Manipulations of listeners' echo perception are reflected in event-related potentials

To gain information from complex auditory scenes, it is necessary to determine which of the many loudness, pitch, and timbre changes originate from a single source. Grouping sound into sources based on spatial information is complicated by reverberant energy bouncing off multiple surfaces and reaching the ears from directions other than the source's location. The ability to localize sounds despite these echoes has been explored with the precedence effect: Identical sounds presented from two locations with a short stimulus onset asynchrony (e.g., 1-5 ms) are perceived as a single source with a location dominated by the lead sound. Importantly, echo thresholds, the shortest onset asynchrony at which a listener reports hearing the lag sound as a separate source about half of the time, can be manipulated by presenting sound pairs in contexts. Event-related brain potentials elicited by physically identical sounds in contexts that resulted in listeners reporting either one or two sources were compared. Sound pairs perceived as two sources elicited a larger anterior negativity 100-250 ms after onset, previously termed the object-related negativity, and a larger posterior positivity 250-500 ms. These results indicate that the models of room acoustics listeners form based on recent experience with the spatiotemporal properties of sound modulate perceptual as well as later higher-level processing.     

Loudness criteria in auditorium acoustics

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Audible Threshold of Early Reflections with Different Orientations and Delays

Early reflection is an important component in an enclosed sound field. Due to the precedence effect, the early reflection may not be the dominant factor in sound source localization; however, it still has obvious influences on spatial position, loudness, timbre, and etc. Till now, there have sparse studies on evaluation of the audible threshold of early reflections with lacking of general and quantitative results. This work investigated the audible threshold of early reflection with a simplified sound field model under various experimental conditions including the combination of eight incidence angles and five time delays. Three-down-one-up adaptive strategy with three-interval three-alternative forced-choice (3I-3AFC) paradigm was used due to its efficiency and robustness. Results indicate that (1) the audible threshold of early reflections decreases monotonically with increasing time delay relative to the direct sound. Furthermore, a linear equation between early reflection threshold and time delay is established with correlation coefficient higher than 0.9; (2) When the direct sound and the reflection locate in the same half-plane, the audible threshold of early reflections decreases with increasing angle deviation between the direct sound and the reflection. Moreover, a front-back symmetry of early reflection threshold is observed for stimuli below 5 kHz; (3) Considerable variations in early reflection threshold are found among individuals, especially at large angle deviation and time delay of early reflections relative to the direct sound.     

Compression and expansion of the temporal envelope: evaluation of speech intelligibility and sound quality.

Sensorineural hearing loss is accompanied by loudness recruitment, a steeper-than-normal rise of perceived loudness with presentation level. To compensate for this abnormality, amplitude compression is often applied (e.g., in a hearing aid). Alternatively, since speech intelligibility has been modeled as the perception of fast energy fluctuations, enlarging these (by means of expansion) may improve speech intelligibility. Still, even if these signal-processing techniques prove useful in terms of speech intelligibility, practical application might be hindered by unacceptably low sound quality. Therefore, both speech intelligibility and sound quality were evaluated for syllabic compression and expansion of the temporal envelope. Speech intelligibility was evaluated with an adaptive procedure, based on short everyday sentences either in noise or with a competing speaker. Sound quality was measured by means of a rating-scale procedure, for both speech and music. In a systematic setup, both the ratio of compression or expansion and the number of independent processing bands were varied. Individual hearing thresholds were compensated for by a listener-specific filter and amplification. Both listeners with normal hearing and listeners with sensorineural hearing impairment participated as paid volunteers. The results show that, on average, both compression and expansion fail to show better speech intelligibility or sound quality than linear amplification.     

Engaging concert hall acoustics is made up of temporal envelope preserving reflections

Strong, exciting, and engaging sound is perceived in the best concert halls. Here, it is shown that wideband early reflections that preserve the temporal envelope of sound contribute to the clear and open acoustics with strong bass. Such reflections are fused with the direct sound due to the precedence effect. In contrast, reflections that distort the temporal envelope render the sound weak and muddy because they partially break down the precedence. The presented findings are based on the earlier psychoacoustics research, and confirmed by a perceptual evaluation with six simulated concert halls that have same monaural room acoustical parameter values according to ISO3382-1.     

