With over 60 years of industry experience, B&C Speakers has designed and built thousands of unique transducers. Each year, our sales and engineering departments work together to develop a comprehensive catalogue. While this catalogue reflects the large majority of our technologies, it is primarily designed to feature some of our more recent additions to the product range.
B&C Speakers - All about the Performance
B&C’s specified Nominal Power Handling is measured according to the AES2-1984 standard. The transducer under test is driven for a two hour period with a pink noise signal having a crest factor of 2 (or 6 dB), and filtered to the working range of the transducer itself. For instance, a 50-500 Hz range is typical for woofer testing. Cone loudspeakers are tested in free air. Compression drivers are coupled to their recommended horn. Power is calculated using the RMS value of applied voltage – averaged over the test period – and the minimum value of electrical impedance within the working range of the loudspeaker. After the test, the transducer must be in working order, without permanent impact on its technical performance. Due to the transient character of most musical programs, whose crest factor is commonly above 8 - 10 dB, it is customary to specify a “Continuous Program Power Handling”, double of the Nominal Power Handling, as a recommended amplifier power in order to fully exploit the thermal and mechanical capabilities of the transducer without any clipping in the amplifier stage. On the contrary, when the amplifier is pushed to its limits and shows frequent saturation, its power specification should be less than the rated Nominal Power Handling of the loudspeaker.
Evolution is a process that affects not only products, but also their technical specifications. Constant advances in research provide more and more precise methods to measure the performance of loudspeakers, and describe their features. Thiele-Small parameters have become the universal language for describing loudspeaker behavior in the small signal domain. Nevertheless, they comment little on the working limits of loudspeakers in the large signal domain. These limits are customarily indicated by Xmax, the maximum linear excursion. This value is typically measured according to the AES2-1984 standard, corresponding to a maximum of 10% total harmonic distortion (THD) with a sinusoidal signal (though most manufacturers, including B&C, now typically provide data for Linear Mathematical Xmax, not measured Xmax). Recent research shows that this method can yield ambiguous results, and even different numerical values for the same loudspeaker. The main limit of this measurement is that it looks at the output signal instead of the physical features of the driver itself. On the contrary, the most up-to-date instruments for distortion analysis can measure the variations in loudspeaker parameters when they are fed with high-level signals. In this way, an excursion limit can be fixed, beyond which the parameter’s variation becomes excessive. The “X var” value reported in our data (generally after the traditional “Xmax” value) is measured this way. Beyond this excursion limit, the magnetic field (Bl) seen by the voice coil, or the total suspension compliance (Cms), or both, drops to less than 50% of their small signal value, producing high distortion levels, strong variations from small signal behavior. The new technique yields different results from the standard measurement based on THD. B&C Speakers believes that this added information gives a more accurate and reliable description of loudspeaker behavior in actual operating conditions.
A known issue of the dynamic loudspeaker is the instability of the average working position for frequencies above the resonance frequency. In this range, because of the phase relationship between force and position, the variations of force factor Bl vs position drive the moving assembly away from the Bl maximum. Intuitively, the loudspeaker tends to “slide” down the slopes of the Bl(x) curve. This is referred to as DC offset.
Since the rest position is the optimal average working point, DC offset leads to several bad consequences: reduced excursion capabilities, increased mechanical stress, and increased distortion. These are due to the loudspeaker working in a region where nonlinearities are larger, and thermal dissipation and power handling are lower. Generally speaking, a large amount of DC offset leads to poor performance and shortened loudspeaker life.
All B&C loudspeakers are designed with DC offset reduction in mind, especially our large excursion subwoofers. Our motors are designed to have a large plateau around the rest position, both through magnet assembly optimization and voice coil design. Reducing the slope of the Bl curve, especially in the central region, will reduce instability and therefore the amount of DC offset. A special winding technique has been adopted in the most critical cases. Our suspensions are also designed to counteract DC offset before it degrades performance.