Phase alignment between subwoofers and mid
Contents
1. 5 Measure the mid highs and save the curve In our example the curve in Figure 15 is obtained Engineering Department 17 Technical Report I SATlive Version 1 31 c T Neumann 2008 SAX p ue ae 8i QE ODE BEN 003 0 Phase one 135 60dB_ en oth Abs af ne ET Wer LL S eee Be i 1 BRN ee eee eee OO B eee ees EE tN ea NPN EE EAV na B 1 XN EB CI eee m B EE ENEN OO mT L JV TT 1 1 L a me ER 20 100 300 500 700 1k OK 3k 5k 7k 10k 15k 20k 45 s HH Sa HIM ei ME V NAS MS oen de 270 N Le 35 20 30 50 70 100 50 300 500 700 7k 10k 15k 20k e mo O D Celan20me be ma m E E E _ Default Fig 15 VT and phase response for the mid high box 6 Mute the mid highs and unmute the subwoofer output 7 Do not use Delay Finder again i e do not synchronise the reference signal to the measured signal again Remember that we are comparing phase on both outputs 1 e we are measuring the difference in time arrival between the two signals as a function of frequency Therefore the synchronisation delay for the reference signal should not be changed on the measurement software again Keep in mind that w2. mT U Lu AE If MON IN Vera AA BH ALA Aa md E ALA er a RAA T ty WON AWA ons i H e E ISSN ee ne ee EEE eee 1248 Cursor PAREN ae REE 288 bu el GR BER 184 1k 1 5 10k 15k 20k n CENSET IAE INN LINE LUC LLL LLL NEL Wa A 2 NL ILLE LL uuu Li AAN zz White Phase Hei r 20 0 a LN 300 500 700 1k 1 5 3k 5k Tk 10k 15k 20k El 1 32 Octave Cr 4 E ML HR 6 E ELLE ia ek Fig 22 m can be seen to be aco different the subwoofers and mid highs The Aero 50 and LX216A sub overlap acoustically in the 45Hz to 125Hz region for the level ratio we selected 9 Add or subtract delay from the subwoofer output until the two phase curves overlap around the crossover frequency as seen on Figure 23 Do not forget to save the curves In this example the phase trace for the subwoofer shows a steeper slope at the pass band than the mid high system It is evident that some delay will need to be removed from the subwoofer until the phase curves overlap as much as possible in the band that they share When reducing the delay time for the green curve on Figure 22 it will shift upwards eventually disappearing and appearing again on th
3. L 2L Tre Peete OO B O CS 1 1 MELLHI aaa ZR EUN Ries m m nee BE LU Ll AN E LIS T BN ONIN A IA IA ne B EVAAA T DDA mm B L IAA om B TT AT LES EM Be 20 1 0 150 300 500 700 1 10 15k 20k si ni Phi se 315 180 M ERNIE NI A NAT AE T eN Na see care 45 A AP Vim cci r 100 150 300 500 700 k EC 10k 15k 20k ELT Bj t 1 320ctave ty See e 008 Cm 6 fg QU En E Default Fig 6 This is the magnitude frequency response curve we are trying to improve on A cancellation can be seen around 400Hz and therefore within the frequency band being reproduced by both boxes 4 Mute the subwoofer output and unmute the mid high output in the processor 5 Measure the mid highs and save the curve In our example the curve in Figure 9 1s obtained Engineering Department 10 Technical Report SATlive Version 1 31 c T Neumann 2008 SAX Le C S a E do MESEN 20 A ns ors J Result Phase Offset 180 _ 6148_ AD E wm E 3 z 1848 CLR LL Si NE i BML en M HA REHN MM MA H RTH A IH a RE telle APE EE RAN NP ee me B 5 A uM A L iM ne AAP eee AV eee ovn 12dB REESE Cursor B ll 0 JAM ME EEE 1t EEEN iim KE NN CIN E 20 100 30
4. PA EH Hi mnes ite iv EM 20 30 Y 70 M00 150 300 500 700 1k 15k MOk 15K 20k el te L 5 P ER eas Fig 20 Shown Wwe is the magnitude frequency response that we are trying to improve on It will be shown below that the low coherence above 100Hz is due to the different arrival times for the mid high and the subwoofer with similar levels The same effect would be seen with a single source and a reflection 4 Mute the subwoofer output and unmute the mid high output in the processor 5 Measure the mid highs and save the curve In our example the curve in Figure 21 1s obtained Engineering Department 24 Technical Report SATlive Version 1 31 c T Neumann 2008 SAX tar BO Qu o0 Coe HB 5 da ee 5 CJ Result Gain 0dB 1 29kHz Phase Offset 45 MUA yo Amplitude NEAR A SIS ee eee MERE 171 c m m E titty ft 1 1 NN na nnb mn nnn ennn v EB BEE RAP MERA ge oT Ei AEN ne m _ ViTiMiy VM NAY ILI VILE Smee AE B COH Limit D SEE CEE E 12dB Cursor RENEE pu ME A Pers Ho 2 i C GR P TN 18dB E 20 100 300 500 700 1k 1 5k 10k 15k 20k E LLL LL ELIU
5. that has poor coherence we can change the microphone location Engineering Department o Technical c Report When it comes to adjusting phase we should check the coherence in order to know which parts of the measurement are reliable and which ones are contaminated by reflections reverberation etc Example 1 Scaled down measurements the subwoofer and the migh high box share a frequency band Before trying to do these adjustments for the first time in a real life situation where one may not always have enough time and where conditions are far from ideal scaled down measurements can be handy to get some practice with the procedure We ll assume that you already know how to take transfer function measurements with the measurement system you are using and that the equipment used is adequate To synchronise the reference signal to the measurement signal we need to measure the system s impulse response which is better obtained from the high frequency band and therefore we ll always use the mid high units for synchronisation There will be times when we ll need to shift the subwoofers backwards by adding delay and times when we will need to move them forward with negative delay Since such thing does not exist we ll add an initial time delay that is the same for all bands so that we will be able to add or subtract delay from the subwoofers initial delay time Once the system has been adjusted we
