Measurements and Room Acoustic Analysis with the ITA-Toolbox for MATLAB

Pascal Dietrich, Martin Guski, Johannes Klein, Markus Muller-Trapet,Martin Pollow, Roman Scharrer, Michael Vorlander

Institute of Technical Acoustics, RWTH Aachen University, Neustraße 50, 52056 Aachen

Website: http://www.ita-toolbox.org — Email: toolbox-dev@akustik.rwth-aachen.de

Introduction

During the last years, a MATLAB toolbox for acousticalsignal processing has been developed at the Institute ofTechnical Acoustics (ITA) at RWTH Aachen University[1]. Recently, two new applications have been releasedfor measurements and room acoustical evaluation. In thispaper we describe these two additional open-source ap-plications currently released inside the ITA-Toolbox.

The measurement application includes basic features,such as signal recording, playback and deconvolutiontechniques for noise, sweep or MLS transfer functionmeasurements. The possibility of simulating an entiremeasurement chain including convolution, filters, noise,quantization, clipping and nonlinear transfer character-istics helps to understand artifacts observed in real mea-surements and is considered very useful for didactic pur-poses for academic education during laboratory excer-cises and lectures.

Using the room acoustics application monaural param-eters can be evaluated according to ISO 3382 employ-ing different evaluation methods. The routines help tounderstand the noise detection algorithms and the dif-ferences between the evaluation results. In combination,these two applications provide a powerful tool for uncer-tainty analysis in room acoustics.

About ITA-Toolbox

The ITA-Toolbox was primarily intended for in-houseuse at ITA as basic container for MATLAB routines.This toolbox handles common post-processing tasks inthe field of acoustic research, including data import andexport as well as different graphical representations ofthe data. Professional audio recording interfaces can bedirectly connected to enable data acquisition. Duringthe years the functionality grew and stable routines forvarious signal processing and measurement tasks weredeveloped. A kernel containing basic functionality wasreleased in 2012. Advanced functionality is provided bydedicated optional applications that work hand in handwith the ITA-Toolbox kernel.

A tutorial script to demonstrate the functionality of thekernel is available in the file ita tutorial.m.

Measurement App

The application Measurement features an object orientedapproach including classes for specific measurement ob-jects. In this context the class itaMSTF (Measurement

Setup Transfer Function) is used. This class realizescorrelation measurements involving deconvolution tech-niques. Arbitrary measurement signals might be used.We mainly concentrate on sweep measurements due toadvantages published in [1, 2]. MLS measurements asproposed in [3] are available but not focus of this paper.The measurement objects store the excitation signal thatcan be freely defined by the user. A so-called compensa-tion which is the inversion of the complex spectrum ofthis excitation signal is generated automatically in thebackground. The level of the measurement can be con-trolled in this object by setting the output amplificationin dBFS1. This value is automatically accounted for byreciprocally scaling the measured input signal. Hence,the absolute values of the measured impulse response re-main constant regardless off the actual output amplifica-tion of the measurement. Only the signal-to-noise-ratio(SNR) will change accordingly. As most spectra of ex-

Figure 1: Two different deconvolution techniques (cyclic andlinear), which can be chosen by parameters in the measure-ment object.

citation signals are band-limited regularization might beapplied to reduce the noise influences outside the fre-quency range of interest. This can be controlled by pa-

1This is a value in dB smaller or equal to zero.

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rameters inside the measurement object. Two deconvo-lution techniques are implemented. The first uses cir-cular deconvolution and the second approximates lineardeconvolution. The effect on the impulse response in areal room using a loudspeaker driven in a fairly linearrange is shown in Figure 1. The impulse response forthe linear deconvolution technique has twice the lengthas the measured raw signal is extended in time domainwith zeros prior to multiplication with the regularizedinverse of the excitation signal.

