There is no doubt that listening skills have a considerable bearing on foreign language learning. Therefore, the importance of teaching listening skills and potential hindrances which deter students from comprehension are still a topic of scholar’s interest. According to Rost [1], listening is considered a key factor in language learning since it is an input that allows learners to develop language knowledge. Furthermore, being adept at listening skills also motivates learners to get access to conversations with native speakers since students’ self-confidence can be drastically boosted [2].

Nevertheless, Vietnamese institutions have never paid enough attention to listening skills during the English learning process [3]. Instead, English-majored students are trained to prepare for examinations on grammar, reading and vocabulary in place of focusing on communicative competencies, which generates an abundance of hindrances for students in mastering these fundamental skills [4], let alone happily accomplishing standardized language tests. This situation can manifest itself starkly in developed metropolitan areas in Vietnam, where International English Language Testing System (IELTS hereafter) is increasingly gaining traction over the course of a few recent decades. 

In the light of the fast-paced world, language testing and assessment have been shifted drastically. One of the most striking developments is that test-takers tend to choose standardized language testing systems to ensure an accurate assessment by applying strict and detailed criteria to each test, which allows to precisely reflect candidates’ language levels. As a result, the International English Language Testing System (IELTS) has become an increasingly popular measurement of English language proficiency in Vietnam over the past decades, especially at the tertiary level [5]. The rationales behind the prevalence of IELTS in Vietnam are varying, which are mainly contingent upon: the governmental policy to enhance Vietnamese learners’ English competency; as well as increased opportunities for Vietnamese to pursue a higher degree in English-speaking countries [6]. 

In terms of ‘listening comprehension’, this is not a novel terminology but defined by numerous scholars. Rost [1] defined listening as a complex process of interpretation in which listeners match what they hear with what they already know. Simultaneously, Hamouda [7] emphasized that listening comprehension is a process where listeners interpret the meaning of the oral input through sound discrimination, knowledge, grammatical structures and pronunciation. Nadig’s statements [8] mirrored that of previous studies, which claimed that various processes of understanding and making sense of spoken language can be considered as listening comprehension.

Concerning the hindrances during the listening process, a host of barriers can cause difficulties for learners in the aural comprehension processes. According to Azmi et al. [9], being aware of problems, which would be mentioned below, would allow learners to better their listening comprehension. To be more specific, there are no less than five problems that students may encounter, including the subpar quality of recorded material, the discrepancy in cultures and accents, the speed of listening and unfamiliar vocabulary.

In Azmi et al. [9] research, depend on each problem, learners’ listening comprehension would be adversely affected. To illustrate, 66% of learners mentioned that speakers’ accent was one of the most noticeable factors that prevent learners from comprehending the recordings. Moreover, Underwood [10] believed that the speed of speakers may generate critical issues with listening comprehension, while students’ problems about listening comprehension can be worsened with the presence of inferior grammar and vocabulary deficiency [11].

Among the aforementioned problems, Azmi et al. [9] concurred with Graham [11] that students' interest and motivation would be aroused by frequently used words, which would have a positive effect on their listening comprehension abilities. They explained that students may get confused and were not capable of picking an appropriate context for words with more than one meaning. 

With the above-mentioned knowledge, lexical resources seem to exert profound effects on listening comprehension. Nevertheless, the estimation of vocabulary size, which allows learners to perform better in listening tests, should be thoroughly scrutinized.

Therefore, to clarify the relationship between vocabulary size and listening comprehension, Duong and Chau [12] concluded that English majors’ listening comprehension problems were attributed to various factors, among which unfamiliar words, idioms and colloquial words are typical examples. Meanwhile, students in a vocational school in Thailand acknowledged that apart from local accents, speakers’ speed and cultural differences, lexical resources are one of the most challenging factors which prevent students from listening comprehension [13]. However, because standardized tests, especial the section 4 of IELTS Listening tests, where mainly focus on academic situations, would require a wide range of high-level vocabulary, test-takers who could not achieve equivalent word-level would find it difficult to process the auditory effectively. As a result, the importance of vocabulary preparation, including the breadth and depth of vocabulary knowledge, would increase the ability to comprehend the listening material [14].

On the contrary, the vocabulary knowledge shortage was found to be a core barrier for adequate listening comprehension [15]. Meanwhile, Bonk [16] proclaimed that “participants with a lexical coverage of 90% achieved higher listening scores than the participants recognizing fewer than those having 80% of lexical coverage”. Owing to the aforementioned studies, the sentiment of the significant correlation between vocabulary size and listening comprehension is justifiable.

However, there is no existing method for accurately calculating vocabulary size, along with an effective solution to categorize the enormous variation in the vocabulary size estimates. Therefore, counting words as lemmas or word families are likely to be a reliable standard in studies of second and foreign language acquisition [17].

