Sound Decay Curves Research Articles

Determination of the Reverberation Time Using the Measurement of Sound Decay Curves

In the measurements of reverberation time, measurement methods of different accuracy are used depending on the room. For ordinary rooms, the measurements are made using the interrupted noise method, which consists of determining the decay curve after switching off the excitation source of the room. The measurements are made for different source arrangements and different receiver arrangements, and at least three repetitions are made at each of such points. Due to such a realization of measurements, several dozen different reverberation curves are obtained, from which the reverberation time is read out. This article demonstrates the differences between reverberation time readouts, depending on the averaging method of reverberation curves. The first analyzed method is based on reading out the reverberation times for each obtained curve and on averaging the results obtained in this way. The second analyzed method involves averaging the reverberation curves using the linear regression method and then determining a simple regression on the basis of which the reverberation time is read out. For each method, different average reverberation time values and different standard uncertainties were obtained. The difference for the 500 Hz frequency band in a teaching room for the measurement uncertainty is 0.28 s. The results obtained in the article are extremely important when designing interiors intended for the reception of verbal sound, in particular teaching rooms.

The Effect on Room Acoustical Parameters Using a Combination of Absorbers and Diffusers—An Experimental Study in a Classroom

Several room acoustic parameters have to be considered in ordinary public rooms, such as offices and classrooms, in order to present the actual conditions, thus increasing demands on the acoustic treatment. The most common acoustical treatment in ordinary rooms is a suspended absorbent ceiling. Due to the non-uniform distribution of the absorbent material, the classical diffuse field assumption is not fulfilled in such cases. Further, the sound scattering effect of non-absorbing objects such as furniture are considerable in these types of rooms. Even the directional characteristic of the sound scattering objects are of importance. The sound decay curve in rooms with absorbent ceilings often demonstrate a double slope. Thus, it is not possible to use reverberation time as room parameter as a representative standalone acoustic measure. An evaluation that captures the true room acoustical conditions therefore needs supplementary parameters. The aim of this experimental study is to show how various acoustical treatments affect reverberation time T20, speech clarity C50 and sound strength G. The experiment was performed in a mock-up of a classroom. The results demonstrated how absorbers, diffusers and scattering objects influence room acoustical parameters. It is shown that to some extent the parameters can be adjusted individually by using different treatments or combination of treatments. This allows for the fine-tuning of the acoustical conditions, in order to fulfill the requirements for achieving a high-quality sound environment.

Sound Scattering Properties of Surfaces with Diffusers

This article compares the values of the normal scattering coefficient measured in a model experiment for two types of diffusers placed on a rigid surface. Wooden diffusers of cubic and pyramidal shapes were tested in a scale model of a room with dimensions of 0.7x0.4x0.4 m. Sound decay curves were measured at frequencies of 4kHz and 8kHz. Two large walls were covered with a porous absorber, on the third, in certain combinations, the investigated diffusers with a characteristic size of 3.5 cm were placed, the number of which varied from 0 to 29. The idea of the applied method is that the sound decay curve in a room with a non-diffuse sound field depends significantly from the scattering properties of surfaces. The decay curve was measured with different numbers of the diffusers on the test wall, which made it possible to determine the influence of the shape of the diffusers and their number on the value of the normal scattering coefficient. According to the results of the measurements a high scattering ability of cubic elements was revealed in comparison with pyramidal ones.

Decay of Non-Diffuse Sound Fields in a Room

The paper presents the results of a study of a non-diffuse sound field decay in a room. In a laboratory experiment, a model of a room in the form of a rectangular parallelepiped with a size of 0.7 x 0.4 x 0.4 m is used. Two non-parallel walls are coated with sound absorbing material. Sound scattering elements can be placed on the third wall which is perpendicular to the absorbing ones, which allows to change the field diffusion degree. The sound field in such a room has a strong anisotropy, its energy decays according to the exponential-power law. Sound decay curves were measured at various positions of the sound source and microphone at frequencies of 4 kHz. The decay curves are compared, their dependence on the relative position of the sound source and microphone is analyzed.