6. 8 AAA Nap gaes o 20 30 50 TO N 00 300 500 700 A 3k 5k Tk 10k 15k 20k pue Aes eso Pe E le el m Fig 12 In TER WEM case delaying the subwoofer does not make the phase traces overlap for the entire band 10 Measure the system frequency response and compare it to the initial measurement If phase has been correctly adjusted subwoofers and mid highs will sum in phase and this will be reflected on the magnitude frequency response Figure 13 compares the system combination without adjustment red trace with 22 2766ms delay on the subwoofer green trace and with 18 666ms blue trace Engineering Department 14 Technical CS Report I SATlive Version 1 31 c T Neumann 2008 SAX 09 56 B 5 3 0 A UL ves wa gt RE Phase Offset 180 7 A ff e EET NA Y TEN Ampiitiias OOP PRT TP LLLI CL E 2E I ee i tat at HE EH E LA Ne ELA LL EHA Aa aun 9 RE Og a P ne AN VARA AL KEREN eS m m 1 7 8 nli EE Phas TA T NCELHTII NM LLL INVE INN I VA VALE iz E A WV WN l NHIN Signal Generator Tem Volume 5k 7k 10k 15k 20k Ay OM Na y 20 30 50 TO 100 150 300 500 700 k 1
7. Since the delay is added to the whole band the phase increase will be larger the higher the frequency i e the lower the period Figure 3 shows that the phase difference between traces becomes larger as the frequency increases Engineering Department 3 Technical Report SATlive Version 1 31 c T Neumann 2008 SAX 16 09 m SRN a w i REAA E H REAR EH 85 5 H B E B z amp D he gi meno ce B E VAS n NA RI E A Syon aki al AAA e ae EAN Se Epa TL E Si NE ERE BAM RN mat _ eee aaa 51 a 1 alii gf me ARV AVAN LTN EEE Nee Pa AM ML ne co LW VA w AAT Y wo E il ee LP BEEN B 1 l 3 EMEN MEN eee eee eee 300 500 700 1k 1 5k 3k 5k 7k 10k 15k 20k Phage 135 3 AG Mpg A T i R Y AE ced a poen ae ax 20 50 JN 150 300 500 700 1k 1 5k 3k 5k Tk 10k 16k 20 El E 1 20ctave v EJ Cof 40208ms IT E M MUN QU Default Fi ig 3 Went IE delay to a band the effect on the phase response is larger for higher frequencies where the added delay represents a bigger percentage of the period compared to low frequencies The same thing happens to subwoofers The blue trace in Figure 4 shows the phase response for a double 18 subwoofer while the green trace shows the effect of physically m
8. delay time can be used 2 Let s first work just with the mid high We ll use the Delay Finder utility to add the required delay to the channel with the reference signal 1 e to synchronise the reference signal to the measured signal See the user s manual for SATlive or your analysis software for more information 3 Measure the frequency response for the complete system before doing phase adjustments At worst we will see significant cancellation in the frequency range being shared by both enclosures Engineering Department 23 Technical Report SATlive Version 1 31 c T Neumann 2008 m SANNE AE TE A b H B E H H ESN zj B 5 CJ Result Gain 0dB 1 29kHz mbh E 1248 AA po an ann a ps Me ee enn d TN EAN EE EE ee E SiS Bii LII lllILI E AA OTA EHI B IAT b 1 1L LLL EE LL pda s S B nr V l UE ET TTT 11 1 1 LIILLMI MIL NN uH uU ETE EI me EN NE EE AR are L5 23 8 ett AN ho a NIA PER SLM ER B VM NEN TVN Z TS L ex EE DN EEEN EE ee A EE E A ERR 0 km AE NE EHE 20 100 150 300 500 700 1k 15 3k 5k 7k 0k 15k 20k i ce Phase BI xw E I i N LIENS MAMA TL NV TX NEE CECE NAM um m AN Ignal Generator
9. ll get rid of excess delay as will be seen in the examples Let s now run a scaled down measurement of an 18 subwoofer and a mid high unit Subwoofer cut off frequencies for the real system will be HPF LR24dB Oct 30Hz LPF LR24dB Oct 85Hz Cut off frequencies for the real mid high unit will be HPF LR24dB Oct 50Hz LPF LR24dB Oct 20KHz We ll use two 4 speakers to get some practice with the phase adjustment procedure For our 4 to behave acoustically like our real full scale systems we will need to scale the crossover frequencies up To do that we ll multiply the real system cut off frequencies by the ratio of the real system to our scaled down box 1 we will multiply the cut off frequencies by 1877 4 4 5 Therefore the cut off frequencies for the scaled down measurements which will be entered in the processor for the 4 system will be as follows Engineering Department 8 Technical e Report Cut off frequencies for the scaled down subwoofer system will be HPF LR24dB Oct 30Hz x 4 5 135Hz LPF LR24dB Oct 85Hz x 4 5 382Hz The cut off frequencies for the scaled down mid high system will be HPF LR24dB Oct 50Hz x 4 5 225Hz LPF LR24dB Oct 20KHz We ll leave the low pass filter for the mid high box at 20 kHz Otherwise we would be in the ultrasonic range For these measurements we used a DAS Arco 4 enclosure as the subwoofer lying on its side The box used as a mid high also a DAS Arco
