Meetings Archive – 2008

The Engineering Art Behind the Beolab 5 Loudspeaker

Date: 11 Nov 2008
Time: 18:30

Location: Royal Academy of Engineering
3 Carlton House Terrace
London SW1Y 5DG

Lecture by Gert Munch, Bang & Olufsen

During this lecture Gert Munch will demonstrate how the development of several key technologies, including the development of “acoustical lenses,” led to the design and implementation of the BeoLab 5 loudspeakers.

Gert is based at the Acoustics Research division of Bang & Olufsen, Denmark; he is a specialist in electro-acoustics and has worked at B&O for 30 years. In that time he contributed to the development and design of numerous speaker models, including the subject of this evening’s lecture, the BeoLab 5.

The aims for the BeoLab 5 design included

  • to make the best possible loudspeakers with the most convincing total sound experience
  • to give best possible experience wherever you sit, wherever the loudspeakers are placed
  • to reproduce the whole audible spectrum and dynamic range
  • to make a loudspeaker that didn’t sound like one!

In order to realise the ambition, the following requirements were specified:

  • Adaptive bass control including a moving microphone measurement system
  • Active loudspeaker design using high power ICE power amps
  • Thermal compression compensation (to remove temperature dependency of response)
  • Advanced thermal protection including thermal modelling and monitoring
  • Precise mechanical control and fitting for consistency
  • DSP Processing for response correction and manufacturing variation control

A little history: In the mid-1980s, B&O made the ‘Penta loudspeaker, which embodied the early attempts at B&O to take control of speaker directivity. It had a tapered design, with centralised tweeters, to minimise effects of the floor and ceiling reflections, a factor recognised by B&O engineers as critical to the sound in a real room.

To further understand these reflection issues, the Archimedes project was established (running from 1988 to 1992), and carried out by B&O in conjunction with the Technical University of Denmark and KEF (the UK-based loudspeaker manufacturer). This work led to many ideas about improving loudspeakers and a new, improved unit was designed that, unfortunately, never made it to market.

BeoLab 5 evolution: Also around this time, Sausalito Audio Works was pioneering loudspeaker design incorporating what it dubbed ‘Acoustic Lens Technology’. Despite some initial scepticism, B&O engineers concluded that the speakers from Sausalito actually sounded good.

After several iterations at B&O of the initial Sausalito design, the BeoLab 5 was the evolutionary result. Its distinctive shape (some liken it to a Dalek or a pylon) make it easily recognisable – and it weighs in at a hefty 61kg!

The Acoustic Lens (perhaps a ‘lens’ in the sense that a curved-mirror in a reflector telescope can be a lens) is a mechanical structure that consists of a specially shaped reflector mounted atop an upward facing driver, the special shape being a quarter of an ellipsoid.

An ellipse has two focal points; the drive unit is located at the first so that, by virtue of the shape, all sound passes through the second (assuming a ray-tracing model and an infinitesimal source).

Prior to the building of the speaker, some ray-tracing based simulations were attempted. This simulation technique was later abandoned because such a basic model lacks the ability to predict diffraction effects, a critical factor in loudspeaker directivity.

An audience member asks, ‘Why not place the speaker at the second focal point and do-away with the lens?’. Gert’s answer is that such an approach would not provide any control over the radiation pattern – and it is this radiation pattern control that the ‘acoustic lens’ technology seeks to master.

Later modelling attempts included Boundary Element and Finite Element Analysis.

An animated picture is shown to demonstrate a radiation pattern simulation. The key point is that the response looks the same at a wide range of angles in the horizontal plane. Comparing the two-dimensional Finite element model with the 3-dimensional boundary element model, it is noted that, as presented, they look very similar, providing further confidence in their validity and the concept in general.

Gert points out that, at least initially, the ideal radiation pattern of this speaker appears to be similar to that of a dipole, however, the problem of traditional dipoles loudspeakers is they must be placed at least 1m away from the wall behind to achieve good performance, a restriction which can prove inconvenient in real-world situations, usually due restrictions imposed by one’s cohabitee.

In the BeoLab 5, B&O have aimed to make a design with a forward directivity similar to that of a dipole but, due to the attenuated rear-response, one which can be placed directly against a wall.

