Maximum Sound Level Research Articles

The effect of the angular error on the determination of the sound power levels of industrial coolers

Drycoolers are relevant noise sources in many industrial plants due to their typically exposed positioning. For this reason, drycoolers are often subject to strict acoustic requirements in the form of maximum permissible sound power levels. Thus, the suppliers have to provide proof of compliance with these requirements before delivery. The acoustic verification measurement is usually carried out by measuring the sound pressure level using the enveloping surface method in accordance with EN 13487. According to EN 13487, the measurement is carried out at discrete points on a cuboid measuring surface. Since drycoolers are sound sources that extend over a large area, an angular error occurs when measuring sound pressure levels on a cuboid measuring surface, which leads to a superelevation of the measured sound pressure level, since the acoustic particle velocity on the cuboid measuring surface is not perpendicular to the measuring surface at almost any point. Therefore, the sound power level determined is methodically too high. By measuring the sound intensity levels in accordance with ISO 9614-2 instead of the sound pressure levels, the influence of this angular error can be excluded and brought into good agreement with theoretical considerations and calculations of the angular error.

Acoustic mechanism and noise reduction optimization of globe valve in air conditioning system

The globe ball valve will invariably emit inevitably generates noise during operation. The flow field characteristics and sound field characteristics inside the shut-off throttle were obtained by numerical calculations. In order to suppress the noise, this paper proposes an optimized model for this type of valve. Optimization valves were set up for different simulation groups to identify the appropriate fillet radius. The effect of the two strategies on noise reduction is examined in terms of noise distribution, path lines form, and turbulence distribution. The results show that the main throttling area of the valve generates more noise compared to the secondary throttling area. The sound power level values are highest in the center and decrease from there to the wall. The best noise reduction effect was achieved by rounding the radius of 8 mm in the primary throttling area. The maximum noise reduction in the primary throttling area is 0.45 dB, and the maximum noise reduction in the secondary throttling area is 7.5 dB. For sensitive locations in the secondary throttling area, rounded corners improve local noise reduction, with radius of 5 mm providing the best noise reduction. Maximum sound power level values are reduced by 5.8 dB and 6.86 dB in the primary and secondary throttling areas, respectively.

Evaluation of Noise Pollution in Residential Areas Under the Influence of Truck Traffic in Ramshir, Khuzestan-Iran

Background & Aims: Noise is one of the types of urban pollution that causes human annoyance. This study aimed at investigating noise pollution in residential areas under the influence of truck traffic in Ramshir in 2023. Materials and Methods: In this applied research, sound parameters were measured by a sound meter capable of measuring the minimum sound level, maximum sound level, equivalent continuous sound level (Leq), and sound pressure level (SPL) of sound parameters at 10 stations in the morning and at night with 3 repetitions. In addition, noise annoyance was determined in 100 people in residential areas exposed to urban noise using the ISO 15666 noise annoyance questionnaire. Measurements were performed over three months in two day and night shifts using a TES-1358C analyzer sound meter device (TES Company, Taiwan). The raw data were entered into SPSS 21 and Excel software and then categorized, and the necessary results were extracted, including the average of the data, as well as the maximum and the minimum of the data. Results: The results revealed that the average SPL was 88.25 dB during the daytime on Tawheed Street, so it had the highest measured sound level compared to other investigated areas. Further, the Leq was 82.17 dB at night in the Shariati area, which had a higher SPL than other areas. The highest Leq of the day was related to Naft Street Station, with an average value of 83.40 dB, while the lowest Leq value attributed to Damghani Street Station was measured with an average value of 71.41 dB. The highest Leq of night noise was related to Shariati Street Station, with an average value of 82.17 dB. The impact of truck noise was highly uncomfortable, uncomfortable, and slightly uncomfortable for 13%, 37%, and 36% of people, respectively. Conclusion: In general, in the range of 10 stations, the SPL and Leq were measured higher at night and day than the standards of the Iranian Environmental Organization and the Environmental Protection Agency (55 dB during the day and 45 dB at night for residential areas). According to the sum of the answers, the sound of the trucks in the intended city could hurt people. People are most dissatisfied with the sound of trucks in the morning and at night.

Effect of Ward-Noise-Reduction Management on the Mental Health and Quality of Life of Patients with Inflammatory Bowel Disease.

