Phonetically, stop sounds (
[t d n] etc) are distinguished by a complete closure of the vocal tract. Nasal sounds (
[n]) allow air to escape through the nose; oral/non-nasal sounds (
[t d]) do not.
The nasal/oral distinction is pretty easy to measure phonetically, so phoneticians are happy about that. But the voiced/voiceless distinction is a bit less obvious. The most popular measurement I've seen used is the voice onset time (VOT)—in other words, how much time is there between when the closure opens, and when the vocal cords start vibrating?
- If the VOT is positive (the closure opens, there's a delay, then voicing starts), that's an aspirated stop
- If the VOT is zero or very small (the closure opens and voicing starts at the same time), that's a tenuis or voiceless unaspirated stop
- If the VOT is negative (voicing starts, there's a delay, then the closure opens), that's a voiced stop
So what does it mean for any of these to be unreleased? That means the closure doesn't open at all. So there's no way to have an "unreleased aspirated" stop. But you can have voicing happen during the closure, or not: this creates the distinction between unreleased voiced and unreleased tenuis stops. An unreleased voiced stop is indeed exactly the same as an unreleased "denasalized nasal"—the only difference between phonetic
[n] is whether the nasal passage is open or not.