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Old 18th March 2011, 06:51 PM   #21
Pano is offline Pano  United States
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Away from other objects (reflections) as much as possible. Rigidly mounted, not suspended.
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Old 18th March 2011, 11:10 PM   #22
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Thanks, Pano.

Not an easy task though to clamp it securely yet at the same time keep it away from reflective objects. ......
I would be interested to see how other folk on here do it.


Dave.
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Old 19th March 2011, 06:45 PM   #23
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I thought I read that you could do a blackman burst with ARTA. Perhaps it was done manually, and just called that by whoever.

Speaker driver testing is allegedly professionally done with the driver mounted on a wall facing into an anechoic chamber. The wall is considered an "infinite baffle" since the rear wave can never get into the chamber. The calibrated mic is positioned about a meter out from the diaphram.

In the real world as I know it, you close mic (1-2 inch) the diaphram (at it's center), and if room acoustics are likely to be an issue, you use tone bursts and "gate" the signal from the mic, so on the oscilloscope (or equiv.) you only see the initial acoustic burst output, and not any other reflections of that acoustic output due to the room. At higher frequencies, most of the energy comes from the part of the cone where the voice coil attaches (the center), and it gets very directional.

Another thing to know is that the burst envelop is likely to have spectra other than the burst frequency (energy at other frequencies other than the sinewave in the burst envelop). So when the acoustic burst envelop looks funky, take it with some grain of salt. That's why things like the "blackman burst" exist. Ideally you want something like a gaussian rise and fall for the burst envelop, in order to minimize this effect. Blackman is one of many attemps to achieve this.

Another thing to know is that if you measure a driver that's mounted in a speaker cabinet, there will be "baffle step response", usually in the vacinity of 500HZ (+/- 500HZ), and below that frequency the front surface of the cabinet will no longer reinforce the propagation of the acoustic wavefront, so you'll get a roll-off below that frequency, until walls of the listening room start to reinforce (reflect) the energies, usually down below 200HZ. That's normal, and generally requires EQ (acoustic or electronic) to clean it up. Most people ignore Baffle step, figuring that its a minor thing if the speaker is going to be near listening room walls. Personally I think it's important. Either way, it's good to know about it. The linkwitzlab.com website can tell you details and math about it.
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Old 25th March 2011, 04:17 PM   #24
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I've just started using ARTA and as a newbie to speaker measurement I found it all a bit daunting at first and very complex. Much of the user-guide went right over my head, then I found this easy guide to making gated impulse FR measurements:

http://audio.claub.net/tutorials/FR%...ing%20ARTA.pdf

Now I'm really getting to grips with the program and obtaining some meaningful measurements. I found MLS gives the best result in my smallish room where no boundary is further than 5ft from the test speaker.

This is a typical result with MLS:

Click the image to open in full size.

The response below 200Hz is inaccurate due to the limitations of the room.
As for the Blackman burst, yes ARTA does this.
All in all, ARTA is an impressive piece of work, once you understand how to use it!
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Old 26th March 2011, 11:06 PM   #25
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Quote:
Originally Posted by Humdinger View Post
...Speaker driver testing is allegedly professionally done with the driver mounted on a wall facing into an anechoic chamber...
Actually there are few of those left. Too much easier to use a big space + gating, compared to spending huge money to build a chamber which might only be good down to 100 or 70 or whatever hertz.

Quote:
Originally Posted by Humdinger View Post
In the real world as I know it, you close mic (1-2 inch) the diaphram...
...and the close miking vastly increases the SPL of the direct sound versus any reflections.
BUT lest any newbies get confused, this only works at very low frequencies. And you must take repeated measurement getting closer and closer until the SPL doesn't rise any more, to be certain you're in the near field.
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Old 26th March 2011, 11:21 PM   #26
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Quote:
Originally Posted by Pano View Post
Away from other objects (reflections) as much as possible. Rigidly mounted, not suspended.
You can mount a string with a washer on it:
- One end at the mic position
- One end at the speaker position
By moving the washer all around the room, you can trace out an ellipsoid. If the washer hits something, shorten the string until it doesn't.
Repeat
Repeat
Repeat
...you get the idea-until the washer doesn't hit anything.

Measure the length S of the string. Compare versus the mic-speaker distance M.

Reflection free zone will be S-M. If you measured in meters or convert to meters, the reflection free time is then
T=(S-M)/(343m/s)
which should be good down to a frequency of
f=1/T

As an example, suppose you had the speaker on a stand in the outdoors, such that the speaker is 1m above the floor. If the mic is M=1m from the speaker, the string will end up being about S=2.83m long:
T=(2.83m-1m)/(343m/s)=0.0053s
f=1/0.0053s=187.6 Hz
and you would get valid data points every 188 Hz.
Note: this does NOT mean all the data points shown above 188 Hz are valid!!! Many programs show interpolated values, which are to be taken with a grain of salt.

For example, in the measurement OP shows, a lot of the waviness at low frequencies I believe to be a typical artifact of the measurement. I think it's due to the windowing applied to the FFT.
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Old 1st April 2011, 03:03 PM   #27
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Has anyone tried WaveSpectra ?

It is free and very fast and the results are comparable with ARTA

Available at ???????? ???????????? WaveSpectra
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Old 1st April 2011, 03:15 PM   #28
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Is there a version in english? jer
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Old 1st April 2011, 03:26 PM   #29
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Old 1st April 2011, 06:50 PM   #30
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Thanks, got it!
Works good with my GINA-24 card,I use a second system for my tones. jer
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