On the importance of early reflections for speech in rooms

This paper presents the results of new studies based on speech intelligibility tests in simulated sound fields and analyses of impulse response measurements in rooms used for speech communication. The speech intelligibility test results confirm the importance of early reflections for achieving good conditions for speech in rooms. The addition of early reflections increased the effective signal-to-noise ratio and related speech intelligibility scores for both impaired and nonimpaired listeners. The new results also show that for common conditions where the direct sound is reduced, it is only possible to understand speech because of the presence of early reflections. Analyses of measured impulse responses in rooms intended for speech show that early reflections can increase the effective signal-to-noise ratio by up to 9 dB. A room acoustics computer model is used to demonstrate that the relative importance of early reflections can be influenced by the room acoustics design.     

A simple method to detect audible echoes in room acoustical design

A simple method to detect audible echoes is proposed as an objective criterion for room acoustics. This method evaluates the perceptibility of sound reflections that are generated by an impulsive sound source and identifies from reflectograms harmful reflections perceived as echoes. Particularly with this method, the masking effect of reverberation is taken into consideration, which cannot be treated sufficiently by the existing objective criteria. The applicability to room acoustical design is verified by evaluating the impulse responses measured in real halls where audible echoes occurred. It is shown that the proposed method detects audible echoes at an accuracy of more than 90% and would be suitable for practical use.     

音乐厅音质设计

本文简要介绍音乐厅建筑实践的发展史和四个最近设计成功的大厅。简要叙述现在于大厅内能测量且与主观响应有关的客观参数，即混响时间、初始时间延迟间隙、响度、扩散、侧向反射和早期对后期能量比。最后，简短地报道近来声场模拟方法的发展，用这些方法可以预测某些声学特性。     

空调压缩机噪声测试及其声品质客观参量分析*

针对空调压缩机辐射声场的不同场点噪声声品质的差异,采用心理学声品质参量即响度、尖锐度、总感觉噪度,对压缩机辐射声场的不同场点噪声进行声品质客观参量分析,研究压缩机场点噪声声品质客观参量值变化规律。通过特征响度研究各场点噪声响度在频域的分布,并得出压缩机各场点噪声特征响度峰值所在噪声频带。实验结果表明,对于处在压缩机不同方向上的场点,其噪声声品质客观参量值存在明显差异,但其噪声特征响度峰值却处在相同的噪声频带。研究工作为压缩机全方向降噪提供参考依据也为研究压缩机的噪声声品质探索了一种新思路。     

早晚期混响划分对理想比值掩蔽在语音识别性能上的影响

真实环境中存在的噪声和混响会降低语音识别系统的性能。封闭空间中的混响包括直达声、早期反射和后期混响3部分,它们对语音识别系统具有不同的影响.我们研究了早期反射和后期混响的不同划分方法,以其中的早期反射为目标语音,计算出了不同的理想比值掩蔽并研究了它们对语音识别系统性能的影响;在此基础上,利用双向长短时记忆网络(BLSTM)估计理想比值掩蔽,测试它们对语音识别系统性能的影响.实验结果表明,基于Abel早期反射和后期混响的划分方法,理想比值掩蔽能够降低词错误率约2.8%;基于BLSTM的估计方法过低估计了理想比值掩蔽,未能有效提高语音识别系统的性能。      

The influence of spectral characteristics of early reflections on speech intelligibility

The auditory system takes advantage of early reflections (ERs) in a room by integrating them with the direct sound (DS) and thereby increasing the effective speech level. In the present paper the benefit from realistic ERs on speech intelligibility in diffuse speech-shaped noise was investigated for normal-hearing and hearing-impaired listeners. Monaural and binaural speech intelligibility tests were performed in a virtual auditory environment where the spectral characteristics of ERs from a simulated room could be preserved. The useful ER energy was derived from the speech intelligibility results and the efficiency of the ERs was determined as the ratio of the useful ER energy to the total ER energy. Even though ER energy contributed to speech intelligibility, DS energy was always more efficient, leading to better speech intelligibility for both groups of listeners. The efficiency loss for the ERs was mainly ascribed to their altered spectrum compared to the DS and to the filtering by the torso, head, and pinna. No binaural processing other than a binaural summation effect could be observed.     