This study aimed to explore the effect of ward-noise-reduction management on the mental health and quality of life of patients with inflammatory bowel disease. The medical records of 275 patients with inflammatory bowel disease admitted to the First Affiliated Hospital of Wenzhou Medical University from January 2020 to January 2023 were retrospectively analyzed. Routine care was performed for such hospitalized patients from January 2020 to July 2021. Thus, 124 patients were enrolled in the control group. From August 2021 to January 2023, our hospital implemented ward-noise-reduction management for such inpatients, and 151 patients were included in the observation group. The Athens Insomnia Scale (AIS), the State-Trait Anxiety Inventory, the Inflammatory Bowel Disease Questionnaire (IBDQ), and the noise level at the time of admission and discharge were compared. No significant difference in the State Anxiety Scale (S-AI), Trait Anxiety Scale (T-AI), and AIS and IBDQ scores at baseline existed between the two groups (P > 0.05). After nursing, the S-AI, T-AI, and AIS scores of the observation group were lower than those of the control group, and the IBDQ score of the observation group was higher than that of the control group (P < 0.05). The noise level of the observation group was lower than that of the control group during hospitalization in maximum sound level and average intermediate (P < 0.05). The application of ward-noise-reduction management in the nursing of patients with inflammatory bowel disease can improve their negative mood, improve their sleep quality, and quality of life, and reduce the ward noise level in maximum sound level and average intermediate, which has high clinical value.

Noise analysis of dry scroll vacuum pump based on experiment and CFD

The scroll vacuum pump is a dry vacuum pump with a wide range of applications. The radiated noise of a multi-stage scroll vacuum pump under different suction pressures is tested. The discharge velocity field is measured by the PIV technique. When the suction pressure changes from 101 kPa to 5 Pa, the average sound pressure level of the scroll pump decreases from 80.37 dB to 58.13 dB, which is reduced by 22.24 dB. The maximum and minimum sound power levels are 97 dB and 74.76 dB, respectively. The spectrum characteristics and the distribution law of radiated noise are analyzed. The measured noises at the rear of all operating conditions are the smallest, followed by the positions above the front part, and the next is the directly above position. Aeroacoustics is the main noise source in the scroll pump. Based on the CFD method, the simulation calculation is carried out at different suction pressures and the maximum simulation error is −5.7%. The location and formation mechanism of the aeroacoustics source are analyzed. In different working processes, the main excitation sources of aeroacoustics are different. The results are helpful to the noise source identification and aeroacoustics improvement of the scroll vacuum pump.

Development of Louvered Noise Barrier with Changeable Sound Insulation from Waste Tire Rubber and Investigation of Acoustic Properties

In line with circular economy principles, the recycling and reuse of tire rubber waste are considered highly advanced and environmentally friendly waste disposal methods. Through the repurposing of tire rubber waste, the goal is to minimize environmental impact while creating a louvered noise barrier with sound attenuation capabilities. The acoustic properties of the structure made of used tire rubber granulate are investigated in this research. Firstly, nine rubber granulate plates of different fractions, thickness, and density were produced. Two plates with the best results were selected after an impedance tube analysis of their sound absorption (α) and sound transmission loss (DTL). These plates were used as a filler in the structure of the louvers. The efficiency of the structure and its dependence on the tilting angle of the louvers and the number of plates were investigated in a semi-anechoic sound-absorbing chamber. The maximum sound level reduction observed was 17.3 dB (in the 8000 Hz frequency band), and the maximum equivalent sound level loss (LAeq) was 7.3 dBA.

Evaluation of Noise Pollution Level in Selected Sawmill Factories in Port Harcourt and Environs, Rivers State, Nigeria

