Effects of signal processing on the measurement of maximum sound pressure levels

Maximum sound pressure levels are commonly used for environmental noise and building acoustics measurements. This paper investigates the signal processing errors due to Fast or Slow time-weighting detectors when combined with octave band filters, one-third octave band filters or an A-weighting filter. For 6th order Butterworth CPB filters the inherent time delay caused by the phase response of filters is quantified using three different approaches to establish the following rules-of-thumb: (1) time-to-gradient/amplitude matching occurs when Bt ≈ 1, (2) time-to-peak matching occurs when Bt ≈ 2 and (3) time-to-settle matching occurs when Bt ≈ 4 for octave band filters, and when Bt ≈ 3 for one-third octave band filters. Four different commercially-available sound level meters are used to quantify the variation in measured maximum levels using tone bursts, half-sine pulses, ramped noise and recorded transients. Tone bursts indicate that Slow time-weighting is inappropriate for maximum level measurements due to the large bias error. The results also show that there is more variation between sound level meters when considering Fast time-weighted maximum levels in octave bands or one-third octave bands than with A-weighted levels. To reduce the variation between measurements with different sound level meters, it is proposed that limits could be prescribed on the phase response for CPB filters and A-weighting filters.     

Effects of unattended speech on performance and subjective distraction: The role of acoustic design in open-plan offices

Unattended background speech is a known source of cognitive and subjective distraction in open-plan offices. This study investigated whether the deleterious effects of background speech can be affected by room acoustic design that decreases speech intelligibility, as measured by the Speech Transmission Index (STI). The experiment was conducted in an open-plan office laboratory (84 m²) in which four acoustic conditions were physically built. Three conditions contained background speech. A quiet condition was included for comparison. The speech conditions differed in terms of the degree of absorption, screen height, desk isolation, and the level of masking sound. The speech sounds simulated an environment where phone conversations are heard from different locations varying in distance. Ninety-eight volunteers were tested. The presence of background speech had detrimental effects on the subjective perceptions of noise effects and on cognitive performance in short-term memory and working memory tasks. These effects were not attenuated nor amplified within a three-hour working period. The reduction of the STI by room acoustic means decreased subjective disturbance, whereas the effects on cognitive performance were somewhat smaller than expected. The effects of room acoustic design on subjective distraction were stronger among noise-sensitive subjects, suggesting that they benefited more from acoustic improvements than non-sensitive subjects. The results imply that reducing the STI is beneficial for performance and acoustic satisfaction especially regarding speech coming from more distant desks. However, acoustic design does not sufficiently decrease the distraction caused by speech from adjacent desks.     

Ray-tracing prediction of optimal conditions for speech in realistic classrooms

Recent papers have discussed the optimal reverberation times in classrooms for speech intelligibility, based on the assumption of a diffuse sound field. Here this question was investigated for more ‘typical’ classrooms with non-diffuse sound fields. A ray-tracing model was modified to predict speech-intelligibility metric U₅₀. It was used to predict U₅₀ in various classroom configurations for various values of the room absorption, allowing the optimal absorption (that predicting the highest U₅₀)—and the corresponding optimal reverberation time—to be identified in each case. The range of absorptions and reverberation times corresponding to high speech intelligibility were also predicted in each case. Optimal reverberation times were also predicted from the optimal surface-absorption coefficients using Sabine and Eyring versions of diffuse-field theory, and using the diffuse-field expression of Hodgson and Nosal. In order to validate the ray-tracing model, predictions were made for three classrooms with highly diffuse sound fields; these were compared to values obtained by the diffuse-field models, with good agreement. The methods were then applied to three ‘typical’ classrooms with non-diffuse fields. Optimal reverberation times increased with room volume and noise level to over 1 s. The accuracy of the Hodgson and Nosal expression varied with classroom size and noise level. The optimal average surface-absorption coefficients varied from 0.19 to 0.83 in the different classroom configurations tested. High speech intelligibility was, in general, predicted for a wide range of coefficients, but could not be obtained in a large, noisy classroom.     

