Estimation of Reverberation Time in Classrooms Using the Residual Minimization Method

The objective of the residual minimization method is to determine a coefficient correcting the Sabine’s model. The Sabine’s equation is the most commonly applied formula in the designing process of room acoustics with the use of analytical methods. The correction of this model is indispensable for its application in rooms having non-diffusive acoustic field. The authors of the present paper will be using the residual minimization method to work out a suitable correction to be applied for classrooms. For this purpose, five different poorly dampened classrooms were selected, in which the measurements of reverberation time were carried out, and for which reverberation time was calculated with the use of theoretical methods. Three of the selected classrooms had the cubic volume of 258.5 m3 and the remaining two had the cubic volume of 190.8 m3. It was sufficient to estimate the correction for the Sabine’s equation. To verify the results, three other classrooms were selected, in which also the measurements of reverberation time were carried out. The results were verified by means of real measurements of reverberation time and by means of computer simulations in the program ODEON.

An Influence of the Wall Acoustic Impedance on the Room Acoustics. The Exact Solution

The Fourier method is applied to the description of the room acoustics field with the combination of uniform impedance boundary conditions imposed on some walls. These acoustic boundary conditions are expressed by absorption coefficient values In this problem, the Fourier method is derived as the combination of three one-dimensional Sturm-Liouville (S-L) problems with Robin-Robin boundary conditions at the first and second dimension and Robin-Neumann ones at the third dimension. The Fourier method requires an evaluation of eigenvalues and eigenfunctions of the Helmholtz equation, via the solution of the eigenvalue equation, in all directions. The graphic-analytical method is adopted to solve it It is assumed that the acoustic force constitutes a monopole source and finally the forced acoustic field is calculated. As a novelty, it is demonstrated that the Fourier method provides a useful and efficient approach for a room acoustics with different values of wall impedances. Theoretical considerations are illustrated for rectangular cross-section of the room with particular ratio. Results obtained in the paper will be a point of reference to the numerical calculations.

Prediction of Sound Insulation of Sandwich Partition Panels by Means of Artificial Neural Networks

The paper presents the application of Artificial Neural Networks (ANN) in predicting sound insulation through multi-layered sandwich gypsum partition panels. The objective of the work is to develop an Artificial Neural Network (ANN) model to estimate the Rw and STC value of sandwich gypsum constructions. The experimental results reported by National Research Council, Canada for Gypsum board walls (Halliwell et al., 1998) were utilized to develop the model. A multilayer feed-forward approach comprising of 13 input parameters was developed for predicting the Rw and STC value of sandwich gypsum constructions. The Levenberg-Marquardt optimization technique has been used to update the weights in back-propagation algorithm. The presented approach could be very useful for design and optimization of acoustic performance of new sandwich partition panels providing higher sound insulation. The developed ANN model shows a prediction error of ±3 dB or points with a confidence level higher than 95%.

Multi-Objective Optimization of Acoustical Properties of PU-Bamboo-Chips Foam Composites

In this study, an effective optimization approach was proposed to improve acoustical behaviors of PU foams. The important parameters of PU foams: content of water, silicone oil and catalyst A1 were chosen and their effects on sound absorption coefficient and transmission loss of PU foams were studied by using Taguchi methods. In addition, bamboo chips were incorporated into PU foams as fillers to improve the acoustical properties of PU foams. Four controlled factors: the content of water, silicone oil, catalyst A1 and bamboo chips with three levels for each factor were chosen and Taguchi method based on orthogonal array L9(34) was employed to conduct the experiments. Based on the results of Taguchi’s orthogonal array L9(34), signal-to noise (S/N) analysis was used and developed to determine an optimal formulation of PU-bamboo-chips foam composites.

Recognition of the 24-hour Noise Exposure of a Human

The civilisation progress has caused noise to become one of essential pathogenic and life comfort decreasing factors. There are several legal regulations aimed at controlling the noise influence on humans. Assessment of the twenty-four-hour influence of noises in various environments constitutes an essential problem. The answer can be supplied by 24-hour monitoring of the sound pressure. This paper is an attempt to learn the real loading of humans by noises. A personal noise indicator was used in measurements. The human 24-hour activity was divided into cycles allowing to model noise hazards. The collected data, even though they did not signal exceeding of individual standards, in the 24-hour period indicated the essential noise influence. These results indicate the need of investigations to recognise the 24-hour noise load of a human, with taking into account various forms of their activity and the need of rest.

