Issues for computer modelling of room acoustics in non-concert hall settings

Abstract

The basic principle of common room acoustics computer models is the energy-based geometrical room acoustics theory. The energy-based calculation relies on the averaging effect provided when there are many reflections from many different directions, which is well suited for large concert halls at medium and high frequencies. In recent years computer modelling has become an established tool in architectural acoustics design thanks to the advance in computing power and improved understanding of the modelling accuracy. However concert hall is only one of many types of built environments that require good acoustic design. Increasingly computer models are being sought for non-concert hall applications, such as in small rooms at low frequencies, flat rooms in workplace surroundings, and long enclosures such as underground stations. In these built environments the design issues are substantially difference from that of concert halls and in most cases the common room acoustics models will needed to be modified or totally re-formulated in order to deal with these new issues. This paper looks at some examples of these issues. In workplace environments we look at the issues of directional propagation and volume scattering by furniture and equipment instead of the surface scattering that is common assumed in concert hall models. In small rooms we look at the requirement of using wave models, such as boundary element models, or introducing phase information into geometrical room acoustics models to determine wave behaviours. Of particular interest is the ability of the wave models to provide phase information that is important not only for room modes but for the construction of impulse response for auralisation. Some simulated results using different modelling techniques will be presented to illustrate the problems and potential solutions.

Absorption and impedance boundary conditions for phased geometrical-acoustics methods

“…This is related to the sphericity error for high absorptions at very low frequencies as Lam and Ingard already pointed out 2,27 because only the plane-wave reflection coefficient is employed in the PBTM simulations. As Ingard found, the sphericity error is indeed related to the angle of incidence: The sphericity error for oblique angle incidence becomes most significant for the lowest impedance, BC5, which supports the predominance of the incident energy at near normal directions.…”

“…For single-value errors, these errors are averaged over the entire frequency range and over the receiver locations as shown in Tables I-IV. Two main sources of error in PBTM have been known: errors at off-resonance frequencies due to geometrical tracing and sphericity error. 2,3 The former error influences e 1 mainly for high impedances, whereas e 2 , which is based on the sound pressure levels summed in 1/3 octave bands, is seldom affected by the former error. However, the sphericity error arises at very low frequencies for low surface impedance.…”

“…coefficients in a phased image source model in comparisons with boundary element simulations and geometrical acoustic simulations. 2 The plane wave reflection model was found to have noticeable errors at higher admittance values and at longer delay time, but the accuracy was improved as the frequency increased. Jeong and Ih also used impedance boundary conditions for a phased beam tracing model in comparison with boundary element simulations and acoustic Green's function calculations.…”

“…[1][2][3] Generally speaking, wave based methods are the most reliable methods in calculating transfer functions at low frequencies, whereas the geometrical acoustics methods provide approximate but reliable and fast predictions at frequencies well above the Schroeder frequency where room modes are highly overlapped; thus individual modal characters do not need to be taken into account. In this sense, the phased geometrical acoustics methods are normally considered as medium frequency methods that can bridge between low and high frequency methods.…”

“…It has been reported that phased geometrical acoustics simulations using surface impedances as boundary conditions agree well with measurements and reference calculations. [1][2][3] Suh and Nelson used surface impedance boundary conditions for important surfaces to estimate the plane wave reflection coefficients, whereas real-valued angle-independent reflection coefficients are used for concrete and plaster walls, assuming no phase shift on reflections from these reflective surfaces. coefficients in a phased image source model in comparisons with boundary element simulations and geometrical acoustic simulations.…”