100M Design Challenge

NSMT Loudspeakers Mastering Series 100M

Design Challenge: 
How to scale-up the magic of a great full-range single driver loudspeaker such as the Role Audio Sampan into the performance of larger floor standing loudspeakers.

Role Audio Sampan        NSMT 100M            KEF Reference 205/2           Thiel3.7 

A well designed full-range, single driver, crossover-less loudspeaker such as the Role Audio Sampan can perform most of the magic that is the holy grail for audiophiles and music lovers.

The Role Audio Sampan has been described by the cognoscenti as producing “music to die for,” it images so well that it is, “The greatest disappearing act since Houdini,” and it creates excitement, “that few loudspeakers on the planet can match,” and it is, “capable of deceiving listeners to think they are listening to a large loudspeaker,” because of its humongous soundstage and its bass prowess. The Sampan achieves its magic and coherence in part because it is a time coherent, phase coherent, point source, without the phase and crossover anomalies that afflict multi-driver loudspeakers.

However, full-range, single driver loudspeakers have their limitations. To achieve excellent high frequency performance the loudspeaker designer must use small diameter drivers which limit bass performance. As the loudspeaker designer increases the size of the full range driver to remedy the bass performance problem, the ability to reproduce high frequencies falls off. The designer is faced with the conundrum of how to get truly full-range performance from larger “full-range” drivers which are rarely full range. The Sampan, which uses a single full-range driver that has no natural output below 100 cycles, solves this problem by using a transmission line design to deliver sturdy bass performance. And, because the transmission line design reduces cone movement, it allows the driver in the Sampan to achieve an unusually high level of dynamic performance. However, a small loudspeaker like the Sampan is limited by the laws of physics in the amount of energy it can project into a room.

The challenge

The challenge for loudspeaker manufacturers remains how to design loudspeakers that produce large scale performance while retaining the magic and the purity of sound present in 3.5 inch full-range loudspeakers. The Model 100M is a response to this challenge. And various designs, including the Thiel CS3.7 and the KEF Reference 205/2 loudspeakers can be seen as efforts by designers to recreate or scale up the magic of the full range, single driver loudspeaker into the performance of larger loudspeakers. Like the NSMT 100M, both the Thiel's CS3.7 and KEF's Reference 205/2 use coincident drivers in their designs, as can be seen in the red circles on the picture above. However, there the similarities end, as each manufacturer employs a different solution to reproduce bass in their designs, and this accounts for differences in design, execution, and performance of the three loudspeakers.

The bass output of a loudspeaker is what primarily determines whether or not it can play in a large or small room. And the way bass reproduction is executed in the three designs accounts for the departure from the qualities that produce magic in small, full range, single driver loudspeakers like the Role Audio Sampan.

Lets look at the Thiel CS3.7. The use of three drivers and a passive radiator results in a soundstage that is time coherent, not on the tweeter axis, but six inches below (see Stereophile review). Hence, unlike a single driver loudspeaker, it is not a time coherent point source. Moreover, the CS3.7 is not phase coherent and with an impedance of between 2 and 3 ohms it requires an expensive high current amplifier, which takes away some of the fun of experimenting with a wider range of lower powered amplifiers.

The use of two stacked bass drivers in the KEF Reference 205/2 means that, like the Theil CS3.7, Reference 205/2 is also not time coherent over its entire operating range. Moreover it uses a fourth-order crossover to integrate the response of the mid woofer and the tweeter which means it is also not minimum phase and departs from the performance characteristics of the single driver full range loudspeaker.

In contrast, by using active bass and monitor sub sections, the NSMT100M retains most of the characteristics of the single driver full range loudspeaker in its design and execution. It uses a first order minimalist minimum phase crossover to integrate the response of the tweeter and the woofer in the monitor section. Hence the monitor section is a true first order, minimum phase, time coherent, point source design. Moreover, with a resonant frequency of 26 Hz, the mid woofer is truly full range. And although its low bass response is attenuated, its operates over the entire frequency spectrum of the 100M. Active, crossover, phase, and volume controls are used to bring the low bass into seamless alignment with the output of the monitor section. But while the active bass provides reinforcement, the response of the monitor section defines the soundstage and always operates as a time coherent point source.

With a full featured 200 watt subwoofer amplifier driving the low bass in each 100M, a wide range of low power amplifiers can be used to drive the monitor section. The adjustable active bass also allows the 100M to be used in large and small rooms where many large loudspeakers would simply overload the room. Because of the full range capability of the monitor section of the 100M and the flexible controls available on the amplifier of the subwoofer section, the bass of the 100M is infinitely adjustable which allows the 100M to be played in large and smal.

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