What instrument is unique to brazil?
how would you describe the sound of this instrument?
why is it considered a percussion instrument?

Actually, there is more than one, as Brazil has a mix of cultures (Indigenous, European, African) and this mixture produced many new instruments.

One example is atabaque, which can produce a variety of sounds: from higher to lower (but not very high,) and it can produce sharp and more fluid, sustaining sounds, too.

It's a percussion instrument because the sound is produced  as a result of striking something (and not for example of a flow of air)

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