Low-frequency Sound Level Research Articles

Predicting low-frequency noise using its own bandwidth

Soon after A-weighing began, an article of 1938 mentioned that as a metric it didn't represent true low-frequency loudness. In 1952, a method for “guessing” low-frequency energy was proposed and later, Beranek in 1954 recommended that if the dBC level is greater than dBA, an analysis should be performed below 150 Hz. In 1969, Botsford introduced a criterion for predicting the low-frequency level by [(dBC-dBA) > 10 dB], using the entire audible spectrum, so that this condition applies only if the spectrum is “balanced.” The German Standard DIN 45680 of 1997 proposed a similar criterion, limiting this calculation to one-third octave band frequencies from 8 to 100 Hz. Since 1999, the WHO recommends using the Botsford criterion, but warns that it is not valid if the spectrum contains tones or if it is unbalanced. This paper will demonstrate that the correct way to analyze the low-frequency sound levels is to use its own bandwidth, that to have a traceable noise descriptor the ISO low-frequency definition must be used, it means to use the one-third octave band frequencies between 16 and 200 Hz, to calculate the difference (dBC-dBA) in this range to predict its actual low-frequency sound level.

Analysis of the impact of low-frequency traffic noise in two South American cities

Traffic noise is one of the most important sources of annoyance in South American cities. The obsolescence of vehicles and the lack of controls are two of the main causes of this noisy traffic. Since the legislation considers it as a typical source of urban noise, the metric to analyze traffic noise is the A-weighted dB (so-called dBA); therefore, it is not easy to analyze its real impact on human health because low-frequency sound levels are strongly reduced by A-weighting. This article presents the results of a field study carried out in Lima (Peru) and Montevideo (Uruguay) to analyze the impact of traffic noise focused the lowest frequencies, in the range defined by ISO (1/3 octave bands between 16 and 200 Hz), and the WHO recommendation using the whole audible frequencies. The data for Lima were taken in places where a specific acoustic EIS has been required, while the Montevideo´s one were for its noise map. The spectra were processed looking for a representative low-frequency traffic noise metric, so the values of (C–A) obtained byWHO recommendation and ISO recommendation are compared. The expected impact on human health is much more worrisome when low-frequencyrange are explicitly considered.

Artificial sound impact could put at risk hermit crabs and their symbiont anemones

The sea anemone Calliactis parasitica, which is found in the East Atlantic (Portugal to Senegal) and the Mediterranean Sea, forms a symbiotic relationship with the red hermit crab, Dardanus calidus, in which the anemone provides protection from predators such as the octopus while it gains mobility, and possibly food scraps, from the hermit crab. Acoustic pollution is recognised by the scientific community as a growing threat to ocean inhabitants. Recent findings on marine invertebrates showed that exposure to artificial sound had direct behavioural, physiological and ultrastructural consequences. In this study we assess the impact of artificial sound (received level 157 ± 5 dB re 1 μPa2 with peak levels up to 175 dB re 1 μPa2) on the red hermit crab and its symbiotic sea anemone. Scanning electron microscopy analyses revealed lesions in the statocyst of the red hermit crab and in the tentacle sensory epithelia of its anemone when exposed to low-intensity, low-frequency sounds. These ultrastructural changes under situations of acoustic stress in symbiotic partners belonging to different phyla is a new issue that may limit their survival capacity, and a new challenge in assessing the effects of acoustic disturbance in the oceanic ecosystem. Despite the lesions found in the red hermit crab, its righting reflex time was not as strongly affected showing only an increase in the range of righting times. Given that low-frequency sound levels in the ocean are increasing and that reliable bioacoustic data on invertebrates is very scarce, in light of the results of the present study, we argue that anthropogenic sound effects on invertebrates species may have direct consequences in the entire ecosystem.

Methodological Foundations Protective Structures Development for Shielding Electromagnetic and Acoustic Fields

