Mark Avis - Publications

M.R.Avis, B.M.Fazenda, W.J.Davies, ‘Thresholds of detection for changes to the Q-factor of low frequency modes in listening environments’, JAES submission March 07 (in press).

This paper reports an investigation into the measurement of thresholds of detection for modal Q-factors in rooms at low frequency. Key features of the approach taken include the use of music rather than test tones or noise as program material, and the manipulation of damping conditions for a range of modes over a broad low-frequency bandwidth as opposed to the control of one modal artefact within an array of surrounding, uncontrolled resonances. It is shown that detectability of Q factor changes is directly proportional to the reference Q, and is weakly dependant on the presence and level of higher-frequency reverberation. A threshold value of Q=16 is suggested, below which further changes are unlikely to be detected.

J.Redondo, R.Picó, B. Roig, M.R.Avis, ‘Time domain simulation of sound diffusers using finite-difference schemes’, Acta Acustica submission March 07 (in press).

Since the invention of sound diffusers three decades ago a substantial effort has been made to predict the acoustic behaviour of these structures, for auralisation and prediction purposes as well as in response to the large costs inherent in anechoic measurements. Volumetric methods such as Finite Element Methods (FEM) or the Finite Difference Time Domain method (FDTD) are not often used, due to their large computational cost. However Near Field to Far Field Transformations (NFFFT) can overcome that problem. The main advantages of the FDTD method are that a single calculation is sufficient to study a wide frequency band, and that the time domain behaviour of the reflected sound can be directly inspected. In this paper we present a comparison between the prediction techniques commented above in the context of sound diffusers, paying special attention to the FDTD method. Having demonstrated that the FDTD method can generate results comparable to more established techniques, early results concerning the modelled performance of diffusers in the time domain (‘time spreading’) are reported, opening a new field of research.

H.Lim, S. V.Utyuzhnikov, Y. W.Lam, A.Turan, M.Avis, V. S.Ryaben'kii, S.V.Tsynkov, ‘An Experimental Validation of the Active Noise Control Methodology Based on Difference Potentials’, JASA submission March 07 (in press).

To achieve active noise cancellation over a large area, it is often necessary to get a measure of the physical properties of the noise source to devise a counter measure. This is however not practical in many cases. A mathematical method, the Difference Potential Method (DPM), can provide an alternative solution for active shielding over a large area. In this approach, the DPM-based solution requires only measurements near the boundary surface but not at the source itself, and it does not require any other information including the characterization of noise sources and the surrounding medium. Moreover, the DPM-based solution is applicable for bounded domains in the presence of acoustic sources in the domain to be shielded. This paper reports the validation of the DPM approach in a laboratory experiment. Test devices were designed and constructed according to the requirements of the solution. In the experiment a tripole sound source was used as the control source. While preserving friendly sound, the DPM can cancel out the unwanted noise. In conclusion, the results confirm experimentally that the DPM-based active shielding solution provides for global sound field cancellation without filtering any of the desirable sound levels in the targeted domain.

L.Kelly, Y.W.Lam, M.Avis, ‘The significance of coupling in 3D modelling of acoustic wave propagation in a cavity containing air and porous absorbent’, ???????

Acoustic wave propagation in air and porous media is a well studied topic in the one and two dimensional case. This study examines the problem in three dimensions using numerical models; in particular it looks at the significance of the coupling effect between the air and porous layer. The numerical model consist of a layer of air and a layer of porous absorbent which are modelled as equivalent fluid and are characterised by complex characteristic impedance based on acoustic measurements. A coupled FEM model is compared with an uncoupled BEM model which models the foam with equivalent locally reacting boundary conditions.

M.R.Avis, L.J.Kelly, “Principles of Headphone Design – a tutorial review”, AES 21st UK conference, Cambridge (2006)

Headphone design using lumped-parameter analysis and analogous circuits is described. Specifically a model of a dynamic driver in a circumaural enclosure is developed, but the approach may be applied to other drivers and enclosure types. Models are presented which illustrate key design constraints such as leak behaviour, open / closed back approaches, cup attenuation, cushion compliance and headband tension. Limitations of the lumped parameter approach are discussed, following which an introduction to analytical-modal and numerical modelling is presented.

