Guides / Chassis / Design for Manufacture

Why an Audio Chassis Rings, and What the Metalwork Can Do

September 21, 2026

Every enclosure has resonances. The question for the metalwork is whether the structure puts them somewhere harmless and damps what is left, or whether it builds a set of large flat plates that happen to be excited by the transformer bolted to them.

Where the energy comes from

In a typical audio component there are only a few sources:

• The mains transformer. Magnetostriction in the core makes the laminations move at twice mains frequency, 100 or 120 Hz, plus harmonics. On an EI transformer this is the dominant source.
• Airborne sound from loudspeakers in the room, exciting the panels from outside.
• Cooling fans, where fitted.
• Mechanical contact: a shelf, a rack, footfall through the floor.

The transformer is the one the chassis designer controls. It is bolted to the structure, so it is coupled directly.

The three levers

For a panel, the resonant frequency goes up with stiffness and down with mass. The amplitude at resonance goes down with damping. Those are the only three things available.

1. Stiffness

The cheapest and most effective lever, because panel stiffness scales with the cube of thickness. Doubling a panel from 1.5 mm to 3 mm makes it roughly eight times stiffer in bending, at twice the weight.

Ways to get stiffness without paying for thickness everywhere:

• A fold or a return on the edge of a sheet part. A 15 mm flange turns a floppy plate into a shallow tray and costs one bend.
• Ribs, either machined into a billet part or formed into a sheet part.
• Breaking up the span. A 400 mm unsupported panel is far worse than two 200 mm panels, so an internal bracket at the midpoint is worth more than extra thickness.
• Curvature. A slightly domed or radiused cover is dramatically stiffer than a flat one. This is a design decision with no manufacturing cost in sheet metal.
• Avoiding large unbroken flat areas. A big flat face is the ideal radiator. Any feature that interrupts it helps.

2. Mass

Mass lowers the frequency and reduces displacement. It is also expensive, heavy to ship and only useful where it is coupled to the thing that moves.

Practical version: put mass where the panel actually moves, which is the centre of a span rather than near the supports. A machined boss or a bonded mass in the middle of a cover does more than making the whole cover thicker.

3. Damping

The one most often skipped and usually the most effective per dollar on thin panels.

• Constrained layer damping is the proper answer: a viscoelastic layer between two stiff layers. Commercial sheets exist, or you can achieve it with a bonded doubler plate over a viscoelastic adhesive.
• A single sided damping pad, such as a bitumen sheet, works but far less well, because the layer is only extended rather than sheared.
• Interface damping: a thin gasket between a cover and its mating flange stops the cover and the chassis ringing against each other, and it is nearly free.
• Sand or shot filling in a cavity is effective and heavy, and it creates an assembly and a sealing problem that is worth being honest about before designing it in.

What this means for the drawing

Damping and stiffening are almost always additions to a part, which means they are additions to the bill of materials and to the assembly instruction.

Things that must be on the drawing rather than assumed:

• Rib dimensions and positions. A rib that is too thin to machine or too deep to fill in sheet metal is a redesign discovered late.
• Minimum rib thickness for machining. On aluminum, a rib thinner than roughly 1 mm at any useful height starts to chatter and deflect away from the cutter.
• Bonded doubler position and adhesive specification, including the surface preparation for bonding. A doubler bonded to an anodized surface behaves differently from one bonded to bare metal.
• Gasket part number and groove or landing dimensions.
• Torque on cover fasteners, because a cover clamped lightly and a cover clamped hard have different resonances.

The honest boundaries

Two things worth being plain about, since this subject attracts a lot of confident claims.

What is well established: a thin, large, flat, undamped panel bolted near a transformer will buzz audibly, and stiffening, damping and decoupling reduce it. This is ordinary mechanical engineering and it is worth doing.

What is contested: whether chassis resonance below the level of audible buzz affects the electrical signal in a solid state amplifier. There are strong opinions in both directions. This site is about manufacturing, so the position here is narrow: design out the buzz you can measure and hear, and treat claims beyond that as claims rather than as specifications.

A short checklist

• No unbroken flat panel larger than roughly 200 mm across without a fold, rib, curve or intermediate support
• Cover material at least 1.5 mm, and thicker if the span is large
• A gasket or compliant strip anywhere a cover meets the chassis
• Transformer on a locally thickened or ribbed area, not the middle of a thin span
• Damping treatment specified as a part number and a position, not as a note saying “add damping”
• Fastener spacing along cover seams close enough to clamp continuously, which also helps shielding

Frequently asked questions

Keep reading

• What an Amplifier Chassis Actually Costs to Manufacture
• Billet, Extrusion or Folded Sheet: Choosing How to Build a Chassis
• Specifying and Machining a Heatsink Extrusion for an Amplifier

This guide is part of Audio Chassis Manufacturing: The Whole Picture, one of four sections in the guide library.