In many cases the simultaneous protection against electromagnetic and acoustic fields of wide frequency ranges is required. The complexity of such a task lies in the impossibility of significantly reducing low-frequency sound levels with traditional sound-absorbing materials, as well as the different requirements for materials that shield low-frequency and high-frequency electromagnetic fields. It is proposed to solve this problem by creating a two-layer structure, the front surface of which is solid, and the inner surface is perforated. The front panel can be made of non-conductive material. It is a variant of the Bekeshi panel and is mainly intended to reduce the level of low-frequency sound of a certain frequency with high amplitude. The perforation of the internal (metal) panel is chosen based on the need to provide shielding of electromagnetic and acoustic fields of certain frequencies or frequency bands. A calculator is provided for calculating the required panel parameters (linear dimensions, thickness, hole diameters, their number per unit of surface area). It is advisable to fill the space between the panels with a sound-absorbing material, for example, granulated polystyrene. It provides sound absorption of medium and high frequencies and makes the design broadband. The use of ferromagnetic material of the inner panel provides protection against the magnetic component of the ultra-low frequency electromagnetic field (mainly industrial). The perforation of the panel is calculated based on the waveguide theory of high-frequency electromagnetic waves. The results of tests of the effectiveness of facing metal-polymer material in the form of tiles are presented. The performance of the material is satisfactory for most industrial and domestic conditions. Determination of the mechanical properties of the material (Young's modulus, shear modulus and Poisson's ratio) showed that it is not inferior to known materials even with a large content of shielding substance. In order to rationalize the design of the structure, a priority factor and frequency (frequency band) is selected on the basis of live measurements of the electromagnetic and acoustic spectra. It is shown that the calculations have large volumes, and the solution of two-factor optimization problems is not always possible. Appropriate creation of application software to simplify the process of designing protective structures and rationalizing panel parameters based on the principles of reasonable sufficiency.

Effects of detailed shape of air-shaft entrance hood on tunnel pressure waves

Air-shaft entrance hood model with fence is investigated to reduce micro pressure wave in a single-track tunnel of the high-speed railway. Aerodynamically designed airshaft hoods are classified by 5 cases according to fence types and the micro pressure waves are measured by the low frequency sound level meters installed at a tunnel exit of a moving model test rig with a scale ratio of 1/64. Based on the case of no hood model, 5 different hood models were tested and analyzed for the speed of a model train from 180 km/h to 250 km/h.

A newly designed entrance hood to reduce the micro pressure wave emitted from the exit of high-speed railway tunnel

Micro-pressure waves (MPWs) emitting from the exit of a railway tunnel pose a serious environmental hazard causing vibration and rattling of nearby structures. Consequently, with the advent of ever increasing train speeds, it has become extremely important for tunnel engineers and designers to control the emission of MPWs from the exit of a railway tunnel. The most economical and effective method known till date to reduce the magnitude of MPWs is by installing an aerodynamic hood at the entrance of a tunnel. Therefore, a newly designed entrance hood with air-slits attached on each side is studied in the current work. The air-slits are designed to bio-mimic the ‘ram ventilation’ technique used for respiration by hunter shark gills. These air-slits reduce the maximum value of the pressure gradient of the compression wave generated near the entrance. Thereby, a subsequent reduction in the maximum magnitude of MPWs emitted from the exit of tunnel is observed. A prototype of this entrance hood has been employed on a 1/64.2 reduced scale single track model tunnel and experimental tests were conducted using a model train with the nose shape of EMU-250 at two entry speeds of 180 km/hr. and 250 km/hr.. Pressure on the tunnel wall were measured at six different locations using a piezo-resistive pressure transducer and MPW at the tunnel exit were captured at three locations using a low-frequency sound level meter. A reduction in the pressure gradient of the compression wave and magnitude of MPW of about 56.3% and 78.7% respectively was observed with the installation of this entrance hood at a train speed of 250 km/hr. The obtained pressure transient data and the corresponding MPW values for both the train speeds are presented in detail with clear elucidations on the flow phenomena. Further, an analytical model to predict MPWs was developed using the solution of a vibrating circular piston in an infinite baffle plate. The predicted results of the analytical model when compared with the experimental values show a reasonably good match for the peak pressure magnitude and waveform of the MPW.

Low Frequency Broad Band Acoustic Propagation in Andaman Sea

During November 2017, an active source seismic survey was performed in Andaman sea to study the acoustic propagation characteristics. High power low frequency acoustic signals generated by 20-air gun array onboard ORV Samudra Ratnagar were recorded from INS Sagardhwani at four different depths within 8 km ranges in shallow and deep waters. Low frequency sound levels were estimated using root mean square and power spectral values. Amplitude levels were analysed with respect to arrival time variation with frequency and is presented.

