STEPS tutorial
Contents
1. 10929 1 413 and for our example 14 14 1 413 10 00V This completes the calibration 24 Measurement Setup Measurement System Stepped Sine Generator Dual channel Frequency response Start frequency Hz 20 00 Response channel Left Gin kapeng Ha 20000 00 Sampling frequency Hz 48000 Frequency increment 1 6 octave Min integration ime ms 00 Generator level dB re F5 30 Transient time ms 200 Test frequency Hz 500 I O delay ms 0 Mute generator switch off transients Intra burst pause ms 200 Set current response as overlay E Generate Default Cancel 1 as Figure 3 5 7 Measurement Setup Before beginning the first measurement please bear in mind the following 4 e For robust results distances between the measurement setup and reflecting surfaces should be much greater than the distance between the speaker and microphone see also ARTA Handbook Section 6 3 e Familiarize yourself with the likely limits of the speaker before carrying out maximum level measurements Is mechanical or thermal overload likely to cause failure A preliminary simulation in e g AJ Horn or BoxSim could help here In principle woofers are more mechanically vulnerable while crossover protected midranges and tweeters tend to experience thermal failure see Figure 3 5 11 Inspeakers without protection circuitry crossover components may also be at risk of thermal overload e Carry out a prelimin2. gt Ph 0 xen 25 H sopte 2 H step ireo nem mo os In addition to the above STEPS can record the following special measurements Distortion vs amplitude Section 3 3 Linearity X vs Y Section 3 3 Driver displacement vs distortion Section 3 4 Distortion limited maximum SPL Section 3 5 Distortion vs amplitude Linearity function Loudspeaker displacement distortion Loudspeaker distortion limited SPL When working with STEPS care should be taken to ensure that the stepped sine excitation signal energy level is sufficient to account for noise At the same time output levels should be carefully controlled to protect the DUT and soundcard As many of the commands and function in STEPS are the same as those in ARTA only features specific to STEPS will be discussed in this section 3 1 Setting up in STEPS measurement parameters can be adjusted in Measurement setup Figure 3 1 1 The menu window is divided into Measurement System and Stepped Sine Generator sections A meter for setting signal levels is provided at the bottom of the window Measurement Setup J M X Measurement System single channel Level Left Sampling Frequency Hz 43000 Response channel Min integration time ms Transient time tms LG delay ms Intra burst pause ms Set current response as overlay Stepped Sin
3. R2 G R2 100 0 1187 100 742 45 ohms 820 ohms 100 820 100 0 1087 19 284 Note The Zener diodes shown in grey are not strictly necessary but offer additional protection for your sound card input If you do not like using Zener diodes they can be omitted see also ARTA Application Note 1 The ARTA Measuring Box 1 Audio Devices Setup X M Sound card Soundcard driver fasio E MU Tracker Pre USB Control Panel Input channels 1 2 v Wave Format Output channels 1 2 24 bit z MIO Amplifier Interface ER TA 3245 75 3130 4 Ext left preamp gain 10 channel diff dB 0 143725 Ext right preamp gain 0 1087 Power amplifier gain 12 2 z Microphone V Microphone Used Left Ch Sensitivity mV Pa 7 35356 Figure 3 5 6 Audio Devices Setup The value calculated for the attenuating effect of the voltage divider is entered in the Ext right preamp gain field in Audio Devices Setup After this is done calibration can begin using the setup shown in Figure 3 5 3 To do this use the sine wave generator in Measurement Setup Figure 3 5 7 with the generator set to e g 30dB re FS and a test frequency that can be measured reasonably accurately with your multimeter This will be the signal that 1s sent to the power amplifier and used to adjust the output voltage Ucaib Uame G where G o Generator Level 20
4. Audix TM1 vs NTIM2210 12dB D4 Audix green Figure 3 2 5 Comparison of Audix TM1 and NTI M2210 The comparison demonstrates among other things that the use of cheap equipment is not worthwhile given the precision required for distortion measurements Other factors can also affect results notably room boundary conditions Figure 3 2 6 shows distortion measurements in dB and 46 on a 5 TMT speaker taken under nearfield conditions at 10 25 and 40cm 12 Magnitude dB re 20uPa 120 0 0 ITI co f 1013 3 Hz Magn 104 73 dB f Hz D2 53 08 D3 50 38 dB File NF sw 2010 09 17 19 24 42 NF Magnitude dB re 20uPa 110 0 100 0 90 0 80 0 70 0 60 0 50 0 40 0 30 0 20 0 20 50 100 200 500 1k f 1013 3 Hz Magn 95 13 dB f Hz D2 68 34 D3 49 16 dB File 10cm hsvv 10cm 2010 08 17 19 26 17 Magnitude dB re 20uPa 100 0 5 90 0 E P 80 0 70 0 50 0 50 0 40 0 30 0 Lj m D3 20 0 a 02 10 0 m Magn 20 50 100 200 500 1k f 1013 3 Hz Magn 89 59 dB f Hz D2 72 58 03 51 91 dB File 25cm hswe 2010 09 17 19 28 19 25cm Magnitude dB re 20uPa 90 0 gt D E 5 m D3 m D2 m Magn 20 50 1k f 1013 3 Hz Magn 85 26 dB f Hz D2 69 16 D3 52 84 dB File 40cm hsw 40cm 2010 09 17 19 27 29 Distortion 10 0 70 Im 4 co THD mD2 0 01 20 50 100 200 500 1k f 1013 3Hz THD 0 31126 D2 0 07005 D3 0 30274 96 f Hz File NF sw 2010 08 17 19 25 38 NF Di
