Audible Spectrum Research Articles

DETECTION OF CAVITATION PHENOMENON IN A CENTRIFUGAL PUMP USING AUDIBLE SOUND

One of the sources of instability in a centrifugal pump is cavitation phenomenon. Cavitation within a centrifugal pump can cause more undesirable effects, such as deterioration of the hydraulic performance (drop in head-capacity and efficiency curves), damage of the pump by pitting and erosion and structure vibration and resulting noise. Cavitation can appear within the entire range of operating conditions; therefore it must, by all means, be prevented. To prevent the onset of the cavitation, we have to know the beginning of cavitation phenomenon in the pump. To detect the beginning of the cavitation process, the emitted noise can also be used, among other possibilities. Experiments have shown that there is a discrete frequency tone within the audible noise spectra, at 147 Hz or BPF/2, which is strongly dependent on the cavitation process and its development. Therefore, the discrete frequency tone at 147 Hz was separated from the noise spectra of cavitating pumps and then used to detect the incipient of cavitation and its development. It was also used to determine the net positive suction head required or the critical value, as well as to prevent cavitation in the pump by means of initiating an alarm, shutdown or control signal via an electrical control system.

Speech enhancement based on audible noise spectrum and short-time spectral amplitude estimator

Subtractive-type algorithms are effective in reducing background noise, but always produce musical noise. An algorithm based on an audible noise spectrum is proposed and tested using various noise types and levels. The performance evaluation shows that the residual noise is significantly removed and it renders better quality than those without masking.

Differential effect of near-threshold stimulus intensities on sound localization performance in azimuth and elevation in normal human subjects.

The ability of humans to localize sounds remains relatively constant across a range of intensities well above detection threshold, and increasing the spectral content of the stimulus results in an improvement in localization ability. For broadband stimuli, intensities near detection threshold result in fewer and weaker binaural cues used in azimuth localization because the stimulus energy at the high- and low-frequency ends of the audible spectrum fall below detection threshold. Thus, the ability to localize broadband sounds in azimuth is predicted to be degraded at audible but near threshold stimulus intensities. The spectral cues for elevation localization (spectral peaks and notches generated by the head-related transfer function) span a narrower frequency range than those for azimuth. As the stimulus intensity decreases, the ability to detect the stimulus frequencies corresponding to the spectral notches will be more strongly affected than the ability to detect frequencies outside the range where these spectral cues are useful. Consequently, decreasing the stimulus intensity should degrade localization in both azimuth and elevation and create a greater deficit in elevation localization due to the narrower band of audible frequencies containing elevation cues compared to azimuth cues. The present study measured the ability of 11 normal human subjects to localize broadband noise stimuli along the midsagittal plane and horizontal meridian at stimulus intensities of 14, 22, and 30 dB above the subject's detection threshold using a go/no-go behavioral paradigm. Localization ability decreased in both azimuth and elevation with decreasing stimulus intensity, and this effect was greater on localization in elevation than on localization in azimuth. The differential effects of stimulus intensity on sound localization in azimuth and elevation found in the present study may provide a valuable tool in investigating the neural correlates of sound location perception.

Neurobiological foundations for the theory of harmony in western tonal music.

Basic principles of the theory of harmony reflect physiological and anatomical properties of the auditory nervous system and related cognitive systems. This hypothesis is motivated by observations from several different disciplines, including ethnomusicology, developmental psychology, and animal behavior. Over the past several years, we and our colleagues have been investigating the vertical dimension of harmony from the perspective of neurobiology using physiological, psychoacoustic, and neurological methods. Properties of the auditory system that govern harmony perception include (1) the capacity of peripheral auditory neurons to encode temporal regularities in acoustic fine structure and (2) the differential tuning of many neurons throughout the auditory system to a narrow range of frequencies in the audible spectrum. Biologically determined limits on these properties constrain the range of notes used in music throughout the world and the way notes are combined to form intervals and chords in popular Western music. When a harmonic interval is played, neurons throughout the auditory system that are sensitive to one or more frequencies (partials) contained in the interval respond by firing action potentials. For consonant intervals, the fine timing of auditory nerve fiber responses contains strong representations of harmonically related pitches implied by the interval (e.g., Rameau's fundamental bass) in addition to the pitches of notes actually present in the interval. Moreover, all or most of the partials can be resolved by finely tuned neurons throughout the auditory system. By contrast, dissonant intervals evoke auditory nerve fiber activity that does not contain strong representations of constituent notes or related bass notes. Furthermore, many partials are too close together to be resolved. Consequently, they interfere with one another, cause coarse fluctuations in the firing of peripheral and central auditory neurons, and give rise to perception of roughness and dissonance. The effects of auditory cortex lesions on the perception of consonance, pitch, and roughness, combined with a critical reappraisal of published psychoacoustic data on the relationship between consonance and roughness, lead us to conclude that consonance is first and foremost a function of the pitch relationships among notes. Harmony in the vertical dimension is a positive phenomenon, not just a negative phenomenon that depends on the absence of roughness--a view currently held by many psychologists, musicologists, and physiologists.

