- Student doesn't sound like *[ˈsdjuːdn̩t]
- discharge ......................... *[dɪˈsd͡ʒɑːd͡ʒ]
- spy ................................... *[sbaɪ]
- describe ........................... *[dɪˈsgraɪb]
- system ............................. *[ˈsɪsdəm]
- school .............................. *[sguːl]
you think they do because you're confusing the plosives in your native language with the plosives in English.
English has two bilabial plosives—voiced bilabial plosive /b/ and voiceless bilabial plosive /p/, two alveolar plosives—voiced /d/ and voiceless /t/ and two velar plosives—voiced /g/ and voiceless /k/
In English, they're distinguished through voicing; /p/, /t/ and /k/ are voiceless while /b/, /d/ and /g/ are their corresponding voiced counterparts.
They are phonemes—mental representation of sounds, but can be realised in more than one way. For instance, /t/ has many allophones (realisations):
- in the beginning of a stressed syllable, it's realised [tʰ] (aspirated: with a big puff of air)
- in unstressed syllables, it can be realised [t] (no big puff of air)
- in most American accents, t in the middle of vowels can be realised [ɾ] (alveolar flap as in wa[ɾ]er)
- some dialects of British English use a glottal stop as in wa[ʔ]er.
All these are the different realisations of the same phoneme /t/.
The most common allophones of /t/, /p/ and /k/ are respectively [tʰ, t], [pʰ, p] and [kʰ, k].
- [pʰ tʰ kʰ] → unvoiced and aspirated
- [p t k] → unvoiced and unaspirated
In English, if you use them interchangeably, they won't change the meaning of the word. For example, the word pin is supposed to be pronounced [pʰɪn], but if you pronounced the p without aspiration (i.e. [pɪn]), it wouldn't change the meaning (as Spanish speakers usually do). That is to say, aspiration isn't phonemic in English.
Whereas in other languages like Chinese, aspiration is a distinctive feature for certain sounds (i.e. aspiration is phonemic).
- the letter ⟨b⟩ is used to represent 'unaspirated voiceless bilabial plosive', IPA [p]
- ⟨d⟩ for 'unaspirated voiceless alveolar plosive', IPA [t]
- ⟨ɡ⟩ for 'unaspirated voiceless velar plosive', IPA [k]
- ⟨p⟩ is used to represent 'voiceless aspirated bilabial plosive', IPA [pʰ]
- ⟨t⟩ for 'voiceless aspirated alveolar plosive', IPA [tʰ]
- ⟨k⟩ for 'voiceless aspirated velar plosive', IPA [kʰ]
In words beginning ⟨sp-⟩, ⟨st-⟩, and ⟨sk-⟩, English speakers use 'voiceless unaspirated plosives'. For instance, the p in pie is aspirated, but unaspirated in spy:
- pie → [pʰaɪ]
- spy → [spaɪ] not *[spʰaɪ]
In Pinyin, ⟨sp-⟩, ⟨st-⟩, ⟨sk-⟩ would be represented by ⟨sb-⟩, ⟨sd-⟩, and ⟨sg-⟩, respectively. So you see there's a mismatch between English plosives and Chinese plosives.
That's why you hear sdudent for student, sgool for school and sby for spy.
Whereas in words beginning ⟨p-⟩, ⟨t-⟩ and ⟨k-⟩, English speakers use 'voiceless aspirated plosives'. Examples:
- pie → [pʰaɪ]
- time → [tʰaɪm]
- kite → [kʰaɪt]
In Pinyin, ⟨p-⟩, ⟨t-⟩, ⟨k-⟩ would also be represented by ⟨p-⟩, ⟨t-⟩, and ⟨k-⟩, respectively. So you hear pie, time, kite correctly.
The same thing is going on with affricates (/t͡ʃ/ and /d͡ʒ/). Affricates in some dialects of English are aspirated, but it varies from speaker to speaker. And if you hear [st͡ʃ] as [sd͡ʒ], then there certainly is aspiration which is affected by the preceding [s] (the same way as [s] affects the aspiration of p, t and k).
Or if /t͡ʃ/ and /d͡ʒ/ are unaspirated, then they're only distinguished through voicing in English and voicing in Chinese is not phonemic so Chinese speakers confuse them easily.
- 'Plosives' are the consonants that are produced by stopping the airflow at a particular place of articulation, accompanied by a sudden release of air such as /p, b, t, d, g, k/ etc.
- I've marked incorrect/ill-formed sequences with a preceding asterisk (*)
- 'Bilabial plosives' are the sounds that are articulated with both the lips: /b/ and /p/
- the superscript h [ʰ] represents aspiration; a big puff of air
- I've used /slashes/ for phonemic data, [square brackets] for phonetic data and ⟨angled brackets⟩ for orthography (spelling)
- I don't know how to format it properly, that's why I've used ' ............. ' instead of ditto marks.