Someone said that there is a sound beginning [z], turning into [s] at the end of words like cause. Maybe, this is just a recommendation on how to pronounce English consonants correctly, but if it is not only, then how is this process called in phonetics?
In English and many other languages, notionally voiced obstruents (in other words lenis obstruents) are subject to assimilatory processes involving voicing. More specifically, the 'voiced' obstruents in English - the plosives /b, d, g/; the affricate /dʒ/; the fricatives /v, ð, z/ and /ʒ/ - all undergo devoicing when adjacent to unvoiced sounds, including when next to silence.
They may sometimes be fully devoiced all the way through. However, they tend to be devoiced at the beginning of the consonant if preceded by an unvoiced sound or at the end if followed by an unvoiced sound. In the parametric diagram below the line for vocal fold vibration (voicing) is flat when there's no voicing and wavy when there is.
For the word bond said in isolation, we can see that the beginning of the /b/ and the end of the /d/ are both devoiced. In a more detailed parametric diagram, we might put the little circle currently under the b and the d as a superscript to the right or left of the consonant to indicate which side is devoiced.
If the consonant is at the end of a word but followed by a voiced sound there will be no devoicing. This type of devoicing is an assimilatory effect, meaning that the consonant is taking on qualities from the sounds next to it. If there is no voiceless sound next to the consonant, it won't normally become devoiced.
Let's look at a specific case-study of English [z]. Have a look at the waveform below for the words news items taken from a BBC radio news broadcast:
Here, the dark black fuzz that we see at 3 and 4 (the red numbers on the diagram) shows voiceless sound. The kind of wavy (periodic) line that we can see from the beginning of the word up till the beginning of 2, is voiced sound. That kind of regular repeating pattern there is what gives us the impression that a sound has musical pitch. This, essentially, is what voicing is. Now the transcription for news item is like this: /njuz aɪtəmz/. Notice that there are two /z/s here/. The first is in between two vowel sounds. The second, at the end of the second word, precedes silence. Now, if you look at 1 on the the diagram you will see that the first z is voiced all the way through. That wavy pattern carries on going all the way through the z section. However, if you look at 4, which represents the z at the end of the word next to silence, you will see that it is a messy smudge of black. This is because it is nearly entirely devoiced.
Back to the Original Poster's question, then. For the word cause said in isolation, there is likely to be considerable devoicing of the final [z] because of the adjacent silence. This means that, auditorily, the [z] will likely be voiced at the beginning, but end with an [s]-like quality. Would we want to use an [s] in the transcription here? The answer is no. The segments [s] and [z] do not only have different voicing—they also have different durations, have slightly different articulations, require different amounts of effort in their production, have different intra-oral pressures, different 'pitches' even absent vocal fold vibration, different effects on surrounding sounds and so on and so forth. Even when a lenis consonant becomes devoiced it retains its other lenis qualities
In short, even though we wouldn't transcribe the word cause said in isolation as ending in an [s], the final [z] segment is likely to start with vocal fold vibration, but end without any. It will therefore arguably start as a normal [z] and end with a sound which has an [s]-like quality. This ype of effect is known as assimilation.
The parametric diagram is from the Speech Internet Dictionary.
The waveform is from Swphonectics.com. Courtesy of both Sydney Wood and SWPhonetics.
Both were originally accessed on 21 November 2015.
I am not sure whether cause is really a good example for the process, because
- English usually keeps final voicing and it is a phonemic distinction in English
- It is more probably that the s becomes voiced in word forms like causes, causing and caused (again, for English). Intervocalic s often becomes voiced in modern English.
However, the process is called final devoicing or Auslautverhärtung and it is frequently seen in many languages. German shows it (but not orthographically), Turkish shows it (also in orthography), many other languages have it. Just English isn't among them.
I suggest an alternative question: "Some guy told me that that in English there is a sound [z], turning into [s] at the end of words like cause. What in the world is he talking about?". To be sure, you'd have to ask the guy, but assuming that guy was speaking in a non-technical way (and possibly assuming that he speaks Russian or some other language that doesn't have these spelling problems that afflict Germanic and Romance), I assume he is referring to the fact that the sound (pronunciation) is "z" but it is spelled "s". If you think that the sound "z" should be spelled "z" (I can't argue with that), then there is something additional involved in the spelling/pronunciation of "cause".
Alternatively, he might be an experimental phonetician who knows about this quirk of pronunciations in English (cessation of vocal fold vibration), but since "bus" and "buzz" are pronounced differently most people aren't aware of such details, and I doubt that anybody other than a phonetician would comment on this.
I think there's some confusion here. I don't think the process you describe in the question itself is a general phenomenon. There are special cases where something like that happens, which I'll get to below, but from the comments, it sounds like you're just trying to understand why "cause" and "course" sound different:
well, I'm not sure but, for example, when one pronounces 'of course' there is no such [zs] sound. Btw, this is not the observations of mine, I was told that 'of course' and 'cause' have different sounds at the end
This has nothing to do with any [zs].
Despite both being spelled with "-se", "course" ends in the /s/ phoneme, and "cause" in the /z/ phoneme. This is the same for both the nouns and the verbs, and even under inflection—"courses" is /'kɔrsəz/ while "causes" is /'kɑzəz/; "coursed" is /kɔrst/ while "caused" is /kɑzd/; etc.
Since these are two distinct phonemes, most accents pronounce them with different sounds, [s] and [z]. Even in an accent with Boston-style intrusive R and ɑ/ɔ merging, you'd get [kɔɹs] vs. [kɔɹz].
Many native English speakers are misled by the spelling into not consciously realizing that they're pronouncing two completely different phonemes here, but that doesn't stop them from doing so.
Are there cases where "cause" does end in [zs]?
There is at least one accent that sometimes produces [zs] sounds from final /z/, the Midwestern US accent that pronounces "The Bears" as [də bɛɹzs], as heard in a famous Saturday Night Live sketch. I don't know how this accent pronounces "cause", but I wouldn't be too surprised if it were [kɑzs].
Also, as LjL explains in a comment, cross-syllablic assimilation in cases like "cause to" can lead to something like [kɑzstu].
But in either of these cases, it's still not the [zs] that makes "cause" differ from "course". It's the /z/ vs. /s/ that makes them different; the /z/ being realized as [zs] actually makes them a little less different.