B.M.Fazenda, M.R.Avis, W.J.Davies, ‘Comments on "Perception of modal distribution metrics in critical listening spaces-dependence on room aspect ratios" – Reply’, J. Audio Eng. Soc. 54(5): 412-413 May 2006

F.G.Bechwati, T.J.Cox, M.R.Avis, “Activated carbon – porosity and surface impedance measurements”, Proc. IOA Vol 28 Pt 1 (2006)

This paper outlines some early work into a project concerned with the acoustic properties of activated carbon. Activated carbons are manufactured carbonaceous materials having a porous structure and a large internal surface area. These materials have the property of attracting a wide variety of substances to their internal surface area; a process named adsorption. There are two main forms of adsorption, chemical and physical. In chemical adsorption, the molecules undergo a strong molecular bonding, and a compound might be formed. In physical adsorption, the molecules of the adsorbent are held at the pore walls by weak physical attraction with the carbon molecules; this physical attraction is known as van der Waals effect. The adsorbed molecules fill the pores by forming layers at the pore walls, and if the pores are small enough, the molecules will join to form concave meniscus at the upper surface with a vapour pressure lower than that of the bulk medium, and capillary condensation will take place. In the case of physical adsorption, the adsorbed molecules can be released back into the surrounding medium if enough energy is transmitted to them to overcome the weak molecular bonding; a process called desorption…

T. J. Cox, M. R. Avis and L. Xiao., “Maximum Length Sequence and Bessel Diffusers using Active Technologies,” J.Sound Vib. 289 (2006) pp807-829

Active technologies can enable diffusers to operate over a wider bandwidth than passive devices, by extending the bass response. Active impedance control can be used to generate surface impedance distributions which cause wavefront dispersion, as opposed to the more normal absorptive. or pressure-cancelling target functions. This paper details the development of two new types of active diffusers which are difficult, if not impossible, to make as passive wide-band structures. The first type is a maximum length sequence diffuser where the well depths are designed to be frequency dependent to avoid the critical frequencies present in the passive device and so achieve a significant, finite bandwidth diffusion performance. The second is a Bessel diffuser, which exploits concepts developed for transducer arrays to form a hybrid absorber-diffuser. Details of the designs are given, and measurements of scattering and impedance used to show that the active diffusers are operating correctly over a bandwidth of about 100Hz to 1.1kHz. Boundary Element Method simulation is used to show how more application-realistic arrays of these devices would behave.

B.M.Fazenda, M.R.Avis, W.J.Davies,’Perception of Modal Distribution metrics in Critical Listening Spaces – Dependence on Room Aspect Ratios’, J. Audio Eng. Soc. 53(12): 1128-1141 December 2005

The purpose of this study is to identify the subjective significance of two general types of metrics used to describe the ‘quality’ of a room based on its aspect ratio. Tests were carried out to evaluate differences between three “virtual” rooms that score extreme classifications in each of the metrics. The results of the tests indicate that room aspect ratios do have some effect on the perception of the modal distribution, but the effect is very much dependent on the frequency content of the original signal. This indicates that a room that scores well using a certain metric may still suffer from problems if the frequency content of the driving signal matches one particularly strong modal artefact. More significantly, the results of these tests imply that attempts to rank critical listening spaces based on modal distribution metrics are likely to be highly misleading; and the derivation of difference limen, which would be required for useful comparison, has been shown in practice to be highly problematic and perhaps meaningless.

M.R.Avis, L.Xiao, T.J.Cox, ‘Stability and Sensitivity Analyses for Diffusers with Single and Multiple Active Elements’, J. Audio Eng. Soc. 53(11) 1047-1060 November 2005

In comparison to active absorbers, active diffusers have more complicated target impedance functions, a smaller region of stable control, and are more sensitive to control impedance errors. The issues of stability and sensitivity are theoretically and empirically analysed in this paper. Measurements show comparable performance between active and passive diffusers, while the active solution requires less space.

