英语音标3:是长音还是短音音标3：是长音还是短音

这个音标是长元音,单词举例如下：
9 assert [əˈsɜːt] [əˈsɜt]
10 attorney [əˈtɜːnɪ] [əˈtɜni]
11 avert [əˈvɜːt] [əˈvɜt]
12 bird [bɜːd] [bɜd]
13 birth [bɜːθ] [bɜθ]
14 birthday [ˈbɜːθdeɪ] [ˈbɜθde]
15 blur [blɜː] [blɜ]
16 burden [ˈbɜːd(ə)n] [ˈbɜdn]
17 burglar [ˈbɜːglə] [ˈbɜɡlə]
18 burn [bɜːn] [bɜn]
19 burst [bɜːst] [bɜst]
20 certain [ˈsɜːt(ə)n; -tɪn] [ˈsɜtn]