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Our Application Notes contain step-by-step instructions for solving common tasks with the Analyzer System.

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The Application Note is dedicated to particular properties of measuring telecommunication drivers used in headsets or mobiles. These drivers are designed to be operated over a wide frequency range with highest efficiency to save battery power. The small dimensions as well as low level state signals such as voltage or excursion require special measurement setups. Furthermore practical tips for mounting and recommended hardware components are given.

This Application Note explains the interpretation of scanning results which gives more insight into the relationship between sound pressure output (directivity), cone vibration (natural modes) and design (geometry and material properties). It summarizes our experiences collected in scanning of all kinds of transducers (microspeakers, tweeters, compression drivers, woofers and subwoofers).

Publishing measured results is important to customers and clients. While the KLIPPEL database contains all information about results and setups in one single file, this format may be not decent for non-technical people. Using the KLIPPEL software, it is easy and fast to create HTML-templates based on operations or objects within KLIPPEL databases. In this Application Note the access of the data is shown using windows scripting (VBS). This is a simple and open programming language which can be edited and extended easily by the user. This solution is convenient and extremely flexible at the same time.

In automated production environments the testing time is defined by the cycle time which includes the actual measurement time, the calculation of results, the output and storage of results and the mechanical handling of the DUT (moving, connecting). In this Application Note it is discussed how to fill the available testing time in a most efficient way. There is no single solution for all applications, but the interaction of measurement system and time restrictions are explained and can be applied to similar problems.

The rest position of the voice coil is a very critical parameter of dynamical transducers (speaker, shaker, headphone, ...). An offset may produce additional signal distortion and a DC-displacement derogating the stability of the driver (moving the coil outside the gap). On the other side an offset from the perfect symmetrical position in the gap geometry may partly compensate an asymmetry of the magnetic field. The optimal rest position may be found by measuring the force factor Bl versus displacement. The large signal identification module (LSI) determines this parameter dynamically by operating the driver under normal working conditions. Additional tools are provided to assess the asymmetry of the Bl-curve and to find the optimal voice coil shift.

Using the Large Signal Identification module of the KLIPPEL ANALYZER SYSTEM the nonlinear characteristic of the stiffness (or reciprocal, the compliance) can be measured. This parameter represents the mechanical property of the spider and the surround. In this Application Note a procedure is given, how the total stiffness may be separated into its contributing parts. These valuable information allows the designer to improve the overall suspension system as the mechanical causes of any problem are revealed. Although this procedure is destructive, the designer may now distinguish between the spider and surround and decide where the focus for solving a problem with the suspension should be. An external matrix manipulation program (e.g. Matlab) is used to calculate the surround characteristic. Two examples are investigated. They represent characteristic cases for how to diagnose and improve suspension designs.

The suspension of most transducers comprises a surround and a spider. Both parts contribute to the total compliance and determine the rest position of the coil. Measuring the large signal parameters of the original driver and of the modified driver with partly removed suspension shows the nonlinear characteristics of both parts. This information reveals the physical cause of mechanical limiting and allows balancing asymmetries of spider and surround. A symmetrical compliance characteristic versus displacement is an important goal in driver design to ensure stability of the driver, lower distortion, more robustness (due to reduced stress in the material) and to avoid dynamical generation of DC-displacement that moves the coil from the optimal position.

Using the 3D Distortion Measurement module (DIS) of the KLIPPEL ANALYZER SYSTEM the maximal peak displacement Xmax of a driver is determined by assessing the harmonic and intermodulation distortion in the radiated sound pressure (near field). The new performance-based method is an amendment of the technique AES 2 (1984) and subject of current discussion. It can be accomplished by straightforward techniques defined in the IEC 60268.

The physical causes limiting the voice coil peak excursion Xmax are represented by separate displacement limits XBl, XC, XL and XD corresponding to the dominant driver nonlinearities in the motor, suspension and radiation. These limits are derived from the large signal parameters of the driver measured by the Linear Parameter Measurement (LPM) and Large Signal Identification (LSI) using admissible thresholds of parameter variation defined by the user. The relationship between the separate excursion limits and the peak displacement Xmax determined by the performance-based method is discussed.

