Why is round spiral duct amplifying low-frequency fan noise?

Field 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.

What to check first

• Confirm the noise source: fan, vibration, air velocity, or loose support.
• Check flex connector and equipment isolation.
• Look for loose hangers or duct skin vibration.
• Verify approved acoustic treatment before wrapping.
• Retest after each correction to avoid chasing noise blindly.

Why it matters

Acoustic resonance can make finished spaces sound cheap and trigger owner punchlist complaints.

Ask foreman

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Final direction belongs to the foreman, approved drawings/specs, manufacturer instructions, pressure/material schedule, employer policy, and AHJ/code requirements.