LIMP Program for Loudspeaker Impedance Measurement User Manual Version 1.7.0. Ivo Mateljan Electroacoustics Laboratory Faculty of electrical engineering R. Boskovica bb. 21000 Split, Croatia December, 2010. Copyright © Ivo Mateljan, 2005 - 2010. All rights reserved.
LIMP User Manual Content 1 WHAT IS LIMP? ............................................................................................................................................... 3 2 IMPEDANCE MEASUREMENT THEORY .................................................................................................. 4 2.1 BASIC CIRCUIT FOR IMPEDANCE MEASUREMENT ............................................................................................ 4 2.2 STEPPED SINE AND PINK PERIODIC NOISE GENERATOR ..
LIMP User Manual 1 What is LIMP? The LIMP is a program for the measurement of the loudspeaker impedance and estimation of loudspeaker physical and dynamical parameters (also called Thiele-Small parameters). It is also a general purpose program for measuring impedance in the range from 1 to 200 ohms.
LIMP User Manual 2 Impedance measurement theory 2.1 Basic circuit for impedance measurement The measurement of the loudspeaker impedance is based on the system shown in Fig. 2.1. The referential resistor R is connected between the signal generator and a loudspeaker impedance Z. Figure 2.1 A circuit for loudspeaker impedance measurement Impedance is defined in the frequency domain - Z (f ).
LIMP User Manual This pink multisine has Ak =√2A2k and its spectral magnitudes roll-off 3dB/oct (after some cut-off frequency). In the LIMP, a variable low frequency cut-off frequency can be changed in the 'Signal Generator Setup' dialog box. The pink noise is usually used with a cut-off frequency set close to the frequency where loudspeaker has an impedance maximum (20-100Hz).
LIMP User Manual 2.4 Measurement in a noisy environment The main source of the measurement noise is a loudspeaker that acts as a microphone for the environmental noise and vibrations. Fig. 2.2 shows modified circuit for the loudspeaker impedance measurement, with included the noise generator En. Figure 2.
LIMP User Manual 2.4.1 Lowering measurement noise in stepped sine mode In stepped sine mode LIMP uses "heterodyned" principle to filter all spectral components that are out of the passband which is centered at the measured frequency. The bandwidth of the filter is equal to 1/T where T is integration time of the Fourier integral. For example, if we use integration time 200ms, then the width of the passband of the "heterodyne" filter is 5Hz. 2.4.
LIMP User Manual 3. Hardware Setup The simplest measurement configuration is shown in Fig. 3.1. The soundcard phone-out (or loudspeaker out) is used as a signal generator output. The soundcard left line-input is used for recording the voltage U and the soundcard right line-input is used for recording the voltage U . 1 2 Note: If the soundcard has no phone-out or loudspeaker-out, then we have to use the line-out and a external power amplifier, as shown in Fig. 3.2. Figure 3.
LIMP User Manual 4 Working with LIMP When you start the LIMP you get the program window shown in Fig. 4.1. There are: menu bar, toolbar and a dialog bar at a top of the window and a status bar on the bottom of the window. The central part of the window will show the magnitude and the phase plot. Figure 4.1 Main program window Figure 4.
LIMP User Manual Figure 4.3 Status bar shows the peak level (ref. full scale) of left and right line inputs Generator type Sampling Frequency (Hz) Size of FFT block Averaging type Reset averaging Figure 4.4 Top and right dialog bars Normally, we work with graph windows and dialog boxes. We also need to get the copy of the graph or the graph window picture. Copying of the full window picture is simple. User needs to simultaneously press keys Ctrl+P.
LIMP User Manual 1) In the Edit box user optionally enters the text that will be appended at the bottom of the graph. 2) Check box 'Add filename and date' enables adding text to the graph that shows file name, date and time. 3) Check box ' Save text' enables saving entered text for the next copy operation.
LIMP User Manual 4.1 LIMP Menus Here is a brief explanation of LIMP menus: File New - initializes a new file content Open... - opens an existing impedance file (*.LIM) Save - saves the file with a current name Save As... - saves the file under a new name Info. - shows information about current file Export ASCII - exports the file content in a textual - ASCII file Commented .TXT file - exports textual file with comment and rows containing frequency, impedance magnitude and phase Plain .