Low Frequency Noise Level Assessment in Vienna

The present contribution reports on the results of sound level measurements in a number of locations in the city of Vienna, Austria. Thereby, a primary objective was to determine the degree to which the measurement results agree with corresponding information in the E.N.D. (Environmental Noise Directive 2002/49/EC) maps. Moreover, the relationship between the low-frequency segment of the acoustical exposure to the broad-band data was investigated. The results point to traffic as the main source of urban noise exposure. E.N.D. maps appear to provide a reasonable general overview of the urban noise circumstances. However, measurement results at individual locations can considerably deviate from E.N.D. data. Numeric values of low-frequency sound level range were found to be generally higher than those of the broad-frequency levels. The results revealed also a strong correlation between measurement-based L50R and NR (Noise Rating) values.

Multiscale spatio-temporal patterns of boat noise on U.S. Virgin Island coral reefs

Sound-sensitive organisms are abundant on coral reefs. Accordingly, experiments suggest that boat noise could elicit adverse effects on coral reef organisms. Yet, there are few data quantifying boat noise prevalence on coral reefs. We use long-term passive acoustic recordings at nine coral reefs and one sandy comparison site in a marine protected area to quantify spatio-temporal variation in boat noise and its effect on the soundscape. Boat noise was most common at reefs with high coral cover and fish density, and temporal patterns reflected patterns of human activity. Boat noise significantly increased low-frequency sound levels at the monitored sites. With boat noise present, the peak frequencies of the natural soundscape shifted from higher frequencies to the lower frequencies frequently used in fish communication. Taken together, the spectral overlap between boat noise and fish communication and the elevated boat detections on reefs with biological densities raises concern for coral reef organisms.

Monitoring long-term soundscape trends in U.S. Waters: The NOAA/NPS Ocean Noise Reference Station Network

The National Oceanic and Atmospheric Administration (NOAA)/National Park Service (NPS) Ocean Noise Reference Station (NRS) Network is an array of currently twelve calibrated autonomous passive acoustic recorders. The first NRS was deployed in June 2014, and eleven additional stations were added to the network during the following two years. The twelve stations record data that can be used to quantify baseline levels and multi-year trends in ocean ambient sound across the continental United States, Alaska, Hawaii, and island territories within and near to the United States Exclusive Economic Zone (U.S. EEZ). The network provides multi-year, continuous observations of low-frequency underwater sound between 10 Hz and 2000 Hz to capture anthropogenic, biological, and geophysical contributions to the marine soundscape at each location. Comparisons over time and among recording sites will provide information on the presence of calling animals and the prevalence of abiotic and anthropogenic activities that contribute to each soundscape. Implementation of the NRS Network advances broad-scale passive acoustic sensing capabilities within NOAA and the NPS and is an important tool for monitoring protected areas and marine species and assessing potential environmental impacts of anthropogenic noise sources. This analysis focuses on the first year of recordings and captures the wide variability of low-frequency sound levels among and within individual NRS sites over time. Continued data collection will provide information on long-term, low-frequency sound level trends within or near the U.S. EEZ and will be used to explore the value of using soundscape analysis to inform management and mitigation strategies.

Is low frequency ocean sound increasing globally?

Low frequency sound has increased in the Northeast Pacific Ocean over the past 60 yr [Ross (1993) Acoust. Bull. 18, 5-8; (2005) IEEE J. Ocean. Eng. 30, 257-261; Andrew, Howe, Mercer, and Dzieciuch (2002) J. Acoust. Soc. Am. 129, 642-651; McDonald, Hildebrand, and Wiggins (2006) J. Acoust. Soc. Am. 120, 711-717; Chapman and Price (2011) J. Acoust. Soc. Am. 129, EL161-EL165] and in the Indian Ocean over the past decade, [Miksis-Olds, Bradley, and Niu (2013) J. Acoust. Soc. Am. 134, 3464-3475]. More recently, Andrew, Howe, and Mercer's [(2011) J. Acoust. Soc. Am. 129, 642-651] observations in the Northeast Pacific show a level or slightly decreasing trend in low frequency noise. It remains unclear what the low frequency trends are in other regions of the world. In this work, data from the Comprehensive Nuclear-Test Ban Treaty Organization International Monitoring System was used to examine the rate and magnitude of change in low frequency sound (5-115 Hz) over the past decade in the South Atlantic and Equatorial Pacific Oceans. The dominant source observed in the South Atlantic was seismic air gun signals, while shipping and biologic sources contributed more to the acoustic environment at the Equatorial Pacific location. Sound levels over the past 5-6 yr in the Equatorial Pacific have decreased. Decreases were also observed in the ambient sound floor in the South Atlantic Ocean. Based on these observations, it does not appear that low frequency sound levels are increasing globally.

Low-frequency sound level in the Southern Indian Ocean.

