Report: studio monitor driver technology

The design of these electromechanical devices (drivers) often raises great passions in those that undertake research and development in this area and among the engineers and musicians who ultimately use them for their work. This means that you’ll often find quite a range of opinions and engineering philosophies from the companies working in the field.

Andy Munro has worked in pro-audio and electro-acoustics for 45 years, starting with Shure and then independently since 1980. He was co-founder of Dynaudio Acoustics in 1990 and is currently owner of Munro Acoustics and Form & Funktion. If you’ve ever worked in a commercial studio or post facility, it’s likely that you’ve been in a Munro-designed room. Munro believes that the key elements of any driver are linearity, efficiency and distortion control. “They are mutually conflicting parameters, so something has to give,” he says. “The best HiFi speakers produce 90dB for 1 watt at 1 metre while the best horn loaded compression drivers can produce 110dB. Each has its place, but the vast majority of critical monitoring is done on nearfields using direct radiating domes and horns that have not changed that much over the last decade or two.”

Munro believes that the biggest advance has actually been in amplifier technology that has allowed more power to be placed in smaller boxes with less heat. “Horns are basically band-pass filters and acoustic transformers so they need equalisation and phase correction to work,” he says. “The use of Digital Signal Processing (DSP) has enabled manufacturers to do this cheaply and efficiently. However, most people still prefer the traditional driver designs and we continue to partner with Dynaudio as our preferred manufacturer with monitors such as the Daytona M4+.”

In the past, many manufacturers of studio monitors would fit tried and trusted drivers in custom-designed enclosures. For example, in the 1970s, the BBC’s LS3/5 nearfield monitors featured KEF drive units that were also commonly in domestic loudspeakers, but that approach is now changing. “Like many loudspeaker system designers, we used to take off-the-shelf drivers, have them modified in some way for our needs and then they effectively became ‘ours’ for incorporation into our products,” says Andrew Goldberg, product manager of Studio Monitor Systems at Georg Neumann GmbH. “These days, we do all our driver design in-house using models we developed ourselves. The effort to undertake a design in-house is significant, and the parts cost are similar to ‘bought-in’ products, the drivers’ performance then is optimal for our monitors – which is obviously good for the customer.” Neumann models the magnetic circuit and the surround, cone and ‘spider’, allowing the company to try out thousands of design iterations until an optional result is achieved – before making any physical prototypes. “While there is more work in design, there is less work (if any) to do in making any physical adjustments later,” says Goldberg. “This leads to better confidence in the feasibility of our project time plans and budgets – and the benefit for the end-user is a better monitor.”

Aki Mäkivirta, director of research and development at Genelec says that the company has been using stiff diaphragms in transducers, typically made of metal for mid and high-frequency transducers and of pulp and plastic for woofers. “The motivation for using stiff diaphragms has a direct link to Genelec’s aim to systematically control directivity in our designs,” he explains. “It is technically possible to design an acoustical radiating system with well controlled directivity across the useful frequencies when the transducer creating the sound is systematically behaving in a predictable way across its intended range of operation.” Mäkivirta says that stiff diaphragms also contribute to small variation of the frequency responses across products, enabling the company to pretty much guarantee that any two random Genelec monitors of the same type will work perfectly as a stereo pair. “We are avoiding the softer dome and cone materials that can bring diaphragm resonances in the useful passband, making the directivity slightly unpredictable, increasing the unit-to-unit variation and reducing repeatability of production,” he says. “Stiff diaphragms also enable high resolution designs that deliver ultrasonic frequencies in the tweeters when combined with a wide band signal processing system.”

Goldberg says that monitor drivers are only one part of a complete system. “Almost everything in a loudspeaker system interacts with something else. It is very rare to be able to change something without it affecting another part of the design. Having made the best driver we can, we still have to put it into a cabinet and apply some filtering to shape the response. All these components must work together as one once the product is assembled. Many driver designers are not using modelling to improve results, but those that are constantly try to improve their models so they more accurately reflect what will happen when the drivers are placed in a monitor in a real space.” Goldberg adds that passive or active analogue filtering or DSP processing can also be used to achieve the required results.

“Genelec equalises the transducers individually,“ says Mäkivirta. “This further removes any remaining unit-to-unit variations and enables new ways of optimising the overall electro-acoustic design, supports creation of very low distortion linear systems and creates an electro-acoustic transduction that has flat frequency response and a precisely consistent input-to-output latency. When we combine this with the control of acoustic directivity, our approach to designing products enables us to build rather ideal transducers and overall electroacoustic transduction systems.”

