A drug cartridge to dispense an injectable fluid includes a drug cartridge body with an interior cavity, a neck region, and a bottom region. The neck region is opposite the bottom region and the interior cavity of the drug cartridge narrows in the neck region and is wider than most of the neck region in the bottom region. A transducer is coupled to emit sonic waves into the interior cavity towards the neck region, and the neck region is geometrically oriented to reflect the sonic waves back from the neck region towards the bottom region to be received by the transducer.