It doesn't sound much like a tuba because it's actually playing in the higher register of the instrument, but it is a tuba.

它听起来不太像是低音号,因为它一般,会在更高音域演奏,但它确实是低音号

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Now the instrument is--that is--playing is a low tuba, a low brass instrument.

现在在演奏的乐器,是低音号,一种低音铜管乐器

耶鲁公开课 - 聆听音乐课程节选