Sound Masking Research Articles

Residual inhibition functions in relation to tinnitus spectra and auditory threshold shift

Conclusions: Psychoacoustic functions relating the depth and duration of tinnitus suppression (‘residual inhibition’) to the center frequency of band-passed noise masking sounds appear to span the region of hearing loss, as do psychoacoustic measurements of the tinnitus spectrum. The results (1) suggest that cortical map reorganization induced by hearing loss is not the principal source of the tinnitus sensation and (2) provide a necessary baseline for optimizing residual inhibition in individual cases. Objective: To measure residual inhibition functions and tinnitus spectra using sounds spanning the region of hearing loss. Materials and methods: Three subject-driven, computer-based tools were developed and applied to measure psychoacoustic properties of tinnitus and residual inhibition in 32 subjects with chronic tonal, ringing, or hissing tinnitus. Residual inhibition functions were measured with band-passed noise sounds varying in center frequency up to 12.0 kHz. Results: The depth and duration of residual inhibition increased with the center frequency of the band-passed noise stimuli. Near-elimination of tinnitus for up to 45 s was reported by 8/24 (33%) subjects at center frequencies above 3 kHz (these cases distributed across tinnitus types). Tinnitus spectra covered the region of hearing loss with no preponderance of frequencies near the audiometric edge of normal hearing.

Spatial unmasking of birdsong in human listeners: Energetic and informational factors

Spatial unmasking describes the improvement in the detection or identification of a target sound afforded by separating it spatially from simultaneous masking sounds. This effect has been studied extensively for speech intelligibility in the presence of interfering sounds. In the current study, listeners identified zebra finch song, which shares many acoustic properties with speech but lacks semantic and linguistic content. Three maskers with the same long-term spectral content but different short-term statistics were used: (1) chorus (combinations of unfamiliar zebra finch songs), (2) song-shaped noise (broadband noise with the average spectrum of chorus), and (3) chorus-modulated noise (song-shaped noise multiplied by the broadband envelope from a chorus masker). The amount of masking and spatial unmasking depended on the masker and there was evidence of release from both energetic and informational masking. Spatial unmasking was greatest for the statistically similar chorus masker. For the two noise maskers, there was less spatial unmasking and it was wholly accounted for by the relative target and masker levels at the acoustically better ear. The results share many features with analogous results using speech targets, suggesting that spatial separation aids in the segregation of complex natural sounds through mechanisms that are not specific to speech.

The role of sound masking treatment in the value engineering of office designs

In the design and planning of office spaces, sound masking systems are often considered optional because of an owner/occupier requirement due to an understanding that can create a more pleasing and private acoustic environment or are required to provide adequate speech privacy. Installations are rarely considered on the basis of project value engineering or return on investment. In this paper, the authors argue that sound masking systems should in fact be considered as another important cost component of the office design, and evaluated along with ceiling tile, private office wall construction and workstation design for their mutual impact on the project budget, as well as on their contribution to acoustical performance. The authors provide typical project data to demonstrate masking systems typically result in overall project cost savings.

Forty‐five years of open plan office experience: An unusual case study

The open plan office design concept has been around for at least 45 years. Through research, design, and practical experience, the basic concepts and parameters that impact acoustics and speech privacy have been fairly well developed and documented. The psychological aspects of human response and expectation to open plan environments, and the conceptual designs that emerge from the architect and interior designers are frontiers that still allow for ongoing study. A large open plan project completed during the dot‐com bubble allowed an opportunity to design and experience an unusual open plan environment that utilized exposed sound masking system loudspeakers and minimal sound absorption.

