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I'm trying to find a way to demonstrate to high school and middle school students the importance of the tuba in their ensemble. I want to show them that they really make a difference. I thought maybe find a recording of a phrase without tuba and the same phrase with tuba but that's all I've come up with.
Does anyone have any ideas on how to convince the students that their role in the ensemble is crucial?
Thanks

When I was that age my teacher used to tell us we were the "backbone of the band". He would tune the band from the tuba(s) and took a lot of trouble to tell the band why, as well. We were a fired up group of tuba players in an otherwise typical program.

If you have a stereo system with a subwoofer, just play it for the kids with and without the sub turned on. Then tell them that the Tuba section is the band's subwoofer.
That should illustrate the point nicely.

If you already have a tuba section, challenge them with a piece that has a tuba 'solo' or a dixie quintet section featuring a tuba.
There are some good disney movie transcribed for MS or HS band pieces that have these sections.
There are a number of pop music arrangements done from the TV Show 'Glee' that have decent bass lines. Sure they were originally written for e-bass and transcribed for band so you have to be careful with those, but some pieces are fun for the tuba section and not doubled by other instruments in the band. Since the tuba line is not doubled, you can push them to bring it out.
For HS/MS kids, this can be more fun than alot of other band lit that just give the tuba player an om-pah-om-pah meaningless bass line, or worse, a transcribed orchestra piece where the tuba players hold the tonal note for 4-8 beats at a time with huge stretches of rest between notes (string bass parts).

why no invite a local artist in to play with the group for a rehearsal to demonstrate and evangelize?

Try this online recording of Shostakovitch 5, First Movement about the ten minute mark. That should clear it up.

http://csosoundsandstories.org/new-on-c ... ne-lamsma/

Try playing them some septet music. Most of the time:

three trumpets = voice 1
three trombones = voice 2
one tuba = voice 3, continuo, tuning reference, metronome...

https://www.youtube.com/watch?v=SaOYCfjs5oE