Acoustic and linguistic factors in the perception of bandpass-filtered speech.

Speech can remain intelligible for listeners with normal hearing when processed by narrow bandpass filters that transmit only a small fraction of the audible spectrum. Two experiments investigated the basis for the high intelligibility of narrowband speech. Experiment 1 confirmed reports that everyday English sentences can be recognized accurately (82%-98% words correct) when filtered at center frequencies of 1500, 2100, and 3000 Hz. However, narrowband low predictability (LP) sentences were less accurately recognized than high predictability (HP) sentences (20% lower scores), and excised narrowband words were even less intelligible than LP sentences (a further 23% drop). While experiment 1 revealed similar levels of performance for narrowband and broadband sentences at conversational speech levels, experiment 2 showed that speech reception thresholds were substantially (>30 dB) poorer for narrowband sentences. One explanation for this increased disparity between narrowband and broadband speech at threshold (compared to conversational speech levels) is that spectral components in the sloping transition bands of the filters provide important cues for the recognition of narrowband speech, but these components become inaudible as the signal level is reduced. Experiment 2 also showed that performance was degraded by the introduction of a speech masker (a single competing talker). The elevation in threshold was similar for narrowband and broadband speech (11 dB, on average), but because the narrowband sentences required considerably higher sound levels to reach their thresholds in quiet compared to broadband sentences, their target-to-masker ratios were very different (+23 dB for narrowband sentences and -12 dB for broadband sentences). As in experiment 1, performance was better for HP than LP sentences. The LP-HP difference was larger for narrowband than broadband sentences, suggesting that context provides greater benefits when speech is distorted by narrow bandpass filtering.

Auditory cortical responses to amplitude modulations with spectra above frequency receptive fields: evidence for wide spectral integration.

Auditory neurons typically respond to a restricted range of frequencies and amplitudes of pure tone stimuli. These findings have led to the concept of the classical frequency receptive field. Over the last few years evidence has accumulated that stimuli outside the frequency and amplitude boundaries of a neuron's receptive field can influence responses to stimuli inside the classical receptive field. We could recently show that sinusoidally amplitude-modulated pure tones could excite cortical neurons although all of their spectral components were above the spectral range of pure tones effective to excite the neuron. This result demonstrated that neurons in the auditory cortex integrate over spectral ranges that are much wider than is evident from responses to pure tones. Here, using sinusoidally amplitude-modulated pure tone stimuli we determine electrophysiologically the high-frequency boundaries of the spectral integration capabilities of auditory cortical neurons in anaesthetized Mongolian gerbils under normal conditions and under the influence of the microiontophoretically applied GABAA-receptor antagonist bicuculline. Our results demonstrate that some auditory cortical neurons integrate over the gerbil's entire audible spectrum. Therefore, the classical excitatory frequency receptive field of an auditory cortical neuron, as determined with pure tone stimuli, cannot provide a satisfactory description of its spectral integrative properties.

