Transmission Lines and Ribbon Tweeters: The Best Route to Loudspeaker Design for Accurate Musical Reproduction

 

General Introduction

The professional music and consumer electronic industry has strived for decades to optimise loudspeaker design for time response, frequency response, dynamic and timbral accuracy within the constraints set by cost, size, and positioning limitations. Bass response has historically presented inevitable trade-offs in terms of low frequency extension and control, as well as cabinet size and drive current requirements. Equally, advancements in electrostatic, planar magnetic and orthodynamic headphone technologies have highlighted the less-than-perfect nature of high frequency reproduction from most typical loudspeaker systems. Recent developments in ribbon tweeter technology, combined with a revised approach to bass-frequency reproduction through the refinement of transmission line technology, allow a designer to tackle some of the fundamental limitations of loudspeaker design and sound reproduction, resulting in a level of absolute sonic accuracy as well as a cost/performance ratio that has been previously unavailable.

 

Introduction to transmission line principle and sonic benefits

An acoustic transmission line is essentially a tuned labyrinth (or ‘rear waveguide’) that begins directly behind the main low-frequency driver in a speaker system, and gradually tapers until it reaches its terminus – which typically has a surface area equal to the cone of the driver. Depending on the format of the enclosure, this labyrinth will consist of a number of internal folds to allow the desired line length to be achieved. In smaller designs, this line length can be decreased in conjunction with a reduction of the area of the terminus to achieve similar effects, with the compromise of losing some extension and overall output. In the truest sense, a transmission line would consist of an infinitely tapering chamber, which in practice is a physical impossibility. To this extent, the term is partially misused, and acoustic ‘transmission lines’ work on the basis of being tuned to 1/4 or 1/3 of the wavelength of the resonant frequency of the driver, and are heavily damped with absorptive material. The output from the terminus greatly reinforces content below 80Hz, with the unwanted higher frequencies from the rear of the driver being absorbed by the internal dampening.

The compromises associated with transmission-line designs include a cabinet size that is often larger than typical sealed or bass-reflex enclosures. Furthermore, transmission line speakers have historically been considered ‘low efficiency’ designs, which require more power to achieve the desired output level. In addition, the cabinets are highly complex to design effectively, and equally complex to manufacture. The advantages offered by transmission line designs include a dramatically improved extension in the low-frequency range when compared to sealed/reflex designs, and a far more gradual ‘roll-off’ in this region. In addition, the definition and accuracy of the bass is greatly improved, and midrange response is far purer and less coloured by distortion. This is as a result of unwanted energy being directed away from the cone, removing the non-linear distortion that often occurs in sealed or reflex enclosures. In such enclosures, this distortion typically occurs as a result of the unwanted, rearward directed energy resonating and reflecting inside the cabinet, and either exiting via a port (in the case of reflex enclosures) or reflecting back through the cone itself, introducing non-linear behavior and colouration.

 

Kerr Acoustic has devoted significant research and development time to harness the benefit of transmission lines, addressing their inherent drawbacks to create designs optimally tuned for maximum acoustic performance. This has allowed us to greatly increase efficiency, and to achieve clean and accurate low frequency response extending all the way down to 33Hz, in the case of our stand-mount K300 speakers. With the recent advances in this technology, the sonic benefits of transmission lines must be matched by improved performance in the definition and time response of other components in the loudspeaker system. This is particularly true of the tweeter, which handles the bulk of the critical information we perceive when listening to music. One important consideration is the Fourier transform limit - an important principle of physics that governs a variety of technologies (including optical communications) which dictates that the time response of a system is always limited by its frequency response. Hence, use of a large bandwidth tweeter would not only add additional high-harmonic content and spatial information clues to the sound of speaker, but it would also allow a much faster time response. This is an ideal way to preserve, enhance and complement the fast and accurate bass response of transmission line designs.

Further reading in relation to the history and physical principals of transmission line loudspeaker technology can be found at the following resources:

 

Introduction to ribbon tweeters – a new opportunity for innovative speaker designers

An effectively incorporated transmission line offers the most frequency-complete and time- accurate bass response. This calls for the use of true ribbon tweeters, with their extended frequency response and ultra-fast time response. Ribbon tweeters are another area shrouded by some misconception. While planar tweeters are not uncommon, true ribbon tweeters are rarely featured in ‘affordable’ products, and the term ‘ribbon’ is often misused to describe the majority of planar tweeter designs on the market. The most commonly seen planar tweeters consist of a folded plastic film or membrane, which has a conductor either weaved into the material, or bonded to the material’s surface. This assembly is suspended within a strong magnetic field, and is technically classed as an ‘air-motion transformer’ design. Typically the resultant response is faster and better extended than most dome tweeters, though arguably more ‘abrasive’, particularly when used over long periods of time. This effect is chiefly responsible for listener fatigue, and is the primary reason that they are considered a compromise – particularly in the professional field. In a true ribbon tweeter, the diaphragm is itself the electrical conductor, and is made from an ultra-thin strip of conductive material (typically aluminum). This guarantees that there is no excess mass whatsoever - the only moving mass being the conductor itself. However this solution comes with some additional complexity and cost. While most planar tweeters present similar impedance and sensitivity characteristics compared to dome designs, true ribbon tweeters must incorporate an impedance-converting transformer to match the amplifier and crossover impedance to that of the diaphragm. This impedance matching is essential to allow proper function of a ribbon tweeter, due to the nature of alternating current flowing directly through the large conductor.

