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Multi-Way Conventional loudspeakers with crossovers

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Old 28th June 2018, 09:09 PM   #591
iMat is offline iMat  Croatia
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Default THD with ARTA or STEPS

When you measure THD for 1kHz sine signal, both STEPS and ARTA will give the same results but it is important that ARTA uses high order windows and same sampling frequency as STEPS.
For measurement on lower frequencies STEPS automatically adjust acquisition length, while in ARTA the user has to decide which FFT length will be used.
Ivo
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Old 28th June 2018, 09:12 PM   #592
wushuliu is offline wushuliu  United States
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ok. thanks.
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Old 18th July 2018, 07:03 AM   #593
sottomano is offline sottomano
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I have the possibility to lay my hands on a UMIK-1 from MiniDSP:
Acoustic Measurement Tools : UMIK-1

Since a unique calibration file is available, it looks pretty good to me.

However, it is a USB microphone, that means that essentially two soundcards are operating: One to generate the signal, in my case an ESI Juli@ PCI card, and the other (the USB Microphone "Soundcard") to record it.

While I figured out that it is not an issue in telling ARTA to use different devices for input and output, what about other shortcomings, like different delays, maybe different sample rates (UMIK-1 is fixed at 24bit / 48 kHz) or other things I have no clue about?

Does anyone have already tried such a configuration?

Thanks!
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Old 18th July 2018, 07:22 AM   #594
kimmosto is offline kimmosto  Finland
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Quote:
Originally Posted by sottomano View Post
Does anyone have already tried such a configuration?
Yes. I've tested latency variations with Umik-1 and external sound card (but not with ARTA). Conclusion was that severe timing/phase differences exist between different measurements. Timing may stay quite stable few measurements but then delay might jump ~0.5...>1 ms back and forth. Rebooting or starting another program between measurements was not needed to generate delay jumps, and latency adjustment of sound card did not help.
So, it is not valid gear for advanced speaker measurement needed to simulate multi-way to off-axis, power, DI responses etc. because that requires very stable timing i.e. semi-dual (or dual) channel connection and measurement mode selected in ARTA (or REW).
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Old 18th July 2018, 07:58 AM   #595
sottomano is offline sottomano
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Thank you for the quick and well-founded answer....allowing me to save some money
Well, so I have to search for something different as measurement microphone...

Btw, I have a MCE-2000 capsule in my junk box (I believe it is equivalent to Panasonic WM-60) and a suitable preamp/phantom power circuit, but have no idea how to calibrate...
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Old 18th July 2018, 08:36 AM   #596
kimmosto is offline kimmosto  Finland
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This one has individual calibration: Sonarworks XREF 20 Mic

Soundcard for semi-dual channel mode: Focusrite Scarlett 2i2 2nd Gen

I'm CLIO 11 user without actual knowledge is this package very good or not. Heard just some rumors that it's fully capable.
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Old 18th July 2018, 09:16 AM   #597
sottomano is offline sottomano
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Thanks Kimmo.

Focusrite Scarlett sure is a well engineered device.

I'll have to think about whether to use my phantom mic amp in combination with Juli@ (a good soundcard btw) or to go the "easy" way and buy an "all inclusive" soundcard. ARTA works well with it, however I made no acoustic, only electronics measurements up to now. The next project will be the refurbishing of a two way speaker, and with your (VituixCAD) and ARTA's help, I'm confident it will end up well.
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Old 18th July 2018, 09:43 AM   #598
DBMandrake is offline DBMandrake  Scotland
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Quote:
Originally Posted by kimmosto View Post
Yes. I've tested latency variations with Umik-1 and external sound card (but not with ARTA). Conclusion was that severe timing/phase differences exist between different measurements. Timing may stay quite stable few measurements but then delay might jump ~0.5...>1 ms back and forth. Rebooting or starting another program between measurements was not needed to generate delay jumps, and latency adjustment of sound card did not help.
So, it is not valid gear for advanced speaker measurement needed to simulate multi-way to off-axis, power, DI responses etc. because that requires very stable timing i.e. semi-dual (or dual) channel connection and measurement mode selected in ARTA (or REW).
In another thread I argued that dual channel measurement was required with any sound card to be absolutely certain that there are no timing/phase errors between multiple measurements, such as when measuring multiple drivers to import into a sim such as Vituixcad, but I was ridiculed for it..