Hepatic lesion ablation with bipolar saline-enhanced radiofrequency in the audible spectrum

The authors' purpose was to create larger and more regular liver lesions in vitro by testing a new hyperthermia approach that uses a bipolar saline-enhanced electrode for radiofrequency (RF) in the audible spectrum and a greater power supply. The authors' hyperthermia approach (group A, n = 23) was used in excised porcine livers, and the results were compared with those of a previously described monopolar saline-enhanced electrode procedure (group B, n = 23). In each set of experiments, RF in the audible spectrum current (50 Hz) was provided for 15 minutes with a similar ablation protocol. Electrical variables (impedance, current, voltage, power, and energy), temperatures in the lesions, volume size, regularity ratio of the lesion, and microscopic findings were measured. In group A, the mean volume size and the mean regularity ratio values were 144.8 cm3 +/- 59.8 and 0.78 +/- 0.1, respectively. In group B, the mean volume size and regularity ratio values were 62.1 cm3 +/- 36.4 and 0.62 +/- 0.1, respectively. The values in group B were thus significantly lower than those in group A (P < .01). The lesions in group A were also more homogeneous. No significant differences were found in electrical variables. The new bipolar saline-enhanced electrode produced larger, more regular, and more homogeneous lesions ex vivo than the previously used monopolar saline-enhanced electrode method. Using a greater power supply increased the amount of coagulative necrosis.

Loss of natural soundscapes within the Americas

Species specific calls within an ecosystem evolve to adjust to the physical acoustic constraints of that ecosystem as well as the constraints imposed by the spectal content of the calls of other species occupying that ecosystem. The result is a rich texture of sound spread over both the audible (human) and inaudible spectrum with changing patterns at different times of the day. Based on 30 years of experience in recording natural soundscapes, the author documents the loss of natural soundscapes within the U.S. and other countries of the Americas. The results are demonstrated with field recordings.

An experimental investigation of a novel active segmented partition for sound transmission control

A unique dual diaphragm configuration for individual elements of active segmented partitions has recently been advanced as a practical, efficient means of controlling sound transmission. The configuration incorporates a useful combination of electro-mechano-acoustical actuation and segmentation, enabling effective global control of transmitting surface vibrations. In this work, experimental efforts are reported that explore important active and passive characteristics of such elements while adjoined in a full partition array. Each element is independently and stably controlled using a single-channel filtered-x adaptive controller (single reference input, single error input, single output). Acoustic pressure scans of element cavities and laser vibrometer scans of their transmitting surfaces affirm important properties of this partition construction that have been previously theorized. Other measurements demonstrate that the controlled transmission loss of the lightweight array lies uniformly between 55 and 60 dB over nearly all of the evaluated spectrum (30 to 300 Hz). Such extensive control is generally very difficult to realize over the lower audible frequency spectrum using an extended lightweight structure. Further measurements illustrate that this is comparable to the passive transmission loss of a much more massive benchmark partition of the same cross-sectional area, but constructed of thick, spaced layers of concrete and sand. a)Currently at Crown International, 1718 W. Mishawaka Rd., Elkhart, IN 46515-1000.

Spectral integration of auditory temporal information

Measures of temporal acuity reveal a remarkable ability to integrate temporal information across frequency throughout most of the audible spectrum. This apparent trading of spectral bandwidth and temporal resolution was investigated in three different experiments measuring amplitude-modulation detection, temporal gap detection, and intensity discrimination. Each experiment included six spectral conditions: a single band of noise 1600-Hz wide centered at 3000 Hz; a set of four noise bands 400-Hz wide having center frequencies separated by 1200 Hz (1200, 2400, 3600, 4800 Hz); and four conditions composed of only one of the four 400-Hz bands. The form of the modulation transfer functions did not depend upon total bandwidth or frequency region. Sensitivity to modulation was inversely related to total noise bandwidth. Temporal gap detection thresholds and intensity discrimination were strongly dependent upon total noise bandwidth but were independent of the spectral proximity and the center frequency of the noise bands. Analysis of the three experiments revealed that the inverse relation between intensity discrimination and noise bandwidth may be used to predict the variation in amplitude modulation and temporal gap detection thresholds with noise bandwidth.

Methods for stereo loudspeaker placement

A method for stereo loudspeaker placement consisting of applying an acoustic signal having equal amplitude components spread over at least a portion of the audible sound spectrum to a set of stereo loudspeakers to create an acoustic signal, measuring the combined sound level of the acoustic signals at the principal listening position, and adjusting the location of the loudspeakers to ensure that they are acoustically-equidistant from the principal listening position.

