MACH 1 Acoustics DM-10 Signature loudspeaker Martin Colloms April 1994
In his review of the Mach 1 Acoustics DM-10 Signature loudspeaker (p.131), Dick Olsher correctly perceived somewhat limited low bass from the DM-10, despite a respectably low measured system resonance at 24Hz. With a modest bass Q factor, fair in-room bass should have been available down to 25Hz if the woofer excursion was up to it.
The reason for the discrepancy can in fact be seen in the individual response of the woofer and in the electrical signal actually driving the woofer motor coil, available thanks to the separated crossover design (p.135). The woofer drive signal peaks by nearly 3dB at 70Hz where, ideally, the response should be flat. This is presumably caused by the unwanted interaction between the woofer's motional impedance (the complex reactive electrical load presented to the crossover due to the moving-coil and diaphragm system in the cabinet) and the rather low crossover network frequency of 250Hz. This upper-bass boost is relatively independent of the low-frequency resonance of the driver or bass damping.
In design terms, if the 70Hz boost was suppressed—eg, by complex impedance conjugation or compensation—then the bass level could have been set higher and the true bass output would have then made its presence felt more powerfully.
In practice, the Signature designer may have legitimately rejected this approach on grounds of other aspects of sound-quality impairment.—Martin Colloms