for demographics was that classrooms with lower distortion of frequency-smoothed magnitude values generally had students with higher language scores.

Effects of exposure duration and type of task on subjective performance and perception in noise

The influences of exposure time and the types of tasks on human perception and task performance have been investigated under a variety of noise conditions with differing level, spectral content, and behavior over time. Ten test subjects each completed 38 hours of testing over multiple days. Subjects were exposed to each of six background noise conditions over 20, 40, 80, and 240 minute trials. During the trials, subjects completed three types of performance tasks (typing, grammatical reasoning, and math tests) and answered questions about their perception of the indoor environment. Results indicate that the different exposure lengths used in this study did not have a significant effect on performance scores. Some influence of exposure time on subjective perception was observed, however; shorter time lengths combined with back to back exposures led to greater differentiation in perception. Significant differences in task performance between the different noise conditions were not observed, but differences in perception were found. Additionally, task performance was related to subjective perception; if subjects perceived the noise to be more tonal, they performed worse on all tasks. The performance results were not always negatively influenced by unfavorable quality perceptions, though. For example, as subjects perceived noise to be more roaring in this study, their typing scores decreased but reasoning and math scores improved.

Quantifying the double slope effect in coupled volume room systems

Coupled volume spaces are typically defined as a main volume connected to an auxiliary volume through an acoustically transparent aperture. Given certain architectural parameters, coupled volumes can exhibit a non-exponential sound energy decay, referred to in this paper as double slope effect (DSE). This paper provides an overview and comparison of several quantifiers of DSE. Four numeric DSE quantifiers, T30/T15, LDT/EDT, Decay Ratio, and ΔdB were used to analyze the energy decays produced in a computational model of an existing coupled volume concert hall. Each quantifiers ability to indicate changes in DSE was determined through comparative analysis. LDT/EDT was found to differentiate most between the levels of DSE in the sample decay curves. T30/T15 was also able to demonstrate changes in DSE, although not with the same level of sensitivity as LDT/EDT. Decay Ratio and ΔdB as determined from Bayesian analysis results did not indicate any DSE across the four cases studied; further research is recommended on applying Bayesian analysis to decay curves from realistic coupled volume scenarios.

Performance Review of Indoor Noise Criteria

Currently there are a number of indoor noise criteria used to quantify the level of background noise in rooms, including Noise Criteria (NC), Balanced Noise Criteria (NCB), Room Criteria (RC), Room Criteria Mark II (RC Mark II) and others. This paper reviews the pros and cons of each of these and then compares the values measured in existing classrooms in the Omaha area. The authors are in the process of subjectively comparing these various criteria to determine which method best correlates with human perception. The subjective testing methods and preliminary results are discussed.

Appropriate characterization of background noise levels in the workplace

Overall loudness is not the only quality of indoor background noise that affects occupants. The distribution of noise across frequency (pitch), whether or not the noise contains tones, and whether or not the noise changes over time must also be considered. There are several indoor noise criteria systems available to quantify the background noise in rooms, but many of them do not account for these factors. These systems are commonly used by architects and engineers, and often incorporated into manufacturer’s data, design guides, and standards. There is much debate over which of the criteria systems best reflect how occupants will respond to noise. To examine these issues, an extensive study is being completed at the University of Nebraska investigating the effects of various types of air-conditioning noise on occupant productivity and perception. Results from performance tests and questionnaires are used to evaluate a number of these noise criteria systems. Specific implications of this project on characterizing noise in the workplace will be discussed.