微型扬声器摆动模态对其辐射声场的影响*

与传统电动式扬声器单元相比,微型扬声器单元由于缺少定位支片等部件,更容易受到摆动模态的影响。该文利用激光传感器采集微型扬声器单元振膜各点振动位移,通过理论计算,从辐射声压级、辐射指向性等角度探究矩形微型扬声器单元摆动模态对其辐射声场的影响。经过研究发现,摆动模态会造成微型扬声器单元频响凹陷、中低频存在指向性等现象,对其辐射声场产生明显的影响。     

磁流体扬声器特性的分析

本文分析了磁流体扬声器的特性并给出一些实验结果.磁流体可以提高扬声器的功率容量,防止音圈过热损坏.磁流体扬声器的输入电功率在音圈温度相同的条件下,比同类普通扬声器可以提高2倍多,从而增加了扬声器的声压灵敏度.磁流体的粘滞性具有阻尼音圈振动的作用,它改善了扬声器的频响,降低瞬态畸变.文中还讨论了磁流体扬声器设计中应注意的问题.     

晶体环境下高次谐波谱的截止频率分析

通过数值求解中红外飞秒光脉冲与一维单电子多阱势相互作用的含时薛定谔方程, 研究了晶体环境下的高次谐波发射光谱, 发现了晶体谐波谱的截止位置公式.研究发现, 不同于气体环境下的谐波谱截止频率规律, 晶体谐波谱的截止频率与激光电场的峰值振幅及晶格参数成线性关系. 鉴于准经典力学的三步模型对于气体谐波截止频率的解释, 进一步采用准经典力学方法验证了晶体谐波截止位置规律的正确性. 关键词： 中红外飞秒光脉冲 晶体 高次谐波 截止频率     

强辐照下陶瓷泡沫骨架内的辐射能量传输机理研究

当泡沫材料直接受强光束(如聚集太阳能)照射时,准确描述材料内的热辐射传输过程成为传热模拟的关键。本文数值研究了真实陶瓷泡沫骨架在强辐照下的容积性辐射传输规律。采用离散坐标法求解了半透明氧化铝陶瓷骨架内部的辐射传输,获得了辐射能量的传递、吸收规律。研究表明,辐射在陶瓷骨架内的容积输运本质上表现为散射和吸收两种行为的竞争结果;随着波长的增加,氧化铝半透明骨架吸收的辐射能量逐渐增多。     

光致发光光谱研究金刚石光学中心的振动结构

金刚石中不同的缺陷中心具有不同的振动结构. 间隙原子相关中心在离开零声子线165 meV之外存在强且尖锐的局部振动模; 空位相关中心具有很强的振动耦合, 在其零声子线之后出现很强且宽的声子边带, 而在165 meV之外却观察不到局部振动模, 且对于仅涉及一个空位的缺陷来说, 其振动都与一个能量约为42 meV的声子或一个能量约为67 meV的声子或两声子有关.     

垂直振动床中的能量传递与耗散

本文采用数值方法分析了一维垂直振动床内颗粒动能/温度、能量耗散以及体积分数的分布规律.离散元模拟结果表明:当床底做低频、小振幅振动时,床层内颗粒整体随床底上下运动,沿床高方向颗粒动能逐渐增加;对于高频振动,床层内的颗粒做无规则的运动,沿床高方向颗粒动能逐渐降低.在不同振动频率(高频、低频)下体积分数、能量耗散也表现出不同的分布规律.将离散元模拟结果与动力学理论计算值对比,当系统做高频振动时,两模型所得结果基本吻合;而对于低频、小振幅振动,所得结果存在较大差异.由于低频、小振幅振动时床内颗粒并非做无规则运动,动力学理论的适用性需进一步完善.     

空化诱发的轴流泵振动特性实验研究

运用LMS多通道振动噪声测试与动力学分析系统,对轴流模型泵最优工况流量下不同空化程度时的振动加速度进行了动态多点测量和分析,获得了振动加速度的特征频率以及低频、高频振动水平随汽蚀余量的变化规律。实验结果表明:轴频、叶频以及其高次谐波是泵振动的主要激励频率;空化诱发的振动主要为高频振动;随汽蚀余量的降低,低频振动和高频振动均呈现先上升后下降的趋势;高频振动比低频振动更易被阻尼力削弱,高频振动水平上升的拐点和空化引起泵能量性能下降的临界点相一致。     

一种辐射声场近似计算方法——单元辐射叠加法

提出一种基于表面振速预报辐射声场的近似方法--单元辐射叠加法.研究表明,声传递向量中的每项元素等于对应单元以单位速度振动、其它单元振速都为零时的辐射声压,即刚性障板上活塞面以单位速度振动时的辐射声压.在此基础上,利用刚性障板上单位速度振动活塞面的辐射声压直接建立表面振速与辐射声压之间的传递关系,根据这个传递关系对表面振速进行加权、求和便得到总的辐射声压.计算时采用规则形状障板面去拟合实际障板面,以规则形状障板上振动活塞面的辐射声压去近似实际障板上振动活塞面的辐射声压.相对于边界元方法,该方法在计算速度与存储空间上具有很大的优势.计算表明该近似方法是合理的、可行的.     

