Speaker Design
Speaker Pulse Response
The main parameter of the speaker is pulse response. Parameters of the speakers are almost exclusively measured by a continuous sine wave signal. But continuous sine waves are not identical with regard to music signals. Music consists of many signal types, basic tones, harmonics and noises and are best represented by pulse response. When a speaker is measured by continuous sine wave signal, it has time to swing at higher frequencies, although the speaker has no control over diaphragm mass and motions.
One of the most important parameters in transducer design for Super Live Audio Systems is the removal of unwanted resonances. These resonances are usually caused by the mechanical design of the speaker and its failure to control the diaphragm motions. Resonances reduce overall definition by masking smaller signals and producing tones not related to the original signal.
The figure below shows an original sine signal (red, top) with its sharply defined end and the same reproduced signal (blue, bottom), still oscillating after the signal stops due to poor control of speaker mass. Poor pulse response has a very negative effect on the ability of a speaker to reject feedback. SLA systems feature exceptional feedback rejection and this in part is due to their excellent pulse response.
Effect of speaker’s resonance, original signal (red, top) and reproduced signal (blue, bottom)
Active Impedance Control
Additionally, control over the speaker mass can be very positively impacted by using an active impedance control, (trans-coil) speaker system. This system utilizes a secondary stationary coil, which reduces inductance close to zero and dramatically improves pulse response. Inductance is the main reason for odd harmonic distortion. Odd harmonic distortion is far more audible than even harmonic distortion.
Low Inductance = Low Non Harmonic Distortion
The Active Impedance Control or AIC is an additional fixed, multi turn coil, positioned in the loudspeaker magnetic circuit gap. This coil is almost as long as the gap height and being wound around the pole piece to be very close to the voices coil. A current flowing into this coil generates a magnetic field that is in opposition to the field generated by the moving coil. This cancels out most of the voice coil inductance and reduces the flux modulation and inductance modulation. The AIC device can be seen as an “active” shorted ring in the gap. The two AIC terminals allow driving the additional coil in many different ways according to specific application needs.
Producing very high quality speakers for audio systems with minimal distortion has created the need for enhancement of the electronics for an SLA system design. In reality few manufacturers are able to utilize AIC trans-coil technology, because upon testing they realize that it immediately shows up the fundamental flaws in their own electronics designs. Even greater compromises in audio quality occur when utilizing digital processing to try and correct their acoustic design.
Frequency response and distortion curve with AIC ON and OFF