The acoustic through-metal channel is characterized by strong multipath components caused by the echoing of acoustic energy within the channel. Transmission at high data rates is therefore difficult to achieve with traditional single-carrier systems. This paper applies an adaptive bit-loading technique to the transmission of digital signals through metal barriers using ultrasonic signaling. The multi-carrier approach discussed here allows us to mitigate severe frequency selectivity of the through-metal communication link and improve spectral efficiency by exploiting the stationary nature of the channel. Experimental performance of bit loading is examined in an ultrasonic through-metal channel. Our results indicate that non-power-scaled rate adaptive bit loading significantly outperforms non-adaptive modulation. Adaptive bit loading was shown to adhere to a strict BER constraint while increasing data rates by roughly 240% from values of 5 Mbps to approximately 12 Mbps when compared to narrowband modulation techniques.