The amplitude modulation of a high frequency tone f1 (voice tone) and a low frequency tone f2 (bass tone) is measured by using the 3D Distortion Measurement module (DIS) of the KLIPPEL ANALYZER SYSTEM. The maximal variation of the envelope of the voice tone f1 during one period of the bass tone is represented by the top and bottom value referred to the fundamental response measured without bass tone. Both values reveal only the effects of amplitude modulation caused by Bl(x), Le(x) and radiation nonlinearity but are immune against frequency modulation caused by the Doppler Effect. The difference between the amplitude response of the fundamental component f1 with and without bass tone reveals nonlinear amplitude compression. This measurement is preferred in Automotive Application for showing the impact of AM on the generation of intermodulation distortion.

The weighted harmonic distortion HI-2 is measured by using the 3D Distortion Measurement module (DIS) of the KLIPPEL ANALYZER SYSTEM. The HI-2 Weighted Harmonic Distortion is the ratio of the rms sum of the harmonics weighted by 12 dB per octave rising with frequency relative to the level of the fourth harmonic and the mean value of the fundamental in the pass band of the driver, expressed in dB. The measurement of HI- 2 distortion enables detection of unacceptable distortion, sounding like a "blat" on bass signals

The modulation of a high frequency tone f1 (voice tone) and a low frequency tone f2 (bass tone) is measured by using the 3D Distortion Measurement module (DIS) of the KLIPPEL ANALYZER SYSTEM . The amplitude of the summed and difference-tone components centered around the voice tone f1 shows the effect of all types of modulation (amplitude, phase and frequency modulation) and are expressed as 2nd and 3rd order modulation distortion according to IEC 60268. A series of measurement is performed to reveal the dependency of the distortion on frequency and the amplitude of the excitation stimulus. Intermodulation distortion are a critical symptom of motor nonlinearities represented by a nonlinear Bl(x), Le(x) and nonlinearities in the acoustical radiation (Doppler effect).

The harmonic distortion component of an excitation tone varied in frequency and voltage is measured with the DIS module (3D distortion measurement) of the KLIPPEL ANALYZER SYSTEM. The 3D measurement reveals the complicated relationship between the excitation amplitude (voltage) and the amplitude of the harmonic distortion components which depends on the heating of the voice coil and other nonlinear effects. The connection between common speaker nonlinearities (motor, suspension, etc. ) and the harmonic distortion components is discussed.

The amplitude modulation of a high frequency tone f1 (voice tone) and a low frequency tone f2 (bass tone) is measured by using the 3D Distortion Measurement module (DIS) of the KLIPPEL ANALYZER SYSTEM . The maximal variation of the envelope of the voice tone f1 is represented by the top and bottom value referred to the averaged envelope. The amplitude modulation distortion (AMD) is the ratio between rms value of the variation referred to the averaged value, and is comparable to the modulation distortion Ld2 and Ld3 of the IEC standard 60268 provided that the loudspeaker generates pure amplitude modulation of second- or third-order. The measurement of amplitude modulation distortion (AMD) allows assessment of the effects of Bl(x) and Le(x) nonlinearity and radiation distortion due to pure amplitude modulation without Doppler effect.

The magnetic field penetrating the coil in the gap comprises a DC-component produced by the permanent magnet and an AC-component produced by the current in the coil itself. Thus the force factor Bl(x,i) depends not only on displacement x but also on the current i (flux modulation). The DIS module (3D distortion measurement) of the KLIPPEL ANALYZER SYSTEM is used to check whether flux modulation is a major source of distortion. Flux modulation can be neglected as long as the nonlinear inductance Le(x) is the dominant source of distortion. Different ways for coping with flux modulation are discussed.

Both thermal and nonlinear effects limit the amplitude of the fundamental component in the state variables and in the sound pressure output. The 3D distortion module (DIS) module of the Klippel Analyzer System is used to separate the effects from voice coil heating and from nonlinear parameters varying with displacement.