Discrimination of complex synthetic echoes by an echolocating bottlenose dolphin

Bottlenose dolphins (Tursiops truncatus) detect and discriminate underwater objects by interrogating the environment with their native echolocation capabilities. Study of dolphins' ability to detect complex (multihighlight) signals in noise suggest echolocation object detection using an approximate 265-micros energy integration time window sensitive to the echo region of highest energy or containing the highlight with highest energy. Backscatter from many real objects contains multiple highlights, distributed over multiple integration windows and with varying amplitude relationships. This study used synthetic echoes with complex highlight structures to test whether high-amplitude initial highlights would interfere with discrimination of low-amplitude trailing highlights. A dolphin was trained to discriminate two-highlight synthetic echoes using differences in the center frequencies of the second highlights. The energy ratio (delta dB) and the timing relationship (delta T) between the first and second highlights were manipulated. An iso-sensitivity function was derived using a factorial design testing delta dB at -10, -15, -20, and -25 dB and delta T at 10, 20, 40, and 80 micros. The results suggest that the animal processed multiple echo highlights as separable analyzable features in the discrimination task, perhaps perceived through differences in spectral rippling across the duration of the echoes.     

Intelligibility of reverberant noisy speech with ideal binary masking

For a mixture of target speech and noise in anechoic conditions, the ideal binary mask is defined as follows: It selects the time-frequency units where target energy exceeds noise energy by a certain local threshold and cancels the other units. In this study, the definition of the ideal binary mask is extended to reverberant conditions. Given the division between early and late reflections in terms of speech intelligibility, three ideal binary masks can be defined: an ideal binary mask that uses the direct path of the target as the desired signal, an ideal binary mask that uses the direct path and early reflections of the target as the desired signal, and an ideal binary mask that uses the reverberant target as the desired signal. The effects of these ideal binary mask definitions on speech intelligibility are compared across two types of interference: speech shaped noise and concurrent female speech. As suggested by psychoacoustical studies, the ideal binary mask based on the direct path and early reflections of target speech outperforms the other masks as reverberation time increases and produces substantial reductions in terms of speech reception threshold for normal hearing listeners.     

A new definition of boundary point between early reflections and late reverberation in room impulse responses

The early reflections in the room impulse response are usually defined as those observed within the initial 80 ms after the arrival of the direct sound, after which time the sound field is called reverberant. This number was chosen from measurements of other functions in a limited number of halls. In order to give an objective foundation to this time separation and to establish a physical indicator for it, a new method is proposed that defines a "transition time t(L)," which is the time at which the energy correlation between the direct plus initial sound and the subsequent decaying sound first achieves a specified low value. For various halls this number is shown and its relevance as a new parameter is discussed.     

Study on echo suppression effect and coloration due to periodic-type diffusers

An echo is one of the obstacles in halls and theatres. In order to suppress this undesirable echo, walls and ceilings have been often given absorptive surfaces which absorb sound energy or corrugated surfaces which provide wave diffusion. While it is not difficult to predict the degree of echo suppression effects in the case of absorptive surfaces, it is generally difficult to do that in the case of corrugated surfaces. In addition, if the surfaces have periodicity, unintended effects called coloration can distort tonal characteristics of the sound field. Thus, many acoustic designers would hesitate to employ this kind of periodic-type diffusers. However, there are some cases where it is necessary to suppress echoes without any energy loss. Periodic-type diffusers have the advantages of easy estimation of diffusion properties, simple method of design, and ready availability. In this study, a subjective experiment with simulated stimuli was carried out to clarify the echo suppression effect of the periodic-type diffusers from the viewpoint of reflected energy level. Although coloration interfered subjective judgments in some cases of the experiment, it is shown that the periodic-type diffusers have echo suppression effects if the coloration does not occur. This study is also devoted to discussion about causes and features of the coloration.     

