BeoLab 5 150
Horizontal Angle [°]
100 50 0 −50 −100 −150 100
1000 Frequency [Hz]
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Fig. 2 Directivity of the BeoLab 5. Contours are in steps of 3 dB related to the on-axis response.
dispersion in the vertical plane. In addition to the specific horizontal and vertical directivity characteristics, the Beolab 5 also proved to have a combination of a flatter on-axis magnitude response and smoother power response than was previously seen in B&O loudspeakers. As was mentioned above, the initial filters applied to a prototype loudspeaker under development are the direct result of acoustical measurements. Following this, the loudspeaker undergoes an extensive sound design process where additional filters are applied based on listening to a wide range of recordings in various listening rooms. The final result is then measured once again in The Cube in order to define the master reference specification of the loudspeaker before the start of production. The final on-axis magnitude and power responses of the Beolab 5, seen in Figure 3, show that a smooth power response proved to be more important than a flat on-axis response. This result did not come as a surprise to the development team, since it was a trend that was seen in the development and tuning of previous B&O loudspeakers.
Fig. 3 On-axis frequency response (blue) and power response (red curve) of the BeoLab 5.
The ‘Sharkfin’ Experiment At the end of the ARCHIMEDES project, Gert Munch (the acoustical engineer on the Beolab 5 project) had an idea of building an active loudspeaker capable of simulta-
neously delivering both a flat magnitude response on-axis and a smooth power response. The basic concept was (for example) that a ‘classic’ two-way loudspeaker construction be used with active filtering to deliver the desired on-axis response. An additional dipole assembly aimed 90º away from the on-axis direction would then sit either on top of the loudspeaker (hence the nickname the ‘Sharkfin’) or horizontally-mounted on its back. Either positioning would place the null of the dipole in line with the front of the ‘main’ loudspeaker. The signal from this dipole could then be used to alter the power response of the system without having an influence on the on-axis response of the total system.
Fig. 4 The “Sharkfin” experimental loudspeaker with a top-mounted dipole made with back-toback drivers.
This concept was finally built and tested in late 2012. Measurements in The Cube indicated that the concept performed as predicted in a free field, and so an extended listening session was undertaken. The outcome of this experiment indicated that (in a 2-channel stereo configuration), when changing the contribution of the dipoles, there was an audible effect - not only on the timbral impression of the loudspeakers but also on the spatial presentation of the recordings. Phantom images changed both in perceived width and distance, and envelopment (when it existed in the recording) was also altered, particularly when the loudspeakers were closer to a reflecting surface. This was due to the fact that, although the dipole’s null was aimed at the listening position, it provided an audible (but unwanted) contribution to the early reflections from sidewalls. This was particularly noticeable in a symmetrical stereo configuration when the dipoles faced in similar directions, increasing the phase difference of the lateral reflections, and therefore giving an artificial sense of spaciousness, even in monophonic recordings.
These artefacts led to abandoning the original ‘pure’ concept of the ‘Sharkfin’, but starting a new loudspeaker concept with a variable (or at least selectable) directivity that would give a customer the opportunity to determine the horizontal beam width and thus the balance of direct-to-reflected sound in the listening room. By that time, Munch and Jakob Dyreby had already been collaborating on a research project with two graduate students at DTU, Martin Møller and Martin Olsen (both of whom started working at B&O after they graduated). They, together with Finn Agerkvist, an associate professor at DTU, presented a scientific paper at a convention of the Audio Engineering Society in 2010 called “Circular Loudspeaker Arrays with Controllable Directivity”. In this publication, they showed how a cylindrical enclosure containing 24 small loudspeaker drivers, each with its own amplifier and individualised DSP, could be used to steer a beam of sound in any direction in the horizontal plane with an arbitrary and controlled directivity, with the purpose of creating different individual ‘sound zones’ within a single listening area. This study proved that it is possible to create a loudspeaker that could, using different filters in DSP, range from a narrow directivity (similar to commercially-available dipole- or cardioid-pattern loudspeakers) through to a full horizontal omnidirectional dispersion. Consequently, it was then used as the basis of development on a commercial product with similar capabilities. Beolab 90: Beam Width Control This first prototype of the new loudspeaker, internally code-named ‘Speaker 40’ or ‘S40’, was devised as a scaled-down version of the barrel loudspeaker used in Møller and Olsen’s experiments. It had a hexagonal arrangement of tweeters and midranges and a square arrangement of woofers as can be seen in Figure 5. Each driver was to have its own amplification and DSP with customised filters to correct its time- and frequency-domain charac-
Fig. 5 S40: prototype 1
The Danish loudspeaker 100 year anniversary — 51