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Balanced Armature Transducer

Examples

 

 

  • Hearing aids
  • Headphones
  • In-ear

Applications

 

 

  • Consumer
  • Medical and convenience applications mostly battery operated
  • Hearing aids
  • In ear monitors

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

          American National Standards Institute
          ANSI S 3.3 (R1976) Electroacoustical Characteristics of Hearing Aids
          ANSI S3.7 Method for Coupler Calibration of Earphones
          ANSI S3.22-2003 Specification of Hearing Aid Characteristics
          ANSI S3.25 Standard for an Occluded Ear Simulator

          International Electrotechnical Commission
          IEC 60118 Hearing Aids Measurements of Electroacoustical Characteristics 
          IEC 60318 Simulators of Human Head and Ear

          International Telecommunication Union
          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)

          Basic System
          Standard 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 System (SCN)

           

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

          Transfer Function Measurement (TRF PRO)
          Rocking Mode Analysis (RMA)

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

          Air leakage noise localization 
          (position of modulated noise source)

           

          Air Leakage Stethoskope (ALS)

          Amplitude Response
          (sensitivity, mean SPL, polarity)

          Transfer Function Measurement (TRF)

          3D-Distortion Measurement (DIS)

          Basic System
          Standard System

          Nonlinear Harmonic Distortion
          (THD, components)

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

          Basic System

          Standard System

          Intermodulation Distortion (IMD)

          3D-Distortion Measurement (DIS)

           

          Amplitude intermodulation distortion (AMD)
          (modulation of the fundamental)

          3D-Distortion Measurement (DIS)

           

          Thermal and nonlinear compression
          (fundamental, harmonics, dc displacement)

          3D-Distortion Measurement (DIS) 

           

          Peak Displacement
          DC Displacement

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

          Multi-tone Distortion

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

           

          Phase response

          Transfer Function Measurement (TRF)

          Basic System
          Standard System 

          Minimal-phase, excess-phase, group delay

          Transfer Function Measurement (TRF)

           

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

          Transfer Function Measurement (TRF)

           

          Thermal Characteristics
          (resistances, time constants, capacities)

          Power Testing (PWT)

           

          Accelerated life test, power test
          (durability, temperature)

          Power Testing (PWT)

           

          Coil Temperature

          3D-Distortion Measurement (DIS)
          Power Testing (PWT)

           

          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)

           

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

          Scanning Vibrometer System (SCN)

           

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

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

           

           

          Rocking mode analysis

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