Thanks for the great advice, it made me realise I should have been more specific from the outset and asked the same question in a different way.
Apologies, should have made the question clearer and appreciate if you could have a little patience with this newbie.
The music is Bluegrass (no drummer) for a Appalachian dance group so what I'm trying to achieve is a nice strong thumpy pizz sound that clearly defines the beat. I can achieve this at slower tempos by bouncing along to the music like a demented "Gus Honeybun" (showing my age there) but as the tempo hits about 220 bpm
the notes become too staccato and a dull thud rather than a nice thump. I did wonder if it was because my left hand was too slow to allow the notes to ring out before moving to the next and have been practising to improve this.
Maybe as The Jaywalker and others suggested I'm trying too hard to get a "prominent and loud front end attack" and the issue is that amount of movement in the right hand at high tempos. So the question should have been, understanding faster will be acoustically quieter what techniques do people use to get a thumpy pizz sound and how to maintain that thump at high tempos.
Thanks again
Ian