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I am a range of lengths of waves

Or else how often, marching, braves

No war but brings up spirits as in school,

And not at all small, find that you'll

Seem quite together, not a pop it has

But all that jazz.

'You are a range of lengths? From short to long?'
'Yes, and every length in-between.'
'No wonder you have a short temper and a long face.'
'Be quiet!'

Riddle Answer A Band

Riddle Explanation
I am a range of lengths of waves or else how often - a range of wavelengths or frequencies used in radio transmission.

marching, braves no war but brings up spirits - a marching band.

as in school - a school band.

and not at all small - big band.

find that you'll seem quite together - a musical ensemble.

not a pop it has but all that jazz - a jazz band, while other genres often take different terms, such as a pop group.