Acceptable range of speech level in noisy sound fields for young adults and elderly persons

The acceptable range of speech level as a function of background noise level was investigated on the basis of word intelligibility scores and listening difficulty ratings. In the present study, the acceptable range is defined as the range that maximizes word intelligibility scores and simultaneously does not cause a significant increase in listening difficulty ratings from the minimum ratings. Listening tests with young adult and elderly listeners demonstrated the following. (1) The acceptable range of speech level for elderly listeners overlapped that for young listeners. (2) The lower limit of the acceptable speech level for both young and elderly listeners was 65 dB (A-weighted) for noise levels of 40 and 45 dB (A-weighted), a level with a speech-to-noise ratio of +15 dB for noise levels of 50 and 55 dB, and a level with a speech-to-noise ratio of +10 dB for noise levels from 60 to 70 dB. (3) The upper limit of the acceptable speech level for both young and elderly listeners was 80 dB for noise levels from 40 to 55 dB and 85 dB or above for noise levels from 55 to 70 dB.     

Obtaining calibrated sound pressure levels from consumer digital audio recorders

Audio recording of environmental sound is an increasingly efficient method for autonomously sensing many ecological and anthropogenic processes. The increasing capabilities of consumer digital audio recorders (DARs), especially increases in storage capacity and reductions in power consumption, enable continuous audio recordings exceeding 1 month in duration with packages that are relatively small and inexpensive. To augment the ability of these systems to document the range of sounds present at a location, this paper examines two methods for calibrating recorders to measure sound levels. Compressed audio recorded by a DAR can be processed to yield relatively consistent measures of one-third octave band L_eq values within a limited frequency and dynamic range. This was evaluated by synchronizing data with a Type-1 sound level meter. The calibration is stable over a 23 day deployment outdoors with wide variation in ambient temperature and humidity. When considering aggregate acoustic metrics over time or a wide bandwidth such as an hourly A-weighted L₅₀, the results can be quite accurate (within 1 dBA).     

Logatom corpus for the assessment of the intelligibility in Spanish speaking environments and its relation with STI measurements

A 1000 consonant–vowel–consonant structure logatoms corpus (CVC-structure), grouped in 10 phonetically equally balanced lists of 100 words each, was developed to satisfy the need of subjective assessment of speech intelligibility in American Spanish speaking environments. This corpus was tested and correlated with the Speech Transmission Index (STI) measurements to compare its articulation intelligibility score with other lists’ scores.Through the development of this work it was determined that in two different acoustically poor rooms that have the same STI (with STI < 0.50), the intelligibility score is lower when the articulation test is performed in a quiet room with high reverberation time than when it is performed in a very noisy room with low reverberation time. The final correlation curve of the American Spanish CVC-structure corpus was around 10% points higher than the CVC_EQB curve obtained by Steeneken and Houtgast in 2002.     

Measures for assessing architectural speech security (privacy) of closed offices and meeting rooms

Objective measures were investigated as predictors of the speech security of closed offices and rooms. A new signal-to-noise type measure is shown to be a superior indicator for security than existing measures such as the Articulation Index, the Speech Intelligibility Index, the ratio of the loudness of speech to that of noise, and the A-weighted level difference of speech and noise. This new measure is a weighted sum of clipped one-third-octave-band signal-to-noise ratios; various weightings and clipping levels are explored. Listening tests had 19 subjects rate the audibility and intelligibility of 500 English sentences, filtered to simulate transmission through various wall constructions, and presented along with background noise. The results of the tests indicate that the new measure is highly correlated with sentence intelligibility scores and also with three security thresholds: the threshold of intelligibility (below which speech is unintelligible), the threshold of cadence (below which the cadence of speech is inaudible), and the threshold of audibility (below which speech is inaudible). The ratio of the loudness of speech to that of noise, and simple A-weighted level differences are both shown to be well correlated with these latter two thresholds (cadence and audibility), but not well correlated with intelligibility.     

