Liberty Instruments, Inc.
Production Control (and Pass/Fail) Testing with LAUD
LAUD provides capability for production testing of magnitude of frequency response, impedance and distortion and for receiving inspection tests of the loudspeakerThiele/Small parameters Qe, Qm, fs and Vas.
You can configure automated Pass/Fail tests by writing custom scripts to run the measurements and by providing upper/lower limit data files to be used for evaluating the result as a pass (within limits) or fail (out of limits). Test Scripts for other parameters such as intermodulation distortion or phase (which not provided with a built-in Pass/Fail curve evaluation function) can be accomplished by reading and evaluating the data at specified marker positions.
Production Control tests can, of course, also be conducted manually (without a script), although such an approach usually requires a higher level of production test operator training.
All of the limit file types use ASCII format. These files can be created using any text editor (such as Windows 3.1 Notepad or DOS's 'Edit'), or in many cases can be generated and exported by LAUD's ASCII [File Save ...Ascii] options.
[some technical details follow for those needing more in-depth information ]
Frequency Response and Impedance limit files:
For frequency response and impedance, the file formats are similar. Two files can be supplied for each test, one for the Upper Limit curve and one for the Lower Limit curve. The first text line of each file is generally ignored and can therefore be used to provide a very brief comment or description; you should begin the comment line with a quote (") mark. Successive lines should contain first a frequency in Hertz (beginning in the first column), followed by one or two spaces, then the magnitude, then optionally another space and then the angle in degrees. The angle is not used in pass/fail evaluations, but may be present so that normal LAUD ASCII saved files (which generate this data) can be used as limit files. For frequency response limit files, the magnitude is expressed in dB. For impedance files, magnitude is specified in ohms.
The frequencies can be any value in the audio range and need not be at any particular points or spacing. The frequencies must, however, start with the lowest frequency and be arranged in increasing frequency order. The frequency ranges given will be interpreted as connected and data at frequencies lower than the first given frequency and higher than the last frequency will not be evaluated. If there are "don't care" ranges within other ranges you do wish to have evaluated, set the upper limit file values in that range to very high values and in lower limit files to very low values (such as very negative for frequency response, or zero for impedance). You should have one "dummy" line below the last desired data line (otherwise the last point might not be used in the evaluation). The "dummy" line can be nothing more than a carriage return or some text such as "End of data".
As a simple example, it you wish to test a device's impedance to verify that it never falls below 4 ohms, the Lower Limit file could be nothing more than the following:
"a comment line like this one; start it with a comma; then
the data lines below it:
End of data
Alternately, a device having an overall "worst case" impedance could be measured using LAUD, and the file saved as ASCII for use as a lower or upper Limit file. You may, in that case, still wish to edit extremes of the frequency range out of the limit file (using a text editor) to avoid evaluation at "don't care" or in ranges of very noisy data.
Frequency responses are often tested to be within a certain tolerance (plus or minus so many dB). Limit files for such tests can be generated by measuring a reference device (if it is a measurement from a microphone, use "relative" or non-SPL mode), then adjusting the GAIN parameter ([F8]) up or down the desired amount for each limit file. Save each adjusted frequency response to disk in ASCII format.
If the "Adjust" option is used when the pass/fail evaluation of frequency response is made, LAUD will pass a tested curve if it can be adjusted by a single gain value to fit between the two Limit file curves. In such a case, the dB difference between the two limit curves is what is important, rather than the absolute gain. In other words, there would be no difference between [upper=+3dB, lower=-3dB] and [upper=+6dB, lower=0dB] when evaluating a frequency response curve with "setEval"=1 and "setAdj"=1.
A Script for pass/fail testing of the frequency response and absolute phase of loudspeakers is provided with LAUD, or can be downloaded from our web site. This script provides for automated generation of the upper and lower limit curves (using a reference speaker), specification of the decibel error tolerance allowed, as well as for repeated testing and automated Pass/Fail evaluation of a series of loudspeakers. The Script can be used as a basis for a custom evaluation script, or in many cases can be used as-is without modification.
Distortion Limit Files:
LAUD provides for automated Harmonic distortion testing and evaluation by use of limit files. If a production measurement of intermodulation distortion is desired, this can also be done, but is usually peformed at a set of test point (for a rub/buzz test), so limit file is not then required.
Harmonic Distortion limit files are created using a text editor. The format is similar to that for Impedance and Frequency Response limit files: First there is one comment line. Then lines of limit data expressed as: frequency in Hertz, then a space or two, then the distortion magnitude limit value for that frequency. The frequencies must appear in ascending order. The distortion magnitude is expressed in percent of fundamental level. The values will be treated as if they were connected via straight lines on the log-frequency distortion plot, so that data at any frequency point falling between those specified will be evaluated using the interpolated value. There should be a final "dummy" line after the desired data lines.
The interpretation of the distortion value (as total harmonic, or as one of the individual harmonics) is determined by the type of distortion measurement. If the measurement is for Total Harmonic Distortion, that is what is evaluated per the limit files. Otherwise, the values of the harmonic which is specified as the "A" harmonic in the DIST_AN will be evaluated.
The following is an example of a simple distortion limit file which begins evaluation at 100Hz and has a limit of 0.2% between 100Hz and 300Hz. The limit curve then ramps down to 0.05% at 400Hz and then is a line at that value between 400Hz and 3000Hz. It then goes back up to 0.1% at 3100Hz and stays there all the way up to the 10000Hz limit.
Distortion Limit File
end of data
There is usually no need for a lower distortion limit file, as distortion is typically to be minimized . LAUD is able to use a lower limit file, however, for use by makers of electronic "effect" boxes or euphonically modified electronics who may want to control the amount of a specific harmonic which is generated.
Thiele/Small Parameter Limit Files:
Thiele/Small parameters are evaluated using a single limit file which encompasses both the maximum and minimum values. The parameters Re, fs, Qe, Qm, Qt and optionally Vas may be evaluated. When the evaluation is performed, a series of horizontal "bars" is displayed on-screen. These bars represent the range of acceptable values (between the lower and upper limit) for each parameter. A red marker is shown on each bar to indicate the position of the measured value within (or outside of) each range. If "setEval" is set to "1", LAUD will make a pass/fail decision based on the parameters and the limit values.
If drivers are being sorted in addition to (or instead of) pass-fail testing, LAUD also provides a means for printing T/S parameter reports to hardcopy. You can give a series of drivers a Type_name (for printing on the report) and assign a sequential numerical serial number to each driver in a series.
The format of the Thiele/Small parameter limit file for Pass/Fail testing differs from that for Frequency Response, Impedance and Distortion. The first line in the T/S limit file must start with the number 0, followed by one or more spaces, then an optional comment. Comments can be on all lines after the given value, as shown in the example below. The second line is the Upper limit for the DC resistance Re in ohms; the third is the Lower limit for Re. Fourth is the Upper limit for Qe. The succeeding lines are as described in the following example. A dummy line should be placed at the end. If Vas is not to be evaluated, those values may be left out of the limit file.
0 You can type comments after the number on each line;first line
10 This is the upper limit for the Re
8 Lower limit for Re
53 Upper Limit for fs
45 Lower Limit for fs
0.85 Upper Limit for Qe
0.55 Lower Limit for Qe
7 Upper Limit for Qm
2.5 Lower Limit for Qm
0.65 Upper Limit for Qt
0.53 Lower Limit for Qt
20 Upper Limit for Vas
15 Lower Limit for Vas
end of data (dummy line)