Figure 2 shows basic GUI elements of the measurementapplication and the MATLAB command line windowwith a summary of the measurement object. Parameterscan be easily modified in a GUI using the function .edit.The measurement itself is triggered by the method .run

and directly yields the impulse response as the deconvo-lution is applied to the measurement object. A so-called

Figure 2: Screenshots of GUI elements of the measurementapplication inside the ITA-Toolbox and MATLAB commandwindow showing a summary of the parameters of the itaMSTF

meausrement object.

dummy class is available that emulates an entire mea-surement chain including quantization effects, measure-ment noise, linear and non-linear system characteristics.Details are published in [4]. This class is a very power-ful tool to analyze and understand the artifacts in theimpulse response caused by e.g. noise, quantization ornon-linearities.

A tutorial script called ita tutorial measurement.m

is available to illustrate the functionality of the mea-surement objects and the emulation of the measurementchain.

Room Acoustics App

The application Room Acoustics is based on ISO 3382 [5].It features the evaluation of room acoustic parametersfrom a measured impulse response, e.g. with the mea-surement application of the ITA-Toolbox, or simulatedimpulse response. The following monaural parametersas provided by the standard are: Reverberation times(early decay time, T10 . . . , T60), clarity index (C50, C80),definition (D50, D80), center time. Furthermore, the peaksignal to noise ratio is calculated.

The evaluation of the parameters PAR in a MAT-LAB struct out of multi-channel impulse re-sponses RIR is triggered by the command: PAR =

ita roomacoustics(RIR).

The user can tweak all options regarding the evaluationmethods by specifying additional parameters as docu-mented in the help of the function calls. The evaluationitself is fully automatic and hence no user interactionis required. It directly yields the parameters in an or-ganized structure. First, the beginning of the impulseresponse is detected and then a fractional octave bandfilter is applied. The noise detection and compensationis processed for each frequency band and each channelof the impulse response independently. This approachyields parameters with exact reproducibility and henceit is applicable for uncertainty analysis for research pur-poses. It is possible to analyze large sets of measuredor simulated impulse responses automatically in a shorttime.

Noise Dectection and Compensation

The noise detection algorithms implemented are mainlybased on ISO 3382. An iterative algorithm as proposedby Lundeby et al. [6] provides the estimated peak sig-nal to noise ratio, the noise level, the intersection timewhere the decay of the impulse response intersects withthe noise floor and the late reverberation time. Figure 3illustrates the determination of the parameters. The pa-rameters are used for the subsequent noise compensation.The algorithm delivers a robust noise estimation and hasproven for measured room impulse responses from severalmeasurement sessions in various different rooms.

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detected start ofimpulse response

late reverberationtime estimation

noise level estimation

peak-signalto noise ratio

Figure 3: Automatically detected parameters of the noisedetect algorithm as proposed by Lundeby et al. in theITA-Toolbox yielding e.g. Peak SNR, late reverberation andintersection time

.

As noise occurs in every real-life measurement and asnoise can pertubate the evaluation of the room acous-tic parameters a proper noise compensation is preferred.Five different ways of noise handling have been imple-mented in this application. Three of them are directlycompliant with ISO 3382. For moderate SNRs, e.g. be-tween 40 and 60 dB peak SNR, these methods alreadyyield significantly different results as can be seen for the

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0

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M1: no noise handlingM2: truncation + correctionM4: subtraction + M2

Figure 4: Reverberation time T30 with different noise han-dling methods (M1, M3, M5) in third octave bands.

reveberation times T30 in Figure 4. The uncertaintiesdue to measurement noise for the different noise handlingmethods are studied in more detail in [7].

The effect on the energy decay curve if no noise compen-sation and no noise truncation is applied in to the back-wards integration is shown in Figure 5 (Method M1).Figure 6 illustrates the noise compensation method in-cluding a truncation of the measured impulse response(M3). Finally, the complex algorithm using noise sub-traction, truncation and compensation for the truncatedenergy of the impulse response is depicted in Figure 7.

