If I may ask, how would one go about making an alveolar tap voiceless as opposed to voiced?
What can I say in reply, except that you switch off the voicing as you make the tap? But for some people perhaps that is easier said than done.
One of the first things I teach any beginners’ phonetics class is basic consonant classification: Voicing, Place, Manner, and how to detect and control each of these. I usually start with hearing and making the difference between voiced and voiceless sounds.
I would demonstrate each point myself, before asking everyone in the class to perform likewise. Everyone has to join in making what might seem to be silly noises.
First, make a vowel sound, for example [ɑː]. Feel the vibration of your vocal folds by putting your thumb and fingers on your Adam’s apple (the outside of the larynx). Then make a voiceless sound, for example [sss], and feel the larynx again. Notice the difference.
More dramatically, cover your ears with your hands. Say [ɑː] again and notice how the buzzing fills your head. Do this again with [sss] — no buzzing.
Then alternate a pair of sounds such as sss — zzz or fff — vvv. (These pairs are fine if you’re a speaker of English. If not, or if your language doesn’t have these sounds, we may have to use other ones.) Do this as you cover your ears, and note the difference inside your head.
Then make mmm. Is it voiced or voiceless? Is there buzzing in your head as you say it? (Yes, there is, It must be voiced.) What about lll? and ʃʃʃ?
Then make the same sound mmm, but without the voicing. Just breathe out through the nose, with the lips firmly together. You’re doing m̥m̥m̥. Alternate mmm — m̥m̥m̥.
Do the same with n — n̥n̥n̥. If you had a cleft palate you might pronounce six as n̥ɪʔn̥. Do it!
Try ɑpɑ ɑpɑ ɑbɑ ɑbɑ. It may be more difficult to detect voicing or voicelessness here, because the consonantal articulation is much quicker: we just bring the lips together for a moment, then release them. (In m, on the other hand, we hold the articulatory position for a longer time.) Do ɑkɑ. Is the k voiced or voiceless?
You can try other experiments. Any consonant you can make voiceless you ought to be able to make voiced, and vice versa. What is the voiced counterpart of k? What is the voiceless counterpart of ð?
If you can make a voiceless velar fricative, xxx, then simply add voicing to get the voiced counterpart, ɣɣɣ.
Say ˈɣala (Greek for ‘milk’), and ˈlweɣo (Spanish for ‘then’).
Similarly for every voiced sound you can make: just switch off voicing to get the voiceless equivalent. Try a voiceless [l̥] Do ɑlɑ — ɑl̥ɑ. The only special difficulty with the voiceless tap ɾ̥ is that the sound is extremely short. As d is to ɾ, so t is to ɾ̥.
I don’t think you can usefully discuss phonetic classification unless you have mastered this sort of thing: not just intellectually, but practically.