Permissible number of synchronous averaging times to obtain reverberation time from impulse response under time-variance conditions
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In the measurement of room impulse response, the synchronous averaging technique and such new methods as the MLS and the swept-sine methods are being widely used to improve the signal-to-noise ratio. In actual measurement conditions, however, the air in a room is continuously moving and the temperature is changing to some degree. The measured value of the reverberation time in such a room tends to be shorter at higher frequencies when applying the synchronous averaging. Therefore, the assumption of a time invariant has to be carefully considered, and, on this point, some research has been conducted to date. We also have reported various research results concerning the impulse response measurement under the time-variance conditions. In this paper, the permissible number of synchronous averaging times for reverberation measurement is studied through some field experiments. In each field, many time impulse response measurements were taken between a fixed pair of sound source and receiving positions by the swept-sine method, without averaging. After the measurements, the characteristics and the extent of the time-variance under measuring were estimated by a short-term running cross-correlation function between each impulse response. The influence of the time variance on the synchronous averaging result was studied based on the estimated time variance.Keywords:
Impulse response
Sine wave
Impulse noise
For various audio and acoustical applications, it is useful to modify the reverberation time of a measured room impulse response. For example, it can be interesting for creating various modified impulse responses for convolution reverbs in video games or sound designs. With this in mind, it is proposed to modify an impulse response reverberation time while keeping the other reverberation characteristics, such as the frequency response, unaffected. Knowing how a typical impulse response is related to the room characteristics, we propose a new method. First, the impulse response of a room is measured. Then, the reverberation time is evaluated. This reverberation time is next approximated by Sabine-Eyring equation, resulting in an absorption coefficient. A new absorption coefficient is then manually provided to reach a new target reverberation time. Finally, the late reverberation of the original impulse response is convolved with a gaussian white noise to the desired length and stitched with appropriate decaying envelopes to the end of the initially measured impulse response. This paper illustrates that the method works based on simulations and that the modified impulse response is realistic and can be related to the actual room.
Impulse response
Room acoustics
Convolution (computer science)
Finite impulse response
Architectural acoustics
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A video conferencing situation combines the acoustical properties of two rooms. The resulting convolution of the two room impulse responses leads to a total impulse response with a reverberation, which is not a classical exponential decay. As a consequence, relationships between parameters such as clarity and reverberation time will be significantly different from those in single rooms, and this will furthermore affect the combination's suitability for speech communication. In this study, a measurement survey is presented from 11 rooms with video conferencing equipment. Their volumes ranged from 24 to 117 m3, and their mid-range reverberation times were between 0.29 s and 0.70 s. Median values were 82 m3 and 0.41 s, respectively. Impulse responses were measured in all rooms and in a subsequent analysis stage, impulse response pairs were convolved, simulating a connection between the corresponding rooms. Those convolved IRs were analyzed in terms of clarity and reverberation time. Recommended parameter values for single rooms were used as guidelines to understand the qualities of convolved rooms.
Impulse response
Convolution (computer science)
CLARITY
Room acoustics
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Impulse response
Architectural acoustics
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The computer simulation of electroacoustic reverberation enhancement systems for variable acoustics is presented. All involved room impulse responses are predicted using the image source model, complemented by a diffuse field extension for the late part. The electronic units included in the reverberation enhancement system are also specified by their impulse responses. The corresponding frequency responses are processed using a matrix formulation which yields the total room impulse response of the hall with the reverberation enhancement system installed. From this, measures which describe the system performance can be calculated. In the simulation study presented, a typical medium-sized hall which seats 450 persons has been modeled and nine different two-channel configurations have been simulated. The current simulation approach proves more flexible than previous methods since also the early part of the total impulse response can be predicted. Transducer directivities and electronic reverberation can be included and it is shown that these, along with the system delay, to a large extent influence the early part of the total impulse response. The risk of feedback is also discussed.
Impulse response
Room acoustics
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Scale model
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Enclosure
Impulse noise
Impulse response
Sound exposure
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Analysis Method for Estimating Diffuseness of Sound Fields by Using Decay-Cancelled Impulse Response
An analysis method for estimating diffuseness of sound fields by measuring the time variation in reflected sound energy of impulse responses is proposed. In this method, first a decay-cancelled impulse response is obtained by removing the reverberation decay from the impulse response using a Schroeder decay curve. The degree of diffusion of the sound field is determined by evaluating the time variation in the reflected sound energy of the decay-cancelled impulse response. By using this method, the frequency characteristics of diffuseness in sound fields can be analysed from the impulse response measured at a single point. The average degree of diffusion in a room can also be evaluated by averaging the analysis results at several points in the room, similar to the analysis of reverberation time. In order to verify the proposed method, the impulse responses in rooms having different types of diffusers were calculated by the wave acoustics computer simulation. The frequency characteristics of diffuseness were analysed from the calculated impulse responses, by using the proposed method. The results showed that the frequency characteristics of diffuseness change depending on the size of the diffusers. Thus, the proposed method can be used for evaluating the effect of diffusers on the degree of diffusion in a sound field.
Impulse response
Sound energy
Room acoustics
Architectural acoustics
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Computer simulation of electroacoustic reverberation enhancement systems (RES) for auditoria is described and compared with actual measurements. The simulation proceeds in two stages: First, the room impulse responses (from the source and all loudspeakers to the receiver and all microphones) are predicted using the mirror image method, which is then complemented by a diffuse field extension for the late part. The electronic units (amplifiers, delays, and reverberation units) are specified by their impulse responses. Second, the impulse responses are convolved and added using a matrix formulation. This yields the total room impulse response of the hall with the RES installed and objective criteria can then be calculated. Auralization is also possible by convolving the total impulse response with anechoic source signals. More complex systems can be studied than was possible with previous methods since transducer directivities and individual channel reverberation can be included. It is shown that these parameters, along with the system delay, to a large extent influences the early part of the total impulse response. The risk of instability of complex systems can thus also be studied.
Anechoic chamber
Impulse response
Architectural acoustics
Room acoustics
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The acoustical properties of two rooms that are one-way connected electroacoustically, e.g., in a telephone/video conference, can be analyzed through the total impulse response from a source in one room to the receiver in the other room. The total impulse response is a convolution of the two involved room impulse responses, and such a model is analyzed in this paper. The room impulse response model used here facilitates convolution analysis as the model is quite simple and composed of two terms only, a direct sound term and an exponentially decaying random Gaussian noise term. Analytical expressions have been derived for the energy decay function, leading to estimates of room acoustical parameters like clarity and the modulation transfer functions for such convolved impulse responses. Background noise expressions are also introduced to allow signal-to-noise ratio studies. Estimates of acoustic parameter values have been compared with measurements to evaluate the model used and verify the results achieved.
Impulse response
Impulse noise
Convolution (computer science)
Architectural acoustics
Room acoustics
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The two methods of reverberation enhancement; assisted resonance (AR), a frequency divided system, and multiple channel reverberation (MCR), a space-divided system, are previously discussed mostly as methods of increasing reverberation time. The influence on sound intensity level, on sound distribution, and on impulse responses may be of great importance and will be discussed on the basis of accepted room acoustic criteria.
Room acoustics
Impulse response
Architectural acoustics
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