10. 100 150 300 500 700 1k 1 5k 3k 5k 7k 10k 15k 20k El 1 320ctave v Ot 4 m 26 ew 16 i QA EL Default Fig 24 ET aE response before blue trace after mem phase alignment 11 Take the lowest delay value and subtract it from the subwoofer and mid high so that at least one of the outputs has a delay time of Oms The DAS Aero 50 mid high is a 3 way line array system with external amplification Resulting delay times are as follows DAS LX216A Subwoofer 14 456ms DAS Aero 50 Low 20ms Mid 25 9167ms High 26 0104ms What we are really after is the time difference for the outputs to be phase aligned Therefore the lowest delay of all bands 14 458ms in this case needs to be subtracted from each of the bands Final delay times will thus be Engineering Department 28 Technical Report DAS LX218A Subwoofer Oms DAS Aero 50 Low 5 542ms Mid 11 4587ms High 11 5524ms Once the final delay times have been entered it is good practice to run the measurement again to check that everything is correct Before adjusting a real system for the first time it makes sense to practice this procedure as often as possible and with whatever combination of gear we happen to lay our hands on until we have mastered the technique Using scaled down systems will allow us to become familiar with the procedures until we are confident to try larger systems Joan La Roda DAS Audio Engineering Departm
11. 32 Octave LJ azim mE Only fe 6 3 M d Default Fig 13 In this case the subwoofer and the mid high box sum optimally when we take away delay from the subwoofer It can be clearly seen that the best sum occurs for the 18 666ms subwoofer delay 11 Take the lowest delay value and subtract it from the subwoofer and mid high so that at least one of the outputs has a delay time of Oms At this time there s a delay of 20ms on the mid high and 18 666ms on the subwoofer Since we had added 20ms just as an arbitrary amount to be able to add or subtract from that as needed once the adjustments have been made we no longer need that excess delay subtract the lowest delay time from the two outputs so that one of them has Oms In our example the mid high output will end up with 20ms 18 666ms 1 334ms The subwoofer output will have 18 666ms 18 666ms Oms Example 2 Scaled down measurements the subwoofer and the migh high box crossed over at the same frequency Subwoofer crossover frequencies for the real system will be HPF LR24dB Oct 30Hz Engineering Department 15 Technical Report LPF LR24dB Oct 85Hz Crossover frequencies for the real mid high unit will be HPF LR24dB Oct 85Hz LPF LR24dB Oct 20KHz Crossover frequencies for the scaled down subwoofer system will be HPF LR24dB Oct 30Hz x 4 5 135Hz LPF LR24dB Oct 85Hz x 4 5 382Hz The crossover frequencies for the scaled dow
12. SE x Ne A i ARL HH He ze 300 500 700 1k 1 5k Tk 10k 15k 20k MW 135 Signal Generator 4 a af Volume White iei E H r3 E Me a i Hi HY 1 32 Octave v Delay fo 40 208ms m ide A Y cem Default Fig 2 The green trace shows the effect of a bell type filter on the phase response Since equalization affects the phase response outputs should never be EQed once phase alignment has been done especially around crossover frequencies Otherwise we would modify the phase of the output being equalized hence affecting the relationship between phase responses which is what we are trying to adjust for when adding delays EQing the input on the processor itself or on a graphic equalizer or the mixing desk will not affect phase alignment since it happens before the crossover b Adding delay to a band or physically displacing it backwards such as moving a subwoofer away from the measurement microphone will have the same effect on the phase response Figure 3 shows the effect on the phase response of adding delay to the mid high cabinet The blue trace corresponds to the mid highs before being delayed while the green one shows the result of adding 0 0313ms 0 0313ms The phase increase can be calculated from the equation Aq 360f It can be clearly seen that the change in phase varies with frequency and as a function of the amount of delay added
13. on the SATLive phase graph decreasing its slope in the pass band 10 Measure the system frequency response and compare it with the initial measurement If phase has been correctly adjusted subwoofers and mid highs will sum in phase and this will be reflected on the magnitude frequency response Figure 24 compares the system combination before blue trace and after red trace adjustment Cancellation and poor coherence that were previously seen around 125 Hz have disappeared As noted above low coherence was due to the fact that the same frequency band was arriving at different times which is equivalent to a reflection of similar SPL Once the correct amount of delay is added the shared band from the two systems arrives at the same time and coherence goes back to normal Engineering Department 27 Technical Report fil SATlive Version 1 31 c T Neumann 2008 F gt 7 7 3 ed A 7i 16 23 m TA m AER esum E Sal Feedback gH B s 3 H B E H A A A 3 Result e 5 60dB 1 29kHz PAL SR IL CLEAN TENE LARSEN V WP T ALLUN LL LLL AEN E iA LL HT STEIN E T Ll HAM aA TCC SACO Te 21 NN A re AAW 91 d TE TU MAN UI NAAN NM MI 11 E ZA L c TL EL _ Phase e HN T A TE LLL ET RAHI HHH NN NE RAMIS Nod BERE SR SD NM 0 AMAA A LESS Y White D TO