Experimental investigation on the sound pressure level for a high thermal capacity burner during a running cycle

In many research or technical expertise studies the maximum noise level of a boiler is associated with the maximum thermal load of the burner. However, this type of air injected burners presents a complex running cycle with different functioning periods, where different parts (engine, fan, flame) of the burner are running separately or in the same time. In this study we are focused on the analysis, by experimental measurements, of the entire functioning cycle of a boiler by pointing out the noise differences and their importance when doing an experimental acoustical investigation. The entire 1/3 octave spectrum of the sound pressure level (SPL) was recorded during a complete running cycle by means of logging software associated to the sound meter. The sound equivalent level was calculated for each period of the running cycle and compared to the norms and with two theoretical prediction models that take into account the heating power and the boiler room volume. It was found that the most accurate data are obtained when the measurements are done in one-third octave. The maximum noise level was established to be not for the maximum thermal load period, but for the ventilation period of the boiler (before gas injection) with 82.1 dB at 125 Hz. A shut down delay was detected at the end of the cycle with 13 s for higher frequencies, due to the vibration of the boiler parts. Two 3D graphical representations point out the most important frequencies characterizing each running state of the burner. Compared to the noise curve (NC85) the minimum differences between the admissible values and the ones produced by the burner were found to be around 5.5 dB and therefore no acoustical treatment was needed. The results of the SPL prediction models matched the experimental data only for some of the boiler cycle periods and for only some of the frequencies. This type of detailed experiment investigation of the burner noise highlights the periods of the running cycle and the frequencies where the noise level requires acoustical treatment.     

Relationship between Electroencephalogram variation and subjective annoyance under noise exposure

The relationship between Electroencephalogram (EEG) variation and subjective annoyance was investigated with 70 dBA white noise and pure tones at 160 Hz, 500 Hz and 4000 Hz being selected as exposed noise sources. The results indicate that when the duration of noise was less than 6 s, Average Power of Electroencephalogram (APEEG) varied irregularly. When the noise lasted for 5 min, the sum of the relative APEEG of θ wave and the relative APEEG of α wave increased with the subjective annoyance increasing under noise exposures. The maximum power of θ wave appeared in the frontal region, while the maximum power of α wave appeared in the occipital region. Up to the fifth minute after noise exposure, more than two APEEG maximums of θ wave appeared, and the time points of maximum occurrence shifted forwards slowly following the increase of exposed noise frequency. The interval between two time points of maximum occurrence was reduced with the increase of the exposed noise frequency.     

Improved Statistically Optimal Nearfield Acoustical Holography in subsonically moving fluid medium

Statistically Optimal Nearfield Acoustical Holography (SONAH) can be used to reconstruct three-dimensional sound fields by projecting two-dimensional data measured on a “small” aperture that partially covers a composite sound source in a “static” fluid medium. Here, an improved SONAH procedure is proposed that includes the mean flow effects of a moving fluid medium while the sound source and receivers are stationary. The backward projection performance of the proposed procedure is further improved by using a wavenumber filter to suppress subsonic noise components. Through numerical simulations at Mach 0.6, it is shown that the improved procedure can accurately reconstruct sound source locations and radiation patterns: e.g., the spatially averaged reconstruction errors of the conventional and improved SONAH procedures are 15.40 dB and 0.19 dB, respectively, for a monopole simulation and 21.60 dB and 0.19 dB for an infinite-size panel. The wavenumber filter further reduces spatial noise, e.g., decreasing the reconstruction error from 1.73 dB to 0.19 dB for the panel simulation. An existing data measured in a wind tunnel operating at Mach 0.12 is reused for the validation. The locations and radiation patterns of the two loudspeakers are successfully identified from the sound fields reconstructed by using the proposed SONAH procedure.     

Evaluation of speech transmission in open public spaces affected by combined noises

In the present study, the effects of interference from combined noises on speech transmission were investigated in a simulated open public space. Sound fields for dominant noises were predicted using a typical urban square model surrounded by buildings. Then road traffic noise and two types of construction noises, corresponding to stationary and impulsive noises, were selected as background noises. Listening tests were performed on a group of adults, and the quality of speech transmission was evaluated using listening difficulty as well as intelligibility scores. During the listening tests, two factors that affect speech transmission performance were considered: (1) temporal characteristics of construction noise (stationary or impulsive) and (2) the levels of the construction and road traffic noises. The results indicated that word intelligibility scores and listening difficulty ratings were affected by the temporal characteristics of construction noise due to fluctuations in the background noise level. It was also observed that listening difficulty is unable to describe the speech transmission in noisy open public spaces showing larger variation than did word intelligibility scores.     