Evaluation of noise pollution level has been carried out in in some selected Sawmill factories in Port Harcourt and environs using noise level and frequency meters in seven locations. The mean values of maximum and minimum sound levels in Mile2, Mile3, Marine base, Oluwood were 81.98±0.43 and 79.28±2.00, 82.450±0.66 and 80.28±0.80, 81.50±4.73 and 74.75±1.10, 81.40±0.40 and 78.03±2.02 respectively. Also the mean values of maximum and minimum sound levels at Igwuruta, Rumosi, Rumuji were found to be 82.00±0.64 and 79.40±2.00, 81.53±0.60 and 78.66±1.63, 81.78±0.40 and 77.95±1.86 respectively. The average noise equivalent (Leq) ranged from 77.20 to 82.25 dB whereas the average noise pollution level (Lnp) ranged from 96.13 to 101.23 dB. Therefore, the indication is that the Sawmill at Igwuruta has the highest average Lnp whereas Rumuosi has the least average when compared with the recommend permissible value by the Nigerian Federal Ministry of Environment (FMEnv). This study has determined the noise level generated in study area through set objectives which included but not limited to measuring the sound level, investigating noise some meters away from the source and of course comparing results with best known standards. The results therefore suggested that the noise level is high at the saw mills and the day exposure to this unwanted sound could threaten the health of the factory workers and the residents at proximity to the noise sources.

Establishing a definition for maximum hourly sound level

The use of a maximum hourly sound level is common within modern standards and design requirements, referenced in ANSI and ASHRAE standards, CHPS and LEED design guidelines, and is included in proposed language for draft ANSI standards, to name a few. Due to its wide adoption, its use is proscribed for education, hospitality, residential, retail, office, affecting nearly every design sector. The problem with this is that the metric lacks formal definition, leaving designers and evaluators without an agreed-upon approach to determine if an assessment is sufficient. Since 2014, the authors have presented multiple works assessing a proposed definition for maximum hourly sound level for vehicular sources. In this work, the authors will be presenting a continuation of the dataset. Long-term rooftop monitoring of the noise from an interstate was used to create an hourly max calculation noise model. The calculations are based on the statistical spread of the data and the learnings can be used to better design the structures for assessing environmental and traffic noise.

Research on Volume Array Based on Curved Disk Transducers

A curved disk transducer with a dual stack structure with a resonant frequency of 800Hz is used as the array element, and the mutual radiation effect between the transducer elements is utilized to form a densely arranged volume array. Based on the formation structure, establish corresponding finite element models in COMSOL software to simulate and analyse the acoustic performance of different volume arrays. Finally, prototype production and array acoustic performance testing were conducted, and the measured data basically matched the simulation results. The results show that compared to a single transducer, utilizing the mutual radiation effect between the transducer elements can effectively reduce the resonant frequency of the volumetric array and expand its operating bandwidth, while also improving the maximum sound source level after formation. In addition, with a reasonable number of array elements, keeping the spacing between the elements constant, the more elements there are, the lower the resonant frequency of the volumetric array, the smaller the quality factor, and the higher the emission voltage response.

Environmental Impact Assessment of Onshore Wind Power Plants: A Case Study of a 50 MW Wind Power Plant in Northeastern Thailand

This research aims to assess the environmental feasibility of a wind power plant by investigating its noise disturbances, shadow flicker, and zones of visual influence. The model is applied as a case study for a 50 MW wind power plant, located in the Nakhon Ratchasima province of northeastern Thailand. The acoustic noise emissions were analyzed using the sound propagation and absorption models under the wind conditions on the site studied. The shadow flicker around each wind turbine generator, in terms of the number of hours per year, was also simulated along with the analysis of the zones of visual influence according to the number of wind turbines that can be seen by an observer from a certain distance. The results show a maximum sound level of 47 dBA, within the allowed limits of the 50 dBA legislation of the Department of Pollution Control of the Royal Thai Government. Similarly, the shadow flicker within 1 km of the wind turbines is 10 hours/year, well below the international standard of 30 hours/year. Results of the zones of visual influence indicate that between 15 and 20 turbines are visible from observation points surrounding the potential wind power plant. The results applied to this case study suggest that the potential wind power plant is well-suited regarding its environmental impacts and should typically not incur negative impacts for the local communities. Studies like these are vital to gaining the trust of the communities living near wind power plants to address their concerns and minimize opposition.

Aerodynamic noise characteristics of a centrifugal fan in high-altitude environments.

In high-altitude areas, the air is thin and the atmospheric pressure is low, which can affect the performance of centrifugal fans and aerodynamic noise. In this paper, steady and unsteady simulations of a centrifugal fan flow field are performed at altitudes of 0, 1000, 2000, 3000, 4000, and 5000 m, and the Ffowcs Williams-Hawkings equation is used to predict the aerodynamic noise of the fan. The results indicate that the tonal and broadband noise generated by the fan decrease with increasing altitude, and the A-weighted sound pressure level of each frequency band of the fan decreases when the air volume is held fixed. The maximum sound power level Lwmax, sound pressure pulsation interval, and total noise sound pressure level Lp decrease linearly with increasing altitude. For every 1000 m increase in altitude, Lwmax and Lp decrease by 0.45 dB and 1.05 dB respectively. The fan noise characteristics, performance parameters, and human auditory perception are the main factors that affect the establishment of fan noise standards in high-altitude areas.

