Summary - Basic System


  • Examining the performance of the drivers in a baffle provides useful information about its fundamental performance - this may generate comparative information for benchmarking or some QC function
  • Putting the driver into the headphone assembly alters the performance dramatically and forces us to think about how the driver contributes to performance differently to the infinitely baffled state, which is more relevant to direct radiator loudspeakers
  • In a closed back headphone, thinking simply, the driver is interspersed between a sealed front volume and a sealed rear volume.  The sound pressure is determined not by the volume acceleration supplied by the driver diaphragm, but by its volume displacement.  This means that sampling pressure in the front acoustic volume (where the ear goes), we can see that the bass efficiency is high because the sound waves are not radiated away into free space, but contained within this front volume.  Without leaks, this efficiency continues down to DC (0Hz)
  • The following models also illustrate the use of volume absorbency models to simulate damping in the frame vents and leaks
  • The volume of the front enclosure space where the ear sits is inversely proportional to bass efficiency.  The shape of this volume (round, elliptical or other) determines the density and distribution of resonances at higher frequencies
  • Looking independently at the influence of the front and rear enclosured volumes.  When a rear enclosure is added to the driver (radiating forwards into half space), the output is that of a 2nd order HPF where the resonance is significantly shifted higher due to the rear air acting mechanically as an "air spring".  Looking at adding only a front volume (and sampling SPL within this) with no rear enclosure, we see that again the additional stiffness of air due to the enclosed front volume raises the resonance, but this time the LF SPL is not diminished
  • The role of damping in the frame vents becomes clearer in the context of the full system.  Here it is observed that the bass port activates when frame vent damping is applied and if the front volume is leaky, a boost at ~60Hz can be seen in the frequency response.  This damping also reduces the quality factor of the driver resonance which in the system is at ~900Hz