The effect of acoustical remedies changing the reverberation time for different frequencies in a dome used for worship: A case study

Acoustic studies were carried out in a dome-shaped sacral room of the Pentecostal Congregation in Katowice in Silesia, Poland. The investigated room is used for religious purposes, and in terms of acoustics it facilitates the propagation of speech and musical sounds. In the study, acoustic measurement methods were adopted to assess sound reverberation. The “in situ” method was used to determine sound decay curves, and reverberation time was determined in line with EN ISO 3382-1: 2009, EN ISO 3382-2: 2010. Participants of the services complained about poor reception conditions of verbal and musical sounds and flutter echo in some places of the room. Acoustic studies confirmed the listeners' impressions. The room had long reverberation time. To improve the acoustic parameters of the interior, the application of spray-deposited insulating material was proposed. The selected material was subjected to laboratory tests on sound-absorbing properties. The sound absorption coefficient of the material was determined as a frequency function. Then the material was applied on the inner surface of the dome. Acoustic measurements of the adapted interior were repeated. Based on the measurements, the STI was calculated. In effect, a significant improvement in reverberation conditions of the investigated interior was obtained.

Increasing the Accuracy of Reverberation Time Measurement Using Wavelet Thresholding

Reverberation time measurement is used in building acoustic analysis and determination of acoustic characteristic of a material such as sound transmission loss and sound absorption index. This process measures the time needed for sound to decay 60 dB since the steady sound in the room is switched off. Usually, the reverberation time is estimated from sound decay curve. In the real reverberation time measurement, the smooth sound decay curve is hard to find because the original sound wave combines with reflection sound wave. In this paper, two level Daubechies wavelet transform is performed to filter noise in the sound decay. The applied wavelet thresholding successfully filtered out the maximum and minimum peak in the sound decay curve, therefore the estimation of reverberation time can be done more accurately in high and low frequency region.

Sound Field Diffusivity Assessment in Non-Stationary State by Measuring Nonuniformity of Sound Decay Curve

Sound Field Diffusivity Assessment in Non-Stationary State by Measuring Nonuniformity of Sound Decay Curve

Variation of mean free path length over time and its effect on room impulse response

In their previous paper (Forum acusticum 2005, Proceedings, 2399-2404) the authors presented the local effect found in a large sports hall, described as significantly shorter reverberation time in the first 400-500 ms than that in the rest of the hall's long impulse response. To analyse this phenomenon, a new measure - mean free path length (MFPL) variation in time over the impulse response - was introduced. It is identified as the function of time obtained as an ensemble average in some fixed interval traced along the impulse response duration. The ray tracing simulation was used to calculate ray paths approaching the receiver in time. Analyzing MFPL variations over time is a new insight into the structure of room impulse response, and this paper presents its main characteristics, its dependence on geometrical properties of rooms, and also its relation to statistically defined mean free path length (4 V/S). The paper also presents a correlation between MFPL variation over time and the sound decay curve. The MFPL variation over time reveals the causes of the local effect introduced in the previous paper, but it is generally useful in explaining the sound decay monotony deviations.

Experimental evaluation of radiosity for room sound-field prediction

An acoustical radiosity model was evaluated for how it performs in predicting real room sound fields. This was done by comparing radiosity predictions with experimental results for three existing rooms--a squash court, a classroom, and an office. Radiosity predictions were also compared with those by ray tracing--a "reference" prediction model--for both specular and diffuse surface reflection. Comparisons were made for detailed and discretized echograms, sound-decay curves, sound-propagation curves, and the variations with frequency of four room-acoustical parameters--EDT, RT, D50, and C80. In general, radiosity and diffuse ray tracing gave very similar predictions. Predictions by specular ray tracing were often very different. Radiosity agreed well with experiment in some cases, less well in others. Definitive conclusions regarding the accuracy with which the rooms were modeled, or the accuracy of the radiosity approach, were difficult to draw. The results suggest that radiosity predicts room sound fields with some accuracy, at least as well as diffuse ray tracing and, in general, better than specular ray tracing. The predictions of detailed echograms are less accurate, those of derived room-acoustical parameters more accurate. The results underline the need to develop experimental methods for accurately characterizing the absorptive and reflective characteristics of room surfaces, possible including phase.