Nonlinearities inherent in the transducer produce a DC component in the voice coil displacement by rectifying the AC signal. Magnitude and direction of the dynamically generated DC component depend on the type of nonlinearity and on the frequency and voltage of the excitation signal. The DIS module (3D distortion measurement) is used to measure the DC component versus voltage and frequency. The results reveal the stability of the driver, the cause of distortion and complicated interaction between driver nonlinearities.

The movement of the voice coil in a magnetic field can become unstable for excitation tones above the resonance frequency. The instability has the tendency to push the coil out of the gap. Using the DIS software module (3D distortion measurement) of the Klippel Analyzer System the most critical excitation frequency is determined in order to measure the corresponding dynamically generated DC displacement. Various ways for improving the stability of the driver are discussed.

A mechanical suspension having an asymmetrical compliance characteristic Cms(x) will partly rectify the signal and will produce a DC component in voice coil displacement. This DC part may move the coil away from the optimal rest position of the coil in the magnetic field and deteriorate the performance of the speaker. Using the 3D Distortion Measurement (DIS) of the Klippel Analyzer System a simple test can be performed for checking the asymmetry of the suspension. Different ways for improving the suspension are discussed.

Multi-tone excitation signals are optimal for the measurement of speakers similar to normal working conditions. Like a regular audio signal it generates harmonic and all kinds of intermodulation distortion. Using the module Linear Parameter Measurement (LPM) of the Klippel Analyzer System a multi tone excitation signal is generated and the voltage, current, voice coil displacement and radiated sound pressure may be measured and analyzed simultaneously. Typical distortion pattern produced by factor Bl(x), compliance Cms(x), inductance Le(x), Doppler and nonlinear radiation are discussed. They may be interpreted as fingerprints of the dominant nonlinearities in transducers.

Using the Large Signal Identification (LSI) module of the KLIPPEL ANALYZER SYSTEM the nonlinear characteristic of drivers can be measured. Objective measurements can confirm that the parameters acquired with the LSI describe the nonlinear speaker characteristic correctly. Several approaches are possible to validate nonlinear parameters such as FEM/BEM approach or direct measurement of Bl over x with a magnetic field probe. However this note presents an easy and straightforward approach. To ensure that nonlinear parameters coincide with the real loudspeaker behavior a comparison between well-known distortion measurement (Harmonic and Intermodulation) and a prediction of distortion based on the measured nonlinear parameters is given. Due to the independent determination of the transfer responses (see graph below) the presented procedure gives an objective proof of the accurate description of loudspeakers using nonlinear parameters measured by the KLIPPEL ANALYZER SYSTEM. A detailed discussion of several aspects on accuracy and agreement of results is given at the end of this application note. Since the Distortion Analyzer acquires sound pressure level (microphone) and displacement (by laser displacement sensor) validity checks of the driver behavior based on both states may be performed. This procedure yields in the given example a very good agreement proving the validity and accuracy of the measured nonlinear parameters. In addition to that the nonlinear simulation module is proved to be reliable too.

The lumped parameters of the thermal equivalent circuit are measured by using Power Test Module (PWT). The high-speed temperature monitoring makes it possible to measure voice coil resistance RTV and the capacity CTV of woofers, tweeters, headphones, tele-communication drivers and other transducers having a very short time constant. The regular monitoring with adjustable sample rate also allows to measure the parameters of the magnet and frame having usually a very long time constant. The temperature monitoring is based on the measurement of the electrical impedance at 1 Hz.

Traditional modeling describes the heat flow in loudspeakers by an equivalent circuit using integrators with constant parameters (Application Note AN 18). This simple model fails in describing the air convection cooling which becomes an effective cooling mechanism if the velocity of the coil and/or the velocity of the forced air in the gap becomes high. Eddy currents generated in the conductive material close to the voice coil directly heat the pole piece and the shorting ring and generates a bypass for the heat. This effects are considered in an extended thermal model. This application note presents a simple measurement technique to identify the convection parameter rv, the power splitting parameter á and the other thermal parameters.