Estimating the model-specific uncertainty of aircraft noise calculations

Aircraft noise contours are estimated with model calculations. Due to their impact, e.g., on land use planning, calculations need to be highly accurate, but their uncertainty usually remains unaccounted for. The objective of this study was therefore to quantify the uncertainty of calculated average equivalent continuous sound levels (L_Aeq) of complex scenarios such as yearly air operations, and to establish uncertainty maps. The methodology was developed for the simulation program FLULA2. In a first step, the partial uncertainties of modelling the aircraft as a sound source and of modelling sound propagation were quantified as a function of aircraft type and distance between aircraft and receiver. Then, these uncertainties were combined for individual flights to obtain the uncertainty of the single event level (L_AE) at a specified receiver grid. The average L_Aeq of a scenario results from the combination of the L_AE of many single flights, each of which has its individual uncertainties. In a last step, the uncertainties of all L_AE were therefore combined to the uncertainty of the L_Aeq, accounting also for uncertainties of the number of movements and of prognoses. Uncertainty estimations of FLULA2 calculations for Zurich and Geneva airports revealed that the standard uncertainty of the L_Aeq ranges from 0.5 dB (day) to 1.0 dB (night) for past-time scenarios when using radar data as input, and from 1.0 dB (day) to 1.3 dB (night) for future scenarios, in areas where L_Aeq ⩾ 53 dB (day) and L_Aeq ⩾ 43 dB (night), respectively. Different uncertainty values may result for other models and/or airports, depending on the model sophistication, traffic input data, available sound source data, and airport peculiarities such as the specific aircraft fleet or prevailing departure and arrival procedures. The methodology, while established for FLULA2 on Zurich and Geneva airports, may be applied to other models and/or airports, but the partial uncertainties have to be specifically re-established to account for individual models and underlying sound source data.     

Sound field characteristics of underground railway stations – Effect of interior materials and noise source positions

In the case of most underground railway stations, no acoustical solutions are used to reduce train noise. Because the reflecting features of train noise in an underground station are not known, appropriate methods for controlling these features have yet to be established. The aim of this study was to clarify the sound field characteristics of underground stations by putting a sound source and receiver on the railway track and platform, respectively. The impulse responses for two vacant underground stations were measured to clarify the effects of the interior materials of the station (Comparison I), and the sound source was put in each station and tunnel to clarify the effect of the noise source positions (Comparison II). Results showed that the sound fields were similar between the stations whose lateral walls were covered with either metallic or fire-resistant wooden panels (Comparison I), and that the sound field for the sound sources near or in the tunnel presented a higher strength (G) by 5.1 dB and longer reverberation time (EDT) by 0.7 s compared to the sound source in the station (Comparison II). The sound sources in the tunnel presented strong and long reverberations at around 500 Hz due to the convergence effect of the tunnel. Therefore, this study proposes a platform screen with doors to limit noise transmission into the platform.     

Effects of doping with nanoscale Co3O4 particles on the superconducting properties of powder-in-tube processed MgB2 tapes

Nanoscale Co₃O₄ particles were doped into MgB₂ tapes with the aim of developing superconducting wires with high-current-carrying capacity. Fe-sheathed MgB₂ tapes with a mono-core were prepared using the in situ powder-in-tube (PIT) process with the addition of 0.2–1.0 mol% Co₃O₄. The critical temperature decreased monotonically with an increasing amount of doped Co₃O₄ particles for all heat-treatment temperatures from 600 to 900 °C. However, the transport critical current density (J_c) at 4.2 K varied with the heat-treatment temperatures. The J_c values in magnetic fields ranging from 7 to 12 T decreased monotonically with increasing Co₃O₄ doping level for a heat-treatment temperature of 600 °C. In contrast, some improvements on the J_c values of the Co₃O₄ doped tapes were observed in the magnetic fields below 10 T for 700 and 800 °C. Furthermore, J_c values in all the fields measured increased as the Co₃O₄ doping level increase from 0 to 1 mol% for 900 °C. This heat-treatment temperature dependence of the J_c values could be explained in terms of the heat-treatment temperature dependence of the irreversibility field with Co₃O₄ doping.     

