Sound radiation and sound quality characteristics of refrigerator noise in real living environments

The characteristics of various types of refrigerator noise were investigated in an anechoic chamber and in a real living environment - a 100 m² apartment which is a common size in Korea. It was found that the sound pressure level of the refrigerator noise in the real living room was about 10 dB higher than the level in the anechoic chamber at the same position (1 m in front of refrigerator). In addition, a tolerance level for refrigerator noise was determined by subjective evaluation experiments. Refrigerator noise was presented by a loudspeaker placed in the kitchen where the refrigerator is normally located. Level 2 responses to the subjective evaluation (“hardly perceivable”) corresponded to a sound pressure level of about 26 dB(A), for which 90% of participants were satisfied with the level of refrigerator noise. A semantic differential test using various adjectives was also conducted to evaluate the sound quality of refrigerator noise. With the semantic differential and the factor analysis, adjectives used in this experiment were grouped into three factors. From the results of correlation and multiple regression analyses on the psychoacoustical parameters and subjective evaluations of 30 kinds of refrigerators, sound quality index which predict the subjective rating score were proposed.     

Soundscape evaluation in Han Chinese Buddhist temples

In this study, surveys were conducted at four typical Han Chinese Buddhist temples. These surveys were then analyzed to identify the subjective and objective factors of soundscape evaluation. Field measurements of the four temples’ sound levels were taken over the course of an entire day, and the representative sounds in temples were recorded. Soundscape evaluation questionnaire surveys were distributed at the temples. The analytical results of the questionnaire and measurement data showed that the sound preferences in temples are significantly correlated with sharpness value of the sounds in terms of psychoacoustic parameters, and the average sound levels at the four temples over the course of an entire day were between 47.0 and 52.7 dBA, and approximately 70% of those surveyed tended to evaluate the temples’ soundscapes as comfortable and harmonious. Regarding the objective factors, there was a significant correlation between the measured sound levels and the soundscape evaluations. When the sound level of a temple was higher than 60 dBA, respondents were more likely to feel uncomfortable, and the correlation between the sound level and the evaluation of acoustic comfort substantially increased. Regarding the subjective factors, the respondent’s age, occupation, religious belief, purpose, frequency of visiting the temples, and educational level significantly correlated with the soundscape evaluation with correlation coefficients ranging from 0.13 to 0.35.     

Measurement of noise associated with model transformer cores

The performance of a transformer core may be considered in terms of power loss and by the noise generated by the core, both of which should be minimised. This paper discusses the setting up of a suitable system for evaluation of noise in a large model transformer core (500 kV A) and issues associated with noise measurement. The equivalent continuous sound pressure level (LAeq) was used as a measure of the A-weighted sound level and measurements were made in the range 16 Hz–25 kHz for various step lap core configurations. The selection of optimum sound insulation materials between core and ground support and for enclosing the transformer is essential for minimisation of background noise. Core clamping pressure must be optimised in order to minimise noise. The use of two laminations per layer instead of one leads to an increase in noise arising from the core. Provided care is taken in building the core, good reproducibility of results can be obtained for analysis.     

Adjustment on subjective annoyance of low frequency noise by adding additional sound

Structure-borne noise originating from a heat pump unit was selected to study the influence on subjective annoyance of low frequency noise (LFN) combined with additional sound. Paired comparison test was used for evaluating the subjective annoyance of LFN combined with different sound pressure levels (SPL) of pink noise, frequency-modulated pure tones (FM pure tones) and natural sounds. The results showed that, with pink noise of 250-1000 Hz combined with the original LFN, the subjective annoyance value (SAV) first dropped then rose with increasing SPL. When SPL of the pink noise was 15-25 dB, SAV was lower than that of the original LFN. With pink noise of frequency 250-20,000 Hz added to LFN, SAV increased linearly with increasing SPL. SAV and the psychoacoustic annoyance value (PAV) obtained by semi-theoretical formulas were well correlated. The determination coefficient (R²) was 0.966 and 0.881, respectively, when the frequency range of the pink noise was 250-1000 and 250-20,000 Hz. When FM pure tones with central frequencies of 500, 2000 and 8000 Hz, or natural sounds (including the sound of singing birds, flowing water, wind or ticking clock) were, respectively, added to the original sound, the SAV increased as the SPL of the added sound increased. However, when a FM pure tone of 15 dB with a central frequency of 2000 Hz and a modulation frequency of 10 Hz was added, the SAV was lower than that of the original LFN. With SPL and central frequency held invariable, the SAV declined primarily when modulation frequency increased. With SPL and modulation frequency held invariable, the SAV became lowest when the central frequency was 2000 Hz. This showed a preferable correlation between SAV and fluctuation extent of FM pure tones.     