Reduced auditory efferent activity in childhood selective mutism

Background Selective mutism is a psychiatric disorder of childhood characterized by consistent inability to speak in specific situations despite the ability to speak normally in others. The objective of this study was to test whether reduced auditory efferent activity, which may have direct bearings on speaking behavior, is compromised in selectively mute children. Methods Participants were 16 children with selective mutism and 16 normally developing control children matched for age and gender. All children were tested for pure-tone audiometry, speech reception thresholds, speech discrimination, middle-ear acoustic reflex thresholds and decay function, transient evoked otoacoustic emission, suppression of transient evoked otoacoustic emission, and auditory brainstem response. Results Compared with control children, selectively mute children displayed specific deficiencies in auditory efferent activity. These aberrations in efferent activity appear along with normal pure-tone and speech audiometry and normal brainstem transmission as indicated by auditory brainstem response latencies. Conclusions The diminished auditory efferent activity detected in some children with SM may result in desensitization of their auditory pathways by self-vocalization and in reduced control of masking and distortion of incoming speech sounds. These children may gradually learn to restrict vocalization to the minimal amount possible in contexts that require complex auditory processing.

Reduced auditory efferent activity in childhood selective mutism

Background Selective mutism is a psychiatric disorder of childhood characterized by consistent inability to speak in specific situations despite the ability to speak normally in others. The objective of this study was to test whether reduced auditory efferent activity, which may have direct bearings on speaking behavior, is compromised in selectively mute children. Methods Participants were 16 children with selective mutism and 16 normally developing control children matched for age and gender. All children were tested for pure-tone audiometry, speech reception thresholds, speech discrimination, middle-ear acoustic reflex thresholds and decay function, transient evoked otoacoustic emission, suppression of transient evoked otoacoustic emission, and auditory brainstem response. Results Compared with control children, selectively mute children displayed specific deficiencies in auditory efferent activity. These aberrations in efferent activity appear along with normal pure-tone and speech audiometry and normal brainstem transmission as indicated by auditory brainstem response latencies. Conclusions The diminished auditory efferent activity detected in some children with SM may result in desensitization of their auditory pathways by self-vocalization and in reduced control of masking and distortion of incoming speech sounds. These children may gradually learn to restrict vocalization to the minimal amount possible in contexts that require complex auditory processing.

Performance comparison of above ceiling and through ceiling sound masking systems

Sound masking systems are an important tool for providing speech privacy in offices and similar spaces. These systems typically use an array of loudspeakers radiating masking sound above an acoustical tile ceiling. The ceiling acts to diffuse the sound laterally in order to minimize localization on loudspeakers by the listeners below. The diffusion also helps keep the speech-to-noise ratio relatively uniform spatially, which may help minimize variations in privacy conditions. Unfortunately, the ceiling material, the plenum, and the space below are rarely homogeneous acoustically. There are usually openings in the ceiling for HVAC return air and lighting fixture ventilation, and different parts of the ceiling often use a variety of materials and elevations for aesthetic reasons. Large ducts and structural elements may compartmentalize the plenum. These variations pose substantial acoustical challenges for the designer and additional costs for the owner, not all of which can be satisfactorily resolved. Recognizing these issues, several manufacturers have recently introduced masking systems that might be characterized as direct field systems. These systems use masking loudspeakers that radiate directly into the occupied space rather than into a plenum. This paper contrasts the typical performance and other characteristics of the two basic types of systems.

Design variables for speech privacy in offices

Open plan office environments have a lower potential for speech privacy and freedom from distraction than conventional enclosed offices. The three areas of concern for the acoustical designer are the source, path and listener environments. Given a source spectrum, level and directivity, design elements, which influence attenuation paths, will be addressed with evaluations of their relative importance. The spectrum, level, uniformity and incoherence of background sound at the listener are critical to establishing adequate signal-to-noise ratio for high privacy criteria. Case studies will be addressed comparing challenges, solutions, and results for various open and closed plan spaces. These will be compared to a computer model, which evaluates the relative contribution to privacy of the most important open plan office design variables. Sound masking was shown to be the most powerful and cost effective design tool for establishing acoustical environments with varying privacy criteria.