Traditional measurements of harmonic distortion performed on loudspeakers reveal not only the symptoms of the nonlinearities but also the effect of linear loudspeaker parameters, radiation into the sound field and the interactions with the room. Thus, the interpretation and comparison of results are difficult if the acoustical conditions change. This problem can be solved by transforming the harmonic distortion measured in the sound pressure into equivalent distortion at the voltage input. The equivalent distortion are almost independent of the radiation, sound propagation, room acoustics and the linear properties of the sensor (Laser, microphone). The equivalent harmonic distortion are not only a minimal set of information but make it possible to predict the traditional harmonic distortion according (IEC standard) at any point r in the sound field by performing a simple filtering with a linear transfer function.

Asymmetric Bl(x) shapes cause critical, instable DC offsets at about twice the resonance frequency. High 2nd order intermodulation in the pass band in presence of a bass tone of this frequency tone will be generated. Using the simulation module (SIM), the original, asymmetric Bl shape can be modified and the resulting distortion for an virtually shifted, now symmetrical Bl shape can be predicted. The original Bl-characteristic defined by the magnet structure is maintained but the only rest position of the voice coil is shifted. The intermodulation distortion of a driver with first an asymmetrical and later a shifted Bl(x) shape are simulated and compared to each other. A considerable reduction of 2nd order intermodulation can be achieved.

Disturbances caused by rub & buzz and other defects are unwanted, irregular nonlinear distortion effects. They are caused by mechanical or structural defects such as filings in the gap, scraping of the voice coil at the pole pieces or even lacks of adhesive. Some disturbances of this kind are clearly audible while other effects may be detected only by trained listeners. However, there is a high need to detect these effects not only in manufacturing but also during prototyping and development. The TRF-Pro module provides several possibilities to detect rub & buzz effects. In this Application Note a test is described for checking unique drivers, where no reference drivers (“golden units”) are available. The result is a measure based on the instantaneous crest harmonic distortion. It describes the peaky-ness of the distortion signal in the time domain and exploits the both magnitude and phase information of the higher-order harmonics. To this measure a constant threshold value may be applied to separate good and bad drivers.

Rub & Buzz effects are unwanted, irregular nonlinear distortion effects. They are caused by mechanical or structural defects such as filings in the gap, scraping of the voice coil at the pole pieces or even lack of adhesive. Some disturbances are clearly audible while other effects may be detected only by trained listeners. However, there is a high need to detect these effects not only in the production process but also during the prototyping and development phase. The TRF-Pro module provides several possibilities to detect Rub & Buzz effects. In this Application Note a test is described for a series of drivers, for which a “Golden Unit” is available. Using the information of a “Golden Unit” the system knows about the defined “good” properties of the reference driver(s). This includes linear as well as regular (expected) distortion and also a specific noise distribution. All this information is efficiently used to separate good from bad drivers. The result is a measure Distortion to Noise Ratio (DNR) that shows the deviation from the expected model behavior. To this measure a constant threshold value may be applied to detect defective drivers.

This application note provides a step by step procedure that maximizes the accuracy of the linear parameters measured with the LPM Module. Factors that deteriorate the accuracy are identified and suggestions are made for improvements. Most important is the careful adjustment of the excitation level. If the excitation level is too high distortions are generated and the driver is not longer operated in the small signal domain. Very low excitation levels lead to a poor signal to noise ratio. The LPM visualizes both noise floor and distortion. Using this information a optimal excitation level can be found.

The nonlinear stiffness K(x) and the reciprocal compliance C(x) of any suspension parts (spider, surrounds, cones) and passive radiators (drones) are measured versus displacement x over the full range of operation. A dynamic, nondestructive technique is developed which measures the parts under similar condition as operated in the loudspeaker. This guarantees highest precision of the results as well as simple handling and short measurement time. Suspension parts are fixed in the measurement bench by using a universal set of clamping parts (rings, cones, cups) fitting to any size of circular geometries between 1.5 – 18 inch diameter. Special clamping parts for other geometries can be manufactured at low cost. The working bench excites pneumatically the suspension to vibration at the resonance frequency related to the stiffness and the mass of the suspension and inner clamping parts. The nonlinear stiffness is calculated from the measured displacement by using modules of the KLIPPEL Analyzer System. The measured parameter is required for specifying the large signal properties of the suspension parts and to detect asymmetrical and symmetrical variation which are the cause for instable vibration behavior and nonlinear distortion.