5. D4 t Bone MM1 vs NTI M2210 12dB D4 t Bone green Figure 3 2 4 Comparison of T Bone 1 and NTI M2210 11 The traces indicate that the T Bone is not sufficiently accurate for distortion measurements The Audix TM1 however is a much better candidate with responses very similar to those obtained with the reference microphone Figure 3 2 5 Distortion 96 0 0 0 N 0 1 0 01 m THDo 0 001 THD 20 50 100 200 500 1k 2k 5k 10k 20k f 2324 7Hz THD 0 27381 f Hz File Audix 1 12dB hsw Ovr NTIM2210 InSitu 12dB hsvv 2010 08 26 10 12 06 Audix TM1 vs NTI M2210 12dB THD Audix grey Distortion 96 10 0 0 m 4 eo 0 1 0 01 9 p3o 0 001 20 50 100 200 f 2324 7Hz D3 0 07681 96 20k f Hz File Audix TM1 InSitu 12dB hsw Ovr NTIM2210 InSitu 12dB hsvv 2010 08 26 10 15 04 D3 Audix TM1 vs NTI M2210 12dB D3 Audix red Distortion 10 0 70 ITI co 0 1 0 01 m D20 0 001 02 20 50 100 200 500 1k 2k Sk 10k 20k f 2324 7HZ 02 0 25747 f Hz File Audix TM1 InSitu 12dB hsw Ovr NTIM2210 InSitu 12dB hsw 2010 08 26 10 13 56 02 Audix TM1 vs NTI M2210 12dB D2 Audix blue Distortion 96 10 0 0 m 4 co 0 1 0 01 040 0 001 m D4 20 50 100 200 500 1k X Sk 10k 20k f 2324 7Hz f Hz D4 0 04343 File Audix 1 12dB hsw Ovr NTIM2210 InSitu 12dB hsw 2010 08 26 10 16 02 D4
6. Measurement parameters are set under Excitation sine voltage range frequency start and stop values linear or logarithmic voltage increase number of steps The maximum output voltage is recorded in the Stop value field This is calculated from the gain of the power amplifier with a safety margin to a maximum of 3dB see also section 3 2 Before measuring check whether the DUT is likely to be damaged by the voltages that will be applied 15 Figure 3 3 2 shows distortion in a small power amplifier at 1kHz as a function of voltage Notes on measurement and setup can be found in Section 5 4 of the ARTA Handbook SISTER is 100 0 uu bg 0001 EE 0 01 0 1 1 0 10 0 Crst11 8 TAD 1 0 Voltage rms 4 1 Ohm va 8 2 Ohm Figure 3 3 2 THD vs voltage 1kHz for a small power amplifier Distortion 96 Distortion 96 10 0 7 20 50 100 200 500 1k 2k 5k f 3038 1Hz THD 0 21267 f Hz 05 1 0 j Crsr 6 0Y THD 0 7796 Voltage V rms B110 THD vs Voltage B110 3v Figure 3 3 3 Left THD vs voltage at different frequencies right THD at 3 volts 7U ITI ca Figure 3 3 3 left shows THD at different frequencies in relation to vo
7. range dB re 20uPa section e Top dB Defines the upper level limit in dB e Range dB Defines the level range in dB Excitation Voltage dBV section e dBV Defines the upper voltage limit in dB e Range dBV Defines the voltage range in dB Show excitation voltage The measured excitation voltage is plotted when this is activated Note If the Microphone used on checkbox in the Audio Devices Setup menu Figure 3 5 6 1s not activated Distortion Limited Level is shown in dBV otherwise Distortion Limited SPL is shown in dB Graph Margins Frequency range SPL range dB re 200 Excitation voltage dBV High Hz 5000 Top 98 140 98 40 Low Hz 2 Range dg 100 Range 98 50 V Show excitation voltage Update Default Cancel Figure 3 5 2 Graph Margins menu The measurement setup for THD limited maximum SPL is shown in Figure 3 5 3 For the maximum SPL measurement determination of the voltage being delivered by the power amplifier is not absolutely necessary right input but it does provide additional control over the excitation level used during measurement For this reason the complete measurement set up 15 described here power Right out amplifier loudspeaker microphone soundcard Right input st Left input e preamplifier Figure 3 5 3 Setup for THD limited maximum SPL measurement It takes a lot of power to get 10 THD from a speaker As an example F
8. to swap multiple cables is replaced by the simple flick of a switch see ARTA Application Note 1 1 Multimeter A multimeter for use with STEPS is not strictly necessary but it is nevertheless indispensable for the calibration of the measurement equipment to be used Besides a good meter is a useful tool for all manner of other measurements If you do not have a multimeter you should ideally opt for a true RMS type There are plenty of options on the market for well below 100 Cables Several cables are required all of which should be of good quality Poor connections inadequate shielding etc can interfere with measurements see also ARTA Handbook Section 6 1 1 Keep all cables as short as possible 1 3 Pin assignment for cables and connectors STEREO JACK Sleeve earth GROUND SHIELD Pin 1 earth GROUND SHIELD Tip Pin 2 Ring Pin 3 Figure 1 3 1 Pin assignment for connecting cables 2 Calibrating STEPS The calibration of STEPS is no different to the calibration of ARTA see the ARTA Handbook Features specific to STEPS e g maximum displacement and distortion limited maximum SPL are covered in the relevant sections of this document 3 Measuring with STEPS STEPS measures the frequency response and harmonic distortion of a loudspeaker by using a stepped sine wave The major commands and operations in STEPS can be found in the top menu bar He Overlay Edt Record Setup Hep M