Currently the speech transmission index (STI) is al-ready included. The calculation of binaural features, e.g.IACC, and room acoustic parameters using a figure-of-eight microphone, such as LF and LFC, will be addedsoon. Furthermore, this application contains a functionto simulate rectangular rooms with rigid boundary con-ditions by modal superposition as published in [8]. Thecombination of this analytic simulation and the evalua-tion of parameters is considered very powerful in termsof uncertainty analysis and testing the robustness of theparameters against pertubations.

A tutorial script called ita tutorial roomacoustics.m

is included to show the evaluation process of room acous-tic parameters.

System requirements and License

A copy of the ITA-Toolbox can be freely downloadedat www.ita-toolbox.org. It is published under the origi-nal BSD-License, allowing the use of the software, anymodifications and the redistribuation for commercial ornon-commercial purposes, as long as the copyright state-ment is preserved and credits to our institution are given.The ITA-Toolbox is compatible with any operating sys-tem (MS Windows, Linux, Mac OS X) running MAT-LAB R2010a or higher. The signal processing toolboxis required for spectral band-bass or fractional octaveband filtering and time windows. The majority of theITA-Toolbox routines runs even without this additionaltoolbox.

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Figure 5: Illustration of impulse response with and withoutnoise (top) and energy decay curve (bottom) for these impulseresponses without noise detection and compensation (M1).

impulse response

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reverberation time estimation

compensation for truncation Ccomp

Figure 6: Energy decay curve with algorithm that truncatesthe impulse response and applies a correction for the energydiscarded with this truncation (M3).

impulse response

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noise level estimation Nest

Figure 7: EDC evaluation method using noise subtraction,trucation and proper correction for the truncation (M5).

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Acknowledgments

The authors would like to thank the co-workers at the Insti-tute of Technical Acoustics and all users of the ITA-Toolboxfor their valuable feedback and contributions. We would liketo thank the developers of PortMusic, especially PortAudio,and playrec which is used for communiation with sound cards.The ASIO SDK by Steinberg was additionally used for thiscommunication on Microsoft Windows systems.

References

[1] P. Dietrich, M. Guski, M. Pollow, B. Masiero, M. Muller-Trapet, R. Scharrer, and M. Vorlander, “ITA-Toolbox –An Open Source MATLAB Toolbox for Acousticians,” in38th German Annual Conference on Acoustics (DAGA),Darmstadt, Germany, March 2012.

[2] S. Muller and P. Massarani, “Transfer-function measure-ment with sweeps,” Journal of the Audio Engineering So-ciety (JAES), vol. 49, pp. 443–471, 2001, printed.

[3] D. D. Rife and J. Vanderkooy, “Transfer-function mea-surement with maximum-length sequences,” Journal ofthe Audio Engineering Society (JAES), vol. 37, no. 6, p.419, 1989.

[4] P. Dietrich, M. Guski, and M. Vorlander, “Influenceof loudspeaker distortion on room acoustic parameters,”in 40th Italian (AIA) Annual Conference on Acousticsand the 39th German Annual Conference on Acoustics(DAGA), 2013.

[5] ISO, Acoustics – Measurement of Room Acoustic Param-eters – Part 1, ISO Std., 2009.

[6] A. Lundeby, T. E. Vigran, H. Bietz, and M. Vorlander,“Uncertainties of measurements in room acoustics,” ActaAcustica united with Acustica, vol. 81, no. 4, pp. 344–355,1995.

[7] M. Guski and M. Vorlander, “Measurement uncertaintiesof reverberation time caused by noise,” in 40th Italian(AIA) Annual Conference on Acoustics and the 39th Ger-man Annual Conference on Acoustics (DAGA), 2013.

[8] M. Pollow, P. Dietrich, and M. Vorlander, “Room impulseresponses of rectangular rooms for sources and receivers ofarbitrary directivity,” in 40th Italian (AIA) Annual Con-ference on Acoustics and the 39th German Annual Con-ference on Acoustics (DAGA), 2013.

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