Feature: Comparing different approaches to driver design

Stephen Bennett asks three monitor manufacturers how they go about making their speakers’ most crucial components.

The first magnetic driver speakers were developed in the 1920s and by the middle of the twentieth century sophisticated full-range monitors were available that any music lover would be happy to call ‘hi-fi’.

You’d think then that innovation in driver development might have stopped at that point – we’ve had moving magnet and moving coil, electrostatic, compression, dual-concentric and ribbon drivers coupled with sophisticated enclosure designs, powerful and dynamic amplification and low-noise electronics all aided by Computer Aided Design (CAD) and Digital Signal processing (DSP).

Hasn’t the discerning engineer and listener been able to purchase monitoring systems capable of recording or reproducing the ‘best sounding album of all time’ <insert personal favourite here> for decades? So what is there left to discover about the good ol’ loudspeaker? Well, plenty, if the latest launches from some of the sector’s leading manufacturers are anything to go by. For example, the three companies questioned for this article offer quite different technologies and design philosophies with their monitors and these choices are the product of a long history of technological innovation.

A bit of background

“The historical line of development for Genelec has been to use stiff radiator diaphragms instead of ones that flex significantly and are subject to modal resonances within their intended frequency range of use,” says Aki Mäkivirta, director of research and development at the Finnish firm. “Stiff diaphragms can allow for better control and are more predictable in production. This translates to a more exact control of directivity and more even quality in production.”

Pioneer DJ has been developing speaker products for many years, operating under the sub-brand Technical Audio Devices (TAD) and Takashi Mitsuhashi, general manager, Engineering Department IV, says that the company has a strong history of designing coaxial drivers. “The Coherent Source Transducer (CST) driver, the Acoustic Filter Assisted System Tuning (AFAST) technology and noise suppression processing inside the RM range of speakers are important elements that have been based on technology developed by TAD,” he explains.

ADAM Audio is best known for its X-ART tweeter, a third generation version of the Air Motion Transformer (AMT), according to chairman David Angress. “The AMT was created and first marked in the 1970s by Dr. Oskar Heil, a German physicist who had emigrated to the US.”

One direction

Genelec has also developed two new coaxial designs, nicknamed ‘The Ones’ (see main picture), that they say overcome some of the limitations the company believed was fundamental to previous designs. “The front surface of the transducer is continuous,” says Mäkivirta. “There is no discontinuity-related diffraction colouring the audio. We have achieved controlled neutral response in the off-axis directions too. This is possible because we are building the coaxial transducers in a unique way, using flexible sections that conform to the global shape needed for the directivity waveguide.”

Though different companies may utilise many of the same kinds of physical parts for their drivers – cones, coils and magnets – and all are attempting to achieve the same sonic ends, the way they are implemented in practice can be quite different. “Controlling directivity in loudspeaker system designs has been one of the long-term goals for Genelec,” says Mäkivirta. “Using a stiff or rigid transducer diaphragm is therefore the logical choice in applying this design principle. Not only do we want to control directivity but we want to do this in a way that this enables the loudspeaker to radiate neutral audio on and off the acoustical axis in order to make sure that the audio going into the room’s reverberant field remains neutral in addition to the sound radiated directly toward the listener.”

Angress explains ADAM Audio’s approach: “The technology we use includes a ‘voice coil’ that is printed onto a substrate that is then folded into a diaphragm that looks like a [set of] bellows and is put into a magnetic field. When an audio signal is passed through that voice coil the diaphragm ‘squeezes’ the sound out rather than moving like a piston as a conventional speaker does. The result is a four-times-faster acceleration of the audio, giving the listener excellent transient response, extended high-end response – up to 50kHz – with extremely low distortion.”

Pioneer DJ chose to utilise a different – yet familiar – transducer. “The coaxial driver and the noise elimination technology inside the cabinet are the speakers’ main features,” states Mitsuhashi. “Many speakers can’t reproduce spatial information accurately, but in the RM range the sound that’s reproduced becomes wider around the speakers because the sound source includes spatial information. When the sound reproduction is accurate, the sound fields are reproduced in front of you and you don’t ‘feel’ the speaker’s physical presence. Additionally, Pioneer DJ’s AFAST technology drastically reduces standing waves that can produce a muffled response in the low to mid ranges. An acoustic tube in the cabinet absorbs only the standing waves, ensuring true reproduction of low to mid frequencies.”

Hard work pays off

Mäkivirta is also keen to mention Genelec’s philosophy of systematic improvement through better and deeper understanding of driver design over four decades of continued research and development. “Transducer engineering and system design have resulted in the unique possibility to optimise the system performance for the stringent needs of professional audio monitoring,” he explains. “While Genelec monitors are excellent for enjoying music and high quality recordings, our starting point is not to tailor the loudspeaker sound character in a certain subjectively pleasing fashion, but to offer as accurate a presentation of the electronic input signal as possible.”

And although ADAM’s ribbon tweeter is the most visible feature that defines its range of monitors, it is only one element of speaker design, as Angress notes: “It is important to create an integrated and well-balanced design that considers the other drivers in the system, the correct amplifier power, modern crossover design and properly dimensioned and constructed cabinets.”