9. S shows the absolute current level in single channel mode with no reference Thus if the output voltage of the amplifier is increased or decreased in single channel mode with STEPS the different frequency response levels can be seen This is sometimes useful if you want to determine the sound level measured by the microphone that corresponds to a particular amplifier output voltage Figure 3 2 2 Dual Channel 1dB to 12dB Single Channel 1dB to 12dB FR magnitude dB re 20uPa 2 83V Magnitude dB re 200 100 0 0 950 T E 90 0 850 80 0 750 700 65 0 60 0 55 0 50 0 20 50 100 200 500 1 2k 5k 10k 20k 20 50 100 200 f 20439 3 Hz Magn 70 08 dB f Hz f 2027 3 Hz Magn 93 27 dB STEPS Dual Channel STEPS Single mit 17 09 dB PK 0 1398 m Figure 3 2 2 Frequency amplitude response in dual left and single right channel modes Dual channel mode Figure 3 2 2 left shows the reference level dB re 20uPa 2 83V Any change in the output voltage is back calculated by STEPS to 2 83 or 1 volt depending on the choice of units under View and Sound Pressure Units In single channel mode absolute changes corresponding to the varying output voltage of the amplifier are displayed Figure 3 2 2 right In addition to frequency response STEPS is particularly suited to measuring harmonic distortion Measurements are less prone to noise interference than those taken using the Farina method Chapter 7 1 but take consid
10. STEPS MANUAL i Transfer function distortion measurement Maximum linear displacement Distortion limited maximum SPL Based on the original STEPS Manuals Original tutorial in German by Dr Heinrich Weber Original manuals in English prepared by Dr Ivo Mateljan O Weber Mateljan Translation into English of Version 2 40D ARTA 1 80 Christopher J Dunn Hamilton New Zealand September 2014 Contents l 2 3 4 10 SDE eoteaetttohos besitos onde invo T E E a N 3 he requirement Stee eo eder a a e ues 3 EDU UU T 3 1 5 Pu assienmenttor cablesatd conte C LODS 4 Cabrales TEE S uocum E 6 Measuring Will STEPS po uti bit 6 S STEPSa a qu MEM UI IM UE I M EEUU Y 3 2 Frequency response and distortion measurements with STEPS 9 225 Factors influencing distortion measurements 10 3 3 Voltage or power related measurements with STEPS 15 3 4 Measuring maximum linear cone displacement with STEPS 18 3 5 Measuring THD limited maximum SPL with 20 FREON CTI CES dietis doti chat auc rai
11. an take several minutes because of the need to sum the integration time transient timeline and intra burst break multiplied by the frequency Increment and the number of octaves swept Initial trials should therefore be carried out with conservative signal levels and frequency resolution 1 6 octave Options available in STEPS include amplitude phase Mag Ph M P MHD D amplitude phase amplitude distortion and distortion To see the correct phase relationship the path between the speaker and the microphone should be compensated by a delay It is difficult to determine the exact value for this parameter because of the difficulty in determining the exact acoustic centre of the speaker see Section 6 3 For a reasonable approximation we can calculate the delay as follows I O delay msec 1000 x measuring distance m speed of sound m sec c 344 m sec Thus for a measured distance of 0 5m from microphone to baffle the delay would be 1 4534 msec Start Hz 20 4 steptH 1000 step iz oct Delay ms 1 4534 FR magnitude dB re 200 2 G3 Phase 130 0 c T 175 0 E p 120 0 c 115 0 110 0 105 0 100 0 180 0 95 0 90 0 90 0 0 0 na I 90 0 an 180 0 100 1000 Cursor 1076 3 Hz 120 96 dB 80 0 deg T Hz Figure 3 2 1 Frequency response of a 6 TMT measured with STEPS with 1 12 octave resolution As described earlier STEPS can perform measurements in single or dual channel mode Unlike ARTA however STEP
12. ary test at normal levels in STEPS or in with the Farina method use approximately 10 of the rated power of the speaker This is likely to indicate any potential weak points in the speaker under test e Adjust the lower frequency limit carefully Do NOT start the maximum level measurement at a frequency significantly below the resonant frequency of the speaker This is especially true for midranges and tweeters Bass reflex speakers should not be tested any more than half an octave below the tuning frequency e THD limits have been shown to be between 1 and 3 for hi fi speakers and 5 to 10 for PA systems 4 e Start your measurements as a precaution with a high Power Reduction Factor e g 50 100 This gives the voice coil time to cool down e Take care not to choose full frequency resolution the smallest step and high THD limit for the first measurement as such a measurement will take a long time and will place a heavy load on the speaker e Ifthe peak power handling of the speaker is known and the power amplifier can deliver a peak power Pmax Power Reduction Factor should Pmax P e Assume as a precaution that tweeters will only tolerate 1 2 watts of continuously supplied sine wave power For our above mentioned 25 watt t amp power amplifier the Power Reduction Factor would be 25 2 13 Figure 3 5 8 shows a 3 THD limited maximum level measurement for a 3 5 inch full range driver in a bass reflex enclosure The measurem