An investigation of the sound field above the audience in large lecture halls with a scale model

Measurements of steady-state sound pressure levels above the audience in large lecture halls show that the classical equation for predicting the sound pressure level is not accurate. The direct field above the seats was measured on a 1:10 scale model and was found to be dependent on the incidence angle and direction of sound propagation across the audience. The reverberant field above the seats in the model was calculated by subtracting the direct field from the measured total field and was found to be dependent on the magnitude and particularly on the placement of absorption. The decrease of sound pressure level versus distance in the total field depends on the angle (controlled by absorption placement) at which the strong reflections are incident upon the audience area. Sound pressure level decreases at a fairly constant rate with distance from the sound source in both the direct and reverberant field, and the decrease rate depends strongly on the absorption placement. The lowest rate of decay occurs when the side walls are absorptive, and both the ceiling and rear wall are reflective. These consequences are discussed with respect to prediction of speech intelligibility.     

Equal autophonic level curves under different room acoustics conditions

The indirect auditory feedback from one's own voice arises from sound reflections at the room boundaries or from sound reinforcement systems. The relative variations of indirect auditory feedback are quantified through room acoustic parameters such as the room gain and the voice support, rather than the reverberation time. Fourteen subjects matched the loudness level of their own voice (the autophonic level) to that of a constant and external reference sound, under different synthesized room acoustics conditions. The matching voice levels are used to build a set of equal autophonic level curves. These curves give an indication of the amount of variation in voice level induced by the acoustic environment as a consequence of the sidetone compensation or Lombard effect. In the range of typical rooms for speech, the variations in overall voice level that result in a constant autophonic level are on the order of 2 dB, and more than 3 dB in the 4 kHz octave band. By comparison of these curves with previous studies, it is shown that talkers use acoustic cues other than loudness to adjust their voices when speaking in different rooms.     

Determination of Velocity Autocorrelation Functions by Multiple Data Acquisition in NMR Pulsed-Field Gradient Experiments

An extension of NMR pulsed-field gradient experiments toward the generation, acquisition, and analysis of multiple echoes is presented. In contrast to currently used measurements where a single or double encoding of displacements by gradient pulses is followed by an acquisition of the echo signal at the end of the sequence, sampling and analyzing the intermediately occurring echoes allows a direct distinction between coherent and dispersive contributions to fluid motion without additional referencing measurements. It is shown that a series of gradient pulse pairs, leading to a train of echoes, can be employed to map the time-dependence of the velocity autocorrelation function between displacements within a single experiment for a system undergoing flow or motion.     

Rainforests as concert halls for birds: are reverberations improving sound transmission of long song elements?

In forests reverberations have probably detrimental and beneficial effects on avian communication. They constrain signal discrimination by masking fast repetitive sounds and they improve signal detection by elongating sounds. This ambivalence of reflections for animal signals in forests is similar to the influence of reverberations on speech or music in indoor sound transmission. Since comparisons of sound fields of forests and concert halls have demonstrated that reflections can contribute in both environments a considerable part to the energy of a received sound, it is here assumed that reverberations enforce also birdsong in forests. Song elements have to be long enough to be superimposed by reflections and therefore longer signals should be louder than shorter ones. An analysis of the influence of signal length on pure tones and on song elements of two sympatric rainforest thrush species demonstrates that longer sounds are less attenuated. The results indicate that higher sound pressure level is caused by superimposing reflections. It is suggested that this beneficial effect of reverberations explains interspecific birdsong differences in element length. Transmission paths with stronger reverberations in relation to direct sound should favor the use of longer signals for better propagation.     

Binaural summation and lateralization of transients: a combined analysis

Subjects judged the loudness and the lateral position of dichotic transient signals, which were presented at equal and unequal levels, synchronously and asynchronously, to the two ears. Binaural loudness summation of clicks does not obey a law of linear addition: It is partial at low level and superadditive at high level. Supersummation is greater for interaurally delayed clicks than for coincidental ones. The relation between click loudness and sound pressure (over moderate SLs) can be described as a power function with a greater exponent for the binaural function. Lateral positions spread over a greater range for interaural level differences than for interaural time differences. The time-intensity trading ratio was greater than is typically reported for tones. When sound lateralization was induced by interaural time difference, but not by intensity difference, a virtually perfect negative correlation between loudness and extent of off-center displacement existed.