14. position that can be repeated easily would suffice For instance the microphone can be placed directly in front of the speaker in its centre very close to the grille This provides an easily repeatable measurement position and a clean measurement with no contamination from the environment What makes phase vary a Any variation on a system s magnitude frequency response will have an effect on the phase response For instance the phase curve changes when equalization is added Figure 2 shows the result of adding a bell type filter centered at 5 04 Khz with a width of 0 42 Oct and 10dB gain to the high frequency band on a processor Phase rises just before the centre frequency and falls just after it Engineering Department 2 Technical e 100 300 500 700 k 1 cn k 10k 15k 20k Report I SATlive Version 1 31 c T Neumann 2008 SAX 16 08 HENT EIE i ae H E 2 B B E B z me A P x jm Pd Phase Offset 45 M 18dB ux PRINCI XL x Ta APA ee LL Loss EB vi l EN A a St ttt ot 6 MY D AA ON w SE B f SNH m E tO AE ANN IN VV ma zm VE VY Lev PRINY V NFL ex me LA LL EL LAC MM ELLEN dE TD bw EB IL MN SERRE eee ae Bane ERE ae es RAE RE ERE 1 Phase AA LLL PLL m LE SAYIN A 8 E 5 8 et 2 en Sa ee
15. to add delay Keep in mind that in our example the subwoofer was physically forward with respect to the top box so we could have mistakenly assumed it was the subwoofer that needed delaying Do not forget that filters have an effect on phase and therefore we cannot predict if we need to add or remove delay until we see the measurements Let s see what would have happened if we had increased the delay time to the subwoofer instead of reducing it On Figure 12 delay has been added to the subwoofer output until the most overlap was obtained The subwoofer delay ended up as 22 276ms Phase overlaps in the 250 300 region which is very little Below 250Hz the blue phase trace is below the green one whereas above 300Hz the green curve is below the blue one i e there s phase difference between them Engineering Department 13 Technical Report SATlive Version 1 31 c T Neumann 2008 SAX 10 43 me oh Oee DEE TN eG x mer iE i a us E B JU IW LALNN Lu L U geen B iJ IL EM A LA LLL da c a MS a Ds pf 9 A a B illl IA MW WMA ALT lM T eek Br NN II altij ut Em l7 l Nui ll ld LIMEN REEN RU RR REGERE ea eee Ta ps BELA 0 gr E HE 360 BENN C MUNITA MN CNC _ TAR ppp HHH NA HL EE TA EMME AAA EAN AVAN TE LL
16. 0 500 700 1 1 5k 3k 5k 7k 10k 15k 20k B d Phase 270 BT Af 21 IN AT PNIS e PEE Ne ener V M ON r y 20 30 50 70 100 300 500 700 1 5 10k 15k 20k j Bl E 1 320ctave LJ ems o 5 nS 009 EES I Default Fig 9 Magnitude and phase frequency response for the mid highs 6 Mute the mid highs and unmute the subwoofer output 7 Do not use the Delay Finder again G e do not synchronise the reference signal to the measured signal again Remember that we are comparing phase on both outputs 1 we are measuring the difference in time arrival between the two signals as a function of frequency Therefore the synchronisation delay for the reference signal should not be changed on the measurement software again Keep in mind that we took the mid high box as our timing reference because it is the signal from which the best impulse response can be obtained 8 Measure the subwoofer and compare the phase curve with that of the mid high box The result can be seen in Figure 10 Engineering Department 11 Technical CS Report SATlive Version 1 31 c T Neumann 2008 SAX NE A Ia cm ES H rik MCES H B E Be ze dE cho fet E Result Gain 60dB m Phase Offset 180 e i 18dB 255 WT y 1 im NN 8 emee I M am MEE KEE d 78 R
17. 4 is placed somewhat higher up and some 15cm 6 behind the box being used as a subwoofer as shown in Figure 7 The microphone is placed on the ground at 90cm 3 feet from the simulated subwoofer Fig 7 Side view set up for the scaled down measurements used for examples 1 and 2 In order to notice more easily the difference between aligning the phases or not aligning them it is recommended to set the acoustic levels of the mid high and the subwoofer the same in the band being shared 225Hz to 382Hz in our exercise The procedure is as follows 1 Enter 20 ms as the delay time for each of the outputs in the processor This is an arbitrary value a different delay time can be used 2 Let s first work just with the mid high We ll use the Delay Finder utility to add the required delay to the channel with the reference signal 1 e to synchronise the reference signal to the measured signal See the user s manual for SATlive or your analysis software for more information Engineering Department 9 Technical Report 3 Measure the magnitude frequency response for the complete system before doing phase adjustments At worst we will see significant cancellation in the frequency range being shared by both enclosures The measurement can be seen in Figure 8 SATlive Version 1 31 T Neumann 2008 SAX eee Arda A US Result a 90dB 1 1848
