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Balanced Armature Transducer

Transducer:

Electro-magnetic transducers for hearing aids, headphones, and other portable devices

Applications:

Consumer, medical and convenience applications mostly battery operated. Hearing aids, In ear moitors

Particularities:

Balanced Armature (BA) transducers are based on the electro-magnetic principle and show a significant higher efficiency compared to electro-dynamic transducers. An armature is balanced in a magnetic field produced by permanent magnets. A fixed coil wounded around the armature produces an AC field that causes a mechanical excursion of the armature. A small rod transfers the vibration to a membrane that acts as sound source. The excursion of the armature is quite small but the force is high. Thus, a coupled small, stiff air volume (e.g. the auditory canal of the human ear) can be excited efficiently. Therefore, typical applications are in-ear products such as hearing aids and head phones. 

 

By default BA transducer are operated above their resonance frequency but there are also wide band transducers in the market. Similar to multi-way speakers they also can be cascaded to cover the full audio frequency range. 

  • The dominant nonlinearities are the coil inductance L(x), the transductance parameter T(x) and stiffness Kms(x) or compliance Cms(x)      
  • A well centered rest position of the armature in the magnetic field is crucial to produce low distortion and get highest SPL / highest excursion. Testing the offset is one of the most important tests for BA transducer. ·         
  • Due to the small and sensitive geometry rub&buzz problems may easily occur and needs to be tested. 
  • In case of overvoltage mechanical clipping may occur. The armature hits one of the magnets.  If the restoring force used to keep the armature centered is too low, the armature may stick permanently on the magnet, if touching.  
  • Scanning of the diaphragm is difficult. The diaphragm is usually covered by the housing. It has usually a rectangular shape. 
  • The transduction parameter T depends on rod position (lever). It is similar to the Bl factor in electrodynamic transducers and defines the coupling between electrical and mechanical domain. 
  • Due to the large coil the DC resistance is usually high. 


Critical issues:

  • Asymmetry / offset of the armature position        
  • Mechanical clipping 
  • Rub & Buzz, air leakage noise and other impulsive distortion 
  • Asymmetry of the stiffness nonlinearity Kms(x) of mechanical suspension 
  • High resistance requires high sensitivity measurement hardware 


Standards:

  •  ANSI S3.22-2003 Specification of Hearing Aid Characteristics 
  •  ANSI S 3.3 (R1976) Electroacoustical Characteristics of Hearing Aids
  •  ANSI S3.7 Method for Coupler Calibration of Earphones 
  •  ANSI S3.25 Standard for an Occluded Ear Simulator
  •  IEC 60118 Hearing Aids Measurements of Electroacoustical Characteristics 
  •  IEC 60318 Simulators of human head and ear 
  •  ITU-T Rec. P.57 Artificial ears 

      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)

      QC Standard SystemBasicStandard System

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

       

       Balanced Armature Check (BAC)

      Effective radiation area Sd

      Scanning Vibrometer (SCN)

       

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

      Transfer Function Module (TRF PRO)

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

      Air leakage noise localization 
      (position of modulated noise source)

       

      Air Leakage Stethoskope (ALS)

      Amplitude Response

      (sensitivity, mean SPL, polarity)

      Transfer Function Module (TRF)

      3D-Distortion Module (DIS)

      BasicStandard 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 (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)

      Multi-tone Distortion

      Linear Parameter Measurement (LPM)

       Standard System

      Phase response

      Transfer Function Module (TRF)

      Basic, Standard, Programmable System 

      Minimal-phase, excess-phase, group delay

      Transfer Function Module (TRF)

       

      Time-frequency analysis

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

      Transfer Function Module (TRF)

       

      Thermal Characteristics

      (resistances, time constants, capacities)

      Power Test (PWT)

       

      Accelerated life test, power test

      (durability, temperature)

      Power Test (PWT)

       

      Coil Temperature

      3D-Distortion Module (DIS)

      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)