13. d to quote excessively optimistic sensitivity and power characteristics for their drivers see also Figure 3 5 4 Effects such as compression partial vibration and the limits of displacement of the driver membrane tend to be glossed over and the theoretical capabilities of drivers are often greatly overstated 4 especially at high and low frequencies The determination of THD limited maximum SPL with STEPS solves this problem A pure sine wave at predetermined frequency intervals is used to determine SPL at given distortion levels Caution the procedure is carried out at VERY high sound levels Before carrying out measurements of this type the following should be observed e Use ear protection The sound levels developed are capable of causing irreparable hearing damage e Protect your speakers Read this whole section completely before starting to test e The measurement of THD limited maximum SPL of drivers and speaker cabinets requires suitable equipment and the right measurement environment Ideally these will include a professional measurement microphone with a high maximum SPL capability an anechoic chamber RAR and a high power amplifier Problems with the measurement microphone or environment can significantly affect results see also section 3 2 1 Notwithstanding the above however comparative measurements are of course also possible with semi professional equipment and in normal rooms Figure 3 5 1 shows the Distort
14. e 3 5 5 Soundcard line in specifications ESI UGM96 Instrumenteneingang input specs unsymmetrische 6 3mm Klinke unbalanced 6 3mm jack Max Eingangspegel max input level A gewichtet A weighted Impedanz impedance e Maximum soundcard input voltage max 4 5dBV 1 0 10 4 5 20 1 079 V RMS e Soundcard input impedance Zw 500kOhm e Amplifier power P 25 watts see note and Figure 3 3 2 Note It is not always a good idea to rely on manufacturer s specifications The t amp is rated at 36 watts into 6 ohms but Figure 3 3 2 suggests that in reality this should be 24 watts Thus rather than turning the volume control to its maximum the input sensitivity of the power amplifier and the maximum output voltage of the sound card should be adjusted Back off the volume until with 0 dBFS feeding in from the sound card the amplifier output just stops clipping This should be the maximum excitation voltage for measurement and should form the basis for calculation of the voltage divider components 23 Using Ohm s law the maximum output voltage is calculated for a load impedance Z 8 ohms as follows U SQRT P Z U SQRT 25 8 14 14V Gi Un max Uovur amp max 1 679 14 14 0 1187 18 514 Thus a voltage divider giving about 19dB of attenuation is required Soundcard R1 G R2 R1 R2 1 RI R2 G R2 2 Zin For a value of R2 100 ohms R1 is calculated as follows 2 R1
15. e Generator Start frequency Hz 20 00 Stop Frequency Hz 20000 00 Frequencv increment 1 6 octave 200 Generator level dB re F5 50 200 Test frequency Hz 1000 Mute generator switch off transients v zn Figure 3 1 1 Measurement setup in STEPS The available fields are as follows Measurement System Measurement Mode Response channel Sampling frequency Min integration time ms choose single or dual channel measurement from the drop down menu see also Section 3 2 determines the input channel default channel left ranges from 11025Hz to 192kHz STEPS determines the frequency response of the signal from I O Delay to the beginning of the Transient Time by integrating the sine signal in the time domain This is termed the integration time and its value depends on the lowest i e start frequency to be measured When the lowest frequency to be measured is F Hz the minimum integration time must be 1 F sec so for 20Hz the integration time is 1 20 0 05 sec 50 ms In addition ARTA and STEPS filter the signal by applying Kaiser windowing which requires a minimum of five complete cycles i e 250 ms at 20Hz Faster measurements can be obtained only by increasing the start frequency Note that the integration time relating to the lowest frequency should be doubled when taking distortion measurements i e 500 msec at 20Hz Transient time ms I O delay
16. ent was performed at 20cm and is level corrected Edit Scale The red curve shows the measured maximum level in dB and the grey curve the excitation voltage in dBV 25 Distortion limited SPL dB Excitation dBV 20 50 100 200 500 ik 2K 5k 10k Crsr 10240 0Hz 102 48dB 23 11dBV 0 5 THO Figure 3 5 8 Measurement of a 3 5 full range driver up to THD 3 We know that the amplifier can only provide power up to about 23 dBV or 14 10 volts For this speaker this is obviously insufficient to achieve the chosen THD limit of 396 from about 1 5kHz Figure 3 5 9 shows the same speaker measured at 0 5 1 096 and 3 096 THD The individual curves were generated using the overlay function Here even without the addition of the Excitation dBV axis we can see that around 2kHz the amplifier 1s unable to supply enough power because the red and blue traces are superimposed Distortion limited SPL dB 20 50 100 200 500 1k 2k 5k 10k Crsr 10240 0 Hz 102 59dB 0 4 THO Figure 3 5 9 Measurement of a 3 5 full range driver THD 0 5 1 0 and 3 0 26 Distortion limited SPL dB 20 50 100 200 500 4k 2k 5k 40k Crsr 10240 0 Hz 102 59dB 0 4 THD Figure 3 5 10 Varying frequency resolution and step increments Figure 3 5 10 shows a measurement taken with a 0 5 THD limit at differing frequency resolution and step increments The blue trace was generated with 1 3 octave resolution and 1dB step levels while the red
