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Active Loudspeaker Systems

Examples

 

 

  • Transducers in enclosure with built-in power amplifier

Applications

 

 

  • Home
  • Automotive
  • Multimedia
  • Professional
  • Telecommunication

Particularities

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.


Challenges in the Design

  • 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

Most Important Characteristics

  • 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

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-CEB19 Recommended Loudspeaker Safety Practices 
CEA-2006-A Testing & Measurement Methods for Mobile Audio Amplifiers
CEA-2019 Testing and Measurement Methods for Audio Amplifiers 
CEA-2031 Testing and Measurement Methods for Mobile Loudspeaker Systems
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 – Electro-acoustic Transducers - Measurement of Large Signal Parameters

Institute of Electrical and Electronics Engineers
IEEE 269 Standard Methods for Measuring Transmission Performance of Analog and Digital telephone Sets, Handsets, and Headsets 
IEEE 1329 Standard Method for Measuring Transmission Performance of Speakerphones 
IEEE 1652 Standard for the Application of Free Field Acoustic Reference to Telephony Measurements

International Telecommunication Union
ITU-R Recommendation BS. 775-2: Multi-channel stereophonic sound system with and without accompanying picture
ITU-R Recommendation BS. 1116: Method for subjective assessment of small impairments in audio systems including mulitchannel sound systems


Most relevant Measurements 

Modules of R&D SYSTEM

Modules of QC SYSTEM

Rub & Buzz and impulsive distortion

Transfer Function Measurement (TRF PRO)

Standard, Programmable System
Meta Hearing Technology (MHT)
Air Leak Detection (ALD)

Far field sound pressure on-axis
(sensitivity, mean SPL, effective frequency range)

Transfer Function Measurement (TRF)
3D-Distortion Measurement (DIS)
Scanning Vibrometer System (SCN)
Rocking Mode Analysis (RMA)
Higher Modal Analysis (HMA)

Basic System
Standard System

Directional radiation characteristics
(directivity index, coverage angle, radiation angle, polar and balloon plots, sound power response)

Scanning Vibrometer System (SCN)
3D-Polar Far-Field Measurement (POL)
Transfer Function Measurement (TRF)
Higher Modal Analysis (HMA)

 

Phase response
(minimal-phase, excess-phase, polarity)

Transfer Function Measurement (TRF)

Basic System
Standard System

Group time delay response (total, minimal phase)

Transfer Function Measurement (TRF)

Sound Pressure Task (SPL)
Sound Pressure and Impedance Task (SPL-IMP)

Time-frequency analysis (Wigner, cumulative decay spectrum, sonagraph, wavelet, …)

Transfer Function Measurement (TRF)

 

Nonlinear harmonic distortion
(THD, THD+N, components)

Transfer Function Measurement (TRF)
3D-Distortion Measurement (DIS)

Standard System

Equivalent harmonic input distortion

Transfer Function Measurement (TRF)

 

Intermodulation distortion
(difference-tone and sum-tone IMD)

3D-Distortion Measurement (DIS)

 

Amplitude intermodulation distortion
(AMD, temporal variation of the fundamental)

3D-Distortion Measurement (DIS PRO)

 

Thermal and nonlinear compression
(fundamental, harmonics versus voltage)

3D-Distortion Measurement (DIS)

 

Sinusoidal burst measurement

Transfer Function Measurement (TRF)

 

HI-2 distortion

3D-Distortion Measurement (DIS)

 

Multi-tone distortion

Linear Parameter Measurement (LPM)
Multi-Tone Measurement (MTON)

 

Distributed mechanical parameters
(mechanical vibration scanned on radiator's surface )

Scanning Vibrometer System (SCN)
Rocking Mode Analysis (RMA)
Higher Modal Analysis (HMA)

 

Modal analysis (natural frequencies, shape of modal vibration, modal loss factor)

Scanning Vibrometer System (SCN)
Rocking Mode Analysis (RMA)
Higher Modal Analysis (HMA)

 

3D-geometry scanning

Scanning Vibrometer System (SCN)

 

Accumulated acceleration level (AAL)

Scanning Vibrometer System (SCN)
Rocking Mode Analysis (RMA)
Higher Modal Analysis (HMA)

 

Decomposition into radial and circumferential mode
(indicating rocking mode)

Scanning Vibrometer System (SCN)

 

Rocking mode analysis Scanning Vibrometer System (SCN)
Higher Modal Analysis (HMA)