Assessment of the public health risk from noise pollution of the Saint Petersburg center

The purpose of this study is to assess the risk to public health from exposure to road noise in the historical city center of St. Petersburg., the source of which is automobile (freight, passenger, public) transport. The presented article is devoted to an urgent problem – the assessment of the noise environment and the levels of the emerging risk to the health of the population exposed to traffic noise in dense urban areas. The risk assessment was carried out based on the results of field measurements of noise levels in the zone of influence of traffic flows. The quantitative risk assessment was carried out in accordance with the methodological recommendations of MP 2.1.10.0059-12 «Assessment of public health risk from exposure to traffic noise» and Instructions 2.1.8.10-12-3-2005 «Assessment of the risk to public health from exposure to noise in populated areas». The relevance of the work is confirmed by the fact that motor transport is becoming an increasingly priority source of noise pollution of streets, house territories, residential areas. The analysis of the results of the performed acoustic measurements indicates that the values of the equivalent and maximum sound levels at all four control points exceeded the normative noise exposure levels from 6 to 29.9 dBA. At the same time, the actual values varied from 61.2 dBA to 74.9 dBA for L(A)eq. and from 70.4 to 85.7 L(A)max. The obtained result indicates a significant noise impact of motor transport on the nearest residential area, which can lead to adverse health consequences.

Environmental Noise Mapping: GIS-Based Study of Urban Industrial Zones and Traffic Areas

Urbanization and industrialization present significant challenges to public health, particularly in managing environmental noise pollution. This study addresses the critical issue of noise pollution at the study area by developing a comprehensive noise map that encompasses industrial zones, construction sites, and areas with heavy traffic. Utilizing ArcGIS modeling software, the research visualizes the spatial dynamics of noise pollution, resulting in a detailed map that represents noise levels throughout the area. The analysis reveals that noise levels during weekday peak hours average 70.5 dBA, markedly higher than the 65.5 dBA average recorded during weekend peak hours. During weekdays morning peak, noise levels indicate three monitoring points in the red zone, two in the yellow zone, and five in the green zone. Notably, noise levels at all ten monitoring locations during weekdays peak hours exceeded the maximum permissible sound level of 60 dBA set by the Department of Environment for residential and commercial areas. These findings contribute to the development of targeted noise mitigation strategies and serve as a valuable resource for environmental authorities, policymakers, and urban planners. This research underscores the importance of a comprehensive approach to addressing environmental noise pollution and provides a practical framework for urban areas facing similar challenges.

Production noise on an automated molding line in a foundry

The purpose of the work is to determine the nature of the noise generated during the operation of manual pneumatic rammers, an automated molding line and knockout grids in the foundry shop based on determining the levels of acoustic noise parameters in the mold manufacturing area and ways to reduce their level. To determine the noise parameters, a mode noise spectrum analyzer SL-5868F was used with subsequent digitization of the data obtained on the analyzer. Measurements of noise parameters were carried out at the site for the production of foundry molds with manual pneumatic rammers and an automatic molding line, as well as at the site for knocking out castings on knockout grids. It has been established that the noise generated in the areas of molding and knocking out molds, regardless of the molding method (manual, automatic) and the model of the knockout grid used, is broadband, and there is no homogeneity in the sound field of the generated noise, which is due to the generation of noise from sources characterized by differences in acoustic power levels and the nature of the noise spectrum. The noise from equipment or a technological operation with a shock nature is not constant and has a maximum sound power level in the region of medium and high frequencies generated by the equipment under study. It is possible to reduce the noise level in the foundry shop analyzed in the work by implementing a set of measures to modernize the molding and knocking out equipment or introducing into production an alternative method of manufacturing and knocking out foundry molds, in which, if it is possible to produce castings of a given nomenclature in the required volume with the required level of quality, the above mentioned are excluded and other sources of noise generation of unacceptable levels.

