Passive Loudspeaker Systems
Examples
- Drive units with crossover in enclosure
Applications
- Home
- Automotive
- Multimedia and Professional
Particularities
Passive loudspeaker systems use an external power amplifier and are usually operated over the entire audio band. The transfer behavior of each drive unit (woofer, midrange, tweeter) can be described by an equivalent network model using lumped elements which are valid at the corresponding resonance frequency.
The linear, nonlinear and thermal parameters of the model can be measured by using a new identification technique monitoring voltage and current at the loudspeaker terminals only. In addition to the mechanical and electrical parameters of drive units (see tweeter, woofer), there are a few acoustical parameters describing the port resonance of a vented enclosure, nonlinear resistance of the air in the port and the ratio of the air and suspension compliances.
Small leakages in the enclosure which do not affect the electrical input impedance may cause audible air noise. This can be detected more reliably 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 condition. The acoustical characteristics can also be predicted by FEA and BEA using the mechanical distributed parameters of the loudspeaker drive units measured by a laser scanning technique and the geometry of the enclosure.
Thermal model and the parameters valid for a single drive unit measured in free air may describe approximately the coil temperature and the cooling process in vented box systems. If the drive unit is mounted in a sealed enclosure, the thermal resistance Rtm describing the heat transfer between magnet and ambience is much higher than in free air.
Design Challenges
- Optimal selection of drive units (woofer, tweeter)
- Optimal design of the vented enclosure (alignment, port noise)
- Crossover design and time alignment
- Thermal power handling
Most Important Characteristics
- SPL frequency response (1 m, 1 W on-axis)
- Mean SPL in a effective frequency range (1m, 1W on-axis)
- Long-term maximal input power
- 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)
- Impedance curve (rated impedance)
- Total harmonic distortion
- Intermodulation distortion
- Impulsive distortion (Rub & Buzz) at maximal short-term SPL in stated band
- Sound power response or directivity index
Critical Issues
- 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)
Standards
Audio Engineering Society
AES2 Recommended practice Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement
AES56 Standard on acoustics – Sound source modeling – Loudspeaker polar radiation measurement
Consumer Electronics Association
CEA-2034 Standard Method of Measurement for In-Home Loudspeakers
International Electrotechnical Commission
IEC 60268-5 Sound System Equipment, Part 5: Loudspeakers
IEC 62458 Sound System Equipment – Electroacoustic Transducers - Measurement of Large Signal Parameters