PROCEEDINGS of the23rd International Congress on Acoustics
9 to 13 September 2019 in Aachen, Germany

A methodology and a tool for interchangeable reproduction of soundsamples in listening tests through different sound reproduction
systems
Dominik KISIĆ1; Marko HORVAT2; Kristian JAMBROŠIĆ3
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University of Zagreb, Faculty of Electrical engineering and Computing, Croatia

ABSTRACT
A common practice in the execution of listening tests is to choose the sound reproduction system based
primarily on its availability, with little regard for its potential influence on the results of such tests. To
examine the magnitude of this influence, a methodology is proposed and a tool is developed for presenting
sound samples through different sound reproduction systems. The tool is equipped with a user interface thatenables easy and seamless change of the sound reproduction system, as perceived by the listener. The designof the tool eliminates the need for any kind of intervention that would interrupt the listening session, such as
on-the-fly system reconfiguration or calibration. The goal is to achieve interchangeability between binaural
reproduction using headphones and 3D Ambisonics reproduction using a multichannel loudspeaker system
as a typical spatial audio reproduction system. However, the extension to two-dimensional amplitude
panning systems such as stereo or 5.1 surround is considered as well. The software design of the tool is basedon Cycling ‘74 Max programing language. The tool is to be used in listening tests focused on the subjective
evaluation of overall acoustic comfort, in relation to the quality of sound insulation in closed spaces and theirroom-acoustical properties.
Keywords: Ambisonics, 3D Sound, Listening tests, Headphones, Speaker calibration, Max/MSP

1. INTRODUCTION
Previous studies have shown that there is not a significant difference between the usage ofheadphones and 3D audio reproduction systems while conducting listening tests (1,2). Nonetheless,the research questions investigated in those studies have been broadly stated and have not regarded thelistening tests conducted in the research of acoustic comfort, which includes the subjective evaluationof sound insulation. This article presents a foundation of a broader research with the goal to show thedifferences between the current norms in building acoustics and the human perception, e.g. in thesingle number values evaluation of impact noise and air borne sound transmission index, ordifferences in perception of sound insulation. Since the preparation of the entire listening test requiresa more detailed analysis of the problem (3), the organization and execution of the complete experimentwill not be covered at this time. Therefore, only the technical aspects of the experiment are presented
in this article.
The idea is to take a slightly new approach to the listening test methodology, which tackles thequestion about the optimal choice of the sound reproduction system that best suits the listening test. To
answer that question better insight is needed considering the advantages and disadvantages of using a3D audio reproduction system compared to much simpler and less expensive headphones setups forlistening tests and what situations would be appropriate to use one or the other.
This experimental setup is primarily meant to be used in two similar listening rooms with theAmbisonics systems: the one located in AuraLAB at the University of Zagreb (figures 3, 4 and 5), andthe one in TGM – Fachbereich für Akustik und Bauphysik in Vienna (figure 1). The technical details
and measurements of the AuraLAB are shown in (4).

123dominik.kisic@fer.hrmarko.horvat@fer.hr
kristian.jambrosic@fer.hr

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Figure 1 – Ambisonics listening room in TGM Fachbereich für Akustik und Bauphysik in Vienna

2. METHOD
The system and its calibration procedure are created to allow the participant, once seated inside thelistening room with the headphones on his head, to participate in the listening test in its entirety
without the need for the removal of headphones. This allows the comparison of the individual’sperception of the differences between those systems. The principal block diagram of the system can beseen in the figure 2.

Figure 2 – Principal block diagram of the system
The inputs to the system are individual mono audio samples and control variables which allow thechoice of the reproduction system and the directivity of the listening source. An audio sample gets its
directivity properties inside the system and is passed through filters to assure that the system produces
the equivalent stimuli on all the sound reproduction systems.
Perceived spatial localization properties can be improved by the rotation of the head (5), which
would be disadvantageous for the situations when the stimuli are presented through headphones
compared to the Ambisonics system where the sound field does not depend on the position of the headof the listener. To ensure the head of the participant is fixed during the listening test, the listener will
be instructed to place his or her chin on a chinrest.
Considering the platform and the programing language, there are several different existing ones
used for conducting standardized listening tests today, which is nicely summarized by Jillings et. al.
(6). For this system, the Cycling ‘74 Max programing language is used due to the capabilities for asimple creation of a suitable user interface, the possibility of using third party VST plug-ins, and realtime processing that will be useful for a planned inclusion of extra features to the system.