The effects of high intensity pure tone on visual field, eye fixation, pupil size, and visual false positive and negative errors

Our aim in this study is to evaluate the effects of pure tone on visual field, pupil size, eye fixation, and visual false positive and negative errors. Thirty-two young adult subjects with normal hearing and normal visual acuity were tested in this study. Measurements were performed over two test sessions. In one session visual factors were measured in quiet, and another measurement was performed during presenting a pure tone (1000 Hz, 100-dB HL) binaurally via a headphone. The statistical program SPSS10 was used for all analyses. Fixation loss was significantly lower (better) in quiet condition compared with continuous pure tone condition, and other visual factors had no significant differences in the two conditions. As a conclusion it can be stated that changes in attentional focus which result from altered levels of arousal or autonomic system activity during presenting a high intensity pure tone affect on fixation of the eye. However, it is suggested that these effects are possibly related to stress caused by sound masking the hearing of speech and other wanted environmental sounds, and not from some direct autonomic system arousal by sound.

Computer-based method for the diagnostics and rehabilitation of tinnitus patients

The proposed method is an electronic diagnostic and rehabilitation system for people suffering from internal ear noise, namely tinnitus patients and people with an abnormally high hearing sensitivity—hyperacusis patients. Thanks to the method employing multimedia personal computers to the fitting masking sounds to the patient’s needs, tinnitus patients can be rehabilitated using the masking or habituation method. The subject is asked to answer detailed questions in the electronic questionnaire and next to identify those sounds which most strongly resemble those they perceive as ear noise. Following an algorithm analysis of the results and analysis of the selection of sounds made by the patient, the computer diagnoses the patient as free from ear noise and hyperacusis or classifies them into a risk group. Next, the patient is informed about the result and can then read about the causes of the ailment and the recommended treatment and also can download compressed sounds applicable in therapy to the rehabilitation tool coupled with the personal computer which is a programmable ear noise masker. In the paper the algorithm of analysis of patients answers and acoustic characteristics of tinnitus maskers are discussed. Hitherto obtained results of this method application are demonstrated.

Masking of tinnitus and mental activity.

The aim of this study was to investigate the effects of tinnitus on mental activity. In a mixed design study, the performance of 20 subjects with tinnitus and 20 healthy control subjects was compared on the digit-symbol test, completed in three auditory conditions: silence, masking and intermittent masking. Results showed marked overall differences between patients and controls on all three conditions. Lower scores on the digit-symbol test were observed during the intermittent masking condition compared with the masking condition for the patients. There was no difference between silence (e.g. tinnitus) and the masking condition, or between silence and the intermittent masking condition. The control subjects scored lower on the task when exposed to the intermittent masking compared to both silence and masking. The patients had higher depression and anxiety scores than the controls. These findings suggest that there is a link between masking sounds and how tinnitus affects cognitive capacity.

Noisy offices for simple tasks and quiet offices for complex tasks?

Many current office plans are designed as open environments to increase knowledge sharing, which is supposed to increase performance through more informal interactions between workers. However, according to workplace surveys, poor acoustics in open office environments have been reported to decrease workers performances. The literature regarding performance with quiet, ambient, and masking sound in offices show that the kinds of performed tasks in organizations or groups may be the main factor to decide whether open or closed office environments are appropriate. People engaged in simple office tasks perform better with masking sounds than with quiet or ambient office sound conditions [Loewen et al., ‘‘Cognitive and arousal effects of masking office noise,’’ Environment and Behavior 24, 381–395 (1992)]. However, when people need to use serial-recall functions in their complex tasks, quiet conditions may be better than other sound environments in offices [Banbury et al., ‘‘Disruption of office-related tasks by speech and office noise,’’ Br. J. Psychol. 499–517 (1998)].