There was some research mentioning the average size of vocabulary which genuinely affects students’ adequate listening skills. Nation [18] had a consensus that students need approximately 6000-7000-word families to cope with spoken discourse, while Stæhr [19] estimated that a vocabulary size of at least 5000-word families would facilitate the students’ listening comprehension. A discrepancy can be observed in Van Zeeland and Schmitt’s study [20], where some experiments were conducted to suggest that 2000-3000-word families could be sufficient for students. Despite the difference in the threshold vocabulary size among the above-mentioned studies, students with a larger vocabulary coverage, generally, tend to perform better in their aural comprehension.

In Vietnam, IELTS appears to be one of the most dominant international standardized testing systems that are used for assessing the English language proficiency of individuals who are planning to study or work in English-speaking nations. Tests mainly assess the four language skills including Listening, Reading, Writing and Speaking, where results are reported on a 9-band scale [21]. Among four skills, listening tests require candidates to listen to four separate recorded texts which are relevant to most circumstances in people’s lives, ranging from personal to academic life. To be more specific, the third and fourth sections are conversations and monologues in an academic context respectively [22]. Therefore, an adequate vocabulary size seems to have a profound effect on listening comprehension. Therefore, further research needs to be conducted to indicate the main rationales that lead to students’ difficulties in listening to help students overcome the obstacles during the listening process, especially to deal with part 4 of the IELTS listening tests. 

For the aforementioned reasons, the research questions are:

•  What problems were perceived by students during their listening comprehension learning

• Was students' confidence enhanced after learning vocabulary

• Do varying lengths of preparation time make a difference to learners' performance in vocabulary knowledge and listening results

Based on the above-mentioned research questions, the methodology of research, including participants, research design and data collection instruments are scrutinized to collate the information and cater for the research purposes.

Regarding the selection of the school, a public college where the researcher was working at the time of this study was chosen to partake in this study for purposive sampling. More to the point, the Foreign Language Faculty, which has been training thousands of English-majored students, was the sample for this research. Students at this public college reported that one of the most difficult skills that hinder them from a satisfied studying result is Listening skills. Over the years, although there have been a host of efforts from lecturers as well as students, the percentage of students who failed Listening courses is still sustainable. For this reason, the researcher chose students from a public college to participate in this study.

As regards the selection of the students, a number of 90 Vietnamese junior students, ageing from 21 to 22, took part in the experiment. All participants were English-majored students who have completed eight credit hours (four courses) out of 12 credit hours (6 courses) of Listening skills. At the end of the last academic year, they were all passed a final Listening test which is based on the format of the Preliminary English Test (PET). This reflected that around half of the students at least gained B1 level in listening skills.

With regard to the research design, students were divided into three different groups (Group A-C hereafter). Group A was given a list of vocabulary one week before the listening test and Group B, one day before. Meanwhile, Group C has not received any materials for home study, instead, they are introduced to the list of vocabulary in class for 30 minutes and discussion was encouraged. All students were informed that the vocabulary on the list would be re-appeared in the aural text of the test. The research plan is set out in Table 1. 

Table 1: Research Design

From the participants and research design, data collection instruments and proposed procedures are formed to clarify the answers for the research questions. 

Firstly, data collection instruments include two tests: A Vocabulary Test (VT) and a Listening Comprehension Test (LC), which was based on IELTS Part 4. Secondly, the proposed procedures are functioned with both Listening and Vocabulary tests completed on the same day. Meanwhile, the Vocabulary is commenced at the beginning of the class, followed by the Listening test. The aggregate time for both tests was around 30 minutes including the time for giving instructions and answering the questions. Besides, a post-test questionnaire was delivered to students to investigate student’s common issues in listening. After the questionnaire, nine students who had just completed the questionnaire were invited to participate in an interview to investigate students’ attitudes towards learning Vocabulary prior to completing the listening task.

In terms of the listening test, for the purpose of testing the participants’ listening comprehension, a standardized listening test from IELTS listening tests was used. In this part of the IELTS listening test, ten questions cover one type of task: gap-filling. At the beginning of the test, the test gave 30-second time for the participants to have a look at the questions and the piece of text was played once. Additionally, the accent in the test was British.

In terms of the vocabulary list, ten words from the aural text were included in the vocabulary list.

These words were chosen by different students who were not students in this study to prevent differences of opinion about the difficulty of the words between the researcher and the test-taker. However, the words chosen by volunteers were only used as a guide and the words on the Vocabulary list were chosen by the researcher.

In the matter of the Vocabulary Test, this test applied in this study to test whether participants had learned the assigned words. The test included only one section with 10 multiple choice questions, based on the words in the Vocabulary list that students have learned. However, the words chosen by volunteers were only used as a guide and the words on the Vocabulary list were chosen by the researcher.