An analytical investigation into noise levels in public health sector neonatal intensive care units in the eThekwini District

Background. Noise is a known environmental stressor in the neonatal intensive care unit (NICU), as it may result in adverse effects on preterm neonates because of the unique vulnerability and physiological immaturity of their central nervous systems. Objective. To investigate noise levels in public sector NICUs in the eThekwini District, KwaZulu-Natal Province, South Africa. Methods. An analytical observational study design with purposive sampling of public sector hospitals was used. Noise was continuously measured with a sound level meter in a central location for 48 hours on 2 consecutive days (Sunday and Monday) in the four NICUs. A sample of noise sources, as well as their frequency of occurrence, was identified through direct observation and a frequency spectrum analysis using one-third octave bands. Data were analysed using descriptive and inferential statistics. Results. This study included one tertiary hospital and three regional hospitals in the eThekwini District. Mean noise levels exceeded international recommendations of an A-weighted equivalent continuous sound level (LAeq) of 45 A-weighted decibel (dBA) and an A-weighted maximum sound level (LAmax) of 65 dBA in all four hospitals. The most frequently occurring sources of noise were staff conversations (30.9%, Hospital A), device alarms (21.0%, Hospital B) and closing metal pedal bins (20.0%, Hospital B). Mean LAeqs &gt;45 BA were found in the mid and high frequencies (250 Hz - 6 300 Hz) in all hospitals, particularly during the afternoon. Conclusion. The findings emphasise the need for continuous noise monitoring, awareness and education among healthcare professionals in the NICU. Future research should expand on existing findings and focus on interventions for noise control in NICUs.

Evaluation of the sensory environment in a large tertiary ICU

BackgroundICU survival is improving. However, many patients leave ICU with ongoing cognitive, physical, and/or psychological impairments and reduced quality of life. Many of the reasons for these ongoing problems are unmodifiable; however, some are linked with the ICU environment. Suboptimal lighting and excessive noise contribute to a loss of circadian rhythms and sleep disruptions, leading to increased mortality and morbidity. Despite long-standing awareness of these problems, meaningful ICU redesign is yet to be realised, and the ‘ideal’ ICU design is likely to be unique to local context and patient cohorts. To inform the co-design of an improved ICU environment, this study completed a detailed evaluation of the ICU environment, focussing on acoustics, sound, and light.MethodsThis was an observational study of the lighting and acoustic environment using sensors and formal evaluations. Selected bedspaces, chosen to represent different types of bedspaces in the ICU, were monitored during prolonged study periods. Data were analysed descriptively using Microsoft Excel.ResultsTwo of the three monitored bedspaces showed a limited difference in lighting levels across the day, with average daytime light intensity not exceeding 300 Lux. In bedspaces with a window, the spectral power distribution (but not intensity) of the light was similar to natural light when all ceiling lights were off. However, when the ceiling lights were on, the spectral power distribution was similar between bedspaces with and without windows. Average sound levels in the study bedspaces were 63.75, 56.80, and 59.71 dBA, with the single room being noisier than the two open-plan bedspaces. There were multiple occasions of peak sound levels > 80 dBA recorded, with the maximum sound level recorded being > 105 dBA. We recorded one new monitor or ventilator alarm commencing every 69 s in each bedspace, with only 5% of alarms actioned. Acoustic testing showed poor sound absorption and blocking.ConclusionsThis study corroborates other studies confirming that the lighting and acoustic environments in the study ICU were suboptimal, potentially contributing to adverse patient outcomes. This manuscript discusses potential solutions to identified problems. Future studies are required to evaluate whether an optimised ICU environment positively impacts patient outcomes.

Effective Engineering Noise Control Strategies with the Stochastic Noise Mapping Simulation Method: A Case Study in Palm Oil Mill

Excessive sound levels pose a substantial risk to the hearing health of industrial workers. This study introduces a stochastic noise mapping technique to propose effective engineering control methods for noise mitigation in the palm oil industry. A case study conducted in a Malaysian palm oil mill employed data to predict the noise mapping and assess personal noise exposure levels of workers. Results showed that 29.6% of the area fell within the high-risk zone, with 28.1% in the extremely high-risk zone. Personal noise monitoring revealed that many workers were exposed to excessive noise, with a maximum time-weighted average sound level of 99.2 dBA among the worker groups. Despite the provision of protective equipment by management, regular usage of hearing protectors during work shifts was lacking. To reduce noise levels, two engineering control methods were proposed: erecting noise barriers around high noise emission sources and limiting the maximum sound power to 110 dBA for dynamic noise sources. Implementing these controls resulted in a 10.9% decrease in the extremely high-risk zone and a 17.8% decrease in the high-risk zone. These findings hold significance as they can guide the implementation of noise monitoring practices and engineering noise control in the industry.

