Sellers of wooden phone amplifiers praise the wood’s resonance, while forum posters warn that wood soaks up sound. Both treat the material as an acoustic ingredient. Horn theory treats it as a boundary: the walls are assumed rigid and smooth, and the shape does the work. What separates wood, printed plastic and aluminium is how well each keeps that assumption true on a desk, in the sun and through a dry winter.
What the walls are supposed to do
Webster’s horn equation describes the air inside the horn and takes only one input from the solid part: how the area changes along the path. Kolbrek lists the walls as “perfectly rigid and smooth” among the assumptions behind it. A wall that meets that assumption adds nothing to the sound and takes nothing away. The material shows up only where a wall flexes, leaks, absorbs or changes size.
That turns the choice of phone amplifier material into a short list of practical demands:
- stiff enough at the chosen thickness that the wall does not move with the sound
- a closed, smooth surface on the inside of the duct
- no gaps along joints, so the sound leaves only through the mouth
- dimensions that stay put, above all at the throat and in the slot that holds the phone
- enough mass to keep the amplifier still on a hard desk
Loudspeaker engineers treat cabinet walls the same way. The BBC Research Department measured timber, wood products and other cabinet materials and assessed damping treatments. A phone horn is small and driven by a small speaker, so wall movement matters less there.
Stiffness: how thick each wall needs to be
A flat wall resists bending in proportion to E·t³, where E is the material’s bending modulus and t its thickness. Two walls bend alike when E·t³ is the same, so the thickness a material needs is t = tref · (Eref/E)1/3. The table takes a 3 mm aluminium wall as the reference. That figure is a round number for the arithmetic, not a Braeburn dimension.
| Material | Bending modulus | Wall for equal stiffness | Mass per m² |
|---|---|---|---|
| Aluminium 6061-T6 | 68.3 GPa | 3.0 mm | 8.1 kg |
| Sugar maple, along the grain | 12.6 GPa | 5.3 mm | 3.7 kg |
| Sugar maple, across the grain (tangential) | 0.82 GPa | 13.1 mm | 9.2 kg |
| PLA, printed flat, solid | 3.02 GPa | 8.5 mm | 10.5 kg |
| ABS, printed flat, solid | 1.43 GPa | 10.9 mm | 12.0 kg |
Sources and assumptions: aluminium from the Protolabs data sheet, density 2.70 g/cm³. Sugar maple from the Wood Handbook at 12% moisture content, specific gravity 0.63, taken as 0.71 g/cm³ with the water. Across the grain, Table 5–1 of the handbook puts sugar maple’s modulus at 0.065 of the lengthwise value tangentially and 0.132 radially. PLA and ABS: flexural modulus of printed bars from the UltiMaker data sheets, specific gravities 1.24 and 1.1. The bending modulus is the MOE for metal and wood and the flexural modulus for the printed plastics. The arithmetic ignores Poisson’s ratio.
Along the grain, maple matches the aluminium wall at 5.3 mm and weighs less than half as much. Across the grain the same wall needs 13.1 mm. A horn carved from one block turns through both directions, so its weak spot is wherever the wall runs across the grain. Printed PLA needs 8.5 mm and ABS 10.9 mm, and both walls end up heavier than the aluminium they replace.
UltiMaker printed its test bars with 100% infill. A horn printed with sparse infill has less material in its walls and will be less stiff than the table suggests. The comparison holds for walls of the same span and matches stiffness, not resonance.
Damping: why a wall should not ring
The Wood Handbook describes internal friction, the way a material turns vibration into heat. Expressed as logarithmic decrement, it runs from about 0.1 for hot, moist wood to less than 0.02 for hot, dry wood, with cool wood in between. Wood turns some of any wall movement into heat instead of radiating it. Aluminium has little internal friction and relies on its stiffness and mass instead.
A piano soundboard, usually spruce, does the opposite job. A soundboard is thin, wide and driven through a bridge, and its purpose is to vibrate. A horn wall should stay still and leave the air inside to do the work. Picking a wood because it “resonates” asks it to do the reverse of what a horn needs.
Moisture: the dimension that moves
Wood swells as it takes up moisture and shrinks as it dries. The Wood Handbook puts the movement tangential to the growth rings at about twice the radial movement, while along the grain it is slight. At 21 °C, the handbook’s sorption equation gives an equilibrium moisture content of 6.2% at 30% relative humidity and 12.0% at 65%. Take that as the swing between a dry room and a humid one.
The handbook estimates shrinkage as S = S0 · (MCfs − MC)/MCfs, with MCfs about 30%. Sugar maple shrinks 9.9% tangentially from green to oven-dry, so a swing of 5.8 percentage points moves it by 1.91%. A body 60 mm wide along the growth rings, as across a flat-sawn board, changes by 1.15 mm. A slot cut 10 mm wide in the same direction opens or closes by 0.19 mm. Across the rings the movement is about half, 0.56 mm over 60 mm. Black walnut, at 7.8% tangential shrinkage, moves 0.90 mm over the same 60 mm.
