Johan Ludvig Nielsen

Performance of some linear time-varying systems in control of acoustic feedback

Some linear time-varying (LTV) components used to control feedback in sound systems were tested experimentally in real-time simulators and rooms with and without external reverberation. Gain before instability (GBI) was measured in single channels employing frequency shifting (FS), phase modulation (PM), and delay modulation (DM) implemented on a digital signal processor. FS performed according to the established theory. For PM GBI increased almost monotonically with modulation index β, except for cases with large loop gain irregularities which displayed a reduced GBI for values of β that corresponded to low carrier suppression. Also, GBI was practically independent of the modulation frequency fm already from 0.5 Hz even when this was much lower than the correlation distance of the loop gain transfer function. Rooms with different reverberation times gave different initial (time-invariant) GBI values but these differences decreased by the use of modulation. The GBI increase was larger for cases with exter...

Properties of convolved room impulse responses

In loudspeaking teleconferencing applications, the impulse response from a talker in one room to a listener in the other room involves a convolution of two room impulse responses (RIRs). Here, such a system is analyzed using simple RIR models with a direct sound component plus a white Gaussian noise which is weighted by an exponential window. Expected energy distributions, and modulation transfer functions, of convolutions of such RIRs are derived analytically. The convolution will include a non-exponential term which gives non-classical acoustical parameter behavior. Derivations are presented, predictions are compared with measurements, and the influence of parameters is studied through numerical simulations.

Recent acoustical measurements in the Christchurch Town Hall Auditorium

Two significant changes have occurred in the Christchurch Town Hall during 1996‐97: the original seats have been replaced with a new type and a new Rieger pipe organ has been installed behind the choir seats. A new comprehensive set of acoustic measurements has been made in the unoccupied hall. These are reported and compared in this paper with earlier measurements. A related study on the usefulness of C80 as a clarity measure in concert halls and which makes use of these results is reported in a companion paper. [This work is supported by the Marsden Fund administered by the Royal Society of New Zealand.]

On spatial variability of room acoustics measures

The use of high‐energy measurement signals combined with digital multichannel recording and postprocessing leads to a highly efficient collection of impulse responses in concert halls. During measurements in two similar concert halls, the Christchurch Town Hall, New Zealand, and the Hong Kong Cultural Centre (both described in companion papers), responses were collected in all seats in some areas and in different positions in some single seats, in addition to the usual sampling of all areas. An omnidirectional loudspeaker source was used, and the measurements were repeated in several source positions. The distribution of various parameters in these areas are compared to the results for the whole hall. Based on this comparison the use of mean and standard deviation as measures of location and spread for the parameters is discussed, and the variation of parameters with varying source locations is demonstrated. Approaches toward the reduction of parameter sensitivity to insignificant details in the early sound field are briefly discussed and illustrated using the measured data.

Room acoustical parameters of two electronically connected rooms

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.

An experiment to identify preference groups among concert hall listeners

The task of specifying the acoustical behavior of a concert hall depends upon knowing the preferred subjective acoustical characteristics for the intended program material. Several studies have suggested that the listening population might be divided into groups with respect to their preferred acoustic, with different weightings applied to different subjective factors in each. Our project endeavors to identify and describe these groups, and to make their composition an integral part of hall design. This paper describes a subjective experiment designed to identify and measure individual listening preferences in a simulated concert hall setting, and presents the experiments first results. Following from these results, an attempt is made to place tested individuals into preference groups. The applicability of the experiment is considered in terms of the quality of hall simulation and of the impact of the simulator and experimental design on the listeners discrimination.

The Hong Kong Cultural Centre halls: Acoustical design and measurements

The main performance spaces in the Hong Kong Cultural Centre comprise the Concert Hall, seating 2000 and the Grand Theatre seating 1700. The design of the former is derived from the Christchurch Town Hall which it develops in several important ways, while the latter is a lyric theatre/opera hall with several unique features which are described briefly in this paper. The Centre opened in 1989 following extensive commissioning measurements in the halls. Recently the opportunity has arisen to repeat the measurements using an improved technique in an attempt to document changes following a stage extension. Results from these measurements are compared with the earlier series in each hall. Marshall Day Associates, Auckland, New Zealand was responsible for the room acoustical design of these spaces. [The recent visit to Hong Kong was supported by the Acoustics Research Centre, the Hong Kong Cultural Centre, and the Marsden Fund administered by the Royal Society of New Zealand.]