M. R. Avis, L. Xiao, T. J. Cox, “Active diffusers: stability analysis and multiple active elements”, AES 118, Barcelona, 2005

This paper presents some results concerning active diffusers; in particular it addresses the issues of stability, sensitivity to impedance errors and the use of multiple active elements. The stability of an impedance controller used to virtually extend diffuser wells is theoretically analyzed based on a practically measured plant model. The analysis is extended to diffusers with multiple active elements empirically. A sensitivity analysis shows that the control impedance must be achieved quite precisely with active diffusers, and the error sensitivity varies with frequency. Measurements on an active diffuser with multiple elements show that the scattering performance of the active diffuser is comparable to its passive equivalent, while the active solution requires less space.

L. Xiao, T. J. Cox and M. R. Avis., “Active diffusers: some prototypes and 2D measurements,” J.Sound Vib., 285(1-2), 321-339, (2005)

Diffusing devices are used to improve room acoustics in a wide variety of applications. The dispersion generated by current diffuser technologies is often limited to mid- to high frequencies because low frequency diffusers are usually too large to be easily accommodated. To extend the bandwidth of diffusers to a lower frequency a new approach is proposed, using active control technology. In particular, active impedance techniques have been exploited to create non-absorbing diffusers and hybrid structures that partly absorb while dispersing any reflected sound. This paper presents results mostly from a feedforward structure. Measurements of polar responses provide evidence that the active diffusers can achieve wider bandwidth dispersion than passive devices of similar dimensions. It is found that achieving active dispersion without absorption over a worthwhile bandwidth can be more difficult than using a combination of active techniques and passive absorption. This is due to the more complex target impedances that the controller is required to learn. Boundary element modelling has enabled the design and performance of these structures to be examined in more application-realistic environments.

Cox TJ, D'Antonio P, Avis MR, ‘Room sizing and optimization at low frequencies’, J. Audio Eng. Soc. 52(6): 640-651 June 2004

Modes in small rooms may lead to uneven frequency responses and extended sound decays at low frequencies. In critical listening environments, this often causes unwanted coloration effects, which can be detrimental to the sound quality. Choosing an appropriately proportioned room, and placing listener and loudspeakers in the right places can reduce the audible effects of modes. A new methodology is detailed for determining the room dimensions for small critical listening spaces as well as the optimum positions for sources and receivers. It is based on numerical optimization of the room geometry and layout to achieve the flattest possible frequency response. The method is contrasted with previous techniques for choosing room dimensions. The variations of the room quality for different room sizes are mapped out. These maps include an allowance for constructional variation, which has not been considered previously.

B.M.Fazenda, M.R.Avis, W.J.Davies, F. Jacobsen, ‘Perception of Modal Distribution in Critical Listening Spaces’, 11th Int. Conf, Sound and Vibr. St. Petersburg (2004)

Critical listening rooms, like recording studio control rooms or listening rooms, are carefully designed to provide the listener with the most accurate listening environment. It is well know that in small rooms the eigenfrequencies may occur well separated in the lower range and cause some amplification or attenuation of sound at certain frequencies. This may distract from the optimum perception and the correct judgement of the reproduced sound. The modal distribution is directly associated with the dimensional aspect ratios of the room. There are in literature various design metrics that assess the distribution of these modes in frequency and help the designer to decide on room dimensions that avoid worst situations and aim for an “optimum” distribution of the eigenfrequencies. The purpose of this study is to identify the subjective perception of two general types of metrics, one based on modal spacing and the other based on pressure response, and to attempt to classify its perceptual importance compared to other factors associated with the perception of room modes. Tests were carried out to evaluate differences between three “virtual” rooms that score extreme classifications in each of the metrics. The results of the tests indicate that room aspect ratios do have some effect on the perception of the modal distribution, but the effect is very much dependent on the frequency content of the original signal. This indicates that a room that scores highly on a certain metric may still suffer from problems if the frequency content of the driving signal matches one particularly “bad” frequency region.

