5.1 Elimination of response dips at intermediate frequencies

Measurements as well as simulation results reveal two dips in the sound pressure response occuring at approx. 400 Hz and 900 Hz (see Figure 6). The elimination of these dips is of great interest for loudspeaker manufacturers, since one of their aims is to produce flat axial pressure responses over a wide frequency range. A proposed specification is the deviation to be held within $\pm 2$ dB [9].

A main task of the surround, which is most critical in low frequency applications, is to give effective absorption and termination of the incident energy at the cone edge through to the mid range. If poorly terminated, the incident energy will be strongly reflected back towards the cone apex, which causes standing waves due to interference with the prime outward travelling energy. Furthermore, more complex vibrational modes in the cone are also excited. On the other hand, the surround geometry should be designed so that no significant eigenfrequencies occur. These radial vibrations and eigenfrequencies may produce dips in response at intermediate frequencies [9]. Since the biggest amplitudes of the vibrational modes occur at the rim, the rim contour as well as the surround are critical parameters in the control of these breakup effects.

The aim of these computer simulations has been the elimination of these response dips. Figure 9 shows the comparison of the axial pressure response with original and modified surrounds. The solid line represents the axial pressure response if the loss-factor in the surround is increased by a factor of 2.5. These high loss-factors can be achieved using surrounds consisting of very lossy rubbers, which have been reported in [15]. As can be seen, the increased loss-factor results in a more effective absorption and termination of the outward travelling energy and in reduced response dips. The dash-dot line represents the axial pressure response when a flat section is added to the surround, as shown in Figure (8-a). Due to the modified geometry of the surround, the change of surround mass and compliance results in a modified equivalent circuit and causes the elimination of these response dips. With both modifications the deviation can be held within $\pm 1$ dB over a wide frequency range and, therefore, an improvement in respect to response flatness can be achieved.

  

Abbildung 8: Design Improvements: a) Elimination of response dips by using a surround with flat section added. b) Improvement in efficiency for high frequencies by using an additional metallic cylinder in the magnet gap

  

Abbildung 9: Axial pressure response at 1m with modified surrounds