基于有限差分法刚性圆柱体涡激振动响应特性数值研究

基于尾流振子模型对刚性圆柱体涡激振动响应特性进行数值研究.建立圆柱体结构振子及尾流振子之间的耦合方程,基于二阶精度中心差分格式对耦合模型先离散后迭代进行求解.对不同质量比及不同阻尼比圆柱体涡激振动响应的无量纲位移、无量纲升力、频率比及锁定区间等参数进行分析.结果表明数值方法可以很好地模拟刚性圆柱体的涡激振动响应特性.随着质量比的增加,锁定开始点逐渐延后,锁定结束点逐渐提前,锁定区间宽度逐渐变窄.     

啁啾脉冲放大系统中空间滤波器小孔尺寸的设计

为了获得啁啾脉冲放大系统(CPA)中空间滤波器的小孔尺寸,可以利用傅里叶光学的方法和激光模式理论,通过数学上的合理近似和对能量频谱函数的数值积分,计算激光束中基横模和主要低阶横模的径向空间频谱与空间截止频率,并讨论以基横模的空间截止频率作为空间滤波器的截止频率的可行性,从而确定小孔光阑孔径的大小并得出一个具有普遍意义的结论,即小孔直径约取6.93倍衍射极限长度时,空间滤波器能够较好地滤去TEM00模以外其他横模的大部分高频能量成分,从而提高光束质量,此结果在实验上得到很好的验证。此外,以M2因子为基础,可以得到一个用以辅助描述空间滤波器滤波性能的光束质量改善因子B。     

Acoustic noise simulation and measurement of a gradient insert in a 4 T MRI

High speed switching of current in gradient coils within high magnetic field strength magnetic resonance imaging (MRI) scanners results in high acoustic sound pressure levels (SPL) in and around these machines. Many studies have already been conducted to characterize the sound field in and around MRIs and various methods have been investigated to attenuate the noise generated. In the work presented here a computational vibro-acoustic model was developed based on an iteratively modified and validated finite element (FE) model to characterize the acoustic noise properties of the gradient coil. The simulation results from the computational model were verified through experimental noise measurement for the gradient coil insert in a 4 T MRI scanner by using swept sinusoidal time waveform inputs. Comparisons show that the computational model predicts the noise characteristic properties extremely accurately. There are three dominant frequency bands where the SPL is much higher than those at other frequencies. The SPL in the horizontal direction is much higher than that in the vertical direction due to the excitation to the horizontally placed X coil. The SPL to the inner surface of the coil is higher than far from the inner surface, which proves that the acoustic noise is radiated from the inner surface and primarily caused by the normal vibration of the inner surface. Further verification was conducted by using two types of trapezoidal sequence inputs usually used, which is to simulate real scanning sequences for small animals. Again the accuracy of the developed model is verified. The validated acoustic computational model could be used as an effective method to predict the noise that would be produced by a coil in the design stage. Modification of the structural design or the excitation pulse could be performed to reduce the acoustic noise when the gradient coil is in scanning.     

Relationship between subjective voice complaints and acoustic parameters in female teachers'' voices

The aim of the study was to identify the acoustic correlates of female teachers' subjective voice complaints by recording their voices in their working environment. The subjects made recordings during lessons (N = 10) and breaks (N = 11). The subjects were divided into 2 groups: those with few voice complaints (FC group) and those with many voice complaints (MC group). The speech sample made in the breaks was maximally sustained /a/, from which fundamental frequency (F0), jitter, and shimmer were analyzed. The classroom samples were analyzed for F0, sound pressure level (SPL), and F0 time (the active vibration time of the vocal folds). Additionally, an index for assessing voice loading is presented. The results revealed a tendency of the MC group to have higher F0 and lower SPL and perturbation values than the FC group. The index values correlated moderately with the subjective vocal complaints.     