18. I RME ALE mum oo tt tity NA UA Le ne B j eee LL ovn NN NA AL SE MKS ECRIRE ee EN ti 1 lu d 360 Phase NI IX ALT 4 A MI 20 30 50 TO 100 50 300 500 700 A 3k 5k 10k 15k 20k CHER es 4e Ld Volume v El de 1 32 Octave 0 eaz B Z g Hn HS Default Delay ma Amplitude Only Averages Range Cursor onpeak Lock on peak Fig 10 Capture pem the difference in phase between the subwoofer and the top box for the frequency band being shared 200Hz to 400Hz This explains the cancellation seen near 400Hz and the fact that the level in the rest of this shared band does not increase significantly 9 Add or subtract delay from the subwoofer output until the two phase curves overlap around the crossover frequency Do not forget to save the curves The curve with the steepest slope of the two is the one with the most delay Therefore it seems Clear in this case that we will have to subtract delay from the green curve i e the subwoofer output We ll be able to do this because we initially added a delay of 20ms to the two outputs Remove some of the delay from the subwoofer output and the green curve will loose slope and shift upwards and the two phase traces will overlap within a fairly wide band Engineering Department Technical CS Report fl SATlive Version 1 31 c T Neuman
19. IL LE NEMA AL Dw A pep Lm Volume White hd E ett D Dip YNE Mere 2 UNE 1 i 1 L UL LI LLLA LIE LE LLLA PWA 135 300 500 700 1 5k Tk 10k 15k 20k El ta Wes 1 32 Octave B 9 dm 40 208ms mi m Only m 16 m Rame Grape d x Default Fig 21 Measurements for the mid high box In this case the system normally complemented with subwoofers uses a very low crossover frequency 7 6 Mute the mid highs unmute the subwoofer output 7 Do not use Delay Finder again i e do not synchronise the reference signal to the measured signal again Remember that we are comparing phase on both outputs 1 we are measuring the difference in time arrival between the two signals as a function of frequency Therefore the synchronisation delay for the reference signal should not be changed on the measurement software again Keep in mind that we chose the mid high box as our timing reference because it is the signal from which the best impulse response can be obtained 8 Measure the subwoofer and compare the phase curve with that of the mid high box The result can be seen in Figure 22 Engineering Department 25 Technical e Report SATlive Version 1 31 c T Neumann 2008 SAX mt RB SOD tke eenn RR OR Result Gain 0dB zx okHz ESSE fe B PATE vl VEELS ues plitude ln 1 LLLELL ILL
20. Technical e Report PHASE ALIGNMENT BETWEEN SUBWOOFERS AND MID HIGH CABINETS Introduction FFT based field measurement systems have made it possible for us to do phase alignment at fixed installations as well as at live events where every venue demands a different approach This is particularly important these days since mid high boxes are often flown and subwoofers remain on the ground meaning that phase differences at the listener location can be very significant Given the interest in the subject and the remarkable improvement it can bring to a system it seems like a good idea to write an article that describes the measurement process step by step Before doing that however let s go over the concept of phase Polarity and Phase Polarity only has two discreet values positive and negative Polarity does not change with frequency and may be accidentally inverted when the speaker cables are connected the wrong way or when a signal cable is repaired and pins 2 and 3 get inverted or when one of the bands has its polarity changed on the signal processor by mistake Other times polarity is purposely reversed such as when a passive crossover filter is used Phase may have any value in degrees values are continuous To find out what the phase response is for a given speaker we need to measure it Throughout this article we ll see measurements performed using SATLive Phase curves are shown on the bottom part of the image while t
21. djusted correctly the sum of the mid high and the subwoofer will be better and the magnitude frequency response will reflect this Figure 19 compares both boxes without delay red trace with 20 848ms delay on the subwoofer green trace and with 18 270ms blue trace Engineering Department 21 Technical CS Report H SATlive Version 1 31 c T Neumann 2008 SAX 16 51 S me A 4o 0 wa DOBBE ok is CJ Result Gain 0dB Sm Phase Offset 135 sz 18dB 11 REA PP s Si B 1 A 4 444 4 H H444 mi B yy nee d 1 ll e m m pun MM SERERE ERE RR eee EAN LA da LL LLL Las LL 2 L A ANT me e oi El imit wo TART Wan GIN AAN SA eC LN LLL ne wo WAUMLLINN ILLNM LLL LV eum E 21 Ae Ae eee DW ne _ 1 l eS ROOD EL d T IN TA ui AN Wt o ym m m im D NIU y NL NOT E olume E uA Y V 315 30 50 70 150 BOO 500 700 1 i 3k 5k Tk 10k 15k 20k z t ij E Z a 1 32 Octave 40208 pierre le QU m E a a Default Fig 19 Both UT improve the magnitude frequency response with respect to initial configuration without delay It can be clearly seen that the green and blue traces significantly improv
