Active Loudspeaker Systems

Active loudspeaker systems use an internal power amplifier and additional analogue and digital electronics for digital audio streaming, DA conversion, limiting, crossover, gain attenuation and loudspeaker control (protection and linearization). The electrical terminals of the transducers (e.g. tweeter and woofer) are usually not accessible for the assessment of the final product. 
Therefore, the evaluation of an active loudspeaker system has only access to mechanical or acoustical output signal monitored by using a microphone or a laser sensor. Contrary to drive units and passive loudspeaker systems, the nonlinearities cannot be measured directly by using a nonlinear system identification technique. However, measurement of harmonic and intermodulation distortion and other meaningful nonlinear symptoms (e.g. dc displacement) may give meaningful indications about the physical cause. Small leakages in the enclosure generating audible air noise can reliably be detected by using a sensitive noise demodulation technique. 
Parasitic vibration of the grill, handle or any other part of the enclosure may cause impulsive distortion similar to the irregular Rub & Buzz defects found in the drive unit. The sound pressure response on-axis and off-axis, sound power response and other directivity characteristics can be measured in the near field under arbitrary conditions or in the far field under anechoic conditions. 
The acoustical characteristics can also be predicted by FEA and BEA using the mechanical distributed parameters of the loudspeaker drive units measured by laser the scanning technique and the geometry of the enclosure. Measurements of the fundamental response at multiple input levels reveal the compression of the fundamental and distortion components caused by nonlinearities and the heating of the voice coil. This data may be used to predict the voice coil temperature and to define the maximal sound pressure output for different stimuli.

• Optimal selection of drive units (woofer, tweeter)
• Optimal design of the vented enclosure (alignment, port noise)
• Crossover design and time alignment
• Thermal power handling
• Thermal and mechanical protection
• Additional equalization
• Rub & Buzz, vibration of the grill, air leakage noise

• Maximal short-term sound pressure level (SPL) in stated frequency band (1s, 1m, on-axis)
• Maximal long-term SPL in stated frequency band (1 min, 1m, on-axis)
• Harmonic distortion at short-term maximal SPL in stated band
• Intermodulation distortion at maximal short-term SPL in stated band
• Impulsive distortion (Rub & Buzz) at maximal short-term SPL in stated band
• Small signal sound pressure frequency response (magnitude and group delay at 1m, on-axis)
• Sound power response or directivity index

• Rub & Buzz, loose partials in the drive unit
• Parasitic vibration of the grill
• Air leakage noise
• Hard limiting of the suspension (large values of weighted harmonics HI-2 distortion)
• Excessive modulation distortion (motor instability, Bl(x) and L(x) asymmetries)
• Acoustical cancellation effects (negative directivity index)

• Cables
• Laser
• Microphones
• Accessories of QC System