ACOUSTIC COUPLING EFFECTS IN ST PAUL'S CATHEDRAL, LONDON

In St Paul's Cathedral there are many arches, columns and cornices which enable the internal space to be divided into subspaces. The subspaces may be considered to be acoustically coupled via areas which connect the rooms. Two of the most acoustically important subspaces in the Cathedral are the choir and the space under the dome. The choir, the space within the wooden choir stalls, has more sound absorption than the rest of the building, which is mostly marble and Portland stone. In the model of coupled subspaces an acoustic energy balance equation, applied to a diffuse field, is derived for each subspace. In St Paul's Cathedral the internal space is divided into 70 acoustical subspaces. The initial-value problem which is formulated by the system of 70 acoustic energy balance equations with initial conditions has been reduced to the eigenvalue problem. The decay of sound energy density has been obtained for different locations in the Cathedral and for different positions of the sound source. Experimentally obtained sound decay curves are in good agreement with numerical results. Both the experimental and numerical results demonstrate the fine structure of reverberation.

Experiences with the application of a software for noise prediction in workrooms

For a couple of years, the BIA has been using a computer program for the calculation of room acoustic parameters and the prediction of sound pressure levels at the workplace. Experiences gathered with the program in connection with industrial consulting activities are presented and illustrated by examples. The software is based on a calculation method referred to in the German VDI guideline 3760 (image sound source method) requiring a mean degree of complexity in the description of the room. The software comprises a large library, indicating sound absorption coefficients for all tested materials. The program is mainly used for planning room acoustic improvements and new workrooms. Noise prediction in connection with production changes (replacement of machinery, new productions) is also possible. In some cases of hearing damage, the program was used for a retrospective determination of noise exposure at former workplace. A comparison of the calculated spatial sound decay curves and the thus-derived room acoustic parameters with the measurement readings showed good correspondence. The development of noise maps and the consequent presentation of the achievable noise reduction by means of concrete measures has become an important instrument for motivating and convincing companies.

Prediction of noise in workrooms including extended sources, directivity of radiation, and screening

For many years the mirror image method has been used to calculate noise levels in workrooms. Fittings are taken into account by a fitting density defined by Kuttruff and extended to this application by Jovicic. In a research program financed by BAUA Dortmund it was examined how directivity of radiation and screening can be integrated. A removable screen was installed in different halls and it measured the sound distribution in the shadow region, which was produced by a uniformly radiating loudspeaker radiating white noise. Calculation of the same situation by taking into account screening showed that only low orders of reflection are influenced by the screen. This gives the necessary information for a modification of software programs to include screening. Similar to this procedure, a loudspeaker with high directivity was used as source with known emission to measure sound decay curves in workrooms. The same situation was calculated by mirroring the complete directivity of the source. Measured and calculated levels are in good agreement. With these techniques the German standard VDI 3760 for noise calculation in rooms can be improved in the next years.

Experiments on evaluation of non-exponentially decay reverberations : Relationship of sound decay curve to subjective reverberation time length

Experiments on evaluation of non-exponentially decay reverberations : Relationship of sound decay curve to subjective reverberation time length

Dependence of the curvature of sound decay curves and absorption distribution on room shapes

The curvature of logarithmic sound decay curves in four reverberation chambers of regular and irregular shapes is treated theoretically and calculated curves are shown. The degree of curvature depends on the location of an absorption specimen as well as on the room shape. It is also shown that the curvature can be minimized by suitable arrangement of absorption specimens.

A Study of the Acoustic Significance of the Amplitude and Phase of Harmonics Present in a Source of Sound in a Room

This investigation is pointed toward obtaining some further measure of the acoustic properties of a room. First, a brief subjective study is made of listener response to sounds which have various degrees of modulation on their decay curves and to alternate sounds which differ only in the phase relation of one of the harmonics with respect to the fundamental frequency. Second, an objective study of the modulation of sound decay curves is carried out for rectangular rooms, with emphasis on the effect of the phase and amplitude of harmonics present in the source of sound. In this study, the sound source consisted of a Complex Wave Generator, a power amplifier, and a loudspeaker. The Complex Wave Generator provides output terminals for the fundamental frequency and for the first four harmonics at controlled phases and amplitudes. The power amplifier is a conventional resistance-coupled amplifier combined with a device designed to cut off the output at any predetermined phase of the fundamental frequency and with a minimum switching transient. The loudspeaker and the complete recording system are conventional.