17. erably longer depending on the settings used Different results dB or 96 can be displayed in STEPS by using the buttons on the menu bar M D Magnitude Distortion D Distortion FR magnitude dB re 20uPaly 305 Distortion 96 20 50 100 200 500 1k 2k 5k 10k 20k 20 50 100 200 500 1k 2k Sk 10k 20k f 20438 3 Hz Magn 61 06 dB f Hz f 20438 3Hz THD 0 00932 D2 0 00832 f Hz D2 80 61 dB File D 12dB hsw 2010 08 18 16 44 43 File D 12dB hsw 2010 09 16 16 45 37 Figure 3 2 3 Distortion measurement display options 3 2 1 Factors influencing distortion measurements Distortion measurements may be influenced by both the measuring equipment and the environment with environmental effects increasing with the measurement distance This will limit the extent to which measurements can be compared and leads to the need for some experimentation with measurement distances in order to gauge levels of interference To exclude environmental effects measurements should be taken under nearfield conditions a 2s dI 3 Critical distance rx 0 057 room volume T60 reverberation time sec 10 If nearfield measurements used care should be taken to ensure that the microphone does not distort The AES2 specification 2 states that measurements should be carried out at around 1096 of the rated power of the speaker this is typically gt 90dB at a distance of At this level in a nearfield enviro
18. f 1013 3Hz D2 0 03483 File 40cm hsve Ovr NF hasw D2 NF vs 40cm D2 FF dark blue NF light blue 2010 08 17 19 35 23 Distortion 96 10 0 U m c9 0 1 0 01 0 001 20 50 100 200 500 1k f 1013 3Hz D4 0 00355 File 40cm hsw Ovr NF hsw D4 NF vs 40cm D4 FF green NF brown 2010 08 17 18 36 38 Figure 3 2 7 Distortion traces Comparisons of nearfield and 40cm farfield measurements The examples demonstrate the factors to be considered when making reproducible distortion measurements They include signal and sound pressure levels the quality of the microphone and its distance from the speaker and the contribution of room reflections The measurement setup should account for these factors as much as possible in order to optimise results 14 3 3 Voltage or power related measurements with STEPS In addition to the functions described above in Section 3 2 STEPS has four further special functions under the Record menu These stepped amplitude tests measure system response and distortion as a function of the amplitude of the excitation signal a sine or two sine signal with a user defined frequency The amplitude of the signal changes from the lowest to the highest value in a predefined number of measurement steps These tests are primarily intended for power amplifier audio compressor and automatic gain control systems and include the following e Distortion vs amplitude e Linearity
19. function e Loudspeaker displacement distortion e Loudspeaker distortion limited SPL The distortion vs amplitude function is used to measure voltage or power related distortion in electrical e g amplifier and electroacoustic e g speaker systems Power values must be worked out manually according to P V R where R is the reference resistance and may be added to the X axis by hand Distortion vs amplitude untitled vsd E R JF mi xj File Edit General distortion measurement THD Y Response channel Left Sampling v rate Hz T ay Excitation sine voltage range Frequency Hz 1000 Start value V rms g 1 H Distortion 95 p UO IT oy Stop value V rms 0 1 max en 53v Use logarithmic steps Iv Number of steps 10 Integration constants gt 0 001 3nn 0 2 0 5 1 0 Intg time tms Voltage v rms Transient ti 50 ransient time ms Record i Copy Setup Overlay Cancel 2 Figure 3 3 1 Distortion vs Amplitude window 0 01 The input fields for the measurement parameters are to the left of the window in Figure 3 3 1 while the settings for graphics and overlays are at the bottom Under General Distortion Measurement different evaluation modes can be selected THD IMD DIN CCIF IMD together with the input channel and sampling rate For more information see the STEPS User Manual DIN IMD 250Hz4