Privacy with intelligibility—A novel electroacoustic solution

The concept of electronic sound masking is a well-established and successful solution in helping to achieve adequate speech privacy either within or between office areas or similar spaces. In many instances, separate paging or voice-alarm systems are also installed. Whereas locating loudspeakers in the ceiling void or within a plenum assists sound diffusion and evenness of coverage, this also degrades speech intelligibility and causes significant nonlinear (frequency-dependent) transmission losses. Conversely, installing conventional loudspeakers directly into the ceiling can result in good intelligibility but poor sound diffusion, leading to uneven sound distribution and immediate localization of the loudspeaker as the sound source. The diffuse, substantially decorrelated and wide dispersion characteristics of the distributed mode loudspeaker suggests that systems employing this new class of device may simultaneously satisfy the requirements for both speech privacy and speech intelligibility. The paper presents the results of a study using a range of loudspeaker formats to establish whether these apparently opposing requirements can be achieved.

The relative contribution of onset asynchrony, harmonic ratios and angular separation of sound sources to the cross-spectral grouping of complex tones in a free field

The Rhythmic Masking Release paradigm (RMR) was used to study how onset asynchrony, harmonic ratios and speakers separation interact toward the cross-spectral grouping of concurrent complex tones. In RMR, a regular sequence is perceptually masked by some irregularly spaced masking sounds, which are acoustically identical to those of the rhythm. The identification of the rhythm becomes contingent upon the perceptual grouping of the masking sounds with some flanking sounds of different frequencies. The rhythm-detection accuracy in a 2-AFC procedure was used to estimate the degree of perceptual grouping of 48-ms long masking and flanking tones, each composed of four harmonics. For each of 18 listeners, synchrony of onset and offset between the masking and flanking tones yielded an almost perfect rhythm-detection performance, independent of whether or not they shared a common fundamental frequency and of the spatial separation of their sources. Rhythm-detection accuracy strongly diminished with onset asynchrony and weakly diminished with inharmonicity and speakers separation. Our results suggest that onset asynchrony is the determining factor for the perception of concurrent sounds of a short duration as separate events; inharmonicity and the spatial separation of sources are weak factors, although they appear to reinforce each other toward perceptual segregation.

Masking of complex sound--technical notes for realization of constant signal-noise-ratio.

Based on findings regarding unconscious semantic information-processing for meaningful visual stimuli, this paper describes a standardized procedure for the construction of constant signal to noise ratio for the masking of complex sounds. The proposed algorithm is derived from psychoacoustical laws concerning frequency and time-weighting of sound. Spectral characteristics are taken into account by mixing signal and white noise within frequency ranges defined through critical bandwidth ("narrow-band masking"). Furthermore, time-weighting is taken into account using sound-pressure patterns generated through low pass filters which follow the international specification DIN IEC 651 which provides specification for sound-level meters. Hardware and software circuits are described in detail. This proposal overcomes recognized lack of standardized methods in studies addressing unconscious semantic processing of auditory stimuli.

Noise levels in dental practice

Noise can cause unwanted masking of sounds, interference with speech, even pain and injury, and temporary or permanent loss of hearing. Concerning exposure of noise levels at the workplace, industrial plants and huge machines are usually considered. However, there are ‘‘clean’’ jobs where people are exposed to significant noise. One example, which will be treated in this paper, is dental practice. In this work it is intended to show the results of SPL measurements that were done at the Dept. of Pedodontics in the School of Dental Medicine Zagreb. One- and two-channel measurements were made using a sound level meter and an artificial head (head-and-torso simulator). Noise levels generated by various dental devices, their spectra, and differences between noise levels in left and right ear (in the dentist’s usual working position) were shown. The sound power and subjective sound levels regarding sound source frequency and intensity were determined upon results of these measurements. Special attention was paid to ultrasound and infrasound components, as well as vibrations. Since two Faculty practices with a dozen of dental sets in each were at our disposal, influence of aging and wearing, particularly in mechanical devices, was also considered.