Mäkivirta agrees, saying that complete systems are more important than just the drivers. “Transducers cannot and should not be designed in isolation. To truly optimise the system for the best performance it is vital to consider the three main components simultaneously: the transducers; the acoustic-mechanic system design, including the loudspeaker cabinet and the transducers; and the electronic system, including the power amplifiers and the signal processing, be it analogue or digital. Poor transducers do not turn to gold with any amount of DSP magic.”

Mitsuhashi says that the audio from the point source in Pioneer DJ’s coaxial driver is integrated with the cabinet technology, which then “suppresses noise, creating accurate unequal reproduction.” The company’s monitors feature AB class amplification in a High frequency/Low Frequency bi-amp configuration. “We optimise materials by making hybrids,” adds Mitsuhashi. “For example, in the driver diaphragms we use Aramid fibre combined with paper, which is commonly used to make helmets.”

Genelec manufactures the key transducers in-house to have, as Mäkivirta says, “the best possible grip on quality with a very rapid feedback cycle back with R&D if things are not working in production as expected.” He adds that it helps the company to design products with high endurance and a long expected lifetime. ADAM’s products are designed in-house in Berlin, Germany, while Pioneer DJ’s Research and Development arm is in Japan.

Angress says that ADAM’s new S-Series flagship range feature woofers that are a new ‘ground-up’ design that incorporate a novel symmetrical magnetic circuit, new low-mass honeycomb sandwiched cone material, and cooling technology. “These features result in woofers that have extremely light yet stiff cones for excellent transient response and low distortion, as well as the ability to move more than three times further than their predecessors, giving higher maximum SPL capability at lower frequencies. The bass and midrange components are driven by Class D amplifier modules, and the S-ART tweeter is driven by a Class A/B amplifier of ADAM’s own design that pairs particularly well with this tweeter.”

ADAM Audio’s new S-ART high frequency driver

All part of the process

Mäkivirta says that the healthy way to design high performance systems is to start with well performing transducers, a correctly optimised enclosure and to perfect the system with careful application of the signal processing. “Proper acoustical design is the healthy platform for utilising DSP to further optimise the system performance and offer tools for active compensation of the acoustical influences of the installation room to maintain the neutral system characteristics.”

Mitsuhashi explains that Pioneer DJ has applied acoustic simulation for the basic design of the monitor’s waveguide, the Harmonized Synthetic Diaphragm Optimum Method (HSDOM) diaphragm in the coaxial driver and the shape of the enclosures. “We’ve also used 3D printing for prototyping,” he continues, “which allowed us to achieve a precise design, which we fine-tuned by testing and experimenting with the technology.”

Angress says that ADAM spends a lot of time listening to customers on how their workflows are adapting as recording formats are changing. “These conversations, along with research into latest technologies, drove the decision to provide crossover and equalisation options via DSP using the latest generation of SHARC chips,” he says. “This also ‘future proofs’ these speakers by allowing new configurations to be set up as workflows continue to evolve and new networking standards emerge.”

“Driver design is still an area of innovation, since there are always new and exciting technologies being developed,” states Mitsuhashi. “This is crucial in delivering products that can be lighter in weight and possibly smaller in size. As new materials and driver configurations are designed, innovative products emerge which provide multiple application options for audio professionals.”

Trial and error

In Mäkivirta’s opinion, transducers turning electrical drive into audible pressure variations have been, and still are, in many senses, the limiting factor in loudspeaker design – and all manufacturers must face these issues. Angress then goes on to say that: “Over the past decades many different technologies have been tried and eventually abandoned. The dynamic loudspeaker is still the mainstay of our industry. How the design theory is implemented, which materials are used, and how the designer integrates all the elements are keys to their success.”

“Today, signal processing can have extreme linearity and inaudible noise level,” notes Mäkivirta. “The transducers have, relatively speaking, rather more stringent limitations on neutral and controlled frequency response, linearity at maximum output levels, long-term heat management, and long-term stability of materials and structure. While the fundamental principles of transducer design have been known for a long time, better modelling tools and new materials offer new possibilities of improvement.Mitsuhashi believes that optimum designs will be developed according to the needs of various applications. “Smaller and lighter drivers are always being developed,” he says. “This, in conjunction with the DSP technology of today, means, that driver development will always be a crucial part of speaker design philosophy."

“There has been some study on improving the efficiency of the electro-acoustic transducer during the last few years,” reports Mäkivirta. “This research may result in some radical ideas that may lead to novel designs, but not without changes in the amplifier technology as well. Loudspeaker arrays and systems for wavefield synthesis transduction are developing some very interesting solutions.”

He also says that with the development of advanced modelling methods, these trends will hopefully fuse into more innovative control designs and a better understanding of the optimisation of the variables that create high-quality wideband transducers.

In conclusion

It is clear that while the underlying technology and physics of driver design is well understood, modern design and manufacturing technologies are pushing the envelope of what is possible with transducer design. As ADAM’s David Angress concludes: “We will continue to enjoy incremental improvements in both sound quality and customer value over time. As far as new technologies go – well, we’ll see!”