What you describe is of course worst when the microphone and sound output are completely different sound devices - in that case there is no true time synchronization between them, and not only can there be a varying sample offset from one measurement to another due to software/driver issues, there will also be sample rate drift between them as they won't have a single locked sample rate clock that is usually the case of a single sound card in full duplex mode.

But these kind of unpredictable timing jumps between output and input can also happen due to software/driver glitches and issues even on a single full duplex sound card - I've had this happen to me before. So while you might "get away with" single channel measurement on a good quality full duplex sound device, you cannot rely on it without a lot of cross checking of measurement consistency.

For this reason I now always do dual channel measurements wherever possible, and particularly if I'm interested in capturing the true phase. Then I can know with confidence that my phase measurements are correct and consistent without constantly performing cross checks between measurements.

Full dual channel mode also helps remove some other sources of error such as imperfections in the frequency response of the test amplifier and speaker cable resistance, if the 2nd channel is taken from the driver terminals or crossover input as applicable for the measurement.

You don't need to pay a lot to get a decent full duplex USB sound card these days either. I only have a Behringer UMC204HD which from memory cost me about 65, and it does the job very well.
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Last edited by DBMandrake; 18th July 2018 at 09:50 AM.
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Old 18th July 2018, 10:25 AM   #599
kimmosto is offline kimmosto  Finland
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Quote:
Originally Posted by DBMandrake View Post
In another thread I argued that dual channel measurement was required with any sound card to be absolutely certain that there are no timing/phase errors between multiple measurements, such as when measuring multiple drivers to import into a sim such as Vituixcad, but I was ridiculed for it..
One of the main issues in practice is that common measurement softwares such as ARTA and REW normalize timing by moving IR peak to 0 ms point if semi-dual or dual channel mode is not selected. Software cannot do much more because input data for cross-correlation function is not available in that location (2nd analog input) they expect.

Normally I don't recommend full dual connection due to possible grounding problems and severe damages if power amplifier is not single-ended: common negative potential/rail with line input and speaker output. User just need to know features of power amplifier typically used while measurement. Compensating of everything; speaker cables etc. might be overshoot because equipment IRL have some linear distortion after speaker is designed.

Single channel gear such as simple USB mics are very common today. They have spread from room acoustics measurements to speaker design which is unfortunate because more and more new users end up to bad, inaccurate/random and very slow design methods. This is not my fight, so I simply recommend to ignore messages and authors supporting or advertising such gear from now on.
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Old 18th July 2018, 10:45 AM   #600
DBMandrake is offline DBMandrake  Scotland
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Quote:
Originally Posted by kimmosto View Post
Normally I don't recommend full dual connection due to possible grounding problems and severe damages if power amplifier is not single-ended: common negative potential/rail with line input and speaker output.
Even when using a single ended amplifier direct connection of earths still gives issues with ground loops where current in the negative cable to the speaker/crossover will influence the measurement. This can cause weird artefacts like a tweeter high pass filter rolling off at low frequencies only so far and then appearing to increase again.

Also the crossover output in systems like mine with a lattice all pass filter in the crossover cannot be measured this way.

For these reasons I use a small high quality audio step down transformer to sample speaker level signals to connect to the sound card input in line level mode.

The step down ratio is such that the peak voltage from a 100w amplifier (my amplifier is only 50w anyway) is less than a normal maximum line level signal so no further attenuator or protection diodes are needed.

The transformer is ruler flat from about 10Hz to 40Khz and provides full galvanic isolation, as well as being able to drive the balanced input in the USB sound card correctly. (Which has slightly higher distortion if driven by an unbalanced signal due to the servo input) Distortion is also vanishingly low compared to speaker distortion.

This gives me full freedom to measure anywhere on a crossover including on the output of a lattice all pass filter and completely eliminates any ground loops or other spurious effects.

I use it both for the 2nd channel input when performing acoustic measurements, as well as being the primary input when measuring the electrical output of a crossover. (I actually have two identical transformers so can use dual channel mode here too if I want to)

I definitely recommend this over direct connection using resistors and protection diodes, provided a transformer of suitable quality and step down ratio is chosen.
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