 

Further reading in relation to the physical principals and different functional types of tweeter technology can be found at the following resource:

How Speakers Work: Understanding the Differences in Tweeter Technology – Electronics House - https://www.electronichouse.com/how-to/understanding-differences-in-tweeter-technology/

 

The sonic benefits of using true-ribbon tweeters are clear: they offer a time response and a level of high-frequency reproduction unmatched by other designs. One of the key factors here is the physical mass of the diaphragm/conductor, which is typically just 20-30 microns thick and can weigh as little as 20 milligrams (compared to a typical 170 milligram air- motion-transformer tweeter, and around 0.5 grams for a conventional dome). As an example, the ribbon tweeter diaphragm featured in our K300 model weighs 0.027 grams, and has an effective surface area of 720 square millimeters. For comparison, most conventional domes have a 700 square millimeter radiating surface area, but on average have a moving mass that is eighteen times greater.

This incredibly low mass allows for a uniquely fast response, which is crucial for accurate reproduction of the very highest frequencies. Ribbon tweeters are therefore purported to possess an extremely airy and transparent sound, owing to the absence of voice-coil inductance, linear magnetic field, and extremely low diaphragm mass. Again, the Fourier transform principal is highly relevant to the reason why the perceived sonic presentation from ribbon tweeters is so aurally preferable over alternate technologies.

Historically, these designs have been prohibitively expensive to implement into cost- effective, competitively priced products, and have included compromises including low power handling and reliability issues. Equally, their off-axis response has been historically regarded as inferior to dome designs, owing to the rectangular shape of the diaphragm. The performance and cost related issues have been addressed by a recent breakthrough in design and manufacturing processes, that has made the technology suitable for high-quality, non-prohibitively expensive ribbon tweeters. This is largely as a result of having improved manufacturing tolerances, and far greater access to a wider range of precision tooling for construction, assembly, and testing. Off-axis response has been greatly improved through a combination of ribbon surface corrugation, and incorporation of diffractive and reflective elements within the tweeter enclosure.

Despite this tremendous progress, the process of designing a loudspeaker that fully exploits the benefits of ribbon tweeters and accounts for their residual idiosyncrasies is not trivial, and requires acute balancing of the accompanying components, and very careful crossover management. Ultimately, if one can design a system that fully exploits the benefits of ribbon tweeters, whilst simultaneously managing their behavioral characteristics, they represent the very best in tweeter technology available today. When combined with transmission line technology, they have the potential to unleash unprecedented frequency extension and time response, leading to the most accurately defined reproduction of harmonic content, dynamics, spatial information and - most importantly – timbre, available.

• How Speakers Work: Understanding the Differences in Tweeter Technology – Electronics House - https://www.electronichouse.com/how-to/understanding-differences-in-tweeter-technology/

 

How are Kerr Acoustic speakers different?

Kerr Acoustic has capitalized on the previous absence of a loudspeaker system that fully exploits the combined benefits of transmission lines and ribbon tweeters, while mitigating all the sonic and practical drawbacks that have been associated with such technologies in the past. We have tackled this challenge according to the following design principals:

  • Efficient, transparent, low-loss crossover networks – for ease of drive and dynamic integrity.

  • Frequency and time response accuracy for absolute transparency and fidelity, in both studio and domestic applications.

  • Insensitivity to boundary conditions and linear off-axis response, for maximum flexibility with positioning.

  • Robust and acoustically inert cabinets for minimal colouration and maximum stability.

 

Approach to loudspeaker design

During the intricate design process, a combination of precise acoustic measurement and tuning by ear is employed in an effort to produce loudspeakers with highly linear frequency and time responses. While acoustic measurement is considered an important factor, great emphasis is placed on constant, critical aural assessment throughout design and fine-tuning of the products. An extensive benchmark is offered by designer Jes Kerr’s personal ownership of and experience in listening to a vast number of loudspeakers, spanning every conceivable technology and design topology available. This is reinforced by extensive experience in the fields of musical performance and production, and regularly being in the presence of acoustic instruments and voices in their inception. In addition, Kerr Acoustic has extensive in-house knowledge of bespoke joinery, which allows for extremely cost-effective and time-efficient development and manufacture of new cabinet designs. Below we expand on the methodology behind our approach to design, and explain how each point of differentiation listed above is facilitated and addressed.