Feature Maps of the Acoustic Spectrum of the Voice

The change in the spectrum of sustained /a/ vowels was mapped over the voice range from low to high fundamental frequency and low to high sound pressure level (SPL), in the form of the so-called voice range profile (VRP). In each interval of one semitone and one decibel, narrowband spectra were averaged both within and across subjects. The subjects were groups of 7 male and 12 female singing students, as well as a group of 16 untrained female voices. For each individual and also for each group, pairs of VRP recordings were made, with stringent separation of the modal/chest and falsetto/head registers. Maps are presented of eight scalar metrics, each of which was chosen to quantify a particular feature of the voice spectrum, over fundamental frequency and SPL. Metrics 1 and 2 chart the role of the fundamental in relation to the rest of the spectrum. Metrics 3 and 4 are used to explore the role of resonances in relation to SPL. Metrics 5 and 6 address the distribution of high frequency energy, while metrics 7 and 8 seek to describe the distribution of energy at the low end of the voice spectrum.Several examples are observed of phenomena that are difficult to predict from linear source-filter theory, and of the voice source being less uniform over the voice range than is conventionally assumed. These include a high-frequency band-limiting at high SPL and an unexpected persistence of the second harmonic at low SPL. The two voice registers give rise to clearly different maps. Only a few effects of training were observed, in the low frequency end below 2 kHz. The results are of potential interest in voice analysis, voice synthesis and for new insights into the voice production mechanism.     

Objective acoustic voice-quality parameters in the computer phonetogram

A phonetogram is a plot of the dynamic range of the voice as a function of fundamental frequency. Traditionally, the phonetogram only records the sound-pressure level (SPL) of the threshold of phonation and the upper limit in SPL the voice can reach with sustained phonation as a function of the fundamental frequency (F₀). In this study, the phonetogram is extended by including acoustic voice-quality parameters. Three additional parameters are tested: jitter, shimmer, and crest factor. For each individual voice, the variation in the three parameters is evaluated over the entire phonetogram area. By averaging individual phonetograms, standard or norm phonetograms are obtained revealing general patterns in voice-quality variation. These patterns reflect the complex relations between F₀, SPL, and the acoustic voice-quality parameters just mentioned. Jitter and shimmer distributions over the phonetogram show that large variations in perturbation values can be expected when production conditions are altered. Highest perturbation values are found for the low F₀ and low SPL phonations. For all voices, a gradual decrease of the crest factor is found with increasing F₀, reflecting the degrading of spectral complexity with F₀. The crest-factor parameter can also be used to mark those areas in the phonetogram where the fundamental dominates the waveform and where flow control is the main SPL regulating mechanism in voice production. The strong quality variations within the phonetogram stress the need for accurate control of F₀ and SPL in objective voice-quality measurement.     

Deviant Vocal Fold Vibration as Observed During Videokymography: The Effect on Voice Quality

Videokymographic images of deviant or irregular vocal fold vibration, including diplophonia, the transition from falsetto to modal voice, irregular vibration onset and offset, and phonation following partial laryngectomy were compared with the synchronously recorded acoustic speech signals. A clear relation was shown between videokymographic image sequences and acoustic speech signals, and the effect of irregular or incomplete vocal fold vibration patterns was recognized in the amount of perceived breathiness and roughness and by the harmonics-to-noise ratio in the speech signal. Mechanisms causing roughness are the presence of mucus, phase differences between the left and right vocal fold, and short-term frequency and amplitude modulation. It can be concluded that the use of simultaneously recorded videokymographic image sequences and speech signals contributes to the understanding of the effect of irregular vocal fold vibration on voice quality.     

Voice Changes During Work: Subjective Complaints and Objective Measurements for Female Primary and Secondary Schoolteachers

The purpose of this investigation was to study voice changes during a working day. The subjects consisted of 33 female primary and secondary schoolteachers who recorded their first and last lessons during one school day. The subjects were studied both as one group and two subgroups (those with many and those with few voice complaints). Estimates of fundamental frequency (F0), sound pressure level (SPL), the standard deviations of these values (F0 SD; SPL SD) and F0 time (vibration time of vocal folds) were made. The most obvious change due to loading was the rise of F0 that was 9.7 Hz between the first and last lesson (P = 0.00). F0 increased more (12.8. Hz, P = 0.006) in the subgroup with few complaints.     

The ChatterVox™ Portable Voice Amplifier: A Means to Vibration Dose Reduction?

By speaking loudly for extended periods, teachers are vulnerable to laryngeal and voice changes associated with vocal fold “vibration overdose.” Voice clinicians frequently recommend voice amplification ostensibly designed to reduce vibration dose and improve voice. However, there are few data regarding the degree of vocal loudness attenuation achieved by specific amplification devices. The purpose of this investigation was to examine the effectiveness of the ChatterVox™ Portable Voice Amplification System (Siemens Hearing Instruments) for reducing the sound pressure level (SPL) of a speaker's voice during a simulated classroom lecture. Ten participants were instructed to continuously read one of two phonetically balanced passages while amplified and unamplified. Voice intensity measurements were obtained at three inches from the mouth (i.e., mouth level) and at the back of a classroom in both amplified and unamplified conditions. When amplified with the ChatterVox™, speakers experienced an average decrease in vocal intensity at mouth-level of 6.03 dB SPL (p < 0.002). Furthermore, an average increase of 2.55 dB SPL (p < 0.038) at the back of the classroom was observed. Collectively, these results indicate that the ChatterVox™ amplification device reduced the speaker's vocal intensity level at the microphone, while it augmented the voice heard at the back of the classroom. By inference, this degree of vocal attenuation at mouth level should contribute to a desirable reduction in vibration dose, thus lowering the risk of vibration overdose.     

