Mon Apr 01, 2024 11:13 am
I think this is a consequence of the Hauptwerk sampling process, and I believe I have commented on it before. With a tutti registration, the mixtures sound different than they would if you just record the mixture by itself. This is most pronounced on a Baroque organ, but it is also noticeable on a Romantic organ. Cavaille-coll composed and voiced his mixtures differently than most Romantic builders. He typically used standard flue type pipes, but as he went up the rank, he boosted the lower pitches of the mixture, sometimes even adding pipes lower than the speaking pitch! So what you're hearing isn't totally the pipes, it's the harmonics of the lower pitched pipes interacting with the pitches and harmonics of the higher pitched pipes.
The other side of the equation is that I feel Cavaillé-Coll didn't intend the mixtures to be used the way we would think. Yes, they should crown the plenum, and be able to be added to other stops for Baroque revival, but he always intended them to be quieter than we habitually have them in Hauptwerk. In most real organs, having the plenum registered tends to starve the mixtures a bit of air, and I think this is in general how they are supposed to sound. So, Cavaillé-Coll voiced them, particularly the lower pitched pipes, to emphasize the harmonics to give that shimmering character. However, it's my opinion that in most Hauptwerk organs, the mixtures aren't quite right. When I program a CODM file, I usually starve the mixtures of air at least a little bit using the wind model, and I think that much improves them.
Now, one drawback of both Metz and Caen is that the mixtures aren't typical for a Cavaillé-Coll in a way. For Caen, Jiri from Sonus Paradisi has his opinions about how mixtures should be, and at least the last time I checked they don't quite sound right to me. For Metz, it has a simpler mixture composition that's absent of so many of the lower pitched pipes (which is what Cavaillé-Coll tended to do towards the end of his life, and which Mutin continued). So, it's not easy to get them to sound *exactly* as Cavaillé-Coll intended.
This is all a roundabout way of saying that it's tricky. What I would do, however, is to voice them to have the volume so that it works with the quieter stops, but then boost the harmonics so that they work with both. It won't be perfect, it's better if they respond to the wind model, but that requires some ODF changes (or a CODM file, as has been alluded to). The mixtures won't crown the plenum as aggresively as you're accustomed to, but trust me that is more what Cavaillé-Coll had in mind. He usually slotted his principals, which boosted the harmonics, and he and many organ builders of the time felt it was better to have good harmonics coming from the principals than to try to augment them with a mixture. As a result, many Romantic plenums have rather understated mixtures (at least in real life, they seem to be always more aggressive in Hauptwerk!), but it isn't unusual for them to have boosted harmonic content, and also in real life, more warmth and interesting interplay between a sounding note and the notes/harmonics of other notes played with it. Some Romantic organ builders actually used more narrow (string scaled) pipes for their mixtures, and I think it was to help boost the harmonic content. The downside though is that they are quieter than their flute scaled counterparts! They tried to raise the pressure on them to make them louder, but this made them more shrill.
Of course, that's my opinion, your mileage may vary! I usually play organs as released by the sample set producer, it's only on my own CODM files that I voice them according to how I think they should sound. The mixtures on Metz have always sounded a bit too aggressive to me, but I still love it, and it's the organ I play the most.