 

Crossover networks

Our crossover networks are extremely efficient and sonically transparent, owing to the use of exceptionally high quality components and intuitive design. High-grade polypropylene capacitors are used in conjunction with hand-wound air-core inductors and carbon-film resistors. This combination allows for outstanding component synergy and drive-unit integration, fully optimizing the output bands for each drive-unit without negatively affecting their efficiency or adding a sonic footprint. Extensive comparison and testing of different components has been carried out here, in a focused effort to design the most efficient and effective crossover networks. Other considerations include ensuring that components are able to withstand high-level current peaks without losing linearity or saturating, which is vital for ensuring that transients are preserved and reproduced accurately during dynamic program material. This is a common theme that extends to the drive units also, ensuring that signal peaks will not introduce distortion or damage components. Tolerances are very carefully measured and components are pair-matched for each pair of speakers produced. A detailed record is kept of the exact component values featured in each pair of loudspeakers sold, in the unlikely event of component failure and subsequent servicing being required. With this method, an overall tolerance of less than 1% can be achieved in any given pair of loudspeakers we produce.

 

Insensitivity to boundary conditions

A combination of factors including a front-firing transmission-line terminus, and very low levels of unwanted energy transfer (owing to cabinet rigidity) result in a loudspeaker that is particularly unfussy with regards to physical placement. While all loudspeakers benefit from being placed in an acoustically treated environment, experience has shown that our speakers exhibit great versatility with regards to their integration in different settings. The excellent off-axis response also lends itself to flexibility in this regard.

 

Robust and acoustically inert cabinets

All Kerr Acoustic loudspeakers are crafted from the finest grade Baltic Birch plywood, which is renowned for naturally being strong and dependable as a material. This plywood consists of a sandwich composite of 1mm thick layers (or ‘plys’) of birch wood, with an alternating grain pattern that rotates 90 degrees between each ply. Resin is used to bond these layers during high-pressure, high temperature formation, and this in conjunction with the 90- degree grain rotation results in a remarkably strong and acoustically inert, low-resonance material. Through applying an iterative approach to manufacture, employing outstanding attention to detail and maintaining fine tolerances throughout assembly, our enclosures are sonically optimised for maximum performance and minimal colouration across the entirety of each loudspeaker’s frequency range.

 

The Results

The end result of this methodical and fastidious approach to design and manufacture is a loudspeaker system that performs quite unlike any other. Their response delivers the accuracy and neutrality required for critical studio monitoring, whilst simultaneously employing the transparency, speed and resolution to make them desirable for high-end domestic application.

When in the presence of Kerr Acoustic speakers, listeners can expect to bear witness to uncannily realistic reproduction of well-recorded sources. Particularly in the case of the smaller models, the response and extension belies the physical cabinet size in quite an extraordinary fashion, with a far greater sense of scale and dynamics than would typically be associated with loudspeakers of comparable size, cost and format.

All musical genres are used in the extensive trialing and fine-tuning process for the products, ranging from classical and baroque recordings, through modern electronic compositions, contemporary rock, pop and urban selections, to experimental music and field recordings. Both analogue and digital mediums are used throughout this process.

 

Tips for best performance

For best performance, the loudspeakers should be partnered with the highest quality ancillaries and partnering components possible, regardless of power. By their nature, our loudspeakers are highly revealing of up-stream components, and will expose any weaknesses in a given signal chain. If being listened to for pleasure, the highest quality sources available should be employed. The shortcomings of compressed digital formats are greatly highlighted by Kerr Acoustic speakers, and therefore should be avoided wherever possible.

While our products will benefit greatly from being partnered with amplifiers that have stable power supplies and high damping factor, they have a greater-than-average sensitivity rating, and do not present any particularly difficult impedance loads. To this extent, they can be partnered with a broad range of different amplifier topologies, from multiple hundred watt solid-state amplifiers, to 5-8W single-ended vacuum tube amplifiers. Our speakers will consistently deliver stunningly natural dynamics and frequency response. Obviously, a greater sense of scale and dynamics will be available when partnered with designs with high current stability, that have sufficient headroom to allow for accurate reproduction of dynamic peaks in the program material. As with all frequency and time-accurate components, our speakers will be extremely revealing of the quality of the amplifier used.

Optimal positioning / placement includes ensuring that the tweeters are roughly at ear level, and that the speakers should be positioned almost parallel to one another, with very minimal toe-in. This configuration generally has shown to produce the most accurate and vivid soundstage in a variety of settings.