Reliability improvement of a sound quality index for a vehicle HVAC system using a regression and neural network model

The reduction of vehicle interior noise has long been the main interest of noise and vibration harshness (NVH) engineers. A driver’s perception of vehicle noise is largely affected by psychoacoustic noise characteristics and SPL. Among the various types of vehicle interior noise, the sound of the heating, ventilation, and air conditioning (HVAC) systems is a source of distraction for drivers. HVAC noise is not as loud as the overall noise level; however, it affects a driver’s subjective perception and may lead to feelings of nervousness or annoyance. Therefore, vehicle engineers work not only to reduce noise, but also to improve sound quality. In this paper, HVAC noise samples were taken from many types of vehicles. Objective and subjective sound quality (SQ) evaluations were obtained, simple and multiple regression models were generated, and these were used with the Semantic Differential Method (SDM) to determine what characteristics trigger a “pleasant” response from listeners. The regression analysis produced diagnostic statistics and regression estimates. In addition, neural network (NN) models were created using three objective numerical inputs (loudness, sharpness, and roughness) of the SQ metrics and one subjective output (“pleasant”). The NN model was used primarily because human perceptions are very complex and often hard to estimate. The estimation models were compared via correlations between SQ output indices and hearing test results. Results demonstrated that the NN model is most highly correlated with SQ indices, which led to determination of suggested methods for SQ metrics prediction.     

Index for vehicle acoustical comfort inside a passenger car

Sound quality is among the main factors that influence customers’ preference for choosing good automobile products. It all started more than 10 years ago and grows up so fast due to high competition in the automotive industries. A-weighted noise levels and sound power are usually utilised to measure the noise but they are not adequate to characterise the impact sound inside a car. The most popular approach to determine sound quality of a product is to define an annoyance or specific index, which involves both subjective and objective evaluations. Subjective and objective tests should be studied concurrently in order to determine the sound quality inside a passenger car. This approach is used in this paper to evaluate vehicle comfort index according to most frequently used sound quality metrics, namely; Zwicker loudness, sharpness, roughness and fluctuation strength. As a result researchers of different fields of automotive acoustics investigations can use this index according to the type of road (international road roughness) without any need to perform time-consuming jury tests. The metrics are correlated with jury test results that show which of them and how much has affected the acoustical comfort of the vehicle. The relation between road roughness and vehicle acoustical comfort index is another point of interest in this research.     

Improvement of impact noise in a passenger car utilizing sound metric based on wavelet transform

A new sound metric for impact sound is developed based on the continuous wavelet transform (CWT), a useful tool for the analysis of non-stationary signals such as impact noise. Together with new metric, two other conventional sound metrics related to sound modulation and fluctuation are also considered. In all, three sound metrics are employed to develop impact sound quality indexes for several specific impact courses on the road. Impact sounds are evaluated subjectively by 25 jurors. The indexes are verified by comparing the correlation between the index output and results of a subjective evaluation based on a jury test. These indexes are successfully applied to an objective evaluation for improvement of the impact sound quality for cases where some parts of the suspension system of the test car are modified.     

Testing the accuracy of smartphones and sound level meter applications for measuring environmental noise

This paper reports on experimental tests undertaken to assess the capability of noise monitoring applications to be utilized as an alternative low cost solution to traditional noise monitoring using a sound level meter. The methodology consisted of testing 100 smartphones in a reverberation room. Broadband white noise was utilized to test the ability of smartphones to measure noise at background, 50, 70 and 90 dB(A) and these measurements were compared with true noise levels acquired via a calibrated sound level meter. Tests were conducted on phones using the Android and iOS platforms. For each smartphone, tests were completed separately for leading noise monitoring apps culminating in 1472 tests. The results suggest that apps written for the iOS platform are superior to those running on the Android platform. They show that one of the apps tested – SLA Lite – is within ±1 dB of true noise levels across four different reference conditions. The results also show that there is a significant relationship between phone age and its ability to measure noise accurately. The research has implications for the future use of smartphones as low cost monitoring and assessment devices for environmental noise.     

某乘用车加速工况车内轰鸣声诊断分析及控制方法

该文针对某乘用车加速工况出现的轰鸣声进行诊断分析与控制。首先通过主观评价确定问题工况,通过实车测试发现2400 r/min左右车内存在明显轰鸣声。利用阶次分析、模态分析及错频验证可知,发动机激励经悬置传递副车架,引起副车架的共振,进而传递到车内引起轰鸣声。最后通过传递分析及柔性连接点导纳法,优化副车架橡胶衬套结构。结果表明,优化后在2400 r/min声压总级与2阶声压差值为7.5 dB(A);声压总级较原状态降低2 d B(A),2阶噪声降低7.8 d B(A),声压总级和2阶噪声声压差值增大,轰鸣声明显减小,主观评价可接受。     

公路交通噪声阈限值的分析研究

本文采用客观测量和主观评价相结合的方法研究公路交通噪声对人的影响。通过研究，求得了公路交通噪声扰民的容忍阈值，分析了比较了公路交通噪声各种客观评价量与人的主观评价量的之间的相互关系。