Speech recording

The audio recording that has been used as a source of irrelevant speech across different experiments in this study was provided by Dr Jennifer A. Veitch from the National Research Council Canada, Institute for Research in Construction (IRC). The recording had been used in one of the studies carried out by the IRC on the masking of speech in open-plan offices (Veitch et al., 2002).

The original recording consisted of 17 minutes of almost continuous speech of a single female voice speaking at a realistic speech level. The speech comprised one-sided dialogues simulating one side of telephone conversations, represented by the voice of an

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actress reading scripts of telephone conversations, in which she called job candidates to arrange for interviews, made internal arrangements for new employees and made personal social calls (Veitch et al., 2002). This study is one of the most comprehensive studies in the area of speech masking in open-plan-office and has been used by numerous studies as a reference in determining the preferred masking sound and sound level.

Seven-second long speech signals were extracted from the speech recording to be used as a source of irrelevant speech in the sound level preference tests (Chapter 4) and audio- only and audio-visual preference and perception test (Chapter 5), to match the length of the water sounds. In doing so, care was taken to extract equally meaningful sentences which contained similar amount of information. The 17-minute long speech recording was used in the task performance tests (Chapter 6) as a source of irrelevant speech. Both the 7-second-long and the 17-minute-long speech recordings were separately calibrated to have appropriate sound levels for the purpose of the tests. More information on the calibration process and the spectral properties of the recordings is provided in their corresponding chapters.

3.4 Methodology

Given the broad area that this study covers, it was important to carry out different experiments with different methodologies, to meet the aims and objectives of the study. Overall, five experiments were carried out, four of which required human participations. A brief overview of the methodologies adopted in each experiment is explained below, and detailed explanations on these methodologies are given in their respective chapters.

3.4.1 Experiment 1: Sound level preference test

The main aim of the experiment was to identify the preferred masking level of the water sounds to be used over irrelevant speech, in open-plan offices. The experiment was also designed to look at the likely effect of the type of water sound, and the intelligibility level of irrelevant speech, on the preferred masking sound level.

Two water sounds, a 4-step cascade (CA), and a 37-jet fountain (FTW) were played at five sound pressure levels, 42, 45, 48, 51, and 54, dBA against irrelevant speech played at a constant sound pressure level of 48 dBA. The speech transmission index (STI) was used as an objective measure of the intelligibility level of irrelevant speech. The STI of the irrelevant speech was altered by adding background noise to it, so that the irrelevant

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speech had two STI values, 0.50, and 0.78. This resulted in four test conditions at each masking level:

• FTW played against irrelevant speech with an STI of 0.50; • FTW played against speech with an STI of 0.78;

• CA played against irrelevant speech with an STI of 0.50; and • CA played against speech with an STI of 0.78.

Paired comparisons were used. For each condition, five sound pressure levels were tested, which resulted in 10 paired comparisons per condition1. Hence, the total number of paired comparisons was 40 (104). The test was carried out in the anechoic chamber of the acoustic laboratory at Heriot-Watt University, where participants were invited to listen to all pairs of sounds through headphones, and state their preference. Detailed explanations of this experiment are given in Chapter 4.

3.4.2 Experiment 2: Audio-only and audio-visual preference and perception tests The aim of this experiment was to identify the type of water sound that would be preferred by people to be used as a speech masking sound, and whether people’s preferences were different between audio-only and audio-visual conditions. The experiment also examined the change in people’s perception of their sound environment, after masking irrelevant speech by a water sound. Six water sounds (Table 3.1) were used and their sound pressure levels were fixed at the level determined by Experiment 1. These were all played against the irrelevant speech which had a constant LAeq, 7s of 48 dB, and a constant STI of 0.78.

The reason for not selecting the STI of 0.50 mentioned in Experiment 1, is given in Chapter 5. The experiment was divided into four main parts:

• Audio-only preference test: Tested the audio-only (no animation) preference of the water sound. Paired comparisons were used which compared all water sounds to each other. This resulted in 15 paired comparisons.

• Audio-visual preference test: Tested the audio-visual (with animation) preference of the water feature. The test was similar to the audio-only preference test, but the audio materials were accompanied by visual materials. The test included 15 paired comparisons.

• Audio-only perception test: Tested the change in people’s perception of the sound environment after masking irrelevant speech with each water sound.

1_{Number of pairwise comparisions =} (𝑆𝑆−1)×𝑆𝑆

2 , where N is the number of variables

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• Audio-visual perception test: This test was similar to the audio-only perception test, but the audio materials were accompanied by visual materials.

The tests were carried out in the anechoic chamber of the acoustic laboratory at Heriot- Watt University. Audio materials were played through headphones and the visual materials were presented on a screen. The visual materials consisted of six high quality animations of the water features embedded in an image of a furnished open-plan office. Detailed explanations of this experiment are given in Chapter 5.

3.4.3 Experiment 3: Cognitive performance and subjective satisfaction

The aim of this experiment was to examine the effect that masking irrelevant speech has on people’s cognitive performance and subjective satisfaction. In light of the findings from Experiment 2, a water sound (FTW), was selected to be used as a speech masking sound. Participants were invited to take part in the experiment. They performed a set of 4 cognitive tasks under two background noise conditions, a speech-only condition and masked speech condition. The speech only condition was created through playing the 17- minute-long speech recording as background noise. The masked speech condition was achieved by playing the FTW sound over the speech recording. Two speakers were used to play the sounds: one used to play the speech recording and the other used to play the water sound. The four tasks were a serial recall task, a one-back task, an information matching task, and a reading comprehension task. Cognitive workload and subjective difficulty of the tasks were measured using questionnaires distributed in both background noise conditions. The experiment was carried out in Room G.33 of the Edwin Chadwick Building at the Edinburgh Campus of Heriot-Watt University. Detailed explanations of this experiment are given in Chapter 6.

3.4.4 Experiment 4 Longer-term exposure to a water sound

The aim of this experiment was to measure the longer-term impact of a water feature on people’s perception of their work environment. In accordance with results of the audio- visual preference test in Experiment 2, a water feature was purchased and modified to match the criteria set out by Experiments 1 and 2. This water feature was placed in a small open-plan office (Room 3.16) in the William Arrol Building at the Edinburgh Campus of Heriot-Watt University. A questionnaire measuring different aspects of the work environment, including the sound environment, was distributed before the water feature was placed in the space. After the water feature had been running in the space for 3 weeks, the same questionnaire (with some added questions specific to the water feature) was redistributed and responses given by participants before and after the installation of the

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water feature were compared. Detailed explanations of this experiment are given in Chapter 7.

3.4.5 Experiment 5 Objective measures

This experiment aimed at finding the rate of drop in the STI associated with masking speech in open-plan offices using a water sound. The distraction distance, rD, and the

privacy distance, rP, are two single number quantities recommended by BS EN ISO 3382-

3 (2012) to represent room acoustic parameters in open-plan offices. The rD is the

distance at which STI drops below a value of 0.50, and the rP is the distance at which the

STI drops to a value of 0.20. Both rD, andrP of two open-plan offices were measured

before and after adding a water sound (FTW) to the background noise, and the rate of drop in rD, andrP was recorded. Measurements in this experiment were carried out in

accordance with BS EN ISO 3382-3 (2012). The two open-plan offices were room 2.04 in the Edwin Chadwick Building, and the ground floor of the William Arrol Annexe, both located at the Edinburgh Campus of Heriot-Watt University. Detailed explanations of this experiment are given in Chapter 8.