Estimates of temporal resolution at low and high spectral frequencies

The auditory system must encode dynamic acoustic stimuli throughout the audible spectrum. This coding is limited by the temporal resolution of the ear. Unfortunately, the results of several investigations disagree on the way in which temporal resolution varies with stimulus frequency. While gap-detection thresholds measured with sinusoidal signals change little over the frequency range of 200–2000 Hz, gap and amplitude modulation detection with narrow-band noise is more variable. Eddins [J. Acoust. Soc. Am. 93, 470–479 (1993)] showed that modulation detection is not frequency dependent from 500 to 4400 Hz, however, there are few measures of modulation detection above this frequency range. In the present experiment, thresholds for modulation depth were obtained as a function of modulation frequency. Narrow-band noise carriers had upper- cutoff frequencies ranging from 3200 to 12 800 Hz and bandwidths from 400 to 3200 Hz. Temporal modulation transfer functions (TMTF) demonstrated the typical low-pass characteristic. The form of the TMTF did not vary with noise bandwidth or frequency region from 3200 to 9600 Hz. Sensitivity to modulation increased with increasing bandwidth. For lower modulation frequencies, thresholds were slightly reduced at 12 800 Hz. These results indicate that modulation detection varies little over a very broad range of frequencies.

Predicting the intelligibility of speech transmitted over nonideal channels

Speech is the most efficient auditory code for conveying abstract information. Hence, high-fidelity speech transmission is an important goal for some multimedia applications. Problems may arise when the signal processing used to create the artificial auditory display inadvertently alters the spectral/temporal properties of the speech or when competing signals mask out portions of the speech. In cases where the distortion amounts to linear filtering and/or noise masking, the degrading effect on intelligibility can be predicted using an elaborate calculation scheme known as articulation theory [H. Fletcher and R. H. Galt, J. Acoust. Soc. Am. 22, 89–151 (1950)]. The theory assumes that the audible spectrum can be divided into a number of bands that contribute independently to speech reception; and the band contributions are additive. Acoustical measurements are translated into their perceptual correlates via transformations derived from psychophysical experiments. These terms are combined to give the articulation index a number which is directly related to the expected percent-correct score for an articulation test. The theory will be described and examples presented. [Work supported by NIH R29 DC02127.]

Modified masking period patterns as a function of frequency region and masker bandwidth

The auditory system must encode dynamic aspects of acoustic stimuli at frequencies throughout the audible spectrum. This coding is limited by the temporal resolution of the ear. Recent measures of modulation detection and temporal gap detection thresholds with narrow-band noises of fixed bandwidth indicate that temporal resolution does not vary with noise carrier frequency from 500 to 4400 Hz. The present study investigated temporal processing using a modified masking period pattern technique. Masked thresholds were measured for pure tones of 500, 2000, and 4000 Hz that were 400 ms in duration. Maskers consisted of narrow-band noises centered on the signal frequency that were either unmodulated or sinusoidally amplitude modulated (modulation depth=1.0) at frequencies from 4 to 256 Hz. The noise bandwidth ranged from 200 to 1600 Hz at each center frequency. By comparing masked thresholds for the unmodulated and amplitude modulated noises, one may assess both within-channel (narrowband noises) and across-channel (wideband noises) temporal processing. Preliminary results indicate that thresholds for a given masker bandwidth depend strongly on modulation frequency but vary little with signal frequency over the range tested. The data will be discussed relative to measures of temporal resolution and comodulation masking release.

Speech enhancement based on audible noise suppression

A novel speech enhancement technique is presented based on the definition of the psychoacoustically derived quantity of audible noise spectrum and its subsequent suppression using optimal nonlinear filtering of the short-time spectral amplitude (STSA) envelope. The filter operates with sparse spectral estimates obtained from the STSA, and, when these parameters are accurately known, significant intelligibility gains, up to 40%, result in the processed speech signal. These parameters can be also estimated from noisy data, resulting into smaller but significant intelligibility gains.

