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Apprentice answer

Why is round spiral duct amplifying low-frequency fan noise

3rd YearRound, Spiral & Flat Oval DuctStandard Correction

Large thin-wall round duct can resonate. Check vibration source first, then add approved acoustic isolation, wrap, or stiffening.

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Plain-English answer

A big spiral barrel can behave like a drum skin when fan rumble, vibration, or pulsing air hits it. The fix is not always inside the duct.

Check equipment isolation, flexible connectors, supports, and whether the duct skin needs external acoustic wrap, damping layer, stiffening rings, or a silencer/detail change.

Ask Foreman

The spiral main at [location] is amplifying fan rumble like a drum. I checked supports and isolation. Do you want acoustic wrap/stiffening or equipment isolation reviewed first?

Verify before acting

Use this as training guidance. Foreman direction, approved drawings, project specs, manufacturer instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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