M.R.Avis, L.Xiao, T.J.Cox, ‘Practical Measurements on Active Diffusers’, ICA Japan, 2004

Active room acoustic diffusers can be used to disrupt reflection paths, and can produce more bass scattering than a passive diffuser of the same depth. Impedance-controlled loudspeakers can be used, adapting past active impedance technology developed for absorption, to achieve wavefront dispersion. This can be done with or without absorption as desired. Measurement results from a feedforward control structure are presented. Results for low-loss and semi-absorbing diffusers are given. In the later case, the use of active elements enables dispersion over a wider bandwidth than can be achieved with a passive device.

B.M.Fazenda, M.R.Avis, W.J.Davies, ‘Difference Limen for the Q-factor of Room Modes’, AES 115, New York, 2003

A subjective test study was carried out in order to identify the perceptibility of changes in the Q-factor of room modes. The experimental technique concentrates on the identification of difference limen for three levels of Q-factor referring to modes in rooms used for critical listening. Trends show that changes in higher Q values are more perceptible than those for lower Q values. The results may be applied in decisions for treatment of modes in common listening and control rooms.

Cox, Trevor J.; Avis, Mark R.; Xiao, Lejun., ‘The Potential for Room Acoustic Active Diffusers’, Forum Sevilla, 2002

Diffusing devices are used to improve room acoustics in a wide variety of applications. The dispersion of current diffuser technology is often limited to mid to high frequencies because low frequency diffusers are usually too large to be easily accommodated. To extend the bandwidth of diffusers to a lower frequency a new approach is required; it is proposed to use active control technology. In particular, active impedance techniques are being exploited to create diffusion rather than the more usual absorption. This paper will present a conceptual design for an active diffuser and some simulation results demonstrating its utility.

B.Fazenda, M.R.Avis and W.J.Davies, 'Low frequency room excitation using distributed mode loudspeakers', AES 21st Int. Conf. (St Petersburg 2002)

Conventional pistonic loudspeakers excite the modes of an enclosed sound field in such a way as to introduce modal artefacts which may be problematic for listeners to high-quality reproduced sound [1] . Their amelioration may involve the use of highly space-consumptive passive absorptive devices or active control techniques [eg 2,3,4]. Other approaches have concentrated on the design of the driver used to excite the room. Distributed sources ranging from the dipole [5] to more complex configurations [6] may be expected to interact with the room eigenvectors in a complicated manner which may be optimised in terms of the spatial and frequency-domain variance of the soundfield. Recent interest in distributed sources has centred on the Distributed Mode Loudspeaker (DML), and this paper reports an investigation into the interaction of DMLs with modal soundfields. It is shown that large DMLs may be expected to modify the low-frequency soundfield, and that smaller panels may interact with the room in interesting ways at higher frequencies. Producing useful low-frequency control remains difficult but may be achieved in some circumstances.

[1] Fazenda, B,Davies, W.J., ‘The views of Recording Studio Control Room users’, Proceedings of the IOA, Volume 23, Pt 8, 2001, pp213-220
[2] Avis, M.R., 'The active control of low frequency room modes', PhD Thesis, Salford (2000)
[3] Avis, M..R., ‘IIR biquad controllers for low frequency acoustic resonance', Presented at 111th AES Convention, Preprint 5474, New York (2001)
[4] Darlington, P., Avis, M.R., ‘Time Frequency Response of a Room with Active Acoustic Absorption', Presented at 100th AES Convention, Preprint 4192(H_5), Copenhagen (1996)
[5] Ferikidis, C., Kempe, U., 'Room mode excitation of dipolar and monopolar low-frequency sources', Presented at 100th AES convention, preprint 4193, Copenhagen (1996)
[6] Nelson, P.A, Elliot, S.J., 'Active control of sound', Academic Press, London, 1993

M.R.Avis, ' Q factor modification for low frequency room modes', AES 21st Int. Conf. (St Petersburg 2002)

Low frequency normal modes of an enclosed soundfield introduce unwanted frequency, spatial and temporal artefacts to reproduced electroacoustic signals. A novel control approach has been reported [1,2] based on an analytical modal decomposition, using a low frequency soundfield model in a one-dimensional environment formed from the sum of a number of second order IIR filter sections. In this paper, these techniques are applied to the low frequency resonances of a three dimensional test room. It is shown that significant reductions in modal Q and corresponding reductions in modal decay times may be achieved, leading to smaller low-frequency soundfield variance and decreasing audibility of time-domain modal artefacts.

