if you rotate the ruler so a smaller amount of the ruler is hanging off as it goes “Boing-oing-oing…” the frequency will change, and make a more accurate sound.
Or get a jaw harp (jew’s harp, snoopy harp). Available at music stores that sell instruments and accessories. It’s a little metal instrument that you hold against your teeth and it has a prong that you pluck. As it vibrates, you can change the shape of your mouth to vary the pitch. Move your tongue forward and backward quickly to get a very nice “BOING” sound.
Sorry- “accurate”? You mean, it actually sounds like springing a boner? I guess I’d never realized there was a sound. Maybe I should pay more attention next time.