Munro’s designs comprise of custom-built systems for either film mixing in dubbing theatres or large music control rooms. “They are three or four way active systems with multiple drivers and that gives a very clean and controlled sound at high sound levels,” he says. “We combine monitoring and room acoustics to give a seamless soundscape – they are basically giant HiFi systems. Recent installations include the new Abbey Road Atmos theatre which is the first to use a hybrid switchable horn/soft-dome facility to allow critical listening of both film and purely music soundtracks.”

Goldman says that Neumann are seeing improvements in both the the modelling and the materials used in classic dynamic drivers. “There are potentially great strides to be made in improving linearity of the drivers using signal processing – but it is early days on this front,” he says.

“One of the recent highlights in our work has been the creation of several novel coaxial transducers for The Ones, Genelec’s new line of acoustically coaxial compact three-way monitors,” says Mäkivirta.

“The main shortcomings in traditional coaxial transducer designs have been the intermodulation due to the difference in midrange and tweeter displacement, the gap between the MF and HF transducers necessary to allow cone/dome movement and the acoustical discontinuities when blending the transducer to the enclosure, causing diffraction.” Mäkivirta says that the motivation to solve these problems remained the focus of an exciting long-term research activity that was started in 2007. “We kicked off a research project that took three years to complete and resulted in the 8260 monitor which features a coaxial design that has a smooth acoustically optimised surface with no discontinuities or gaps, acoustically blending to the front cover directivity waveguide.” Mäkivirta says that this creates resolution and stereo imaging that a “Japanese professional user said makes it effectively ‘a microscope of sound’.” The latest phase of the company’s research work has now led to the 8351, 8341 and 8331 products which Genelec describes as ‘the world’s most compact three-way monitors’. “For these, a dual air gap magnet motor system was created and the tweeter and midrange are sharing this motor,” says Mäkivirta. “We continue to have a coaxial design with acoustically continuous, optimised midrange transducer surface blending to the waveguide. The physically small yet powerful magnet motor and the coaxial concept have enabled Genelec to create the most compact coaxial three-way on the market without sacrificing the sonic accuracy and capacity.”

Munro says that new materials and methods are coming out of science laboratories and that the ‘holy-grail’ of driver design would be a more efficient interface between drivers and air. “More than 90% of amplifier power is converted into heat instead of sound,” he says. “Horns are part of the solution, but uneven directivity and non-linearity need to be addressed if they are to be among the best sounding speakers.” Munro uses Dynaudio drivers almost exclusively in his installations. “They are renowned for their sonic accuracy and reliability under high usage,” he says. “Ultra-lightweight hexagonal aluminium wire voice coils allow longer excursion and a bigger cooling area for linearity and power. By fitting eight drivers into a single box, we create a sound that fills the room but with virtually no distortion, lobbing or clipping. As each system is built to order and built into the fabric of the control room, we also eliminate diffraction or reflection induced interference at the mixing position. Munro says that their systems are used in top studios all over the world, which he believes speaks volumes about the quality of the design.

“As all of those working with transducers know, that creating a reliably working design requires much more than just the selection of the materials for the components,” says Mäkivirta. “The final transducer is a well-balanced combination of materials, electro-acoustic design principles and a large amount of prototyping and testing to verify that the design targets can reliably and repeatably be turned into manufacturable devices. The use of DSP gives us an extra level of control on top of the traditional approach of creating very reliable designs. We never use DSP to correct for bad transducer design, but only to enhance the overall system performance. The transducer quality creates the core performance potential of any loudspeaker design.”

A cursory glance at the designs of a range of monitors from different companies clearly demonstrates that there are widely different philosophies at work in the field. With more powerful processing and more sophisticated modelling becoming available, engineers should be able to come closer to what they believe to be their own personal ‘holy grail’ of driver design. However, there still seems to be room in the marketplace for those building monitors with a mixture of personal expertise and a careful selection of components. Whether the current research into solid-state transducers – for example those using Micro Electro Mechanical Systems (MEMS) or Graphene-based technologies – can ever deliver in the way that current drivers can is debatable, but until then, it seems that there is still life left in the old cone-flappers yet.