20. ge range Start value V rms 0 5 Stop value V rms lt 0 0631 H Number of steps 38 Logarithmic step increment THD break value 9 10 p Sine Frequencies Frequency F1 Hz 2600 T 8 5 FL Ul 42 Distortion 96 Integration constants Integration time ms 300 Transient time ms 3t 0 5 1 0 2 0 5 0 10 0 T0 00 5g THD 0 0 766 Voltage Ema Dz 0 051 34 D 3 0 04937555 Record BIW Figure 3 4 2 Loudspeaker Displacement Distortion window Distortion D3 0 1 0 01 0 001 0 001 2 0 5 0 2 0 5 0 05 1 0 10 0 0 5 1 0 Crsr 10 00 0 0786 Voltage vrms Crsr 10 00 THD 0 46496 D2 0 0513496 D3 0 0497596 D2 0 43369 D3 0 1519796 18dB 2 5kHz BdB 2600Hz 18 dB XO 6 dB XO 10 0 Voltage Vrms Figure 3 4 3 Distortion vs voltage for a tweeter with two different crossovers f 2fs 19 3 5 Measuring THD limited maximum SPL with STEPS To assess loudspeaker performance we need in addition to frequency and impedance response information on directivity and the achievable limits to which the speaker can be driven This last factor is particularly relevant to PA loudspeakers Manufacturers ten
21. igure 3 5 4 shows a BoxSim simulation for maximum load and input voltage for a small PA monitor The maximum load capacity is defined by either maximum displacement or electrical load As stated earlier this simulation is somewhat optimistic but it shows that in the region above 200Hz the THD limited maximum SPL measurement of this speaker would need to be carried out at 100 200 watts assuming that the 5 to 10 THD limit is reached before the speaker fails altogether For testing at a low frequency however as little as 10 50 watts would suffice 22 Maximale eff Eingangsspannung vor aktivem Filter fur linearen Hub und max elektr Belastbarkeit 20 AU 100 200 1000 2000 SUDO 10000 20000 hesamtbox H350 1 AbD 2 Figure 3 5 4 Theoretical maximum load of a small PA monitor BoxSim The use of power amplifiers at these levels is guaranteed to overload the right input channel of the soundcard and cause irreparable damage so a voltage divider is mandatory The component values will depend on the amplifier s output Although we know that high power amplifiers are needed to THD limited measurements for practical reasons the following example uses the Thomann t amp discussed in the ARTA Handbook and the soundcard specifications shown in Figure 3 5 5 Hi Z Instrumenteneingang Type unsymmetrisch 6 3mm Klinke Max Eingangspegel 4 5dBV max THD N 0 003 A gewichtet Impedanz 500 kOhm Figur
22. ion Limited Levels window with its functions The main header menus are as follows File e Open Opens binary files with measurement data msp e Save As Saves data as binary msp file e Export Exports data in ASCII or Excel CSV format e B W Switches display between black and white and color e Copy Copies the current graph to the clipboard e Scale maximum level Allows you to scale the trace e g to correspond to measurement at 1 meter adds subtracts the value entered from the trace The footer control buttons are as follows Record Start the measurement Clicking again stops the measurement Copy Copies the current graph to the clipboard in bmp format B W Switch between black and white and color Setup Opens the Graph setup menu Overlay Opens the overlay menu Cancel Closes the window without saving the setup OK Closes the window and saves the current setup 20 Distortion Limited Levels untitled msp File Edit Measurement channel SPL Channel Left NM Distortion limited level dBV Measure excit voltage 130 Excitation sine range THD limit 90 Start freq Hz Stop freq Hz Fr resolution Level step idB Power reduction duty cyde Integration constants Transient time ms Record Bw coy setup overey cancel Oc Figure 3 5 1 Distortion Limited Levels window The controls on the left side of the window have the fol
23. lowing functions Measurement channels SPL channel Selecting the input channel for the microphone default line in left Measure excit voltage Measures the output voltage of the power amplifier when the box 1s ticked Sampling rate Hz Selects sampling frequency Excitation sine range THD limit Enter the THD limit termination criterion Start freq Hz Enter the start frequency Stop freq Hz Enter the stop frequency see note Freq resolution Frequency resolution 1 1 1 6 1 9 1 12 octave Level step dB Choice of resolution level 0 5 1 0 1 5 2 0dB Power reduction factor Input a value between 2 and 1000 describes the ratio of signal duration 4 pause duration to signal duration This ratio is proportional to the ratio of peak power to the total power output during signal generation Integration constants Transient time ms Enter the time to steady state see also Section 3 1 Note STEPS needs at minimum the second and third harmonics in order to be able to calculate THD in this part of the program The upper frequency limit is determined accordingly by the sampling rate of the sound card 96kHz sampling rate upper frequency limit of 12kHz The menu item Setup at the bottom of the window opens the Graph Margins menu Figure 3 5 2 The chart axes are defined here Frequency range High Hz Defines the upper frequency limit Low H2 Defines the lower frequency limit 21 SPL