LIMP User Manual Audio devices - sets current input and output devices Generator - sets generator parameters Measurement - sets measurement parameters Graph - sets graph margins CSV format - opens dialog box for setup of decimal separator character in CSV files Analyze Loudspeaker parameters - Added mass method - shows the dialog box for the estimation of loudspeaker parameters using Added mass method Loudspeaker parameters - Closed box method - shows the dialog box for the estimation of loudspeaker parame
LIMP User Manual 4.2 Soundcard Setup Activate the menu Setup->AudioDevices. You will get the 'Soundcard Setup' dialog box shown in Fig. 4.6. In this dialog box you choose which soundcard will be used as an input or output device. Generally, choose the same card as an input and output device. Figure 4.6 Soundcard setup The 'Audio Device Setup' dialog box has following controls: Soundcard driver - chooses the type of soundcard driver (WDM – windows multimedia driver or one of installed ASIO drivers).
LIMP User Manual 5) In ‘Audio device setup’ dialog click the button ‘Control panel’ to open the Windows ‘Master Volume’ dialog box, which is shown on Fig. 4.8. 6) Click on menu ‘Options->Property’ and select soundcard channel that will be used for output (playback), as shown in Fig. 4.7. 7) Mute Line In and Mic channels in dialog ‘Master Volume’ (Fig. 4.8) 8) Set Master Volume and Wave Out volume to maximum.
LIMP User Manual Figure 4.8 Typical setup of a soundcard output mixer in Windows XP Note: Most professional audio soundcards have their own program for adjustment of input and output channel, or have hardware control of input monitoring, and input and output volume controls. 4.2.2 Vista / Windows 7 WDM driver setup Microsoft has changed their approach in control of sound devices in Vista / Win7.
LIMP User Manual Figure 4.9 Vista Sound Control panel Figure 4.
LIMP User Manual Figure 4.11 Setting the native bit resolution and sampling rate in Vista Note: There are a lot of drivers that do not work stable under Windows 7. In that case please use ASIO driver if it is available for your soundcard. 4.2.3 ASIO driver setup ASIO drivers are decoupled from the operating system control. They have their own control panel to adjust native resolution and memory buffer size. The buffer is used for the transfer sampled data from the driver to the user program.
LIMP User Manual In Limp, the latency is not problem, as it is encountered in software, but it is not recommended to use buffer with size larger than 2048 samples, or smaller than 256 samples. Some ASIO control panels express the buffer size in samples, while other express the buffer size in time [ms]. In that case we can calculate the size in samples using following expression: buffer_size [samples] = buffer_size[ms] *samplerate[kHz] / number_of_channels.
LIMP User Manual Recommendation: For most reliable results use sine generator, but don't push loudspeaker into large displacement (the largest displacement is at frequencies below the loudspeaker resonance frequency). For measurements of bass or mid-bass loudspeakers set the Pink cut-off frequency close to the loudspeaker resonance frequency (20-100 Hz). Press the button Test to monitor input/output levels.
LIMP User Manual Transient time (ms) - enters transient time to allow the system to get to the steady state (before starting the integration). Intra burst pause (ms) - enters time needed for system to release the energy from reactive components. In section FFT mode (pink noise excitation): FFT size - chooses the length of the FFT block ( 32768 or 65536). Type - chooses: None, Linear or Exponential averaging. Max. Averages - enters the maximum number of averaging for the 'Linear' mode.
LIMP User Manual Figure 4.16 Impedance measurements with averaging a) Procedure in Stepped sine mode Measurement procedures for stepped sine mode are almost the same as in FFT mode. The only difference is that in FFT mode user will see the impedance plot for full measured frequency range almost instantly, while in stepped sine mode procedure will be repeated for many frequencies. Cursor will show current progress in measurements, and that process will be very slow. 4.
LIMP User Manual View All – enables the view of all DFTspectrum components that are used in impedance estimation. View Phase – enables a phase plot. Update – updates the graph with a new setup. Figure 4.18 Dialog boxes for a graph colors setup Figure 4.19 Standard Windows dialog for color setup Graph colors can be changed in two ways. The first one is to change the background color from "Black" to "White" by clicking the menu command View->B/W color or by clicking the toolbar icon .
LIMP User Manual The second way to change graph colors is a "user mode". User sets an arbitrary color for every graph element using the 'Color Setup' dialog box, shown in Fig. 4.18. This dialog can be activated by clicking the menu Edit->Colors. Clicking the left mouse button on colored rectangle opens the standard Windows dialog box 'Color' (Fig. 4.19). Button 'Default' restores default colors. A check box 'Use dotted graph grid' enables drawing of grid with dotted line style.
LIMP User Manual Finally, in section Calibrate we enter the Number of averages (recommended value is 1, as we measure high level signals) and press the button ‘Calibrate’. After a second, we have the calibrated system. Attention: If we change the sequence length or a sampling rate, we have to repeat the calibration procedure. The last section shows a report of the calibration procedure. The most important is information is Channel difference in dB.
