Adequate power

The BeoLab 5 is a 4-way powered loudspeaker. For the lower and upper bass systems, the enclosures have small sealed volumes (29 and 5 liters respectively), in order to maintain the desired footprint and height.

At the same time, adequate acoustic power was desired to effectively mimic the loudness of orchestral and rock/heavy metal performance.

The design goal was to obtain full bandwidth musical performance to 108 dB SPL for short-term sustained periods, with peak levels up to 120 dB SPL, for listeners in a large room, at 3 meters from a stereo pair of BeoLab 5 loudspeakers.

This required three separate strategies:

• sufficient power available for each driver in the loudspeaker,
• a strategy to deal with the problems of thermal compression, and
• a strategy to protect the drivers and amplifiers from damage due to overloads.

Power is provided by Bang & Olufsen’s proprietary ICE Power Class D switching amplifiers. These are highly efficient, small in size, require little cooling, and yield high power output. As a result, the actual amplifier power available for each loudspeaker is very high, at:

Tweeter: 250 Watts
Mid-range: 250 Watts
Mid-bass: 1,000 Watts
Lower bass: 1,000 Watts
Total power per speaker: 2,500 Watts
Total power per stereo pair: 5,000 Watts

This configuration provides adequate power output to manage both the desired sustained levels and peak levels mentioned above. These levels deserve some comment. The measured level of 108 dB SPL at 3 meters is done with 2 loudspeakers playing uncorrelated pink noise. Allowing for reverberant energy, one loudspeaker will therefore generate a sustained level of ca. 102 dB SPL at 3 meters, or 111 dB SPL at 1 meter, with peaks as high as 121 dB SPL. This is approximately 15 dB higher than conventional loudspeakers as described in the earlier paper, and roughly equivalent to the concert hall levels encountered with acoustical performance. It is still 10-15 dB below the sustained levels that can be obtained by a rock or heavy metal band in live performance. In listening trials, the BeoLab 5s obtained subjective loudness levels that satisfactorily mimicked (and may have replicated) the sound pressure levels obtained by large acoustical ensembles.

Thermal compensation and protection

Thermal compression occurs as a function of heat buildup in the drivers, causing acoustical output to drop as temperature rises. Each driver has a thermal sensor built in. The output of the two low frequency drivers is increased as temperature increases, by up to 4 dB, in order to make short term compensations for thermal buildup in the drivers.

In addition, a thermal protection model using the same sensors is also in place to attenuate each driver if and when it reaches dangerously high temperatures, for a fail-safe protection system.