Assessment of noise pollution level in a tertiary care hospital of Imphal, Manipur

Background: Noise pollution is a major public health problem. Previous studies reported the presence of noise pollution in various hospital settings around the world. There are few studies conducted on noise pollution in the north eastern part of India especially in Manipur. Therefore, the study was undertaken to assess noise pollution in a tertiary hospital of Manipur for future suggestive measures to reduce noise and associated health impacts. Methods: An observational study was conducted in a public tertiary care hospital of Imphal, Manipur. Data collection for noise level measurement was done by using a digital sound level meter instrument and mobile sound meter app. The data was analyzed using IBM SPSS for Windows, version 21.0. Armonk, NY. Results: The highest sound level of 87.6 dB(A) was recorded in the general OPD counter. Among the OPDs, Medicine OPD recorded the highest sound level of 85.2 dB(A). Parking areas recorded an average maximum of 80.5 dB(A). Hospital wards recorded an average maximum sound of 77.4 dB(A). Conclusions: In this study, the maximum sound level in a week average was found in parking areas of the hospital. Majority of the places around the hospital recorded a noise level which is beyond the recommended CPCB noise level 2000.

A Study on Modifying Campus Buildings to Improve Habitat Comfort—A Case Study of Tianjin University Campus

At present, the design and planning of teaching and living areas on university campuses are relatively straightforward but encounter problems, such as poor ventilation, low indoor air quality, and poor sound insulation. In this study, the teaching building and living area cluster at the Tianjin University campus were selected as the research objects. We verified the effectiveness of the simulation results before and after renovation through onsite testing. To improve ventilation, an atrium and patio were added to the teaching building, and the ventilation of the renovated building was studied. The indoor thermal environment intelligent control system regulates carbon dioxide (CO2) concentration and humidity in the teaching building and changes the thermal comfort of the teaching building. Limiting vehicle speeds near the teaching building and the living area cluster, using muffling materials and muffling equipment, and increasing greenery to reduce noise were factors we studied, considering whether they had a noise-reduction effect. It was found that the average number of air changes in the overall functional space of the first teaching building reaches 6.49 times/h, and the wind speed in the human activity region is below 1 m/s. When using a thermal environment intelligent control system, the indoor temperature throughout the year was within the thermal comfort range 81% of the time. The maximum noise around the teaching building during the daytime was 51.0 dB, the maximum noise at nighttime was 41.5 dB, and the maximum sound level on the facade of the living area cluster was 53 dB. The average noise-reduction rate was 22.63%, which exceeds the noise-reduction rate given in the above research literature.

INVESTIGATING THE ENVIRONMENTAL NOISE IMPACT OF MASS RAPID TRANSIT ON NEARBY COMMUNITIES AT BATU 11 CHERAS, MALAYSIA

Mass Rapid Transit (MRT) in Malaysia is designed to provide commuters with a comfortable journey that is highly functional and safe. Besides, it allows commuters to explore the city with ease and convenience. Due to the extension of the network throughout the years, the noise generated from MRT had increased and this environmental noise tends to annoy the adjacent community. This research aims to evaluate the environmental noise impact generated by MRT on the surrounding community at Batu 11 Cheras Station. A total of 4 monitoring points were selected for environmental noise impact assessment. After that, a sound level meter was set up at these monitoring points to measure the sound level and some parameters such as LAeq, Lmax, Lmin, L10, L50, and L90. For each monitoring point, the noise level was measured for 6 hours continuously from 7:00 am to 1:00 pm. Based on the results, the noise levels at critical monitoring points exceeded 2.71% of the permissible sound level in Batu 11 Cheras Station. While for the maximum sound level, all four-monitoring points A, B, C and D had exceeded 2.75%, 10.13%, 4.38%, and 7.13%, respectively. For community annoyance impact, little levels of environmental impact and subjectively perceived noticeable differences were found at all the monitoring points.