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Low-frequency drive units

Examples:

woofer, subwoofer

Applications:

home, automotive, multimedia and professional

Particularities:

Low frequency drive units are operated below and above the fundamental resonance frequency (usually below 500 Hz) by using a crossover with a low-pass characteristic. Most low frequency drive units contain a moving coil assembly using an electro-dynamical motor principle. The electrical, mechanical and thermal behavior of the drive unit at fundamental resonance frequencies can be described by 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 dominant nonlinearities are the force factor Bl(x), stiffness Kms(x) or compliance Cms(x) and inductance L(x) versus displacement x and the inductance L(i) varying with the input current i. Thermal parameters describe the heating of the coil, the heat transfer to the pole tips, magnet and ambience considering conduction, radiation and convection cooling.

Critical issues:

Maximal peak displacement
High dc displacement (coil is shifted out of the gap)
Motor instability (bifurcation, jumping effect)
Thermal power handling
Rocking modes and circumferential modes
Intermodulation distortion caused by Le(x) and Bl(x)

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

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) , L(i))	Large Signal Identification (LSI) Power Test (PWT)
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 (thermal resistances, time constants, capacities, air convection parameter)	Large Signal Identification (LSI) 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 ok (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 (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)	Transfer Function Module (TRF)
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)	Basic, Standard, Programmable System
Equivalent harmonic input distortion	Transfer Function Module (TRF)
Intermodulation distortion (difference-tone and sum-tone IMD)	3D-Distortion Module (DIS)
Amplitude intermodulation distortion (AMD) (modulation of the fundamental)	3D-Distortion Module (DIS PRO)
Sinusoidal burst measurement	Transfer Function Module (TRF)
Thermal and nonlinear compression (fundamental, harmonics 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)
HI-2 distortion	3D-Distortion Module (DIS)
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)
Voice coil temperature	3D-Distortion Module (DIS) Large Signal Identification (LSI) Power Test (PWT)
Distortion in reproduced audio signal (contribution of Bl(x), Cms(x), Le(x), Le(i))	Auralization Module (AUR) Large Signal Identification (LSI) Power Test (PWT)
Auralization	Auralization Module (AUR)
3D-geometry scanning	Scanning Vibrometer (SCN)
Shape of the surface (scanning geometry)	Scanning Vibrometer (SCN)
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)
Accumulated acceleration level (AAL)	Scanning Vibrometer (SCN)
Decomposition into radial and circumferential mode (indicating rocking mode)	Scanning Vibrometer (SCN)