Summary - Driver

Observations:

  • Measuring the driver in a similar manner to a conventional loudspeaker, in a large flat baffle radiating into free space, permits easy comparison using lumped parameter modelling and returns basic prediction of SPL, displacement and electrical impedance.

  • The driver without damping in the frame vents (felt-paper type covering) has an obvious resonance at ~3kHz resulting in a step up in output.  The bass port appears to have very little influence.

  • Adding the damping layer to the frame vents increases the contribution from the bass port - its effect becoming more marked at the acoustic resonance formed between the rear air volume and the bass port.  Blocking the frame vents completely produced a strong null where the acoustic resonance of the bass port and air volume behind diaphragm, so it is clear that an intermediate impedance sufficiently activates the function of the bass port, but suppresses undesirable mid-band nulls.

  • Looking at the number of frame vents, reducing from 13 to 8 holes revealed a hidden degeneracy between the resonance caused by the frame vents and that caused by the bass port.  Reducing the number of holes reduced the frequency of the corresponent resonance and separated this from the upper bass port resonance, which appeared as a separate feature.  Damping the reduced number of frame vents confirmed which resonance was affected.

  • Looking at the bass ports, the number of holes and their dimensions were shown to determine the bass port tuning at both an upper and lower frequency.  The experimental driver has a plurality of bass port holes stamped into the motor - a greater number of these being activated produced increasing resonance frequencies and greater SPL contribution.

  • A simple absorbency model used to apply damping to the frame vents in the FE model seemed to exhibit similar behaviour between simulated and experimental cases however it was kept in mind that the drivers were different in a number of ways, which affected specific tunings of the acoustic system and a degree of modal degeneracy compunded the interpretation of certain features.  It was therefore difficult to conclude on the accuracy of the model and ideally a new model, more precisely representing the experimental driver could be developed.

  • An alternative viewpoint was to examine the electrical impedance of the experimental driver, which would reflect the driver behaviour as variations in voice coil mobility under the influence of the coupled acoustic load.  From these measurements, we were able to observe the reduction in diaphragm velocity due to the presence of frame vent damping which resulted in a 4 * decrease in Zmax and a noticable drop in fundamental resonance frequency.  Further investigation using a vacuum chamber, capable of a differential pressure of almost 1Bar, it was observed that it was not possible to remove sufficient air to obtain equivalence in electrical impedance between the driver with and without frame vent damping.  It was anticipated that when air was removed, the impedance curves measured from the 2 driver variants would converge because the damping was associated with the friction of air as it oscillates in the frame vent.  It was interesting to note that even small residual airloads can affect the driver mobility.  To confirm equivalence between the drivers, they were measured again with all vents blocked resulting in closely matched data - so in electro-mechanical terms, they were equivalent.

  • Electrical impedance measurements were also used in tandem with acoustical measurements to determine the influence of the bass port when the frame vents were open and closed.

  • Looking at the diaphragm, the influence of the rib features formed into the diaphragm was examined by reducing their density and observing the frequency response.  The ribs may have multiple functions.  They could be influential in controlling the diameter on the diaphragm where bending can occur, a forced hinging that may increase and equalise the radiating areas between forward and rearward strokes at low frequency, resulting in lower resonance and less distortion.  At larger excursion, another purpose could be the alleviation of hoop stresses in the component that can lead to crinkling of the diaphragm.  Examined here was the dynamic behaviour at higher frequency.  Where there were none or few ribs moulded into the torus shape, the diaphragm could be seen to mechanically break up to a high order and introduce non-axisymmetric modes.  Where 40 ribs were present, the mechanical behaviour was more controlled.  Although this did not significantly impact the frequency response, it is believed that nonlinear distortions can result from this.

  • Laser Doppler Velocimeter measurements helped to evidence rocking mode excitation in the driver with a vacuum chamber and determine its principal cause as mechanical imbalance to due leadout wires, rather than from an imbalanced radiation load cause by non-axisymmetric frame vent layout.