Forty-seven adult Spanish-speaking, second-language learners of English (L2; mean age = 30.5 years old, range = 19-57 years; 38 female, 9 male) and 60 adult monolingual speakers of American English (AE; mean age = 27.3 years old, range = 19-50 years; 41 female, 19 male) participated as listeners. All but nine (3 AE and 6 L2) had pure-tone thresholds ≤ 20 dB HL bilaterally at octave frequencies from 250 Hz to 8,000 Hz. The nine listeners had single-sided elevated thresholds (25-30 dB HL) at one of the frequencies.

The L2 listeners were fluent in English but represented a broad range of linguistic history and usage characteristics (Table 1). Briefly, the onset of learning English varied from infancy to 45 years old. Although Spanish was the comfortable language for the majority to speak (n = 30) and read (n = 25), 28 L2 listeners spoke both English and Spanish at home, but none reported to speak only Spanish at work or school.

L2 listeners were also screened for articulation errors in English with Bankson-Bernthal Quick Screen of Phonology (BBQSP) (Bankson & Bernthal, 1990). Consistent error patterns (e.g., /b/ for /v/) noted with BBQSP were used to guide scoring during the listening task. Listeners with random articulatory errors were excluded from further testing. Language background questions (current age, age of arrival in the U.S., onset age of learning English, language use, preferred language for reading/writing and speaking; see Appendix for actual questions) were administered orally. L2 listeners who could not understand/answer these questions were considered not fluent in English and were not tested further. Finally, L2 listeners who scored below 80% on speech recognition in quiet were also excluded (see Stimuli section for speech materials used for this). Additional 35 L2 listeners were recruited, but they were excluded due to heavy accent that interfered scoring, poor speech recognition in quiet, low proficiency in English, hearing loss, or voluntary withdrawal.

Words from the Phonetically Balanced Kindergarten (PBK) Test (Haskins, 1949) and sentences from the Bamford-Kowal-Bench (BKB) Standard Sentence Test (Bench & Bamford, 1979) were used as stimuli. They met two requirements: 1) meaningful yet simple for all listeners, thus minimizing memory load and minor grammatical errors; 2) minimal learning effects between the two materials.

The PBK Test is a word recognition test consisting of four lists of 50 monosyllabic words that are phonetically balanced and familiar to kindergarten-age children. In particular, lists 1, 3, and 4 were found to be equally effective as materials for adults (Harvard PB-50 test) (Egan, 1948; Haskins, 1949). These lists have been used extensively for more than 50 years for clinical assessment of speech perception for children between 6 and 12 years old. The present study used words from lists 1, 3, and 4 (150 words) for the listening task. BKB Test consists of simple everyday sentences that were originally developed in Britain and were subsequently modified for use in the U.S. to evaluate speech perception in noise for children between 5 and 15 years old. All 320 sentences are syntactically and semantically appropriate and contain three to four keywords each. Only 26 words were shared between PBK words and BKB sentences. Although nonsense words would allow more effective examination of acoustic-phonetic cue processing, nonsense materials were avoided to minimize the influence of unnatural phonotactics, unintended similarity to Spanish words, and confusion with real words, which were expected to have greater influence on second-language learners. For assessing L2 listeners’ performance in quiet, 25 PBK words (half of List 4) and 20 BKB sentences (List 1A & B) that were not test stimuli were used.

After consenting, hearing screening and speech and language tests described above were administered to individual participants. A brief language background questionnaire was verbally administered to the L2 listeners. L2 listeners were also screened with 25 PBK words and 20 BKB sentences that were not test stimuli to ensure good performance in quiet. The cutoff score (80%) for both words and sentences was chosen in reference to the critical differences from 95%-correct (1 error) recognition on 25-item speech tests (Raffin & Thornton, 1980).

All speech stimuli were presented to individual listeners in a sound-attenuated booth. Both in the experimental and the inquiet tasks, listeners repeated the speech stimuli exactly as heard, even if they did not make sense. However, listeners were told that all speech stimuli were meaningful. Responses were scored online by an experimenter. PBK words were scored as correct only when the entire word was correct. BKB sentences were scored as correct only when all keywords in a sentence were repeated in correct order. In the case of unclear/ambiguous responses, the intended answer was confirmed by asking to spell or define words or by asking for clarification using the results of BBQSP (e.g., for /f/-/θ/ confusions, “Did you mean /f/ as in ‘off’ or /θ/ as in ‘bath’?”) for some L2 listeners who were not proficient spellers.

Stimuli were presented binaurally via earphones (Sennheiser HD25; Sennheiser, Wedemark, Germany) in a background of speech-shaped noise (ANSI S3.5-1997) (American National Standards Institute, 1997) at a fixed overall rms level of 65 dB SPL. Speech-shaped noise was used as background noise to avoid possible interaction between language of babble and signal for L2 listeners because multi-talker babble in native language masks speech more effectively than the non-native babble (Van Engen & Bradlow, 2007). Stimulus presentation and response acquisition were controlled by Behavioral Auditory Research Tests, a computer program developed at Boys Town National Research Hospital. Presentation order between PBK words and BKB sentences was randomized across listeners. Listener performance was measured as the signal-to-noise ratios for 70%-correct performance (SNR70) using adaptive-tracking method (Levitt, 1971). Specifically, each block started at 10 dB SNR; noise level was increased following two correct responses and decreased following each incorrect response to estimate SNR70; following the first four reversals, step size was decreased from 4 dB to 2 dB. Tracking stopped after four reversals at the smallest step size. The SNR70 was calculated as the mean value of all reversals obtained with the smaller step size. The total number of trials for each listener varied depending on how rapidly the criterion was met for 70%-correct performance. This procedure was repeated six times-three for PBK words and three for BKB sentencesand average SNR70 values were calculated for each listener across three blocks for PBK words and BKB sentences. The three blocks were presented in a random order. The software chose the items for each trial in a random manner. The entire testing time to obtain SNR70 for both materials was less than 30 minutes per listener.