Racetrack Drive Unit - Simulation Results

Simulation of a Racetrack Loudspeaker

 

Below are results at 1 metre distance for the racetrack drive unit, sampled at 10 degree intervals between 0 degrees (on axis) to 90 degrees (on the plane of the infinite baffle).

Frequencies of interest were selected to illustrate the structural and acoustic behaviour across the intended bandwidth of the drive unit; 83Hz, 370Hz, 616Hz, 1.56kHz, 3.74kHz and 4.4kHz.

The charts show that the directional characteristics of the racetrack loudspeaker are different when the output is sampled from 0 to 90 degrees across the major axis than across the minor axis where the off axis level drop-off begins at a higher frequency.  This is expected as larger radiators become directional at lower frequencies - the racetrack therefore has a fan-shaped directivity pattern which may be a useful attribute for acoustic product designers.


 

83Hz - Fundamental Resonance

Structural Response

 

 

Nearfield Acoustic Pressure Amplitude and Phase

 

 

 

 

 

 

 

 

 

Here, the motion of the loudspeaker is that of an oscillating rigid body suspended on its compliant spider and surround.



Response at 370Hz

Structural Response

 

Nearfield Acoustic Pressure Amplitude and Phase

 

 

 

 

 

 

 

 

 

The structural response indicates a resonance where the cone contribution along both the major and minor axes is destructive acoustically, causing a dip in the response at this frequency.


 Response at 616Hz

Structural Response

 

Nearfield Acoustic Pressure Amplitude and Phase

 

 

 

 

 

 

 

 

 

As frequency increases, the near-field acoustic field becomes complicated by numerous components breaking up into increasing orders of resonance.  This isn't an effect that repeats between different models of racetrack loudspeaker, but changes with geometry, material parameters, loadings, restraints etc. and therefore creates a scenario highly resistant to analysis methods other than finite element modelling.


Response at 1.56kHz

Structural Response

 

Nearfield Acoustic Pressure Amplitude and Phase

 



 

 

 

 

 

 

The resonance in the minor cone axis causes a slight rocking of the voice coil.  Typically, the motor unit gap is reduced to minimum clearance to make most efficient use of expensive magnet materials and avoiding mechanical contact between the voice coil and the motor metalwork is clearly important to prevent audible distortion and failure of the driver.  A risk assessment can be made using the data from a finite element model and the model can be "re-balanced" to improve a worst case of out-of-axis movement.


 Response at 3.74kHz

Structural Response

 

Nearfield Acoustic Pressure Amplitude and Phase

 




 

 

 

 

 

 

Some directional behaviour is evident in the acoustic display plane.  It is also interesting to observe the "mixing" of the near-field polarised regions on the cone surface and the resultant sound-wave progressing towards the microphone array.


Response at 4.40kHz

Structural Response

 

Nearfield Acoustic Pressure Amplitude and Phase

 




 

 

 

 

 

 

As frequency continues to increase, the structural response continues to increase in complexity.  The modal behaviour on the long axis of the surround is of a high order due to the high compliance of the component.  In contrast, the dustcap being a small component with a lightweight, stiff material starts to break up into its first "breathing" mode.