There is a difference between vowels and vocoids. Referring to the following link*, the contoid/vocoid distinction is similar to but different from the consonant/vowel distinction.
These new words were introduced by Pike to overcome the difficultly posed by sounds which fit the phonetic definition of either a vowel or a consonant but don't function as that in speach. According to his definition, the contoid/vocoid distinction is "strictly phonetic and the other based on function".
According to Pike "Generally, vowels are syllabic vocoids." The semivowels [j] & [w] are vocoids because they are sounds with "no audible noise produced by constriction in the vocal tract". Semivowels are never syllabic so [j] & [w] are consonants.
Here's another good site which examines what is meant by the terms in question. If it seems that what is being said here contradicts what I've explained above, don't blame me. We're talking about the words as they were used by Pike but I don't have my hands on what he actually wrote, I'm just going by what I'm able to gather from these sites. This second site, however, seems to me to give a better coverage of what is meant by the words "vowel" and "consonant".
As they explain the words "vowel" and "consonant" are actually not all that easy to define. They go on to distinguish three senses in which these very old words are used "(they were first used by the ancient Greeks)". There are "phonetic, phonological, and orthographic vowels and consonants", they say. The orthographic vowels of English are easy to define, they are just the letters "a", "e", "i", "o" and "u"; the 21 letters of the rest of the alphabet are the orthographic consonants.
We might use five vowel letters in English but there are about four times that many vowel sounds. There is still a difficulty when it comes to defining what we mean by "vowel sound" and "consonant sound". As I've mentioned above the problem lies in the distinction between phonetics and phonology. They say "We can look at vowels and consonants from the point of view of what they are (phonetics), or from the point of view of what they do (phonology)."
Pike (1943) used the terms "vocoid", "contoid", "syllabic" and "non-syllabic". A vocoid is a phonetic vowel and a contoid is a phonetic consonant. A sound which is both a phonetic vowel and a phonological vowel he called a "syllabic vocoid". A sound which is both a phonetic vowel and a phonological vowel he called a "non-syllabic contoid".
"Sometimes, however, a phonetic vowel behaves phonologically like a consonant and then we have a non-syllabic vocoid, such as /j/ or /w/ in English. ([j] and [w] are vocoids according to Pike's strict phonetic definition.)" In other words, the "w" and "y" sounds in "wet" and "yet",for example, are non-syllabic vocoids. They are phonological consonants in these words because of the way they act. However, because there is no obstruction to the passage of air they are phonetic vowels.
Similarly, "English consonants are normally non-syllabic contoids, in other words consonants from both phonetic and phonological points of view. In the occasional cases where a phonetic consonant behaves as a phonological vowel we have a syllabic contoid. In English, /l/ and /n/ sometimes behave like this." The "en" and the "le" sounds in "sudden" and "muddle" are syllabic contoids. They are phonetic consonants but act as the centre of the syllable (they are the whole syllable) in these words thus they are phonological vowels.
I am trying to describe four Burmese characters included in table of 33 Burmese consonants: /ya./ /ra./ /wa./ and /ha./ . The Burmese script is an abugida and is closely related to the various Indic scripts. The consonants are presented in a table of 7 rows and 5 columns.
r1: /ka./ /hka./ /ga./ /Ga./ /ñga./
r2: /sa./ /hsa./ /za./ /Za./ /ña./
r3: /Ta./ /Hta./ ...
r4: /ta./ /hta./ /da./ /Da./ /na./
r5: /pa./ /hpa./ /ba./ /Ba./ /ma./
r6: /ya./ /ra./ /la./ /wa./ /tha./
r7: -- /ha./ /La./ /a./ --
The Burmese consonants can be combined to form more consonants (ligature), e.g. /ka./ + /ya./ to form <ka.ya'pin> with the sound /kya./. Only /ya./ /ra./ /wa./ and /ha./ can be used as the second consonant to produce <ya.pin.> <ra.ris.> <wa.hswè:> and <ha.hto:>. I usually describe these four as semivowels, but should I describe them as contoids? vocoids? Or do we need another set of new terms to describe them?
I guess it all depends on exactly what you want to say and how you want to say it. Let me explain what I mean by that.
The first thing that you'd have to do is make a distinction between the characters and the sound they represent. Sorry if this sounds as if I'm just stating the obvious but it's a point often over looked. Strictly speaking, what you have is a table of characters, calling it a table of consonants is liable to cause confusion.
Are you approaching the subject from a strictly phonetic or a strictly phonological point of view? Are you trying to look at both aspects? Are you, on the other hand, not intending to go into the kind of detail that would require the distinction between these two points of view?
The words "contoid" and "vocoid" were coined by a linguist named Pike. It's up to you whether you want to use these terms in your writing. There are other ways of making the distinction between what is a vowel or a consonant from a phonetic or a phonological point of view.
You have to consider your intended audience. Perhaps your audience will be unfamiliar with Pike's words. If so, then you have to decide whether to use them or not. If you still want to use them, you'll have to explain their meaning. Perhaps this explanation would only serve to make what you're saying less intellegible.
From what I've managed to gather, the terms "semivowel" and "non-syllabic vocoid" as pretty much synonymous. The question is "How much detail do you want to go into and what kind of terminology do you want to use?" Are you more interested in what these sounds are phonetically or in how they function in the language/syllable? I don't think that there's anything wrong with calling them semivowels.
Thanks for your interest in my question. May I correspond directly with you? My email: email@example.com
Joe Tun (aka) U Kyaw Tun, retd. professor of chemistry, Taunggyi University, Myanmar, now living in Canada.