Hi,
As others have pointed out if there are harmonics in the wave you might be hearing them instead of the base frequency. If it was a square wave, it would have harmonics that go way up into the kilohertz or higher even with an imperfect transducer.
Your wave looks like it has at least some even harmonics, and the first even harmonic would be at 15Hz, which is just a little below the "standard" 20Hz usually given for human hearing. 20Hz is just a guideline though not a set exact figure that applies to every human that was ever alive on the Earth. The first odd harmonic is at 22.5Hz, which is clearly above that standard, and the volume has something to do with it too as louder sounds will be easier to hear.
It's very hard to get a perfectly clean sine wave out of a transducer however, the transducer would introduce some distortion on it's own even with a perfect amplifier and perfect electrical sine wave. You would probably have to make a special transducer that was designed to have extremely low distortion.