Sandpaper sound on Stainz

Hello all,
Can anyone tell me what voltage is needed to start the "sandpaper"sound on the Stainz?
Also can anyone tell me what voltage will the sound stop?
I am running on 10 aa NiMh rechargable batteries, sound will run on full charge - about 14 volts, but sound goes to a electrical high pitch screech as the voltage reduces.

Any ideas?

Cheers from Andrew

Sandbar & Mudcrab Railway

There isn't by any chance a secondary battery needed for the sound unit? The analog boards in the sound freight cars use a 9v battery to keep the sound going when the track voltage is too low (below 9v, I guess). The sound gets pretty funky at low speed without the battery.

In my steam-sound-power-car I tried to eliminate the 9v battery by connecting to my NiMH pack but the sound board was not at all happy with that arrangement. It sounded a bit like what you described.

My Stainzes run pretty fast at 14v - too fast for 'reality', so I'd hope the sound unit would work at lower voltages.

I simply hooked up the batteries after disconnecting the track power. I thought that the basic sound would still work as before.
Even on the track power the Stainz needed a bit of speed to get the sound working, but then I could slow the loco down & keep the sound going.
I have fitted out the Stainz with the Hobbyking system which controls the speed & direction.
Every thing works great except for the sound problem. May be time for another "mylocosound" unti.
Or even spend some money on the new Revolution reciever with the sound included in the unit!
Depends on price!
Cheers
Andrew