Back-EMF can do that sorta

Decoders with Back-EMF can to some extent sense the load on the locomotive, and the sound volume is usually tied to that, so an engine with such a decoder will get quieter going downhill than uphill, but normally you can't easily modify volume on the move like you are thinking about.  That may be something for new decoder and/or controller designs to take into account.

Down grade driffting

You didn't say what decoder you are using so I'm not sure if this will help.

I'm using Tsunami and while it is true that you can set up the decoder to sense the load on the loco and it will chuff louder under load and softer when the load is light it is still not quite the same as a real loco.

This is what I do, sometimes, when I'm operating. CV 131 is the exhaust volume and you can adjust it on the fly, not easily mind you, but it can be done by using the programing on the main function and changing CV 131. I normally have my loco's exhaust set around 60, when I'm at the base of my "big hill" I program CV 131 to 200 and hit enter just as the engine starts to bite into the grade and notch the throttle down slowly a bit. The sound change is a tad sudden but the overall effect is quite convincing.

Conversely when going down the hill I set CV 131 to 10 and I get just the slightest hint of a chuff as it drifts downgrade. Of course you have to remember to set CV 131 back to it's normal setting when you reach the top/bottom of the grade.

It's a bit convoluted I'll admit but it works. I often use this when we are shooting video and you really want to "play with the sound.

I hope this is of use.

Check LokSound and Zimo decoders

Dear Peter,

The "drifting" effect which you seek is much more complex than just "tweaking a CV",
(although as Rick notes, adjusting the "chuff" volume in realtime could kinda simulate the effect).
it involves being "designed in" as a behaviour within the decoder itself,
as well as specific sound files to match.

From our euro friends, both LokSound and Zimo decoders have "Drift" effects IIRC,

From the LokSound V4 decoder manual, Page 9

http://www.esu.eu/en/downloads/instruction-manuals/digital-decoders/

Fron the LokSound Select manual, Page 9

Might be worth checking in with the LokSound group on yahoogroups?

Happy Modelling,
Aim to Improve,
Prof Klyzlr

Steam chuff

Check this out

the best so far

DC Sr