Named after Mimir , a primal god of Norse mythology who was renowned for his knowledge and wisdom, we present
Enclosure and stuffing
The amount of stufﬁng and port tuning is based on measurements and extensive listening. The cabinet is ﬁlled with 100 g of Acousto-Q. The stuffing is distributed evenly in the box, but kept away from the port opening to allow free movement of air. This to ensure that the airﬂow noise stays as low as possible. The port length is 20 cm including the ﬂanged end and the inner diameter is 5 cm. This gives a port tuning of 42Hz providing a smooth low frequency roll-off.
The tweeter is the SEAS 27TDFC . This is a 27 mm High Definition precoated fabric dome tweeter with a wide, soft polymer surround. The dome and surround materials give high consistency and excellent stability against variations in air humidity. The voice coil is wound on an aluminum voice coil former with adequate ventilating holes to eliminate noise from internal air flow. The voice coil is immersed in low viscosity magnetic fuid, for high power handling capacity and simplifed crossover design. A stiff and stable rear chamber with optimal acoustic damping allows 27TDFC to be used with moderately low crossover frequencies. The chassis is precision moulded from glass fibre reinforced plastic, and its front design offers optimum radiation conditions. For detailed technical parameters on the drive units see the data sheets.
The drivers should be in phase at the crossover frequency, and the sum should be flat when both drivers are at -6dB relative to the pass band level. Now the inevitable vertical off-axis cancellation, which is always present with the use of non-coincident drivers, is at least outside the listening axis (tweeter axis). To accomplish the design goals it was necessary to use a 2nd order electrical filter for the high pass section, with an “l-pad” for attenuation because of the different sensitivities of the drivers. The low pass section was realised with a 2nd order electrical filter and a baffe-step compensator consisting of a parallel connection of an inductor and a resistor in series with the driver, thus providing a flat frequency response. The crossover schematics is shown in figure 2.
As seen in the figure the average sensitivity is 84dB and the response lies within ± 2dB. The black curve shows the response with the tweeter connected with opposite polarity. In this curve we look for a deep and symmetrical notch around the crossover frequency, which indicates that the drivers are in phase in the crossover region. Here the notch is more than 25dB deep, and that confirms that this system has a very good phase response. The blue and green curves show respectively the response of the woofer and the tweeter.
The 2nd order electrical crossover combinedwith the natural roll-off of the drivers gives a 4th order acoustical Linkwitz-Riley slope. The crossover frequency is 2.2 kHz, high enough for the tweeter to operate inside its limits, even at high amplitudes, and low enough so that the woofer doesn’t become too directional, and thus providing a smooth power respons. The off-axis respons of the Mimir is shown below.
This figure shows that the power response is very smooth throughout the whole frequency range, and the controlled off-axis roll off of the tweeter.
The next figure shows the 2nd and 3rd order harmonic distortion with an output of 96dB at 1m. The overall distortion is very low and without any peaks that might become noticeable. This makes the reproduced music very clean and without coloration.