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The sample preparation and Ambisonics decoding is done using IEM Ambisonics VST plug -insuite (https://plugins.iem.at/). For binaural directivity IEM BinauralDecoder uses HRTFs recordedwith the Neumann KU 100 dummy head.
The level and frequency calibration of the system is made using a FabFilter Q2 EQ plug -in and its
frequency match functionality, which uses up to 24 band EQ filters to match the spectrum of twosignals. The use of FabFilter Q2 EQ plugin has been sufficient so far, but alternative non-commercial
options are being considered, and a custom frequency matching Max patch is being developed which
will allow a much better control for the majority of the calibration process.
Although the main tool is made in Max, the usage of third party VST plugins allow the system to be
adoptable to any DAW, such as Reaper or Cubase.

3. SETUP AND CALIBRATIONThis experimental setup assumes a single participant in the listening test at a given time.
Adjustments for a higher number of participants might be possible but would require major changes tothe entire calibration procedure and will not be regarded at this time. The system calibration is aprocess that requires several steps and its goal is to prepare filtering so the sound samples would
represent the equivalent stimuli on each system on which they are reproduced.
The first step is the calibration of the individual speakers of the Ambisonics system using ameasurement microphone (figure 3). A pink noise signal is played over each speaker individually andthe overall level and frequency spectrum of the input signal, recorded with the microphone, iscompared to the output that is being fed to the speakers. These signals are then matched so the ir
difference is reduced to a minimum. The matching is repeated for each of the 16 speakers. This
provides a flat room correction curve in the sweet spot of the room and serves as a reference in thesubsequent steps of the calibration.

Figure 3 – Calibration of the system with a measurement microphone
A calibration procedure continues with the placement of the dummy head on the chair in thelistening position (figure 4), simulating the situation in which the participant will be in during thelistening test. The position of the dummy head’s ears should be at the same height as the middle row of
the speakers in the Ambisonics system. The dummy head used for this calibration process is Bruel &Kjaer head and torso simulator type 4128. A pink noise is then played over the Ambisonics system andthen recorded with the dummy head. This recording is to be used as a reference signal for the rest of thecalibration.
The next step of the calibration is the placement of headphones on the dummy head (figure 5). A
pink noise signal is then played back via the Ambisonics system and compared to the previously

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recorded signal without the headphones on. A frequency correction curve is then generated tocompensate for the filtering introduced with the placement of the headphones. Since the variability ofheadphone placement should be taken into account (7,8), the procedure is repeated several times withthe repositioning of the headphones to create a suitable average for the filter.
The last step in the calibration process generates a correction curve for the playback overheadphones to nullify the coloration that is introduced by the headphone’s own frequency
characteristic.

Figure 4 – Calibration step using the dummy head

Figure 5 – Calibration of the system with the headphones on the dummy head6148

4. DISCUSSION AND FURTHER WORK
Although this system is still in its early stage of development, it will be continuously improved
through further use and implementation of individual listening tests. The addition of a real time headtracking function to this system is something that will be added in the future to remove the need for afixation of the head position during the listening test with a chinrest. The development of theAmbisonics codec and panning algorithms is also being considered to improve the control of theirparameters with which a higher level of reliability could be achieved. Also, a possibility to merge thissystem to the existing universal listening test interfaces, e.g. Hulti-gen (9), could provide a usefulcontribution to the development of such interfaces.
This system is to be used in several listening tests that are currently in preparation. Such listening
tests include: a conduct of the listening tests already carried out on one of the reproduction systems andlooking at the possible differences while using an additional system; a test of the perception of thelocalization from different systems; testing the measured sound insulation parameters of building
elements with different materials and their impact on people, both in situations when people sleep orrest and during work or study scenarios that require a higher level of concentration.

ACKNOWLEDGEMENTS
The authors acknowledge the financial support by the European Commission,(H2020-MSCA-RISE-2015 project 690970, “Advanced physical-acoustic and psycho-acoustic
diagnostic methods for innovation in building acoustics” (PAPABUILD). Also, a big thank you toVanesa Matošević for proofreading this article.

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