This study presents long-term statistics on the ambient sound in the Southern Indian Ocean basin based on 2 years of data collected on six widely distributed autonomous hydrophones from 47°S to 4°S and 53°E to 83°E. Daily mean power spectra (10-100 Hz) were analyzed in order to identify the main sound sources and their space and time variability. Periodic signals are principally associated with the seasonal presence of three types of blue whales and fin whales whose signatures are easily identified at specific frequencies. In the low frequencies, occurrence of winter lows and summer highs in the ambient noise levels are well correlated with iceberg volume variations at the southern latitudes, suggesting that icebergs are a major sound source, seasonally contributing to the ambient noise, even at tropical latitudes (26°S). The anthropogenic contribution to the noise spectrum is limited. Shipping sounds are only present north and west of the study area in the vicinity of major traffic lanes. Acoustic recordings from the southern sites may thus be representative of the pristine ambient noise in the Indian Ocean.

Is low frequency sound level uniformly increasing on a global scale?

Deep water ambient sound level increases have been documented in the eastern North Pacific Ocean over the past 60 years. It remains unclear whether this increasing trend is observed in other regions of the world. In this work, data from the Comprehensive Nuclear Test Ban Treaty Organization International Monitoring System (CTBTO IMS) were used to examine the rate and direction of low frequency sound level change over the past decade in the Indian, South Atlantic, and Equatorial Pacific Oceans. The sources contributing to the overall sound level patterns differed between the regions. The dominant source observed in the South Atlantic was sound from seismic air gun surveys, while shipping and biologic sources contributed more to the acoustic environment at the Equatorial Pacific location. Unlike the increasing trend observed in the NE Pacific, sound levels over the past 5–6 years in the Equatorial Pacific were decreasing. Decreases were also observed for specific sound level parameters and frequency bands in the South Atlantic Ocean. Based on these observations, it does not appear that low frequency sound levels are increasing in all regions of the worlds' oceans. [Work supported by the Office of Naval Research.]

Soundscape study methods of band rehearsal room

Soundscape study methods were applied to a band rehearsal room to understand the acoustical issues involved in this situation. A taxonomy of sound sources, direct observation to identify the specific sonic events and the multiple source and receiver paths involved in the complex listening and performing tasks that occur during band rehearsals were identified. It was found that students spend almost 1/2 of their rehearsal time involved with verbal instruction and discussion. Interviews with conductors and questionnaires administered to music students in three different band rooms were used to determine what musicians are listening for during rehearsals. Source and receiver combinations for physical acoustical measurements were located to study the multiple listening tasks identified in the questionnaires. Musicians were constantly trying to “hear each other” for intonation, rhythm, dynamics, articulation, and tone quality during rehearsals. Statistical models linking the qualitative results of the questionnaires with the acoustical measurements and architectural features of the rooms show that the ceiling height, room volume, area of sound diffusing surfaces, low frequency sound level, early reflected sound energy, and reverberation were related to the ability of musicians to hear each other and the detailed attributes of music.

Characteristics of Low-Frequency Pressure Waves Radiated From a Train Passing an Open Section

Characteristics of low-frequency pressure waves radiated from a train passing a Shinkansen viaduct were examined from field measurements based on the manual for measuring low-frequency noise issued by Ministry of the Environment Government in October 2000, using a low-frequency sound level meter (NA-18 A) manufactured by Rion Co. Ltd. Measurements proved that low-frequency noise comprises a hydrodynamic pressure variation and an acoustic pressure wave. The hydrodynamic pressure variation is designated as the train passing pseudo-sound and the acoustic pressure wave is designated as the low-frequency pressure wave. The low-frequency pressure wave is emitted from a line source and is proportional to the 4th-5th power of train velocity and inversely to the first power of the distance from the source to the observation point.

Characteristics of Low-Frequency Pressure Waves Radiated from a Train Passing an Open Section

The characteristics of the low-frequency pressure waves radiated from a train passing a viaduct of Shinkansen were examined through field measurement based on the manual for measuring low-frequency noise issued by the Ministry of the Environment in October 2000, by using a low-frequency sound level meter (NA-18 A) manufactured by Rion Co., Ltd. The measurement proved that the low-frequency noise consisted of a hydrodynamic pressure field and an acoustic pressure wave. The hydrodynamic pressure field is called train passing pseudo-sound and the acoustic pressure wave is called low-frequency pressure wave. The low-frequency pressure wave is seemingly emitted from a line source. The low-frequency wave is also proportional to the 4-5 th power of train velocity and approximately to the 1 st power of the distance from the source to the observation point.

