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Now the instruments appear from all over even though the soundstage may be nicely wide and defined. The culprit is the speaker and more specifically its radiation pattern. For this particular example a more directive/directional speaker would solve the soloist’s textural problem as long as the listener remains confined to the sweet spot. But this solution becomes less effective when dealing with an orchestral recording which would benefit more from a wide-dispersion speaker to include what now are more benign early reflections. By implication no existing speaker design can be 100% fit for all purposes. In practical use they all must be compromised. Here it means that the earlier mission statement of ‘to reproduce a recorded acoustical event accurately’ must be restated. In Eelco’s view it must become ‘to create a credible illusion of a recorded acoustical event’. Too easily ordinary speakers betray their presence either with amplitude colorations where certain frequencies stick out or simply by distorting the signal. Excessive dispersion causes the omnidirectional effects already mentioned. Grimm’s project brief for the LS1 required easy transport and compact dimensions. These goals plus specific dispersion/directivity controls had an obvious impact on what type of enclosure would be most suitable. Some compromises were inevitable and the final outcome was a wide baffle to confine the omnidirectional or 4pi radiation elements to the lower frequencies since human hearing limits the detection of directional cues below 100Hz. That simply means that it is hard to localize sound sources by reflection and reverberation below that frequency.

Strong room interactions start below 250Hz. Their interaction by resonance breaks up the homogeneity of the recorded signal. Taking this as a given, the next challenge for the Grimm team was to insure that all frequencies above 250Hz would exhibit stable directivity and that all harmonics—not just the higher ones—were subject to the same balance of direct and reflected sound. Though a wide baffle is ideal to evenly disperse the middle and high frequencies, baffle edges cause diffraction. When a driver moves outward, it pushes and thus briefly pressurizes the air. This pressure wave travels forward bounded by the baffle to form a half dome until the edge of the baffle. Suddenly there is no resistance and the pressure drops. This is smeared in time since not the entire pressure wave reaches the edge at the same time. Once literally over the edge, pressure drops by about 50%. In the listening seat there’s first an increase in air pressure and a fraction later a decrease to create a secondary virtual sound source albeit with a phase differential or time delay versus the original sound. The result is a smeared vague image.

Smoothing the baffle edge minimizes this effect. Substantial smoothing can eliminate the effect by diffusing the transition. That’s how the Grimm LS1 does it. Its two drivers are spaced closely in the vertical plane and embedded in a wide baffle with almost semi-circular edges as wide as the cabinet is deep. What to do about the horizontal upper edge though? Round that off the same? This would have taken real effort to fluidly and aesthetically join a semi-circular tube or half cylinder to other 90° angle.

Such diffraction control with a woofer is less critical since the woofer already is a source of diffraction for the tweeter due to its form. This warranted the reversal of the driver layout. With the woofer on top, the bottom edge of the cabinet could receive the matching semi cylindrical edge now neatly between the same edges of the cabinet’s cheeks. And the cabinet top could remain flat. By elongating the vertical semi cylinders, they turned into legs or a built-in stand. As a compact system with a relatively high-mounted woofer, provisions to integrate a subwoofer had to follow. Standing on two legs already, the ideal place for the sub was between them. Firing upwards to push against the floor, a relatively high crossover transition would further unburden the midrange for distortion-free high output. With the basic geometry fixed, the next step in R&D were the electronics. The enclosure had already been conceptualized to be optimal for the intended acoustical requirements. To not compromise this concept, the only possible way to insure repeatable results in various environments was active control. Thankfully the Grimm team had expert knowledge of DSP, ADC/DAC, clocks and amplification electronics in their own house.

The starting point for the electronics was designing a DSP crossover but plainly not a straight-forward off-the-shelf affair. That’s because most if not all current such electronic filters employ brute-force corrections which make the cure worse than the disease. Grimm only tackles the impulse response and most critically is focused on a minimum phase sum. Deviations not specific to the listening seat are carefully corrected not by a standard algorithm but manually. To do that both drivers are equalized flat to a certain extend across a very large bandwidth exceeding the passband each driver is meant to handle. Once equalized woofer and tweeter need to operate in specific bands. Here Grimm Audio found no other filter as suitable as a 4th-order Linkwitz-Riley set at 1.550Hz. 360° phase rotation warranted a driver cross at -6dB. Finally the drivers should be time aligned where the tweeter was delayed to match the woofer’s time arrival. These three compensation steps in DSP software are preceded by a non-interfering 2nd-order inverted all pass to insure that phase alignment is correct to begin with. For use in combination with a subwoofer, be it from Grimm or a third party, DSP also handles the necessary high pass settings. Without a subwoofer, the default cut-off is 40Hz. With a subwoofer it can be higher—80Hz for Grimm’s own LS1s—and the accompanying Q is lowered by 0.2 to 0.5 compared to the setting with no subwoofer to insure flat amplitude response. Other settings like time and level alignments too are adjustable in software. Adding the LS1s with the optimized settings transforms the LS1 monitor into a true three-way active system.

All these sophisticated non-intrusive mild DSP corrections are intended to control the drive units in the leggy speaker for perfect phase integration. These units are sourced from Seas and are the 22cm Excel W22EX001 magnesium-coated mid/woofer and 27mm DXT-equipped tweeter. Built quality, availability and consistency were prime factors for these specific choices. The tweeter’s DXT diffraction expansion technology helps to achieve the desired dispersion. In house each woofer and tweeter is submitted to examination which includes individual measurements. These become the input data for individualized correction curves stored in firmware. That’s how any production deviations are nulled. Does it come as any surprise that Grimm Audio would calibrate their measuring microphone with an in-house built spark gap?

One of the LS1 legs contain all the electronics. That’s the case for both left and right speaker. Which leg becomes the active leg is up to the user as is whether connections end up in the front or back. A single module containing the board for all three major electronic functions is located as far away as possible from the woofer and tweeter magnets and their mechanical vibrations at the very bottom of the leg. As we will see later, with the LS1s subwoofer in place a little adjustment is recommended. Input and signal routing occurs on the DSP board electronics with the central clock and all AD/DA conversion hardware. The clock is basically what’s also found in the Grimm CC1 but its sampling rate is now set to 93.75kHz, a frequency not likely found in any incoming source clock signal to prevent interference with the sample rate converter. It’s a clever trick to secure a clean signal. The same clock is used by the ADC and DAC chips, the SRC and DSP. Clearly Guido Tent’s expertise controlled this part of the design process.