Audible performance of smoke alarm sounds

This paper concerns a study aimed at selection of alarm sounds with improved audible performance characteristics for older listeners over current conventional residential smoke detectors. Many current residential smoke detectors possess alarms that have their primary frequency peak in the 4000-Hz region of the audible spectrum. Additionally, many of these alarms are constant instead of providing temporal modulation of the signal. This study analyzed a variety of alternative sounds for selection as a better choice for an ‘age-sensitive’ smoke alarm signal. The study presented a battery of candidate sounds to pairs of subjects aged 65 years and older with varying levels of hearing impairment (0–45 dB) in their own homes to see which sounds performed best in terms of detection, localization, and perceived attention-getting value. Subjects were placed in various location- and masking-based conditions within their homes during listening periods and subjected to sounds played at a constant level. A computerized system collected response data as the battery of stimuli was presented. The data showed a fairly predictable positive trend in detection and localization performance level as the frequency of the stimuli decreased from 4000 to 500 Hz. The data also showed that pulsed signals were more detectable than steady alarms.

Audible noise of transmission lines caused by the corona effect: Analysis, modelling, prediction

The paper presents the results of investigations of audible noise of high voltage transmission lines of alternating current (a.c. UHV) caused by the corona effect. The investigation results concern the 400 kV lines in two basic layouts existing in Poland: single horizontal and double vertical. Environmental investigations in real conditions were executed in a whole year cycle with the short-term method. Apart from the results of the environmental investigations the paper contains also the laboratory investigation results. The influence of constructional design and environmental parameters on the sound level distribution along a span was analysed. The influence of the conductor surface state and weather conditions on the level and shape of the emitted audible noise spectrum was particularly taken into account. The evaluation of the influence of the line audible noise on the acoustic climate of the environment was carried out. The analysis of possibilities of the application of well known computational procedures to the prediction of the line audible noise caused by the corona effect in Polish conditions was executed and an original form of a line acoustic model with the audible noise prediction possibility as a function of rainfall was presented.

Prediction of articulation scores

Harvey Fletcher developed articulation theory to enable the prediction of the speech transmission capacity of imperfect communication systems, thereby eliminating the need for the time‐consuming speech recognition testing that is otherwise required to differentiate among systems. Two assumptions underlie the theory: (1) a speech signal in each of a number of contiguous frequency bands comprising the audible spectrum contributes independently to the articulation score (percent‐correct for lists of nonsense syllables); and (2) the separate contributions are additive. The approach of Fletcher and his colleagues was to define the articulation index, A, an entity for which these assumptions hold true. The equations for calculating A contain variables representing hearing acuity, critical bandwidth, masking, loudness, perception, and frequency importance weights so that the communication system’s frequency response and gain are translated into perceptual terms. In addition to the derivation of the theory, Fletc...

Audible noise in speed‐controlled inverter‐fed medium‐sized induction motors

AbstractThe audible noise produced by three‐phase, squirrel‐cage induction motors, is caused by the space harmonics of the flux‐density distribution in the air gap of the machine. When the motor is supplied by a frequency inverter in order to control the speed, time harmonics in both current and voltage are present, yielding new components in the audible noise spectrum. In the paper, a number of motor‐inverter combinations are compared for medium‐sized induction motors (90 kW). As a reference case, the motors are supplied with a generator, delivering a variable frequency without harmonics. Theoretical explanations for the increase of the audible noise level and the additional frequency components are given.

Audible Performance of Smoke Alarm Sounds

This paper concerns a study aimed at selection of alarm sounds with improved audible performance characteristics for older listeners over current conventional residential smoke detectors. Many current residential smoke detectors possess alarms that have their primary frequency peak in the 4000 Hz region of the audible spectrum. Additionally, many of these alarms are constant instead of providing temporal modulation of the signal. This study analyzed a variety of alternative sounds for selection as a better choice for an “age sensitive” smoke alarm signal. The study presented a battery of candidate sounds to pairs of subjects aged 65 and older with varying levels of hearing impairment (0 to 45 dB) in their own homes to see which sounds performed best in terms of detection, localization, and perceived attention-getting value. Subjects were placed in various location- and masking-based conditions within their homes during listening periods and subjected to sounds played at a constant level. A computerized system collected response data as the battery of stimuli was presented. The data showed a fairly predictable positive trend in detection and localization performance level as the frequency of the stimuli decreased from 4000 Hz to 500 Hz. The data also showed that pulsed signals were more detectable than steady alarms.