A slot that holds a phone snugly in one season can bind or rattle in the other. The handbook notes that varnish or lacquer slows moisture change but does not prevent it, so a finished wooden body still needs clearance. Aluminium expands by 23.6 µm per metre per kelvin. A 60 mm aluminium body warmed by 20 K grows by 0.028 mm.
Heat and layers: where printed plastic gives way
UltiMaker gives a heat deflection temperature of 58.8 °C for printed PLA and states that it is not suitable for parts exposed to more than 59 °C. Its ABS reaches 86.6 °C, with a stated limit of 87 °C. A PLA horn on a desk is well inside that limit. Near a heater outlet or on a sunlit car dashboard it can reach it, and ABS has the extra margin.
Printed parts are also weakest across their layers. Upright test bars of PLA break at 33.1 MPa, against 45.5 MPa for flat ones, and ABS falls from 33.9 to 19.0 MPa. Each time the phone goes in, it loads the lips of an upright-printed slot across the layers. Printing the part so the lips lie flat, or making them thicker, reduces the risk.
The layer steps themselves are acoustically negligible at this size. UltiMaker’s test profile used 0.15 mm layers, while the wavelength at 10 kHz is 34.3 mm, about 229 times longer. The same argument applies to the tool marks on a milled duct, as the page on machining the aluminium body explains.
Aluminium: what the original bodies used
The Braeburn record describes an “all aluminum CNC unibody construction”, cut in machining centres the firm owned and operated. The numbers above show why that suits a horn: the thinnest wall for a given stiffness and dimensions that barely move with temperature. At 8.1 kg per square metre the aluminium wall is heavier than maple of equal stiffness along the grain and lighter than printed plastic.
A coating changes the inside dimensions of a metal horn more than heat does. Paint and powder have thickness and build up on edges, as the page on finishes shows. The same applies to varnish in a wooden throat. The guide to horn flare and cutoff shows how to size the flare and the mouth in the first place. The passive phone amplifier guides place the material among the other choices.
| Material | Strength | Watch for |
|---|---|---|
| Machined aluminium | Thin stiff walls, stable size, no seam in a unibody | Coating thickness in the throat |
| Hardwood | Stiff and light along the grain, internal friction | Thin walls across the grain, a slot that moves with humidity |
| Printed PLA | Exact geometry from a file, easy to iterate | Thick walls needed, heat limit near 59 °C, layer direction |
| Printed ABS | More heat margin than PLA | Less stiff than PLA, so walls even thicker than PLA’s |
Common questions
Does a wooden amplifier sound warmer than a plastic one?
Not because of the wood itself, if both walls are stiff and the duct has the same shape. The shape sets the frequency response. Differences appear when one wall is thin enough to vibrate or when the two horns have different geometry. A comparison is only fair with the same horn shape in both materials, measured bare against cradled at the same distance.
Is a 3D-printed phone amplifier as good as a wooden one?
It can be, if the walls are thick and printed solid. Printing gives exact control of the flare, which is harder to carve by hand. PLA needs walls of about 8.5 mm to match 3 mm of aluminium in stiffness, and it softens near 59 °C. ABS tolerates more heat but needs even thicker walls.
Should a wooden amplifier be varnished inside?
A thin, smooth finish keeps the inside of the duct closed and slows moisture change. Keep it thin in the throat, where every layer eats into an area of only a few tens of square millimetres. Finish all faces, and leave clearance in the phone slot, because a finish slows moisture change without stopping it.
Why were the Braeburn cradles machined from aluminium?
The record states the construction, an aluminium unibody cut on the firm’s own CNC machines, and the finishes offered. The engineering case follows from the numbers above: a thin stiff wall, dimensions that hold and no seam along the duct. Owning the machines also allowed a new body for each phone generation without new moulding tools.
Sources
- Wood Handbook: Wood as an Engineering Material, USDA Forest Products Laboratory, FPL-GTR-282. Chapter 4: Tables 4–2 and 4–3, shrinkage, sorption equation. Chapter 5: Tables 5–1 and 5–3a, internal friction.
- UltiMaker PLA technical data sheet. Printed-bar moduli, strength, heat deflection.
- UltiMaker ABS technical data sheet. The same for ABS.
- Protolabs, Aluminum 6061-T651/T6 material data sheet. Modulus, density, thermal expansion.
- H. D. Harwood and R. Mathews, “Factors in the design of loudspeaker cabinets”, BBC Research Department.
- B. Kolbrek, “Horn Theory: An Introduction, Part 1”, audioXpress. Assumptions of horn theory, rigid walls among them.