市街地の音環境を聞こえる音の種類で評価するための手法の研究 : 沖縄県那覇市市街地を例として

Focusing on the factor of sound environment, in this research we have surveyed sound levels (L_<Aeq>), subjective appraisal, audible sound types and frequency analysis along boulevard, commercial and residential areas of Naha city. The research areas covered the newly developed city, the old city and the Naha city streets during the no-traffic designated periods. From the results of the frequency analysis it was observed that during no-traffic periods the low frequency sound level falls and in turn decreases the annoyance response. In the research of the newly developed city and old city the characteristics of the results of the audible sound types also appeared in the results of the frequency analysis. It was also observed that it was possible to determine the quantity of sound environment.

A Study of a Measurement Method to Obtain Waveforms of Pressure Variations Due to Train Passage

Low-frequency pressure variations observed at the time of train passage have been measured so far by a low-frequency sound level meter. The waveforms measured by this instrument are different from their original because of the influence of the frequency characteristic of the instrument. Therefore, it is desired to establish a method to grasp the original waveform of pressure variations. To obtain the original waveform, a method which restores the original waveform from the measured data obtained by the existing instrument is considered. To restore the original waveform, frequency characteristics of the low-frequency sound level meter is estimated through a frequency response function. The problem of restoring the original waveform is formulated as designing a digital inverse filter whose frequency characteristic corresponds to the inverse of that of measuring system. Through the application of the digital inverse filter to the data measured by the low-frequency sound level meter, effectiveness of the method to obtain original waveforms is verified.

Sound and the developing infant in the NICU: conclusions and recommendations for care.

To present the conclusions and associated recommendations for care developed by the Physical and Developmental Environment of the High-Risk Infant Center, Study Group on Neonatal Intensive Care Unit (NICU) Sound, and the Expert Review Panel. A multidisciplinary group of clinicians and researchers reviewed the literature regarding the effect of sound on the fetus, newborn, and preterm infant and developed recommendations based on the best evidence. An Expert Review Panel reviewed the data and conclusions. The following recommendations are developed from the review of the literature and are clarified in the body of the article. (1) Women should avoid prolonged exposure to low-frequency sound levels (< 250 Hz) above 65 dB(A) during pregnancy. (2) Earphones or other devices for sound production should not be used directly attached to the pregnant woman's abdomen. (3) The voice of the mother during normal daily activities, along with the sounds produced by her body and those present in her usual surroundings, is sufficient for normal fetal auditory development. The fetus does not require supplemental stimulation. Programs to supplement the fetal auditory experience cannot be recommended. (4) Infant intensive care units should incorporate a system of regular noise assessment. (5) Sound limit recommendations are to maintain a nursery with an hourly Leq of 50 dB(A), an hourly L10 of 55 dB(A) and a 1-second Lmax of 70 dB(A), all A-weighted, slow response scale. (6) Infant intensive care units should develop and maintain a program of noise control and abatement in order to operate within the recommended permissible noise criteria. (7) Care practices must provide ample opportunity for the infant to hear parent voices live in interaction between parent and infant at the bedside. (8) Earphones and other devices attached to the infant's ears for sound transmission should not be used at any time. (9) There is little evidence to support the use of recorded music or speech in the environment of the high-risk infant. Audio recordings should not be used routinely or left unattended in the environment of the high-risk infant. The recommendations, if followed, should provide an environment that will protect sleep, support stable vital signs, improve speech intelligibility for the infant, and reduce potential adverse effects on auditory development.

Field study of the annoyance of low-frequency runway sideline noise

Noise from aircraft ground operations often reaches residences in the vicinity of airports via grazing incidence paths that attenuate high-frequency noise more than air-to-ground propagation paths, thus increasing the relative low-frequency content of such noise with respect to overflight noise. Outdoor A-weighted noise measurements may not appropriately reflect low-frequency noise levels that can induce potentially annoying secondary emissions inside residences near runways. Contours of low-frequency noise levels were estimated in a residential area adjacent to a busy runway from multi-site measurements of composite maximum spectra of runway sideline noise in the one-third octave bands between 25 and 80 Hz, inclusive. Neighborhood residents were interviewed to determine the prevalence of annoyance attributable to runway sideline noise at frequencies below 100 Hz, and of its audible manifestations inside homes. Survey respondents highly annoyed by rattle and vibration were concentrated in areas with low-frequency sound levels due to aircraft operations in excess of 75 to 80 dB.