Taking the power average from nine measurements made at random room positions, yields some kind of loudspeaker power response. Other measurement techniques have been tried, but this power averaging technique, Gert reports, shows better correlation with subjective testing.

Efficiency of loudspeakers is generally low and the BeoLab 5 is no exception. Free-field, 200W of electrical power input might yield 1W of acoustic power. The BeoLab 5 contains amplifiers capable of supplying around 2.5kW of power!

Gert notes there can be huge changes in power response at around 100Hz for differing speaker placement, so a filter is introduced to equalise the power response positioning-room. A normal tone control can never compensate for this kind of problem; much more precise control is provided in the BeoLab 5 using Digital Signal processing.

The BeoLab 5 includes a formidable array of signal processing. The crossovers are performed digitally, and much more besides.

During factory test, the response of each driver is automatically equalised to compensate for manufacturing tolerances. Overall equalisation is also applied to achieve the overall target frequency response. This production testing employs a total of 6 microphones – four at the front (one close to each of the drivers) and two at the rear. A reference speaker provides the target for the equalisation process . Each production speaker is adjusted to match the frequency response of this reference unit with a target error of less than 0.5dB.

Temperature and air pressure can alter the measurements significantly, so these are monitored during this phase.

Using an in-built, motorised microphone which slides out from under the speaker, automatic correction of low-frequency response up to around 300Hz can be invoked by the user to reduce the effects of the room in which the speakers are placed. Gert points out that this correction is not a modal correction – it’s more like a general equalisation, with the filter response being smoothed during the measurement process.

Interestingly, the target response for this ‘auto-correction’ system is not, as one might expect, a flat response, but rather a response that has been determined empirically through critical listening.

The thermal monitoring uses a combined technique of feed-forward modelling in conjunction with average temperature measurement of the driver mechanical assembly. Each driver also has thermal modelling, arranged such that should, on average, too much power be applied to any driver, progressive attenuation is applied to its output (and also to outputs to all drivers of higher frequency to maintain a consistent tonal balance).

A “party test” is also carried out which runs the speakers at full-power for three days!

The BeoLab 5 is a no-compromise design that might at first appear to be at the more esoteric end of hi-fi. But many thousands of units have been sold, proving that many consumers still aspire to achieve great audio reproduction and are prepared to buy-in to new technology to achieve it.

It was fascinating to hear about the design philosophy and gain some insight into the processes. On behalf of all present I’d like to extend thanks both to Gert for the presentation, and to B&O for making it possible.

Report by Nathan Bentall

An interview with Bob Stuart

Date: 9 Dec 2008
Time: 18:30

Location: Royal Academy of Engineering
3 Carlton House Terrace
London SW1Y 5DG

Conducted by Keith Howard

A recording of this lecture is available here.

Bob Stuart has been a major figure in the British audio industry for over 30 years. Best known as Chairman and co-founder, with Allen Boothroyd, of what is today Meridian Audio Ltd, he has done much more than steer the company through challenging times to its current high-profile position manufacturing some of the most sophisticated audio equipment available. A pioneer of active and then DSP-equipped loudspeakers, he was quick to recognise the potential of CD and, as part of the ARA, to push for a version of DVD dedicated to high-resolution multichannel audio. Meridian’s own lossless compression algorithm, MLP, was developed in anticipation of this and selected by the DVD Forum for DVD-Audio in a technology shoot-out against stern competition. In expanded form it remains the basis of the Dolby TrueHD lossless compression scheme used in Blu-ray Disc. With a long-standing interest in psychoacoustics, which he studied alongside electronic engineering at Birmingham University, Bob is one of very few creators of high-quality audio equipment to have explored the fundamentals of sound perception and generated computer models of human hearing to help guide the design process. In recent years, in collaboration with Peter Craven, he has investigated the effects of digital anti-aliasing and reconstruction filters, one intriguing result being that Meridian’s latest flagship CD player – the 808.2 Signature Reference – uses minimum-phase rather than linear-phase output filtering.

These subjects and many others will be covered in this interview, with Bob presenting supporting material to clarify the issues.