24. ltage for a 5 woofer The right panel shows distortion as a function of frequency at about 3 volts The distortion readings at 3 volts in the right hand panel for all frequencies correspond to those in the left hand panel where each individual frequency trace crosses 3 volts on the X axis 16 IT ranty Function untitled vsl eee is rmi Hes h nic steps 2 WO cer dH TET ee F pce iis ie F es 10 i H COT ep Lio Me E Integration constants 0 0001 0 001 Intg time ms L Figure 3 3 4 Linearity Function window X vs Y Under Linearity Function the relationship between two quantities can be measured Available options are shown under Measurement Channels Figure 3 3 4 Both left and right channels can be selected for either excitation or recording Figure 3 3 5 shows a simple linearity test for a cheap onboard soundcard Response M rms Linearity test 1 0 0 1 Doo 0 0001 0 001 0 01 0 1 1 0 Crsr 0 000435 v 0 060814 Excitation rm Figure 3 3 5 Onboard soundcard linearity test 17 3 4 Measuring maximum linear cone displacement with STEPS The maximum linear displacement of a driver determines its maximum undistorted SPL across a given frequency range AES2 2 defines this as The voice coil peak displacement at which the linearity of the motor deviates by 10 Linearity
25. may be measured by percent distortion of the input current or by percent deviation of displacement versus input current The manufacturer shall state the method to be used This recommendation has been extended by Klippel 3 and is now included in the standard 62456 Sound system equipment electroacoustical transducers Measurement of large signal parameters The same standard has been implemented in STEPS since release 1 4 The measurement setup for the determination of peak linear displacement is shown Figure 3 4 1 soundcard power amplifier microphone loudspeaker preamplifier Figure 3 4 1 Klippel Light measurement setup Figure 3 4 2 shows the Loudspeaker Displacement Distortion window Although full functionality is described in ARTA Application Note AP7 1 a safety feature not included in Distortion vs Amplitude is discussed here This is the THD break value whereby a voltage cut off can be set to protect the system under test Figure 3 4 3 shows distortion vs voltage for a tweeter with two different crossovers 18dB 6dB slope at 2 6 kHz Here the tweeter has handled the applied voltages better than expected and the break value of 1 has not been reached 18 ES Distortions 18dB 7600Hz dx File Edit Measurement channels Response channel Left Sampling rate Hz asooo Hse displacement sensor on other channel i Sensitivity mmm Excitation sine volta
26. ms Intra burst pause ms Stepped Sine Generator Start frequency Stop frequency Generator level Frequency increment Mute generator switch off transients the sine signal should be at steady state before measurements start The time to steady state depends on the resonance behaviour of the system or on acoustic reverberation For room measurements the transient time should be at least 1 5 of the reverberation time For typical rooms this will be between 100 ms and 200 ms for outdoor measurements the transient time may be set to 50 ms to 100 ms this is the signal delay from the speaker to the microphone t s c it must be accounted for if useful phase responses are to be obtained after a measurement has taken place the system must be allowed to settle before the next measurement begins this is the intra burst pause As a rule of thumb it may be assumed to be 1 5 of the reverberation time value of the start frequency in Hz value of the stop frequency in Hz enter the generator output voltage in dB re FS size of frequency step eliminates clicks at the end of the signal when checked This prolongs measurement time a little 3 2 Frequency response and distortion measurements with STEPS STEPS generates frequency response measurements similarly to ARTA The main difference lies in the excitation signal see STEPS User Manual and the duration of the measurement Depending on the choice of parameters measurements c
27. n i iate cit de aee 28 Foreword Because of the increasing scope and complexity of the ARTA Handbook this guide is now presented separately to those for the other programs in the ARTA family STEPS is a program for measuring transfer functions and distortion in loudspeakers and audio equipment STEPS also provides tools for specialised measurements including maximum linear displacement in accordance with IEC 62458 and distortion limited maximum SPL This handbook aims to familiarise users with the STEPS part of the ARTA family of programs It is not however intended to be a substitute for the original user manual with which the reader should familiarise him or herself Further information can be found on the ARTA website which contains the most up to date releases and application notes Although we aim to continue to supplement and update this handbook in line with the continuing development of the ARTA software we would ask for your patience with any discrepancies that may arise from time to time Suggestions for improvements and corrections are always welcome 1 Introduction to STEPS 1 1 Installation requirements To use the ARTA suite of programs you will need e Operating system Windows 98 ME 2000 XP VIS TA Windows 7 Windows 8 e Processor Pentium 400MHz or higher memory 128k e Soundcard full duplex Installation is very simple Copy the files to a directory and unzip them That s it registry en