22. e chose the mid high box as our timing reference because it is the signal from which the best impulse response can be obtained 8 Measure the subwoofer and compare the phase curve with that of the mid high box The result can be seen in Figure 16 Engineering Department 18 Technical Report SATlive Version 1 31 c T Neumann 2008 SAX OEE nz A U o3 Phase I B RATEN LAAT P A CT hr Ampe ESS B i ieee B a xs Hio tT Th AL n Ld LL Lb tt i aac NA RoE A 1 asus Pee m A LA MND Ok ne zm A WUWLLIMNL TTA LL LS NE EET ovn EM E eee Ld a eN EE NE E 30 50 7 10p 150 300 o 1k OK 3k 5k 7k 10k 15k 20k Inl ja d AE LUNDI MALUI INN LLL LE LUC CE PE IEN eel pA BENEN LZ B PALO ULL OUI ess Eo Lun LL AL LLLI EM 20 50 70 VIR 500 ANA u 3k 5k Tk 10k 15k 20k ERE M Odwe v LJ MESE ee ER iei s RE uet Fig 16 Phase can be seen to be different at the crossover region so it is clear that the sum between subwoofer and mid high can be improved upon 9 Add or subtract delay from the subwoofer output until the two phase curves overlap around the crossover frequency Do not forget to save the curves It is not very obvious in this example whether th
23. e lower part of the graph We need to bear in mind that the phase graph only shows values between 180 and 180 If a larger range were to be used the traces would not zigzag as they do here Engineering Department 26 Technical CS Report fi SATlive Version 1 31 T Neumann 2008 sax 16 19 u a CJ Result Gain 0dB 1 20kHz Phase Offset 45 ces z ee E B uuwIYL I Wal LiLL EL LL La LLL LL Pl a EE ES if WENT OMA aa a VL A lel 0 93v w l MLM ERR NAA CAL ENE EE EEN MARIE BEN Es L E LILL LE E ENEN 8 1 5k 10k 15k 20k Lie EE E m 4 H E mnd EN MAL 21 JN LLL IN Mw aH LLLI EHI WIL LL ett NT ES ed n am 20 50 7o M 1 LN 300 500 700 1k 1 5k 3k 5k Tk 10k 15k 20k 1 32 Octave flame cl Z os il Amplitude Only Averages Y Range Cursor on peak Lock on peak II EN LE a SE NNI Soa ane al LI 5 Hd 74 il Dv 54 Et 6 We X N d Reset E E 9 H B E H 2 2 a wmm P i 3 amp 3 lt gt 25 7cm white Fig 23 When TOTE the delay time on the subwoofer output the green trace shifts upwards and reappears on the lower part of the scale
24. e subwoofer needs delay to be added or subtracted Let s try both and see which one works best Option a Remove some of the delay from the subwoofer output The green curve will shift upwards Engineering Department Technical Report SATlive Version 1 31 c T Neumann 2008 SAX 16 38 Ei nie ade mater 0 0 n OE z yer 1 E BNR NAA PPS eee ee a ieee ee NN eine B A PT LL SSCS WS ARS dr Le KNK ME RENEE IT NR ene _ EUST TIMES ne Br A VAV IM AN LA HEEREN AEN ME ES SAN mnm by ANN ROO See ee RED ER a E 505 WL 1 50 TO 10p 150 300 iGO 10 1 ok 5k 10k 15k 20k M j MIN J 7 BOE IANS PTT Ee EEE E 1L Ub DANA LAWN a EO nal Hu b uir EOSTN BEE 315 20 30 50 70 IN 300 500 IN I u 3k 5k Tk 10k 15k 20k E E E ray E x 4 i NE Hh 1 32 Octave oT Mc cete Default Fig 17 Mid TP a subwoofer responses once delay time on the latter has been adjusted such that the phase traces cross each other at the acoustical crossover frequency which is 400Hz in this case The delay on the subwoofer output ended at 18 270ms We have adjusted the delay time until the phase traces cross each other at the acoustical crossover frequency This way they sum perfectly in phase at that freque
25. e the magnitude frequency response However there is no significant difference between them 11 Take the lowest delay value and subtract it from the subwoofer and mid high so that at least one of the outputs has a delay time of Oms Let s say we pick the blue curve There s a delay of 20ms on the mid high and 18 270ms on the subwoofer Since we had added 20 ms just as an arbitrary amount to be able to add or subtract from that as needed once the adjustments have been made we no longer need that excess delay subtract the lowest delay time from the two outputs so that one of them has Oms In our example the mid high output will end up with 20ms 18 270 1 73ms The subwoofer output will have 18 270ms 18 270ms Oms If we wanted to run the same exercise with speakers of different size we would just need to find the ratio between the subwoofer we mean to simulate and the speaker to be used for the measurement Engineering Department 22 Technical nas Report Example 3 Measurements on a real system When measuring a real PA for live or installed use only one of the arrays should be measured and the microphone should be placed approximately halfway between the source and the maximum distance to be covered assuming reasonable coherence is obtained at that location When we choose this centre point for phase alignment we make sure that significant phase change will not occur at any other listening position with
26. ent Copyright 2009 Engineering Department 29 Technical Report Engineering Department 30