Sound insulation property of Al–Si closed-cell aluminum foam sandwich panels

Al–Si closed-cell aluminum foam sandwich panels (1240 mm × 1100 mm) of different thicknesses and different densities were prepared by molten body transitional foaming process in Northeastern University. The experiments were carried out to investigate the sound insulation property of Al–Si closed-cell aluminum foam sandwich panels of different thicknesses and different densities under different frequencies (100–4000 Hz). Results show that sound reduction index (R) is small under low frequencies, large under high frequencies; thickness affects the sound insulation property of material obviously: when the thicknesses of Al–Si closed-cell aluminum foam sandwich panels are 12, 22, and 32 mm, the corresponding weighted sound reduction indices (R_W) are 26.3, 32.2, and 34.6 dB, respectively, the rising trend tempered; the increase of density of Al–Si closed-cell aluminum foam can also increase the sound insulation property: when the densities of aluminum foam are 0.31, 0.51, and 0.67 g/cm³, the corresponding weighted sound reduction indices (R_W) are 28.9, 34.3, and 34.6 dB, the increasing value mitigating.     

Taking account of loudness constancy for the loudness criterion for concert halls

One of the surprises from analysis of results of an objective and subjective study of British concert halls (1988 Acustica 66, 1–14) was that the subjective judgement of loudness in concert halls is influenced not only by sound level but also by the source–receiver distance. This response implies that the same sound level is judged louder at positions further from the orchestra platform. Whereas level decreases with distance in actual halls, loudness is judged more-or-less independent of position in average halls (except at positions close to the platform and seats overhung by balconies). As an observation it ties in with evidence from experimental psychologists for loudness constancy throughout a space. The sound strength G is the sound level in an auditorium normalised to the sound power level of the source; the traditional criterion of acceptability for level is that G ? 0 dB. The paper proposes that, on the basis of subjective evidence and objective behaviour in auditoria, the criterion for G should not be a unique value of G but rather a function of source–receiver distance.     

Measured light vehicle noise reduction by hedges

The acoustical effects of hedges result from a combination of physical noise reduction and their influences on perception. This study investigates the physical noise reduction so as to enable estimation of its relative importance. Different in-situ methods have been used to measure noise shielding by hedges. These include a statistical pass-by experiment where the real insertion loss of a hedge could be measured, three controlled pass-by experiments using a reference microphone at close distance, and transmission loss measurements using a point source. Thick dense hedges are found to provide only a small total A-weighted light vehicle noise reduction at low speeds. Measured insertion losses range from 1.1 dBA to 3.6 dBA. The higher noise reductions are found to be associated with an increased ground effect.     

Influence of coding strategies in electric-acoustic hearing: A simulation dedicated to EAS cochlear implant,in the presence of noise

This work deals with electric-acoustic stimulation (EAS), which keeps the low frequency acoustic information and electrically codes the high frequencies of the signal. One of the goals of the coding strategies is to limit the phenomenon of channel interaction, which can occur in CIs. The “N-of-M” strategy, where only a subset of electrode channels is stimulated, may be of advantage. Generally, this processing is associated with a pre-emphasis filter. Two important parameters for the N-of-M strategy are the number of active channels (N) and the updating rate; the latter corresponds to the stimulation rate. M is the number of electrical channels.The goal of this study was to investigate the influence of these parameters on speech intelligibility in EAS. The signal was presented, in simulation, to normal-hearing (NH) subjects in acoustic (A), electric (E) and electric-acoustic conditions. Recognition performance was measured in quiet and in the presence of background noise (cafeteria noise).Signal-to-noise ratios (SNRs) ranged from 0 to +12 dB. Fifteen listeners participated in the experiment. The N values ranged from 2 to 10 (out of 10); M was 10. The frame updating rate was 250 updates per second (ups) and 1000 ups.Results showed that increasing N from 2 to 10 improved speech intelligibility, especially in the presence of the background noise, under E and EAS conditions. In noisy situations, 2/10 coupled with a high-pass pre-emphasis filter led to results similar to the 10/10 condition. Changing the frame rate from 250 ups to 1000 ups did not modify the performance.Future investigations on patients using EAS are now needed to validate the performance seen with NH listeners. Above all, in the strategy 2 out of 10, the number of pulses per second can be divided by 20, and when the pre-emphasis is used only a slight decrease in performance is expected; this is of interest when interaction between the electrodes corrupts the performance.