LIMP User Manual Textual files have extension .txt or .zma. They contain lines of text with three numeric literals: frequency (in Hz), magnitude (in ohms) and phase (in degrees). The line ends with new line character. The only difference between .txt and .zma files is that .txt files can contain comment lines. Comment lines start, after arbitrary number of space or tab characters, with nonnumeric characters, while data lines start with digit or dot character.
LIMP User Manual 5 Loudspeaker parameters This chapter gives some definitions and measurement procedures for the estimation of loudspeaker parameters. 5.
LIMP User Manual Using these analogous circuit elements, Thiele and Small [6, 7] introduced dynamical loudspeaker parameters. They are defined in the Table 5.1.
LIMP User Manual 5.2 Estimation of Thiele-Small parameters It is easy to estimate Thiele-Small parameters if we have measured data for the loudspeaker impedance. At low frequencies the influence of a voice coil inductance and eddy currents is small and an expression for the loudspeaker input impedance has the following form: Bl 2 Z RE RMS sS 2 QMS 1 sTS / QT s 2Ts RE 2 s 1 sTS / QMS s 2Ts S2 S s 2 QMS where s = j, Ts = 1/s.
LIMP User Manual QMS fs f 2 f1 r0 r1 2 r1 1 2 2 Now we can define a step-by-step procedure for the measurement of Q factors: 1. Measure a voice coil resistance RE with a dc ohm-meter 2. From impedance curve find fS and Zmax . Define ro=Zmax/RE. 3. Choose some impedance magnitude RE<|Z1|
LIMP User Manual 5.3.2 Closed Box Method In this method we first measure the impedance curve and estimate Thiele-Small parameters fS, QMS and QES, for the loudspeaker mounted in a free air. Then we mount the loudspeaker in a closed box, of known volume VB, measure impedance curve and estimate Thiele-Small parameters fC, QMC, QEC . From these we find VAS VB ( f C QEC 1) f S QES Then, by using equations that are defined in Table 5.
LIMP User Manual Figure 5.2 Dialog box for the estimation of loudspeaker Q-factors 5.4.1 Added Mass Method To estimate all physical and dynamical loudspeaker parameters we must make two impedance measurements of the loudspeaker in a free air. In one of the two measurements, the membrane must be loaded with an additional mass. That is what we need: 1. 2. 3. 4. 5. 6. measured impedance data, impedance data measured with an additional mass on the membrane (as in Fig. 5.
LIMP User Manual Then we activate the menu Analyze->Loudspeaker parameters – Added mass method to get the 'Loudspeaker Parameters' dialog box, shown in Fig. 5.4. Figure 5.4 Dialog box for the estimation of loudspeaker parameters Finally, we enter values for Voice coil resistance, Membrane diameter and Added mass. Press on the button 'Calculate' gets the report shown in the left edit box. This report can be pasted to the clipboard by pressing the button 'Copy'. 5.4.
LIMP User Manual Figure 5.5 Impedance curve (the one with higher resonance frequency is obtained by mounting the loudspeaker in a closed box) Then we activate the menu Analyze->Loudspeaker parameters – Closed box method to get the 'Loudspeaker parameters – Closed Box Method' dialog box, shown in Fig. 5.6. Figure 5.6 Dialog for the estimation of loudspeaker parameters using Closed Box Method Finally, we enter values for Voice coil resistance, Membrane diameter and Closed box volume.
LIMP User Manual 6 RLC measurement The LIMP can be used to measure value of resistors, capacitors and inductors, simply by calculating resistive, inductive or capacitive parts of the measured impedance. For example, Fig. 6.1 shows impedance curves of an inductor with nominal value of 1.5mH. Figure 6.1 The impedance graph of a 1.5mH inductor By clicking menu command Analysis->RLC Impedance value at cursor position we get the dialog box with report as shown on Figure 6.2.
LIMP User Manual 6.1 Importance of calibration When measuring impedance and capacitance it is very important to calibrate the system before the measurement, and it is best to make the calibration with the impedance connected (DUT). Why? Even if there is a very small difference between channel sensitivities (i.e. 0.1dB) the LIMP can give very erroneous result, because inductor impedance has phase close to 90 degree, and capacitor impedance has phase close to minus 90 degree.
LIMP User Manual Not all LIMP users will have this problem. As explained before, the problem exists when probe for impedance voltage V2 has higher sensitivity than it is the sensitivity of probe for generator voltage V1. To circumvent this condition we can change probes sensitivity or we can simply exchange input channels and also change the reference channel in the LIMP measurement setup.
