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Micro-speakers and headphones

Low-frequency drive unit


Transducer:

Micro-speaker, transducers for headphones, analogue and digital telephones, handsets and headsets

Applications:

telecommunication, consumer and convenience applications including wireless and mobile communication devices


Particularities:

Microspeakers and transducers for headphones are transducers reproducing not only speech signals but more and more full band audio signals. Some of the transducers are also used below the resonance frequency ranging from 100 Hz – 2 kHz. Most high frequency drive units use moving coil technology dispensing with a separate spider and surround. The moving coil assembly is prone to rocking modes, to irregular vibration modes of the wire and the rim zone and to hard limiting at maximal excursion causing
Rub & Buzz and other impulsive distortion. The electrical, mechanical and thermal behavior of the drive unit at fundamental resonance frequencies can be described by the equivalent network comprising lumped elements with linear and nonlinear parameters. Distributed parameters are required to describe the vibration and radiation behavior at higher frequencies where the radiator (diaphragm) does not vibrate as a rigid body and bending and longitudinal waves propagate in circumferential and radial direction. The linear parameters comprise the Thiele-Small parameters and visco-elastic parameters (creep factor). The inductance and the losses due to eddy currents are negligible. The dominant nonlinearities are the force factor Bl(x), stiffness Kms(x) or compliance Cms(x) and the nonlinear damping Rms(v) versus velocity v. The nonlinear variations of the inductance L(i) and L(x) versus input current i and x are usually negligible. Direct mechanical measurements of the mechanical vibration by optical sensors (e.g. laser scanning techniques) are required to measure the mechanical parameters (T/S) including the effective radiation area Sd reliably. Perturbation techniques are difficult to apply because an added mass cannot be attached to the diaphragm, and a precise value of Sd is not available for the enclosure technique. Thermal parameters describe the heating of the coil, the heat transfer to the pole tips, the magnet and the ambience.  


Critical issues:

  • Rocking modes and other irregular vibration modes
  • Rub & Buzz, air leakage noise and other impulsive distortion
  • Heating of the coil and thermal power handling
  • Asymmetry of the stiffness nonlinearity Kms(x) of mechanical suspension
  • Nonlinear mechanical resistance Rms(v) dominates total damping
  • Asymmetry of the B field and voice coil rest position


Standards:

  • IEC Standard IEC 60268-5 Sound System Equipment, Part 5: Loudspeakers
  • IEC Standard IEC62458 Sound System Equipment – Electroacoustic Transducers - Measurement of Large Signal Parameters
  • IEEE Standard 269 IEEE Standard Methods for Measuring Transmission Performance of Analog and Digital telephone Sets, Handsets, and Headsets
  • IEEE 1329 Standard Method for Measuring Transmission Performance of Handsfree Telephone Sets
  • ANSI S3.22-2003 Specification of Hearing Aid Characteristics
  • ANSI S3.7 Method for Coupler Calibration of Earphones
  • ANSI S3.25 Standard  for an Occluded Ear Simulator
  • ITU-R Recommendation BS. 775-2: Multi-channel stereophonic sound system with and without accompanying picture. International Telecommunication Union (ITU), 2006

Most relevant Measurements 

Modules of R&D SYSTEM

Modules of QC SYSTEM

Linear lumped parameters

(resonance frequency, Q-factors, Thiele/Small, creep, inductance)

Linear Parameter Measurement (LPM)

Impedance task

Motor + Suspension Check (MSC)

Effective radiation area Sd

Scanning Vibrometer (SCN)

 

Loudspeaker nonlinearities

(Bl(x), Kms(x), L(x) , Rms(v))

Large Signal Identification (LSI Tweeter)

 

Single-valued nonlinear parameters
(Xmax , XBl , XC , Voice coil offset Xoffset , Suspension asymmetry AKms)

Large Signal Identification (LSI Tweeter)

Motor + Suspension Check (MSC)

Thermal parameters

(resistances, time constant, capacities, air convection cooling)

Large Signal Identification (LSI Tweeter)

Power Test (PWT)

 

Irregular loudspeaker defects

(Rub & Buzz, loose particles, wire beat, bottoming, air leakage noise)

Transfer Function Module (TRF PRO)

Standard, Programmable System

Meta-Hearing-Technology

Leak Detection Module

Air leakage noise localization

(position of modulated noise source)

 

Air Leakage Localization Module

Amplitude response

(sensitivity, mean SPL, polarity)

Transfer Function Module (TRF)

3D-Distortion Module (DIS)

Basic, Standard, Programmable System

Nonlinear harmonic distortion

(THD, components)

Transfer Function Module (TRF)

3D-Distortion Module (DIS)

Basic, Standard, Programmable System

Intermodulation distortion (IMD)

3D-Distortion Module (DIS)

 

Amplitude intermodulation distortion (AMD)

(modulation of the fundamental)

3D-Distortion Module PRO (DIS)

 

Thermal and nonlinear compression

(fundamental, harmonics, dc displacement)

3D-Distortion Module (DIS)

 

Peak displacement

DC displacement

3D-Distortion Module (DIS)

Transfer Function Module (TRF)

Large Signal Identification (LSI)

 

Multi-tone distortion

Linear Parameter Measurement (LPM)

Multi-tone task in Standard System

Phase response

Transfer Function Module (TRF)

System task in Standard System

Minimal-phase, excess-phase, group delay

Transfer Function Module (TRF)

 

Time-frequency analysis

(Wigner, cumulative decay spectrum, sonagraph, wavelet, …)

Transfer Function Module (TRF)

 

Accelerated life test, power test

(durability, parameter variation)

Large Signal Identification (LSI)

Power Test (PWT)

 

Coil temperature

3D-Distortion Module (DIS)

Large Signal Identification (LSI)

Power Test (PWT)

 

Distortion generated by dominant nonlinearities

Bl(x), Cms(x), Le(x), Rms(v) in reproduced audio signal

Auralization Module (AUR)

Large Signal Identification (LSI)

Power Test (PWT)

 

Accumulated acceleration level (AAL)

Scanning Vibrometer (SCN)

 

Decomposition into radial and circumferential mode

(indicating rocking mode)

Scanning Vibrometer (SCN)

 

Polar radiation response analysis (far field SPL response at spatial resolution), directivity index, sound power

Scanning Vibrometer (SCN)

 

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

Scanning Vibrometer (SCN)

 

Auralization

Auralization Module (AUR)