27. gh pass filter as well as a Bessel one with identical cut off frequency Engineering Department 5 Technical CS Report fi SATlive Version 1 31 T Neumann 2008 Land 16 11 Lad E B Sal H 3 4 8 8 B B z A H fa Result Gain 0dB z parek Phase Offset 45 1848 LEA v PAP S c OA PO Amplitude 8 2 AI NEN A ME Y 2 1 ES B SSS JL MEN a A A AA um m SLM Med OP A i NA me En Aha ARE e wo LLL LM YIN HL HE DW rr ERE i an SERIE Rima EE ESSE 20 30 50 70 100 150 300 50f 700 1k OK 3k 5k Tk 10k 15k 20k ia v 180 EI 25 1L LE LIUINIM LL Ne 90 Ni 1 uM p REUS mt N Signal Generator Nt 90 135 100 150 300 500 700 Tk 10k 15k 20k El E 40208ms em Only nM INS cae Default Fig 5 If the ETE um is changed on a processor the magnitude as well as the phase frequency response changes The figure above shows the effect of an L R 24dB oct high pass filter blue trace and a Bessel 24dB oct one both with the same cut off frequency 1410 Hz 5 1 32 Octave v What do we mean by phase alignment What we are after is a sum of the subwoofers and the mid highs that result in maximum achievable sound pressure level 1 e no cancellation part
28. gnal using the Delay Finder on SATLive or the equivalent function in other systems In this case it will be seen that coherence for the high frequencies is very low as seen in Figure 6 fl SATlive Version 1 31 c T Neumann 2008 2 E 5 Hd 1 WT Dl G 4 B B E E 12 43 H HR E Result 1 22kHz ASM BN uu rv iil B J Jxtj0lz1llz Iri 1 nbi a Ht i i 0 i i T Gain 0dB Phase Offset 45 Amplitude BB Hi T n A B o UA 1l LL B o ilL M W lii LLL J a ee D 700 1k 1e5k 3k if 4 6 i 1 32 Octave M Delay 31 271ms v Amplitude Only Averages x Range Cursor on peak Lock on peak 10k 10k see En eee en sjen A B ld sl d siu 15k 20k 2 b bh a sbboo p E N HH Setup Back Help e m m mn Signal Generator Default 5 Fig 6 SATLive s scale can be seen on the right hand side ranging from 0 to 1 This curve s coherence trace has been stored and loaded with the Trace Manager utility and is shown as a thinner blue trace 2 Reflections These will cause coherence in some frequency bands to be low We should not trust measurements on those bands If we are interested in measuring a part of the spectrum
29. he magnitude frequency response will be at the top of the image Figure one shows a typical subwoofer phase response in blue with polarity reversal in green A 180 shift can be clearly observed at all frequencies Engineering Department 1 Technical Report fi SATlive Version 1 31 T Neumann 2008 J 16 07 m MELT RR i ee A ed mea 2 7 3 4 El IH EI HI z a un Phase Offset 457 6108 Amplitude M ig B ULM LLLI my Wit il MI oT EE TTT LLL UT hE CWT WT TT EE A Nee I LI eee eee 139 Lu s uL MCh IAA Noh ell Ne AA ong oen EAT EE ee eee eee ae R ANR EEE RENNERS REE ERR I ne EEE NE ERE A ES EE ea CREER RE ees ae ER RAT ERR RI DW Ea MERRIE ERE D E D D E REE EEE EE eee ae AA 204 30 50 70 100 150 300 500 700 1k 1 5k 3k 5k Tk 10k 15k 20k B Phase B iN ae mn 4 le id JN NV e 135 20 30 50 70 100 150 300 500 700 1k 1 5k 3k 5k Tk 10k 15k 20k Eel 1 32 Octave v p 27 10 208118 25 e an AEN 16 ES Par Default Fig 1 Comparison of two phase curves one with opposite polarity Running a phase measurement on our systems and storing it for reference can be useful for checking for correct polarity after maintenance Since we are only looking for a comparison any measurement
30. ial or total in the crossover region To accomplish that goal we need to get the phase traces to overlap Sometimes we will reach complete overlap whilst other times we will not as we shall see in the examples but there will normally be an improvement as compared to a system that has not been phase aligned A final magnitude frequency response measurement will always be required after delays have been applied so improvements can be checked against the curve for the system without delays When complete overlapping is not reached with the use of delay alone we can improve upon the results if our processor provides phase filters To simplify the understanding of this technique however our examples will only use delay Coherence Curve The coherence curve that Fast Fourier Transform based measurement systems provides indicates the probability that the measurement is reliable It s very common to find a coherence curve ranging from O to 1 or from 0 to 100 depending on the Engineering Department 6 Technical Report measurement system with low values in part of the spectrum We should not trust the magnitude or phase frequency responses for those bands for which our measurement system shows low coherence There are two main reasons for poor coherence 1 Reference signal is badly synchronised with the measured signal We can test for this easily if we initiate a measurement without having first synchronised the measurement si
31. in the audience area unless we are really close to the array be it closer or further away from the speakers Make sure the floor reflection does not create poor coherence at the crossover frequencies This is a common occurrence when using a microphone stand In this example we are aligning phase between a DAS Aero 50 line array and some DAS LX218A subwoofers The level difference between the two will make the shared frequency range narrower or wider In our example the two systems overlap in the 45Hz to 125Hz range The DAS LX218A subwoofer is a self powered system incorporating signal processing crossover and equalization but we will use an external processor in order to add delay for alignment to the mid highs A common mistake 1s to filter a self powered system at the same crossover frequencies used in the equivalent passive system this way the slope corresponding to the external processor adds to the slope provided by the integrated crossover SO we end up for instance with a sort of 48 dB oct filter instead of a 24 dB oct one In our example no filtering is used at the external processor Only the filtering that 15 built into the subwoofer is used The DAS Aero 50 mid high is a 3 way line array system Each of its 3 bands has a factory specified delay time applied on an external processor The procedure is the same as for the previous examples 1 Enter 20 ms as the delay time for each of the outputs in the processor a different
