What is the reason that I don't lose the sense of tonality in these measures

Question

The question itself is rather subjective hence the 1st person pronoun. However, I believe that many of you reading this question agree with me and hear 'Bb major'. If you hear it in a different way, please comment on it. That could also be a survey.

The following measures are from the last movement of Trilogy Sonata by Philip Glass. It's an arrangement, so the piece was not written for the piano originally. But, it's not just 'another' arrangement; the composer himself knows about this arrangement. I also re-wrote it on Sibelius and played with the help of NotePerformer, and I captured my screen. So you can listen to the measures on loop.

As you see, we have D natural and D flat in the same measure, and the same 'trick' continues for two measures. I first listened to the piece without the score, then when I looked at the score I was simply shocked. We have both D major and D flat in the same measure -which makes the chord both Bb major and Bb minor- and I heard (and I still do) it as Bb major, despite the D flat tone(tone=note).

How is that possible? I think the primary reason is the excellent voicing and the fast tempo. But I've never encountered such a thing before -except in Glass's other works.- How can we explain this aesthetic approach? What is the reason that we don't lose the sense of tonality here?

Here's the video

And the picture (the source is Philip Glass The Piano Collection, Wise Publications):

Answer 1

I think you've hit the nail on the head with "voicing".

The tendency for the human ear is to focus in on the highest pitches. Because of that tendency, the passage aurally breaks down into a "lead part" (what would be the melody in a different style of music) and a "supporting part".

Here is how my ear is focused in this passage from mm. 67 – 70:

The Bb major dyad of Bb and D-natural is so well established by

• register,
• rhythmic regularity, and
• metric regularity

that the ear readily accepts the entire passage as Bb major. The remaining notes either support Bb major or, in the case of the Db's, don't occur in a way that competes with Bb major.

Note that while topmost register always includes D naturals, the Dbs are always lower. Also, in the right hand, the D-naturals occur every other beat, which the Dbs occur only every third beat. Similarly, in the left hand, the D-naturals occur on every beat, while the Dbs occur only every other beat.

Glass's technique here reflects an organizing principle in common-practice Tonality: sufficiently establish a key, and then one can wander from it without loosing the central tonality. Establish C major, for example, and one can throw in some G major chords without the ear "changing key" to G major.