Micro-speakers and headphones

• Telecommunication
• Consumer and convenience applications including wireless and mobile communication devices

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.

• 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

• 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

AN 24 Measuring Telecommunicatoin Drivers 
AN 32 Effective Radiation Area SD