Jamie A. S. Angus

Volumetric diffusers: Pseudorandom cylinder arrays on a periodic lattice

Most conventional diffusers take the form of a surface based treatment, and as a result can only operate in hemispherical space. Placing a diffuser in the volume of a room might provide greater efficiency by allowing scattering into the whole space. A periodic cylinder array (or sonic crystal) produces periodicity lobes and uneven scattering. Introducing defects into an array, by removing or varying the size of some of the cylinders, can enhance their diffusing abilities. This paper applies number theoretic concepts to create cylinder arrays that have more even scattering. Predictions using a boundary element method are compared to measurements to verify the model, and suitable metrics are adopted to evaluate performance. Arrangements with good aperiodic autocorrelation properties tend to produce the best results. At low frequency power is controlled by object size and at high frequency diffusion is dominated by lattice spacing and structural similarity. Consequently the operational bandwidth is rather small. By using sparse arrays and varying cylinder sizes, a wider bandwidth can be achieved.

Lüke and power residue sequence diffusers

Conventional Schroeder diffusers have been successfully used for many years. However, their frequency range is limited by the flat plate effect that occurs when all the wells radiate in phase. This occurs at harmonics of p times the design frequency f(0), where p is the small prime that is used to generate the structure. A typical diffuser, using p=7 and f(0)=500 Hz, has an upper frequency limit of only 3.5 kHz. Achieving a first flat plate frequency above 20 kHz requires a prime equal to at least 41 and results in diffusers that are too big to be practical in most applications. This paper suggests an alternative approach using number theoretic sequences that, although short in length, are based on large integers. Two new sequences, Type-II Luke and power residue, have this desired characteristic. They are investigated using both simple models and the more exact boundary element method. The results show the flat plate effect is moved to much higher frequencies as expected. For Luke sequences at certain frequencies, redirection rather than dispersion is achieved. Modulation techniques can be used to mitigate these problems. Power residue sequences perform the best, providing good diffusion and a flat plate frequency outside the audible range.

Comparison of measurement and simulation of the scattered pressure distribution from partly absorbing surfaces.

When diffusion or scattering from a surface if discussed usually it refers to the distribution of pressure reflected from a rigid surface while the amount of reflected energy is not discussed. In this paper the scattered pressure distribution from surfaces that consist of absorbing and reflecting elements has been measured following the ISO 17497‐2. The requirements of the standard were not perfectly met as the signal to noise ratio was lower than required due to the absorbing nature of the surface and high‐low‐frequency noise in the measuring system. The measured results were then compared with simulations using boundary element modeling. The comparison showed good agreement. Finally the issue of the reflected energy was discussed.

Orthogonal coding sequences for multiple-source study of outdoor noise propagation

This paper describes recent developments in the design of a coded acoustic signal for the study of outdoor sound propagation. Low signal‐to‐noise ratio is a common limitation in outdoor propagation investigations, and time variance of the air limits the amount of averaging. Recent work by the authors presented a coded acoustic signal approach consisting of a frequency carrier biphase‐modulated by a specially designed pseudorandom code sequence to overcome these limitations. The “inner and outer” code sequence was specially designed for environmental sound propagation investigations, combining simultaneous fine time resolution and large range ambiguity, together with an ability to average and probe the propagation path. This approach is extended to multiple acoustic sources for the study of outdoor sound propagation, with the design of signals using orthogonal correlation sequences. The results illustrate that combinations of orthogonal coding and multiple carrier frequencies can permit continuous sound sp...

The effect of diffusers on low frequency modes

Small listening rooms are becoming prevalent, due to the availability of production facilities on personal computers, and the financial pressures faced by the industry. In these rooms, modal behaviour at low frequencies significantly affects the quality of the reverberant decay, due to their non‐diffuse nature compared with other frequencies. Diffusion and absorption can help and may be a useful way of improving the low frequency performance of such rooms. This paper discusses the properties of these rooms at low frequencies, in particular the effect of diffusing boundaries on the modal behaviour of such rooms. The paper will first discuss what is meant by a mode and modal decay. It will then go on to examine the effect of diffusing boundaries on the frequency and density of modes. In particular, it will examine the effect of the scale of the diffuser on its efficacy in this task. For ease of visualisation this will be done using a two dimensional model and, for accuracy a finite element, element simulati...