In addition to listening and vocabulary tests, a post-test questionnaire and the interview are also employed. More to a point, the six-item questionnaire dictated to students’ hindrance when listening, followed by the interview with nine questions mentioning students’ attitudes towards learning vocabulary and their confidence after preparing vocabulary before completing the test.

After the preparation of questions for tests and interview, the research can be commenced with the aggregate number of participants was 89 and divided into three groups. Out of three experimental groups, Group A registered the highest time for vocabulary preparation, in one week, followed by Group B with preparation time being significantly lower, in one day. The lowest preparation time was recorded in Group C, whose preparation time for Vocabulary was 30 minutes before the Vocabulary test. Each experiment group was first given the vocabulary test before doing the listening test. After the tests were finished, they filled out the questionnaire through the assistance of Microsoft Form. 

In connection with data Analysis, an add-in to augment Microsoft Excel users was employed to analyze the students’ test scores. By employing an add-in in Microsoft Excel, a variate analysis using the ANOVA model was conducted to evaluate the impact of vocabulary preparation on learners’ vocabulary knowledge, listening comprehension, confidence level and learners’ listening proficiency.

The questionnaire responses were analyzed manually with descriptive statistics, including frequencies (F) and percentages (%), for the collated data on learners’ listening problems were subsequently calculated using Microsoft Excel.

The interviews were recorded and later transcribed. Content analysis was used to analyze the data in three steps. The analysis process commences with the data being typed into a form and read the transcripts repeatedly to get acquainted with the content before coding and recording the collected data and culminating the process with grouping the coded information into larger categories. Finally, the authors examined the categories and themes in order to determine any relationships and connections.

Based on the data collection, findings for the research including listening problems, students’ confidence and the effects of different preparation time on students’ listening ability can be clarified. 

Firstly, the first purpose of this study was to investigate the hindrances experienced by junior students who are studying the Listening course in their fifth semester at a public college in Vietnam. The data for this study was collated from the questionnaire to answer the first research question ‘What problems were perceived by students during their listening comprehension learning?’

Result: This part presents the problems that students in three experimental groups have when listening. The participants were required to mark each problem based on a five-point Likert scale (1= never; 2 = rarely; 3 = occasionally, 4 = frequently; 5 = always).

As can be inferred from Table 2, the percentage of students admitting that they missed the next part of the listening test while thinking about the meaning of the earlier part top the list with the figure at being 79.12%, followed by the percentage of students whose listening issues were fast speech rate and unfamiliar words or phrases, being slightly lower, at 79.07 and 72.52%, respectively. Meanwhile, around 58% of aggregate participants concurred that they cannot recognize the sounds or mistake one word for another in the listening test. At the same time, the lowest figure was recorded in the percentage of students who find it hard to concentrate during the listening test, at only 45.05%. 

In brief, the majority of participants experienced some perception problems that adversely affect their listening comprehension. In particular, the participants failed to recognize many of the sounds in the listening test while the fast speech rate caused them to miss the latter parts of the test. Additionally, an abundance of students had struggles with unfamiliar words or phrases. Meanwhile, most of the students also lost concentration while listening.

Secondly, the second research question: ‘Was students' confidence enhanced after learning vocabulary?’ was answered by interviewing nine students from each group, which has a different length of preparation time. 

Table 2: Questionnaire Results of Students’ Problems when doing Listening Test (n = 89) 

Result: Throughout the interview, all students affirmed that they did not complete well on the test. Specifically, when the Listening test finished, nine interviewees believed that the results of their Listening test would be below 50%. They explained that the speech rate was the key factor that hinders them from having better results in the listening test.

However, all interviewees asserted that their confidence significantly enhanced after having time for vocabulary preparation. Meanwhile, out of nine interviewees, seven participants stated that they might have done the Listening test poorly without studying the vocabulary. As stated by the sixth Student Interviewee (SI6), she agreed to employ this method in her studying in the future; in particular, “[…] to improve my listening skills, I used to open the textbooks and complete as many exercises as possible in the expectation that my skills would be gradually improved […] But from this time, I would analyze the audio script and learn new words prior to starting to listen to the audio.”

An added feature found in the interviews was students’ attitude towards time preparation for Vocabulary. Surprisingly, all interviewees extracted from three experimental groups showed their satisfaction with time preparation. According to SI3, the quality of time for vocabulary preparation is not of much importance; for example, “[…] the amount of time is not important […] actually, many of my friends thought that other classes (group) have more time to prepare and they may remember the words better, which leads to better listening results, but 10-minute preparation is enough.”

Furthermore, half of the interviewees suggested that teachers should not separate the meaning of new words from their pronunciation. To be specific, the SI9 mentioned that pronunciation plays a pivotal role in helping him to understand the listening; for instance, “[…] pronunciation is vital since the incorrect use of pronunciation inevitably leads to the fact misunderstanding the speakers’ utterances.”