Identification of Harmonic Musical Intervals: The Effect of Pitch Register and Tone Duration

An experiment was conducted to explore the effect of the pitch strength of pure tones constituting a dyad on the accuracy of musical interval identification. Pitch strength was controlled by presenting the intervals in different frequency regions and varying their duration. The intervals were organized into 18 blocks made up by a combination of three octaves: the second (65.4-130.8 Hz), the fourth (261.6- 523.3 Hz), and the sixth octave (1047-2093 Hz), and six tone durations, ranging 50-2000 ms in the second octave, and 10-500 ms in the two higher ones. The results indicate that interval identification improves with increasing pitch strength of the interval’s component tones. The identification scores were much lower in the second octave than in the two higher ones and in all octaves identification worsened as the interval’s duration was shortened. The intervals were most often confused with intervals of similar size rather than with their inversions and intervals of similar sonic character. This finding suggests that the main cue for the identification of harmonic intervals is the pitch distance between two tones. However, in the low pitch range, when the tone pitches are less salient, the impression of consonance may become a helpful, although not very effective cue.

Using PWE/FE Method to Calculate the Band Structures of the Semi-Infinite PCs: Periodic in x–y Plane and Finite in z -direction

This paper introduces the concept of semi-infinite phononic crystal (PC) on account of the infinite periodicity in x-y plane and finiteness in z-direction. The plane wave expansion and finite element methods are coupled and formulized to calculate the band structures of the proposed periodic elastic composite structures based on the typical geometric properties. First, the coupled plane wave expansion and finite element (PWE/FE) method is applied to calculate the band structures of the Pb/rubber, steel/epoxy and steel/aluminum semi-infinite PCs with cylindrical scatters. Then, it is used to calculate the band structure of the Pb/rubber semi-infinite PC with cubic scatter. Last, the band structure of the rubbercoated Pb/epoxy three-component semi-infinite PC is calculated by the proposed method. Besides, all the results are compared with those calculated by the finite element (FE) method implemented by adopting COMSOL Multiphysics. Numerical results and further analysis demonstrate that the proposed PWE/FE method has strong applicability and high accuracy.

Prototype of Wideband Air Sonar Based on STM32

The paper presents a project of a low-cost prototype version of a wideband air sonar and the problems occurring during its development process. The aim of this work was to design a short range air sonar capable of creating the reflectivity map of its surroundings using an arbitrary shape wideband signal. The transmitting and receiving transducers that have been chosen for this application typically work in pulsed operation modes. Therefore, dedicated output and input circuits had to be designed to expand its capabilities without information losses. The signal generation, transmission, reception and storage are managed by a Discovery evaluation board with a STM32 microcontroller. The receiving data are saved on a SD card and are processed off-line.

Spectral Properties of Circular Piezoelectric Unimorphs

The piezoelectric unimorphs are essential resonant components of many oscillating systems including electroacoustic devices. The unimorph spectral properties are namely dependent on geometric dimensions, applied materials and mounting. Preliminary dimensioning and optimization of unimorph shape are usually carried out prior to comprehensive design work mostly based on finite element method. Simple analytical model is a suitable tool for initial design phase. This paper presents a derivation of calculation model describing natural vibrations of a circular unimorph with the piezoelectric layer diameter smaller than the elastic layer diameter. The system of equations with closed-form solution is instrumental to calculation of resonant frequencies and mode shapes for unimorphs with clamped, simply supported and free circumference. The theoretical results are compared with vibration velocity measurement of clamped unimorph sample in a wide frequency range. Analytical model derived in this paper is used to assess the effect of the thickness tolerance on unimorph resonant frequencies.

Analysis and Optimization of Micro Speaker-Box Using a Passive Radiator in Portable Device

With the rapid development of multimedia devices such as smart phones and tablet PCs, microspeakers have been recently increasingly used for audio equipment. Improving the acoustic performance of a microspeaker is always a main concern, especially in the low frequency range. To avoid sound cancelation, a microspeaker unit is usually inserted into a speaker box. A passive radiator is also used in speaker boxes to improve the sound performance in the loudspeaker system. However, passive radiators have not been applied into microspeaker system. In this study, a speaker box with a passive radiator was analyzed and optimized to achieve a higher Sound Pressure Level (SPL) in a microspeaker system. The Finite Element Method (FEM), two-degree-of-freedom (DOF) vibration theory, and a plane circular piston sound source were used to study the electromagnetic, vibration, and acoustic characteristics, respectively. Optimization was conducted by changing the mass, stiffness, and size of the passive radiator. Based on the optimized parameters, a new sample was manufactured. The experiment results show that the SPL of the optimized speaker box with a passive radiator is improved by 5 dB at 200 Hz compared with the one without a radiator. The analysis results also matched the experiment results.