[1] Avis, M.R., 'IIR biquad controllers for low frequency acoustic resonance', Presented at 111th AES Convention, Preprint 5474, New York (2001)
[2] Avis, M.R., 'The active control of low frequency room modes', PhD Thesis, Salford (2000)

L.D.Copley, M.R.Avis and T.J.Cox, 'Distributed mode loudspeaker arrays', AES 112 (Munich 2002)

The usefulness of Distributed Mode Loudspeakers (DMLs) in arrays has been investigated. The design goal is an array that evenly distributes energy over a hemi-disc. A model has been developed to predict trends of DML array radiation and compared with measurements. This model enables the performance of established array technologies to be tested. When several panels are positioned in an array, spatial aliasing results, as would be expected. Conventional array techniques, such as number theory modulation, can improve the radiation characteristics.

M.R.Avis, 'IIR Biquad Controllers for Low-Frequency Acoustic Resonance', AES 111 (New York 2001) Preprint 5474

Low frequency normal modes of an enclosed soundfield introduce unwanted frequency, spatial and temporal artefacts to reproduced electroacoustic signals. A novel control approach is presented based on an analytical modal decomposition, which incorporates a low frequency soundfield model formed from the sum of a number of second order IIR filter sections. It is shown that within constraints determined by the model accuracy, IIR controllers may be constructed and may be applied to control tasks such as point pressure cancellation and the reduction of modal quality factor.

M.R.Avis and L.D.Copley, ‘Modelling DML panels using classical theory’, Proc. IOA Vol 22 Pt 6 (2000) pp 39-46

Much published work exists in the literature regarding the vibrations of finite plates under a variety of boundary conditions to excitation forces applied at a point, and over more complex geometries. This work reports the application of classical plate theory to panels used for the radiation of acoustic energy, with the aim of obtaining a better understanding of the mechanisms of plate radiation.

The dependence of surface velocity on mass density, Young’s modulus and loss factor is well known, but measuring the latter physical constants is not necessarily straightforward. Using measurements of the velocity response of a thin panel with well-understood boundary conditions, a modal decomposition model is used in a multidimensional optimisation in order to provide a ‘fit’ to the measured data. This process is proven using an aluminium plate with low loss-factor, and extended to materials with much higher damping such as are typically used in DML applications, where conventional measurement techniques are shown to be highly prone to error.

Preliminary extensions of the model for acoustic radiation are compared with practical pressure measurements made in an anechoic environment. Conclusions are then drawn as to the suitability of the method for the prediction of the radiated pressure due to arrays of DMLs.

P.Darlington and M.R.Avis,’Time/frequency response of a room with active acoustic absorption’, AES 100 (Copenhagen 1996) Preprint 4192 (H-5)

Decay spectra are presented which describe the control of normal modes of an enclosed sound filed using active absorbers. The visualisation techniques are introduced in the context of a resonant waveguide and are extended to investigate the acoustics of a small room. Results demonstrate that active absorbers offer a practical solution to the problems introduced by discrete low frequency modes.

P.Darlington and M.R.Avis, ‘Noise control in resonant soundfields using active acoustic absorbers’, Proc. Internoise ‘96 pp 1121-1126

P.Darlington and M.R.Avis,’Modifying low frequency room acoustics 2: Global control using active absorbers’, Proc. IOA Vol 17 Pt 7 pp87-96

M.R.Avis and P.Darlington,’Modifying low frequency room acoustics 1: Local active dereverberation’, Proc. IOA Vol 17 Pt 7 pp77-86

P.Darlington and M.R.Avis, ‘Improving listening conditions in small built spaces using active absorbers’, Proc. Active ‘95 (1995) pp519-528