Piloting

In this research, three pilot perceptual experiments were carried out for two different purposes. First, two trial listening tests were conducted to ensure the intelligibility of the noise-free clearly spoken and conversationally spoken utterances, respectively. For these two pilot testings, two complete lists of the 48 stimulus sentences were created. One list contained all the sentences spoken clearly (Clr-N) by the two groups of speakers. For the other list, all the sentences were produced by the Cantonese and English speakers in a conversational style (Con-N). Half were true utterances and half were false with a balance in the speakers’ L1. The specific sentences from each speaker were the same for these two lists. This preliminary examination of the clean speech (i.e., without noise) seemed to be necessary; any subsequent analysis of listeners’ scores on the noisy utterances in the actual experiment would be pointless if the noise-free stimulus sentences were unintelligible at the outset. Second, before the multibabble noise was digitally mixed with the speech signals for the actual perceptual experiment, an appropriate level of noise had to be determined. The proper level was selected with the underlying rationale that the noisy sentences would neither be too easy nor too difficult for the listeners to verify. In the

subsequent piloting sessions, an S/B ratio was established that would lead to a moderate amount of degradation in which between 70% and 80% of the noisy sentences still remained partially or completely intelligible at the selected level of the masking noise (Munro, 1998). The stimuli for the last pilot study contained both Clear speech and Conversational speech mixed with the 12-talker babble; a full list of 48 trial utterances was made up of sentences selected from both Clr-N

and Con-N. Again, half were true utterances and half were false with a balance in the speakers’ L1. All sentences were saved as computer audio files so that they could be randomly presented to the listeners through ExperimentMFC, a Multiple Forced Choice listening experiment program available in Praat.

The babble noise added to the sentences was extracted from a

30-second-long sample, the amplitude of which was adjusted in accordance with the trial S/B ratios using Praat. The mixing of the speech signals with the babble noise at any particular noise level was done using GoldWave. Details will be discussed later in this section.

A sentence-verification task was used to assess the intelligibility of the sentences throughout the three pilot studies (Li, 2000; Munro, 1998). Details are given in section 3.1.5, as this task was also used in the actual perceptual

experiment. Briefly, upon listening to the stimulus sentence once, listeners in all the pilot tests had to determine the truth value of the sentence by selecting one of three buttons (“True”, “False”, or “Unknown”) on the computer screen. The verification scores were based on the number of correct true and false responses (Munro, 1998).

The first pilot listening task pertained to the intelligibility of the clean Clear speech. A male native English listener who was an undergraduate student at Simon Fraser University voluntarily participated in this trial session. The subject was 21 years of age and passed a pure-tone hearing screening (250, 500, 1000, 2000, and 4000 Hz at 25 dB HL) bilaterally prior to performing the perceptual experiment. It was found that for the clean Clear speech, the overall correct

verification score (pooled over the two speaker groups) was 95.8%. As already mentioned, in this list of 48 Clr-N sentences, half of them were produced by the native English speakers, while the rest were produced by the Cantonese

speakers of English. The subject failed to correctly verify two (out of 24)

sentences produced by the Cantonese speakers, while he obtained 100% correct verification score on the native-produced utterances.

For the second pilot session, a new male native English listener served as research participant. The subject was 21 years of age and was an undergraduate student. He passed the pure-tone hearing screen prior to the listening task. This time, the subject was asked to verify the truth value of all the clean utterances spoken conversationally by the Cantonese speakers of English and native English speakers. It was found that he correctly verified all the 24 Con-N sentences spoken by the non-native English speakers and those produced by the native English speakers (i.e., overall 100%). From the results of these two pilot experiments, the degree of intelligibility of the clean Clear speech (Clr-N) and clean Conversational speech (Con-N) produced by the Cantonese speakers of English and the native English speakers could be considered acceptable for this research.

The final piloting session was used to evaluate the intelligibility of the noisy sentences at three different S/B ratios: 0, -1, and -2dB. In other words, the RMS amplitudes (dB) of the multibabble were 62 dB, 63 dB, and 64 dB,

respectively, with the speech signals being fixed at the target level of 62 dB. A complete list of 48 noisy sentences was used as stimuli. For each S/B ratio, 16

sentences were mixed with the appropriate level of babble noise. Within each of the three sets of 16 sentences, the truth value (True vs. False), the speaking style (Clr vs. Con), and speakers’ L1 (Cantonese vs. English) were all

counterbalanced.

Two new native English listeners (one female, one male) served as research participants in this last trial experiment. Both were undergraduate students at Simon Fraser University. The female listener was 35 years of age, and the male participant was 18 years of age. Both of them passed the pure-tone hearing screening (25 dB HL at octave intervals between 250 Hz and 4000 Hz).

Again, neither of them had participated in any speech experiment involving noise.

The mean percentage score (pooled over the two listeners) for the

verification task at 0 dB S/B ratio was 78.1%. The values at -1 dB and -2 dB S/B ratios were 59.4% and 34.4%, respectively. On the basis of these results, it was decided that in order to meet the target of 70 – 80% correct verifications, the S/B ratio should be set at 0 dB. In other words, the RMS amplitude of the babble should be identical to that of the speech signal. To achieve this, they both were set at 62 dB.