Finally, the third research question dedicated to the varying lengths of preparation time to differentiate the differences between learners' performance in vocabulary knowledge and listening results. 

Result: This question examines the amount of time given for vocabulary study in relation to the listening result. To ensure the effects of vocabulary knowledge on listening tests, a one-way between subject’s ANOVA was conducted to compare the effect of varying preparation time, including 1-week preparation, 1-day preparation and 30-minute preparation, on students’ vocabulary knowledge. Table 3 indicates that there was a significant effect of different preparation time on the vocabulary results at the p<0.05 level for the three conditions [F(2, 87) = 4.3, p = 0.02]. Post hoc comparisons using the Tukey HSD test indicated that the mean score for Group A with 2-week preparation (M = 8.78, SD = 2.43) was significantly different from Group B with 1-day preparation (M = 7.2, SD = 1.91). However, such significant discrepancies cannot be witnessed between Group B and Group C (M = 7.5, SD = 1.87) which had only 30 minutes to prepare for the vocabulary. Taken together, these results suggest that the lengths of preparation time do have an effect on students’ vocabulary knowledge. To be more specific, the abovementioned results demonstrate that the more time students used for learning vocabulary, the better they remember vocabulary. However, it should be noted that the length of preparation time must be high in order to see an effect. Conversely, medium lengths do not appear to significantly increase vocabulary knowledge.

Table 3: The Effects of different Preparation Time on the Vocabulary Results

From the above-mentioned results of students’ differences in vocabulary knowledge, the researcher was confident to analyze students’ listening results to ensure the effects of vocabulary preparation on listening results. Table 4 presents the effects of different vocabulary preparation time on participants’ results of the listening test. On the whole, the level of vocabulary does not have a significant impact on the results of the listening test. A between-groups ANOVA was performed to compare the impact of vocabulary level on listening results. 

Table 4: The Effects of different Vocabulary Preparation Time on Participants’ Results of Listening Test

The independent between-groups ANOVA yielded a statistically significant effect, F crit (2.87) = 3.1, p =0.05 is much smaller than F (2,87) = 0.32, p = 0.72. Thus, the null hypothesis of no differences between the means was rejected. Therefore, there was not a statistically significant difference between groups as determined by one-way ANOVA. From these statistics, the research can conclude that although students have more time to learn vocabulary, there would not be any differences between their listening results.

This study scrutinizes how three different periods of vocabulary learning time influenced students' listening comprehension performance, coupled with how these preparation characteristics affected their confidence and their test performance.

According to the findings of both the questionnaire and the interviews, the bulk of students had at least one listening issue during their listening process. To begin with, these students had some perception issues because they could not understand or regulate the pace of the spoken content. It can be inferred that the first Language (L1) probably influences second Language (L2) acquisition. Furthermore, students also have some problems with unfamiliar words or phrases. Therefore, teaching vocabulary should be combined with teaching pronunciation so that students can become familiar with the pace of the spoken materials. 

From all results of the survey, a large majority of students in all three experimental groups agreed that learning vocabulary was beneficial to their comprehension and many of them were able to use the given words to predict potential material, location, or even the main character if proper nouns were provided. However, if the pre-task preparatory work is going to be used in a classroom listening test, it's crucial to give learners enough time to plan. It is because the lengths of preparation time do affect students’ vocabulary knowledge, which was proven in the ANOVA test. For all these considerations, we have to keep in mind that our overall aim is to give learners the experience of achievement and to help them gain the competence to understand listening contexts. Furthermore, a considerable number of students also said that they couldn't adapt the sound of a phrase to the written form. When students are practicing listening, they should be allowed to say phrases that they are unfamiliar with. 

Together with the questionnaire and interviews, vocabulary tests and listening comprehension texts based on IELTS part 4 listening tests in the hope to find the effects of vocabulary preparation on listening tests. However, although the lengths of vocabulary preparation do affects students’ vocabulary knowledge, the discrepancies in vocabulary knowledge do not lead to the difference in listening results. Perhaps, there are still other factors that would affect students’ listening comprehension, among which pace and pronunciation are exemplars. 

The effects of vocabulary preparation time on L2 listening were studied in this review. However, there was one drawback: it was exceedingly difficult to keep track of how much time students spent preparing. While students who were given one week to prepare did not complain about not having enough time, it is unknown how much time they actually spent preparing. With this constraint in mind, a recommendation for future research is that a timed classroom study will enhance the understanding of how vocabulary training time affects L2 learners' LC in order to better monitor the allotted time. Last but not least, the sample size is inadequate to allow generalizations to other cases. This is attributable to the fact that only 89 English students from a Vietnamese college took part in the study.

Therefore, a group of bigger participants could be employed to generate more accurate results. 

Acknowledgements 

We would like to express our gratitude to the study participants for their assistance in collecting data for this article. This paper would not have been possible without their support.

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