32. n 2008 SAX EE E ER GAEREN ee 180 an y ALARA NAG A PTT aT UL 3ES B 0 D j 1 193 eee B ji l mnths hD E EN a SH I AEH Hy EOL f Ne daa NL 1 1 ees ad LL Ne mm 0501 LUI lu ne Br Ye WAM Nn MA Na FIN DW EM WV ll ll mmm EMBEE MN MEREN EE EE m ERE ES 20 190 500 700 10k 15k 20k WOOT E NS VALL ME SNE fC A NAA PAUP B m E Za 8 EM m men rc N NA el T de Cd 45 Volume Y i r 100 300 500 700 k 115 3k 5k 10k 15k 20k Z e 1232 0ctave P MS 2 mE o E 6 d M m Default Fig 11 Top box and subwoofer responses with phase adjustment It can be seen that phase overlaps in the shared frequency band which means they will be summing perfectly in phase The delay on the subwoofer output ended up at 18 666ms From 150Hz to 400Hz the two curves overlap i e they are in phase within the entire band they share Therefore if we compare two phase curves and we want to minimize the difference in phase between them we need to remember the following if a curve has a steeper slope than the other it s arriving late and we need to take away delay If a curve has a gentler slope than the other it s arriving early and we need
33. n mid high system will be HPF LR24dB Oct 50Hz x 4 5 225Hz LPF LR24dB Oct 20KHz Place the boxes as for example 1 and follow the same procedure 1 Enter 20 ms as the delay time for each of the outputs in the processor a different delay time can be used 2 Let s first work just with the mid high We ll use the Delay Finder utility to add the required delay to the channel with the reference signal 1 to synchronise the reference signal to the measured signal See the user s manual for SATlive or your analysis software for more information 3 Measure the magnitude frequency response for the complete system before doing phase adjustments At worst we will see significant cancellation in the frequency range being shared by both enclosures Engineering Department 16 Technical Report LA TP PRA eT AACS Te OE HER m B 7 i E 800 a EE ee wae SLEE IK TEE ie eee _ _ ANT ERES ttt tt EH t A RE LLEDE VO IEA 11 7 MAAN ee 2 hai th b 1 pro o a 4 a i d 2 o Wo eu 4 4 Mute the subwoofer output and unmute mid high output in the processor 14 This is 4 OS a OA Y ic
34. ncy Above and below that frequency there is phase difference that needs to be evaluated when comparing the different sums for the boxes with the different delay times Option b Add delay to the subwoofer output until phase values are the same at the acoustical crossover frequency Phase matching is better above the crossover frequency than below it Again we will evaluate the result on the next step Engineering Department 20 Technical Report SATlive Version 1 31 c T Neumann 2008 SAX EN Mms Enz XO X gr m Mm E 1 1 1 5 aud Nn E EE im Ind Aldi SAT Sedi ESA NN NUUK Va NUENEN CLASE m _ BERE AE ee ne Ml A T AVY LAC LC RR FS aaa ey ew Sees Ss E Res v SaaS RR RISE E eee As mT TT TE TE E ar Nu ee ee E 20 30 50 70 10p 150 300 1k ok 3k 5k 7k 10k 15k 20 A M UN J i ERNA LLL INN LLLI LL 2 LL NUN LL EI NL B MA E 20 50 70 VIR 300 500 zd u 3k 5k 7k MOk 15k 20k E L DN NU EUN PO EE NEN x Fig 16 In this when delaying the phase overlaps at the acoustical crossover frequency and above it but not below it 10 Measure the system magnitude frequency response again and compare it to the initial measurement If phase has been a
35. oving it back 1 7m about 5 6 ft The delay physical in this case increases the slope of the phase curve in the pass band Again the effect increases with frequency Engineering Department 4 Technical Report SATlive Version 1 31 c T Neumann 2008 SAX oee s a ee eee xe Em 6008 AAW Ampiithoe E S ETNA ER BANAAL LL a a ANY AMAN TIT eV T ee ee ee PD LZ PS I o B TAN ERR RC PE 7 RR DS mu B IJ lia a ne B SSAA E F mE B Pc a ae ae aes EN eee m B LL i MI LLELLCE E 28 T BEAMER E a E 2 30 50 70 1100 150 300 500 700 1k 10k 15k 20k 7 Phase T SIN HEMA IH AO NI T T BNSC li dowd de pies l VIN ae Aa 20 30 100 150 300 500 700 10k 15k 20k Bl ds 2 Octave LJ ty 40 208ms 2 82 HE AE 6 5 8 zm SN me Default Fig 4 Moving a source behind its initial placement has the same effect as adding a delay The blue trace corresponds to the initial position while the green one shows the phase curve of a subwoofer that has been shifted 1 7m behind c If the type of crossover filter is changed the phase will change too since the different filter types and their correspondingly different slopes will have their own effect on phase Figure 5 shows the responses of a Linkwitz Riley 24dB oct hi
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