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High-frequency Drive Units

Examples:

tweeter, horn compression driver

Applications:

home, automotive, multimedia and professional

Particularities:

High-frequency loudspeaker drive units are operated at higher frequencies by using a highpass of a crossover system. Most of the transducers use a moving-coil assembly based on the electro-dynamical transduction principle. At the resonance frequency (about 1 kHz) the radiator (soft or hard dome, diaphragm, membrane) vibrates as a rigid body, and the transfer behavior of the drive unit can be modeled by an equivalent network comprising lumped elements with linear and nonlinear parameters. The linear parameters comprise the Thiele-Small parameters, visco-elastic parameters (creep factor) and electrical parameters describing the lossy inductance at higher frequencies. The maximal acoustical output is limited by the heating of the coil, regular nonlinearities and irregular defects, such as Rub & Buzz. Thermal parameters describe the heating of the coil, the heat transfer to the pole tips, magnet and ambience considering conduction and radiation. The dominant nonlinearities are the stiffness K_ms(x) or compliance C_ms(x) depending on the voice coil displacement x and the resistance R_ms(v) varying with velocity v.

This drive unit is usually operated at higher frequencies where bending waves and longitudinal waves are generated on the radiator. Distributed parameters are required to describe the vibration and radiation behavior and can be measured by laser scanning techniques. Modal analysis and other decomposition techniques reveal the modal density, loss factor of the material and radiation problems.

Critical issues:

Flat SPL response on-axis and off-axis in the far field
Maximal peak displacement limited by suspension nonlinearities
Heating of the coil and thermal power handling
Rocking modes causing voice coil rubbing
High local displacement on particular points on the diaphragm causing nonlinear distortion

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
AES2-1984 AES Recommended practice Specification of Loudspeaker Components Used in Professional Audio and Sound Reinforcement
AES56-2008 AES standard on acoustics – Sound source modelling – Loudspeaker polar radiation measurement

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) Transfer Function Module (TRF)
Loudspeaker nonlinearities (Bl(x), Kms(x), L(x) , L(i), Rms(v))	Large Signal Identification (LSI Tweeter)
Single-valued nonlinear parameters (X_max , X_Bl , X_C , Voice coil offset X_offset , Suspension asymmetry AK_ms)		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
On-axis sound pressure amplitude response (sensitivity, mean SPL, effective frequency range)	Transfer Function Module (TRF) 3D-Distortion Module (DIS) Scanning Vibrometer Analysis Software (SCN)	Basic, Standard, Programmable System
Directional characteristics (directivity index, coverage angle, radiation angle, polar radiation measurement)	Scanning Vibrometer (SCN) 3D-Polar Radiation Measurement (POL) Transfer Function Module (TRF)
Sound power response	Scanning Vibrometer (SCN) 3D-Polar Radiation Measurement (POL) Transfer Function Module (TRF)
Phase response (minimal-phase, excess-phase, polarity)	Transfer Function Module (TRF)	Basic, Standard, Programmable System
Group time delay response (total, minimal phase)	Transfer Function Module (TRF)	System task in Standard System
Time-frequency analysis (Wigner, cumulative decay spectrum, sonagraph, wavelet, …)	Transfer Function Module (TRF)
Nonlinear harmonic distortion (THD, THD+N, components)	Transfer Function Module (TRF) 3D-Distortion Module (DIS)	Standard, Programmable System
Equivalent harmonic input distortion	Transfer Function Module (TRF)
Intermodulation distortion (IMD)	3D-Distortion Module (DIS)
Amplitude intermodulation distortion (AMD) (modulation of the fundamental)	3D-Distortion Module (DIS PRO)
Thermal and nonlinear compression (fundamental, harmonics, dc displacement versus voltage)	3D-Distortion Module (DIS)
Voice coil displacement (peak, bottom, dc component, Xmax)	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
Accelerated life test, power test (durability, parameter variation, maximal input power)	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)
Auralization	Auralization Module (AUR)
Distributed mechanical parameters (mechanical vibration scanned on radiator's surface )	Scanning Vibrometer (SCN)
Modal analysis (natural frequencies, shape of modal vibration, modal loss factor)	Scanning Vibrometer (SCN)
Shape of the surface (scanning geometry)	Scanning Vibrometer (SCN)
Accumulated acceleration level (AAL)	Scanning Vibrometer (SCN)
Decomposition into radial and circumferential mode (indicating rocking mode)	Scanning Vibrometer (SCN)
3D-geometry scanning	Scanning Vibrometer (SCN)