28. nment the microphone would register a level of around 120dB which is enough to drive most inexpensive models beyond recommended limits The following example illustrates this point Three microphones were compared an inexpensive model MM 1 T Bone costing around 35 a mid range model Audix TMI around 300 and a class 1 reference microphone NTI M2210 cost around 1100 Figure 3 2 4 compares THD D2 D3 and D4 traces for the T Bone and M2210 Distortion 96 10 0 70 ITI co 0 1 0 01 m THDo 0 001 m THD 20 50 100 200 500 1k 2k Sk 10k 20k f 3483 0Hz THD 0 88935 96 f Hz File t Bone 1 12dB hsw Owr NTI M2210 InSitu 12dB hswv THD T Bone MM vs NTI M2210 12dB THD t Bone grey Distortion 96 10 0 T E 10 S 04 0 01 D30 0 001 a 20 50 100 200 20k f 3483 0Hz D3 0 21094 f Hz File t Bone MM1 lnSitu 12dB hsw Owr NTIM2210 InSitu 12dB hsw D3 t Bone MM1 vs NTI M2210 12dB D3 t Bone red Distortion 96 10 0 70 ITI co 0 1 0 01 m D20 mD2 20 50 100 200 500 1k 2k 5k 10k 20k f 3483 0Hz D2 0 85058 f Hz 0 001 File t Bone MM1 lnSitu 12dB hsve Ovr NTIM2210 InSitu 12dB hsw D2 t Bone MM1 vs NTI M2210 12dB D2 t Bone blue Distortion 96 10 0 0 IT A co 0 1 0 01 m p4o 0 001 20 50 100 200 20k f 3483 0Hz f Hz D4 0 06962 File t Bone MM1 lnSitu 12dB hsyw Owr NTIM2210 InSitu 12dB hsw
29. stortion 10 0 7n m 4 ceo THD m D2 20 50 100 200 500 1k f 1013 3Hz THD 0 35188 D2 0 03828 D3 0 34817 96 f Hz File 10cm hsw 10cm 2010 09 17 19 26 37 Distortion 96 10 0 70 m 4 co 0 01 20 50 100 200 500 1k f 1013 3Hz THD 0 25606 D2 0 02322 D3 0 25377 96 f Hz File 25cm hsw 25 2010 09 17 19 28 36 70 IT 4 co 0 1 0 01 20 50 100 200 500 1k f 1013 3Hz THD 0 23291 D2 0 03483 D3 0 22807 96 f Hz File 40cm hswv 40cm 2010 09 17 19 27 50 Figure 3 2 6 Distortion amplitude and nearfield and at 10 25 and 40cm from top With increasing distance the room influence becomes more noticeable in both frequency and distortion traces Figure 3 2 7 shows direct comparisons of distortion traces for nearfield and 40cm measurements Increasing distance not only makes the traces noisier but it also contributes to overall apparent distortion 13 Distortion 96 10 0 7U ITI co 0 1 0 01 20 50 100 200 500 1k f 1013 3Hz THD 0 232891 f Hz File 40cm hsvve Ovr NF MF vs 40cm THD FF red NF blue 2010 08 17 19 34 44 Distortion 96 10 0 7U ITI co 0 1 m 030 0 01 20 50 100 200 500 1 f 1013 3Hz D3 0 22807 f Hz File 40cm hsvv Ovr NF hsw D3 NF vs 40cm D3 FF red NF dark blue 2010 08 17 19 35 52 Distortion 96 10 0 7U Im co 0 01 20 50 100 200 500 1k
30. trace shows 1 12 octave resolution and a 0 5dB step level The improved resolution and closer approximation to the defined THD limit is obtained at the price of a much longer measurement time and a greater load on the device under test Have fun taking THD limited maximum SPL measurements but do make sure that your tests do not end up causing the kind of damage shown in Figure 3 5 11 Figure 3 5 11 Test victims 27 4 l 4 References ARTA Support Internet cited 2014 Sep 21 Available from http www artalabs hr support htm AES AES2 2012 AES standard for acoustics Methods of measuring and specifying the performance of loudspeakers for professional applications Drive units 2012 Klippel GmbH Measurement of Peak Displacement Xmax performance based method Internet 2012 cited 2014 Jul 26 Available from http www klippel de nc know how application notes html sword_list 5B0O 5D an4 Four Audio Monkey Forest MF Audio Measuring System Internet 1999 Dec cited 2014 Jul 27 Available from http www four audio com en products monkey forest html 28
31. tries are automatically saved at first start up 1 2 Equipment The following is a brief summary of the equipment required accompanied by some basic directions and cross referenced to more detailed information elsewhere Soundcards There are three types of soundcard e Standard onboard soundcard found typically on a computer motherboard e Plug in cards for PCI or ISA bus e Soundcards connected via USB or firewire Essentially all three types are suitable for use with STEPS if they have an output channel Line Out and two input channels Line In Note however that onboard sound cards fitted to laptop computers often have a single mono channel only identified as the microphone input Mic In Amplifier A power amplifier with linear frequency response and power 5 10 watts is adequate The output impedance should be 0 05 Ohms An inexpensive solution that meets these requirements and is small and easily portable is the Thomann t amp PM40C see also ARTA Handbook Section 5 4 For the measurement of linear displacement Section 3 4 or distortion limited maximum SPL measurement Section 3 5 you will need something more powerful Depending on whether hi fi or PA speakers are being tested an output of 200 watts or more may be required Measuring Box The ARTA Measuring Box is not absolutely necessary but it does make life a lot easier When switching between acoustic and electric measurements the annoyance of having
Download Pdf Manuals
Related Search