Ultrasound absoprtion by polypropylene beaker for HF acid
Polymers, like other materials as well, attenuate / absorb acoustic waves.  Beside reflection (echo, similar to active sonar), this renders ultrasound suitable for non-destructive inspections.  The extent of absorbing ultrasound depends on several factors (e.g., temperature, density/crystallinity of the sample, deployed frequency) and may be expressed as attenuation and by acoustic impedance.

The following table is part of a lecture about medical diagnostic applications of ultrasound:



(source, slide 5.)

A more detailed listing, specifically addressing polymers, may be found here. From data like 

|---------------------------+----------+---------+-----------+-------------|
| Plastic material          | Velocity | Density | Impedance | Attenuation |
|                           |    [m/s] | [g/qcm] |           |     [dB/cm] |
|---------------------------+----------+---------+-----------+-------------|
| Polyethylene, low density |     1950 |    0.92 |      1.79 |         2.4 |
| Polypropylene, Profax     |     2740 |    0.88 |      2.40 |         5.1 |
| Polypropylene, white      |     2660 |    0.89 |      2.36 |        18.2 |
|---------------------------+----------+---------+-----------+-------------|


some differences in polymers of same name, but different brand (and degree of polymerisation, etc.) become apparent. (Interestingly, Teflon/PTFE is not mentioned in their listing, yet according to this study possess a much higher acoustical impedance than PP.)

Hence, beside chemical resistance against HF, you likely have to test the efficiency of the ultrasound waves passing through the walls of your beaker anyway.