Speech Intelligibility and Annoyance

Speech intelligibility and sound annoyance studies are two subjects for which au-ralisations are often used. The studies concerning these subjects and the use of auralisations will be discussed in this section.

3.7.1 STI

STI stands for Speech Transmission Index and can be considered as the represen-tation of the preservation of dissimilarities of speech sound [58]. This parameter is

considered a reliable measure for speech intelligibility [58]. The use of auralisation for the prediction tests of STI can have major advantages. Yang and Hodgson [59] state that auralisations make it possible to perform STI tests with humans. These tests give more realistic results than measurements or predictions of objective metrics.

Disadvantages of such tests are the required number of test subjects and the fact that these tests can only be performed in rooms that are already built. Auralisations make it possible to perform those tests easier with a relatively high number of test subjects during the design phase.

For their research, Yang and Hodgson [59] compared the results of the same STI-tests for real and virtual classrooms. 300 words of Modied Rhyme Test list (MRT [60]) were recorded in an anechoic chamber. Next, a model of the classroom where the measurements were performed was built in CATT-Acoustics v8.0. This software was thereafter used to simulate the room's BRIR and to convolved the recorded MRT words and babble noise. As mentioned earlier on page 5, the paper only focussed on the dierence between the results of the listening tests of the real and virtual classroom. The authenticity of the used auralisations was not assessed.

To compare the results of both objective and subjective speech intelligibility between measurements and CATT/ ODEON models, Hodgson et al. [61] used a MRT test.

For the 'real' situation, anechoic recordings of the used samples were played from a speech source for the test subjects in a classroom. Tests were performed with both a 'noise on' and 'noise o' condition. The room was modelled in both CATT [62] and ODEON Acoustics [13], where the predicted BRIRs were used to convolute the same MRT samples. The same MRT-test was subsequently performed with these convo-luted samples, which during the tests were presented to the test subjects through headphones. Although it is possible to measure personalised HRTF for every test subject, the same generalized HRTF (based on a KEMAR dummy head) was used for the convolution. It was expected that it would not aected the speech intelligibility signicantly. For the objective comparison, the RT , EDT , C80and Lp were assessed.

The research concludes that the prediction of acoustical parameters and modelling of the models remains dicult. The results of the speech intelligibility tests

con-rms their prediction, which was that scores from auralised tests underestimate the speech-intelligibility of low reverberant rooms.

3.7.2 Annoyance

Vorländer et al. [63] mentioned the potential of auralisation and Virtual Reality concept for new investigations of annoyance and comfort in the architectural design process. However, many studies on annoyances that make use of auralisations are focussed on trac and vehicle noise ([45] [47] [46] [52] [64]). Especially the trac noise annoyance studies do not fall within the scope of this literature review since this study focusses on indoor acoustics. The term annoyance in the context of indoor auralisation is mostly found on the answering scale of the double-blind triple-stimulus and MUSHRA tests ([29] [65]). For the research of Lokki and Pulkki [65], test subjects were asked to judge both the dierence between the reference sample and sample A and the reference sample and sample B. The answering scale reached from "very annoying" (1.0) to "imperceptible" (5.0).

Thaden [66] performed a study on the auralisation of impact sound insulation. For

their research, they performed listening tests to evaluate the annoyance of impact noise. This is a subjective measure, instead of the single number quantity found with standardized sources such as tapping machines. Since recording impact noise in real buildings is very time consuming, it would be preferable to use auralisations for these tests. The goal of this research was thus to obtain an IR that describes the path between the impact noise source in one room and the receiver in an adjacent room. The paper concludes that an auralisation system has been developed and tested with the sound of a tapping machine. For further work, more dierent noise sources (such as jumping children and walking people) have to be recorded. Listening tests were not yet performed, but were planned for the future. A study that uses interior auralisation and that is closely related to trac annoyance is that of Asakura et al. [67]. In their research, they propose an auralisation method to investigate the transmission of noise through sound insulation constructions.

3.8 Conclusion

There are four common listening test methods that are used for all types of auralisa-tion studies: double-blind triple-stimulus, Multiple Stimulus with Hidden Reference and Anchors and Three/Two Alternative Forced Choice. All but the 2AFC-method have the possibility to check the reliability of the test subjects with the use of refer-ences, which can increase the trustworthiness of the results [29].

The usability of the auralisation authenticity tests will depend on the level of dier-ence between the auralisations and the measurements. For big dierdier-ences, the direct scaling MUSHRA-test has the most potential [29]. However, the MUSHRA-test com-pares a multitude of samples to one reference sample. This situation is not common when performing listening tests on auralisations since most auralisation studies only compare measurements and auralisation samples with the same S/R-combination.

This limits the number of used stimuli per question to 2 (or 3 when using a hidden reference). For smaller dierences, the indirect listening tests 3/2AFC and AB com-parison are more suitable [17]. The chosen method will thus depend on the level of similarity of the auralisations and recordings. The similarity can be dened by the listening test results of the test subjects. Another important aspect is the level of acoustical expertise of the test subjects. Without training or experience, it can be dicult for the test subjects to judge several 'abstract' acoustical attributes such as the timbre and the spaciousness of the samples.

The preferred tests are an SDT-test and a double-blind triple-stimulus test. One of the drawbacks of using these tests may be the amount of time it takes to complete all the questionnaires. Fatigue of the test subjects may have a negative eect on the trustworthiness of the results. Bech and Zacharov [28] state that single test sessions of 20 minutes are a good length to prevent fatigue and boredom. The complete test can be longer, but in that case, it is preferable to split the test into multiple sessions with a suitable break in between. Since test duration is limited to 20 minutes, signals should be kept relatively short. This coincides with ITU Recommendation 1284-1 [38], which states that audio excerpts used for tests should be no longer than 15-20s.

When comparing the results of auralisation / measurement comparison studies that used dierent types of stimuli, it is clear that broadband signals such as drum and

especially noise stimuli make for the most audible dierences. It is questionable though, if the noise signal is a signal that has to be used for auralisation studies since a human speech or music signal is more likely to be encountered in a real situation. A drum signal may be a good compromise, as it is a naturally occurring sound with a broadband frequency range (see Fig. 3.3).

4 | Measurements

The measurements to recorded the BRIRs required for this study will be described in this section. These were performed inside a Student Sport Center Eindhoven sports hall (see Fig. 1.4). The measurement plan and the used equipment will be described in the rst two sections. Afterwards, the results of the measurements will be discussed.