100% 3D printed noise cancelling headphones

This article gives an update on our project to design a pair of noise-cancelling headphones that can be made 100% using an FDM 3D printer.

THESE HEADPHONES ARE NOT CERTIFIED - THEY CANNOT REPLACE AN PROTECTIVE EQUIPEMENT

Gobal view

The headphones consists of two shells covering the ears and a hoop holding the shells to the head.

The shells are made up of two shells (in the illustration: brown) separated by a visco-elastic porous material (grey), a porous material (blue) inside the shell and a gasket (grey).

The seal and the visco-elastic porous material should be printed in TPU, a flexible and visco-elastic material.

The other components should be printed in PLA or another rigid material.

Conception

A. Headphones - head interface

The watertightness of the interface between the headphones and the head is essential for high levels of acoustic insulation. The same applies to earplugs.

If the assembly leaves a free space between the helmet and the head, then the noise will mainly pass through this free space.

Individuals have different head shapes and sizes. The helmet must therefore be able to adapt to these differences, as it is not made to measure.

The shells are linked to the roll-bar by ball-and-socket joints connected to an adjustable slide. So when they are pressed against the head, the shells follow its shape.

In addition, a flexible gasket (grey, in the shape of a ‘3’) allows the shell to follow the shape of the head. Normally, this function is performed by dense foam. The fact that the shell is 100% 3D printed led us to opt for a flexible gasket. The central bar of the seal slides between two walls of the helmet's inner shell, ensuring that the seal is as thin as possible once the helmet is placed on the ears. The outer bars of the seal give it its springy effect thanks to their S shape.

Finally, to reinforce insulation at the joint, a porous resonator (blue) is placed along the path of the acoustic wave entering the headphones via the side of the joint. It's the same concept as putting a strip of porous material on the edge of a window air intake channel. See our blog article on air inlets --> https://sil-et-add.fr/reduire-le-bruit-des-entrees-dair

B. Shell insulation

The shell insulates against noise by applying the double wall principle. This principle is used in many applications: building walls, engine encapsulation, etc.

The shell consists of two solid walls/shells (brown) separated by an open porous material (grey).

The porous material between the two walls is printed in TPU. This material is flexible and viscoelastic. The porous material attenuates the acoustic resonance between the two walls thanks to its resistivity (small pores) and attenuates the mechanical resonance of the walls thanks to its viscoelasticity. In addition, to improve vibration decoupling between the inner and outer shells, contact between the porous material and the inner shell is limited to a very small area. This is achieved by a closely spaced pattern on the surface of the porous material.

Finally, the porous material is printed flat, like a fabric, to simplify printing.

C. The ‘sound of the sea in a shell’ effect

The inside of the shell is covered with a porous material with open porosity (blue).

Its role is to attenuate the acoustic resonance of the cavity formed between the head and the rest of the shell thanks to the acoustic losses it generates. In other words, it's like putting foam inside a shell so that you can't hear the sea when you put the shell to your ear.

For a better understanding of this phenomenon, see our blog article --> https://sil-et-add.fr/entendre-la-mer-dans-les-coquillages

3D printing using molten wire deposition

The entire noise-cancelling headset is manufactured using 3D printing, a method known as Fused Deposition Modeling (FDM).

In its current state of design, it requires 300 grams of PLA and 100 grams of TPU.

The porous materials (PLA or TPU) are not modelled in the CAD. They are generated in the slicing software from their contours and filling pattern parameters.

The porous PLA materials (image on the left) are printed directly with the inner shell, with a fill ratio of 25% (75% porosity) and a layer height of 250 µm.

The porous TPU materials (image on the right) are printed flat, with a fill rate of 50% (50% porosity) and a layer height of 250 µm. The last millimetre in height has a fill rate of 10%.

These properties of the porous materials give them acoustic properties suited to their respective roles.

For more information on the numerical prediction of the acoustic behaviour of porous materials produced by 3D printing, see our blog article --> https://sil-et-add.fr/des-panneaux-acoustiques-en-semelles-de-chaussures

Improvements

Send us your suggestions for improvement using the contact form in the ‘Contact’ section of the site

Ideas for improvement :

• Reducing the amount of TPU needed to reduce product mass and 3D printing costs: refining the porous viscoelastic material

• Widening the joint to improve sealing

• Optimise dimensions to reduce product mass and 3D printing costs while maximising insulation: thickness of shells, porous materials, fill rate of porous materials - using digital simulation