Pseudorandom sequences for binary amplitude diffusers.

Binary amplitude diffusers create a mixture of absorption and diffusion and are used to improve room acoustic conditions. They require a two dimensional binary pseudorandom array to state where absorbing and reflecting patches should be. Consider the binary array to have dimensions A×B. Ideally, A and B should be similar to maximize performance. Such an array can be produced by folding a one dimensional sequence into a two dimensional array using the Chinese Reminder Theorem. This theorem requires A and B to be coprimes. However, there is a limited set of optimal one dimensional sequences that can be generated from number theory with suitable length AB. In the past maximum length sequences have been used because they display desirable autocorrelation characteristics, but their period is 2m−1, where m is an integer and so there is a limited number available. Consequently, it is necessary to examine other ways of generating optimal two dimensional arrays. This paper looks into other number theoretic sequences, and other construction techniques for forming binary arrays. The suitability of these for constructing two dimensional binary amplitude diffusers is examined. Boundary element modeling is used to evaluate their performance.

A coded signal technique for the study of environmental noise propagation

This paper describes the design of a coded acoustic signal for the study of environmental sound propagation. An increasing awareness of environmental noise problems, and the need to better specify their needs and extent, means that a better knowledge of the effects of weather on sound propagation is necessary. Low signal‐to‐noise ratio is a common limitation in outdoor propagation investigations. A further problem is time variance. A coded acoustic signal approach consisting of a frequency carrier biphase‐modulated by a specially‐designed pseudo‐random code sequence is proposed to overcome these limitations. This so‐called ‘‘inner and outer’’ code sequence is specially designed for environmental sound propagation investigations, combining simultaneous fine time resolution and large range ambiguity. Experiment measurements are used to illustrate how the amplitude of the transfer functions between the receivers together with accurate times of flight may be calculated from the cross‐correlation of the measur...

Spread spectrum sequence design for the study of outdoor noise propagation

The design of carrier‐modulated code sequence signals that can be used for the study of outdoor propagation, are presented. In particular, the construction of these signals such that they are robust to the time‐variance of the outdoor acoustic path is discussed. The design of these signals so that they can also occupy known bandwidths is also described. The result is an audible frequency signal that is biphase‐modulated by two pseudorandom code sequences in a way that is specially designed for simultaneous fine time resolution and large range ambiguity. A particular advantage of this signal is that an estimate of the time‐varying Doppler can be derived by recovering the suppressed carrier from the received signal, and various means of extracting the Doppler are explained. Extensions of these signals to allow multiple simultaneous measurements are also described. [Work supported by EPSRC UK]

Environmental noise measurements using a coded signal

Application of a coded acoustic signal to the investigation of environmental sound propagation is presented. The problem identified is that of obtaining adequate signal‐to‐noise ratio in environmental measurements, since the sound power level of the source is often required to be limited so as not to cause a noise disturbance. An established method of increasing signal‐to‐noise ratio is by means of coded signals such as maximum length sequences. However, this method cannot be applied outdoors because of the time‐variant nature of the atmosphere. Consequently, a coded acoustic signal design is proposed for the study of environmental sound propagation, involving the modulation of multiple acoustic carriers by orthogonal correlation sequences. An experiment is described in which the correlation properties of the coded signal are demonstrated under outdoor conditions typical of environmental noise measurements. The results illustrate that a coded signal approach can provide researchers, engineers, and environ...

An acoustic spread spectrum technique for the study of outdoor noise propagation

An investigation of the properties of a directly carrier‐modulated code sequence modulation signal is presented for the study of outdoor propagation. The audible frequency carrier is biphase‐modulated using a pseudorandom code sequence specially designed for simultaneous fine‐time resolution and large‐range ambiguity. An experiment is described in which the correlation properties of the spread spectrum signal are demonstrated, and the amplitude of the transfer functions between the receivers, together with accurate times‐of‐flight, are calculated from the cross‐correlation of the measured acoustic spread spectrum signals. The results illustrate that an acoustic spread spectrum technique can provide significantly improved ways of measuring sound propagation outdoors. In particular, the fine‐time resolution for time‐of‐flight suggests an application in acoustic tomography, for example in the investigation of nocturnal boundary layers, horizontal gradients, and of turbulence structures in the atmospheric sur...