The Modulation of Sound Decay Curves

The irregularity of decay curves has long been noted as a difficulty in the measurement of the reverberation time of a room. It has been suggested that these irregularities may be an important factor in the evaluation of the subjective acoustic qualities of a room. The departures from an exact exponential decay arise from three causes: (1) Beats are produced by interference among the resonance frequencies of the room. (2) Multiple decay rates result from the shape of the room or the non‐uniform distribution of absorption on the walls. (3) The direct waves from the source and its first images are important for large or highly absorptive rooms, and at high frequencies. To investigate these phenomena, objective measurements have been made on nine rooms of widely differing sizes and shapes. Apparatus has been designed which compensates for the exponential slope of the decay, leaving only the remaining variations to be recorded by a high speed level recorder. Experimental data are presented for the nine rooms.

The Modulation of Sound Decay Curves

The irregularity of decay curves has long been noted as a difficulty in the measurement of the reverberation time of a room. It has been suggested that these irregularities may be an important factor in the evaluation of the subjective acoustic qualities of a room. The departures from an exact exponential decay arise from three causes: (1) Beats are produced by interference among the resonance frequencies of the room. (2) Multiple decay rates result from the shape of the room or the non-uniform distribution of absorption on the walls. (3) The direct waves from the source and its first images are important for large or highly absorptive rooms, and at high frequencies. To investigate these phenomena, objective measurements have been made on nine rooms of widely differing sizes and shapes. Apparatus has been designed which compensates for the exponential slope of the decay, leaving only the remaining variations to be recorded by a high speed level recorder. Experimental data are presented for the nine rooms.

On Non-Uniform Acoustical Boundary Conditions in Rectangular Rooms

An exact solution is obtained for the behavior of sound in a rectangular room whose walls are covered non-uniformly with acoustical materials. The sound field in the chamber is decomposed into normal modes of vibration, each having its normal frequency, decay constant, and pressure configuration affected by the distribution of acoustical materials. The normal modes of vibration are considered to be composed of traveling waves undergoing both specular reflection and diffraction. In most cases the non-uniform boundary may be replaced, through a perturbation method, by an equivalent uniform boundary whose acoustic admittance is simply related to the properties of the actual boundary. Additional effects of diffraction, when not negligible, may be superposed. The diffraction patterns for several simple cases are computed and compared with the predictions of conventional free wave analysis. The present theory is adequate for precalculation of sound decay curves in rectangular rooms for any given arrangement of acoustical materials. It can, in addition, be used for guidance in selecting distributions of material leading to particular results, such as linear decay curves or maximum “apparent absorption coefficient.”

Analysis of Sound Decay in Rectangular Rooms

Sound decay in a rectangular room, each wall of which is uniform, is analyzed in terms of the damping of normal modes of aerial vibration, as influenced by the acoustical impedance of the boundary surfaces. From two to seven decay terms may be required in the complete decay equation under different circumstances, each term representing a group of the excited modes of vibration having common properties. The analysis is given in terms of r.m.s. pressure measured for specified locations of source and receiver and includes explicitly the relative dimensions of the room. The results of the mathematical analysis are presented in form to allow determination of the complex wall impedance from consideration of the initial and final slopes of the sound decay curve, and conversely. Experimental decay curves confirm both the detailed predictions of the theory and the sufficiency of the wall impedance as a unique characterization of the influence of the wall on the behavior of sound in the room.

Recording Instruments for Frequency and Intensity

An inexpensive recorder which employs a spark to puncture a moving strip of chart paper has been constructed. One of the sparking electrodes is a light pointer attached to a short-period D'Arsonval galvanometer element. The other is a curved metal guide which bends the chart paper to the arc described by the tip of the pointer. In conjunction with the logarithmic vacuum-tube voltmeter already described (Rev. Sci. Inst. 4, 672, 1933) this instrument comprises a level recorder having a uniform decibel scale (60 db) and capable of following sound decay curves as steep as 150 db/sec. A new electronic frequency meter has been developed which is capable of following rapid variations of frequency. This device employs gas-filled discharge tubes and yields an output that is strictly linear with frequency from zero to 8000 cycles and independent of the input intensity over a range of at least 35 db. The instrument is flexible and any portion of the frequency range may be fitted to the indicating instrument permitting high sensitivity to small frequency changes. In combination with the spark recorder, or any other type of recorder, this instrument places the delineation of time variations of frequency on an equal footing with time variations of intensity. For example, the melodic curves or intonation of speech and music, vibrato, and attack may be recorded directly.