I'm really not sure of a better way to phrase my question. If the tenor and alto voices are both on a B, and the tenor voice moves down to A, does it count as parallel octaves if the soprano voice moves down to an A on the next downbeat? (I've attached a screenshot of the measures in question.)
Well, it depends on your classification system. The aural result in this case would definitely sound a bit like parallel octaves, even though the octaves only occur by the confluence of bits from three different voices.
This occurs in this particular example because there's a voice overlap in soprano and alto, where the soprano leaps below the last alto note. That's generally not good voice-leading in four-part chorale style (though it may be acceptable in other stylistic contexts).
There also is what's known as direct fifths or hidden fifths between the soprano and tenor, where they both move in the same direction toward an octave, which will accent the octave a bit. But they are generally not considered a serious problem unless between outer voices.
So, basically, you have the combined problems of voice overlap in S-A with the direct fifths in S-T, which end up also creating parallel octaves within the texture as a whole. Whether or not you want to call these truly "parallel octaves" (which often has the implication of two specific voices moving in parallel), there are enough voice-leading problems to make some changes here.
Do parallel octaves “count” across multiple voices?
Parallel motion is between two voices.
But it isn't clear what you mean by "count."
By definition the movement in question is not parallel octaves and in that sense it doesn't "count."
If "count" means "something to avoid," then the question is about style. For that concern you need to both identify the motion type properly and understand what motion is considered good in a particular style.
From Kostka, Tonal Harmony an example of misidentified parallel fifths which would be acceptable for 18th century style voice leading...
So, the movement you asked about is not parallel octaves, but there are other problems with voice leading in your example:
- parallel octaves between bass and soprano in the first two chords
- unbalanced/incomplete chord tones in the second chord
- crossed voices between soprano and alto
- direct fifths in outer voices to the
It's a drag to write something, then proof it against of voice leading prohibitions. I suggest getting a few harmony or counterpoint textbooks and review the part writing procedures. Better books (for me that was Piston's Harmony first edition) will what you should do and why rather than list prohibitions. Procedures like...
- voice chord completely
- hold common tones
- move voices by step, etc.
Combine that with a review of the rule of the octave (an old method of teaching harmony, short and simple but goes a long way for understanding) and you should get a handle on 18th century voice leading and harmony.