MASKING OF COMPLEX SOUND--TECHNICAL NOTES FOR REALIZATION OF CONSTANT SIGNAL-NOISE-RATIO

Based on findings regarding unconscious semantic information-processing for meaningful visual stimuli, this paper describes a standardized procedure for the construction of constant signal to noise ratio for the masking of complex sounds. The proposed algorithm is derived from psychoacoustical laws concerning frequency and time-weighting of sound. Spectral characteristics are taken into account by mixing signal and white noise within frequency ranges defined through critical bandwidth (“narrow-band masking”). Furthermore, time-weighting is taken into account using sound-pressure patterns generated through low pass filters which follow the international specification DIN IEC 651 which provides specification for sound-level meters. Hardware and software circuits are described in detail. This proposal overcomes recognized lack of standardized methods in studies addressing unconscious semantic processing of auditory stimuli.

Field experience in assessing open plan office acoustical privacy

Experience continues to show that the significant factors affecting open office plan speech privacy include available masking (background), sound barrier height and absorption, distance between sound source and receiver, ceiling sound absorption, furniture layout, speech effort of occupants, etc. These factors are adequately quantified by currently available metrics, and methods for predicting speech privacy prove to be practical, easy to use, and quite accurate. Case studies of open plan offices will be presented including available systems and approaches for electronic sound masking in both large and small office configurations.

Release from masking due to spatial separation of sources in the identification of nonspeech auditory patterns.

A nonspeech pattern identification task was used to study the role of spatial separation of sources on auditory masking in multisource listening environments. The six frequency patterns forming the signal set were comprised of sequences of eight 60-ms tone bursts. Bursts of masking sounds were played synchronously with the signals. The main variables in the study were (1) the difference in spatial separation in the horizontal plane between signals and maskers and (2) the nature of the masking produced by the maskers. Spatial separation of signal and masker ranged from 0-180 degrees. The maskers were of two types: (1) a sequence of eight 60-ms bursts of Gaussian noise intended to produce predominantly peripherally based "energetic masking" and (2) a sequence of eight 60-ms bursts of eight-tone complexes intended to produce primarily centrally based "informational masking." The results indicated that identification performance improved with increasing separation of signal and masker. The amount of improvement depended upon the type of masker and the center frequency of the signal patterns. Much larger improvements were found for spatial separation of the signal and informational masker than for the signal and energetic masker. This was particularly apparent when the acoustical advantage of the signal-to-noise ratio in the more favorable of the two ears (the ear nearest the signal) was taken into account. The results were interpreted as evidence for an important role of binaural hearing in reducing sound source or message uncertainty and may contribute toward solving the "cocktail party problem."

Shielding against noise interfering with quantitation of insect infestations by acoustic detection systems in grain elevators

Acoustic devices used to detect hidden insect infestations must be shielded from noise in most practical applications. One device developed specifically for use in a noisy environment, the Acoustic Location ‘Fingerprinting’ Insect Detector (ALFID), counts the numbers of insects present in grain samples from shipments being graded for export at commercial grain elevators. This report considers the performance of ALFID's noise-shielding components, which include an enclosure for passive reduction of ambient noise, and an electronic system for active detection and masking of sounds originating outside the grain sample container. Sound pressure levels (SPLs) of ambient noise are reduced inside the enclosure by 60–90 dB at frequencies between 1 and 10 kHz, with a reduction of ~6.5 dB per octave (frequency doubling). The active noise-masking system protects ALFID from loud ambient sounds not sufficiently attenuated by the enclosure. If the output from one of four sensors mounted on the outside of the grain sample container rises above a preset amplitude threshold, a signal is triggered that inhibits acquisition of insect sound data from sensors inside the container. In tests of the complete ALFID system at a grain elevator with ambient noise of 73 ± W dB re 20 μPa SPL, the mean rate of noise-mask triggering was 5.5 s −1, inhibiting acquisition of insect sounds for only 3.9% of the total testing period. This level of performance is sufficient to enable successful operation of ALFID under such noise conditions.