Vocal loudness and frequency capabilities of the voice

A phonetogram is a graph showing the sound pressure level (SPL) of softest and loudest phonation over the entire fundamental frequency range of a voice. A physiological interpretation of a phonetogram is facilitated if the SPL is measured with a flat frequency curve and if the vowel /a/ is used. It was found that in soft phonation, the SPL is mainly dependent on the amplitude of the fundamental, while in loud phonation, the SPL is mainly determined by overtones. The short-term SPL variation, i.e., the level variation within a tone, was about 5 dB in soft phonation and close to 2 dB in loud phonation. For two normal voices the long-term SPL variation, calculated as the mean standard deviation of SPL for day-to-day variation, was found to be between 2.4 and 3.4 dB in soft and loud phonation. Speakers who raise their loudness of phonation also tend to raise their mean voice fundamental frequency. Measures obtained from speaking at various voice levels were combined so that typical pathways could be introduced into the phonetogram. The average slope of these pathways was 0.3–0.5 st/dB for healthy subjects. Averaged phonetograms for male singers and male nonsingers did not differ significantly, but averaged phonetograms for female singers and female nonsingers did, in that the upper contour was higher for the female singers. Averaged phonetograms for female patients with non-organic dysphonia showed significantly lower SPL values in loudest phonation as compared to healthy female subjects, while no corresponding difference was seen for males in this regard. With respect to the SPL values for softest phonation, male dysphonic patients showed significantly higher SPL values than healthy male subjects, while no corresponding difference was seen in female subjects. The subglottal pressure mirrored these phonetogram differences between healthy and pathological voices. The averaged phonetograms of female patients after voice therapy showed an increased similarity with those of normal voices. For the male patients the averaged phonetogram did not change significantly after therapy.     

Acoustic Measures and Self-reports of Vocal Fatigue by Female Teachers

This study investigated the relation of symptoms of vocal fatigue to acoustic variables reflecting type of voice production and the effects of vocal loading. Seventy-nine female primary school teachers volunteered as subjects. Before and after a working day, (1) a 1-minute text reading sample was recorded at habitual loudness and loudly (as in large classroom), (2) a prolonged phonation on [a:] was recorded at habitual speaking pitch and loudness, and (3) a questionnaire about voice quality, ease, or difficulty of phonation and tiredness of throat was completed. The samples were analyzed for average fundamental frequency (F0), sound pressure level (SPL), and phonation type reflecting alpha ratio (SPL [1-5 kHz]-SPL [50 Hz-1 kHz]). The vowel samples were additionally analyzed for perturbation (jitter and shimmer). After a working day, F0, SPL, and alpha ratio were higher, jitter and shimmer values were lower, and more tiredness of throat was reported. The average levels of the acoustic parameters did not correlate with the symptoms. Increase in jitter and mean F0 in loud reading correlated with tiredness of throat. The results seem to suggest that, at least among experienced vocal professionals, voice production type had little relevance from the point of view of vocal fatigue reported. Differences in the acoustic parameters after a vocally loading working day mainly seem to reflect increased muscle activity as a consequence of vocal loading.     

Effects of vocal training on the acoustic parameters of the singing voice

Vocal training (VT) has, in part, been associated with the distinctions in the physiological, acoustic, and perceptual parameters found in singers' voices versus the voices of nonsingers. This study provides information on the changes in the singing voice as a function of VT over time. Fourteen college voice majors (12 females and 2 males; age range, 17–20 years) were recorded while singing, once a semester, for four consecutive semesters. Acoustic measures included fundamental frequency (F₀) and sound pressure level (SPL) of the 10% and 90% levels of the maximum phonational frequency range (MPFR), vibrato pulses per second, vibrato amplitude variation, and the presence of the singer's formant. Results indicated that VT had a significant effect on the MPFR. F₀ and SPL of the 90% level of the MPFR and the 90–10% range increased significantly as VT progressed. However, no vibrato or singers' formant differences were detected as a function of training. This longitudinal study not only validates previous cross-sectional research, ie, that VT has a significant effect on the singing voice, but also it demonstrates that these effects can be acoustically detected by the fourth semester of college vocal training.