Mystim E-Stim Ribbed Urethral Sound The Tickleberry Finn is Mystim’s sound with ribs for extra sensation, now available @ Mister B. This slim and ribbed urethral sound is perfectly suited for the special stimulation of the penis. With some practice its 8 mm can be inserted easily into the urethra, and then create a stimulation like you have never felt before. As with all urethral insertables, please use plenty of (sterile) lube and work hygienically. Length 19.5 cm, diameter 8 mm.