The timpanist is tuning his kettledrums before the orchestra performance.

kettledrums 是 kettledrum 的复数形式。kettle 意思是"水壶"，drum 意思是"鼓"。kettledrum 字面意思是"壶形鼓"，指一种铜制半球形鼓，也就是"定音鼓"。