In Ethereum, ultrasound money refers to a proposed upgrade to the Ethereum protocol's fee market mechanism. The upgrade is designed to improve the predictability and stability of transaction fees by automatically adjusting the fees to balance the supply and demand for block space. This is achieved by introducing a "base fee" that is adjusted based on the amount of block space used by recent transactions. The goal of ultrasound money is to create a more efficient and fair fee market that incentivizes efficient use of block space and discourages spam transactions. The proposal for ultrasound money is still in development and has not yet been fully implemented.