Effect of Rib Features on Diaphragm

One common feature observed on many headphone drivers are the 40 or so ribs formed into the diaphragm body.  The purpose of these may be to add structural rigidity to the diaphragm, which is only around 40um thickness.  Normally, such features mitigate the effects of hoop stress as the diaphragm is moved forwards and backwards.

 

The frequency response shows the driver only as if mounted in an infinite baffle and measured at 10cm distance.

 

The animations show how the diaphragm (other driver features hidden) of the headphone transducer deforms at the upper range of frequencies and the influence of the 40 rib features.  The ribs offer some reinforcement preventing bending in torus part of the diaphragm, which is effectively the suspension.

At resonance (~200Hz), this forces bending to occur towards the outer edge of the diaphragm which means that a greater mass of diaphragm oscillates - this is evidenced by the slightly lower resonance frequency where a greater number of ribs are included.  A possible further benefit of this stiffening is to improve the symmetry of the effective radiating areas between the forward and rearward strokes, which are known to be different in cases where the suspension component is large and is a significant source of harmonic distortion in headphones.

 

40 Ribs on Diaphragm

 3.9kHz  5.2kHz
  10.3kHz  16kHz

 40 ribs appear to resist non-axisymmetric bending deformations.

 20 Ribs on Diaphragm

 3.9kHz  5.2kHz
  10.3kHz  16kHz

 20 Ribs means that non-axisymmetric bending can occur at the upper range of frequencies.

10 Ribs on Diaphragm

 3.9kHz  5.2kHz
  10.3kHz  16kHz

 10 rib features appears to produce rather chaotic bending, where higher order axisymmetric and non-axisymmetric resonance can be observed in the diaphragm.

Ribless Diaphragm

 3.9kHz  5.2kHz
  10.3kHz  16kHz
 

No rib features results in little excitation of non-axisymmetric bending, but the axisymmetric modes are of far a higher order compared to the case with 40 ribs.

Reference:

[1] Rowan Williams and Kelvin Griffiths, "Investigating Headphone Driver Modelling Using the Finite/boundary Element Method", Proc. IOA Reproduced Sound, 2014