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New Method for Automated Polar Measurements... Feasability Question!
New Method for Automated Polar Measurements... Feasability Question!
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Old 5th July 2020, 12:58 PM   #1
BlacK_Chicken is online now BlacK_Chicken  Germany
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Default New Method for Automated Polar Measurements... Feasability Question!

Hi all,

I am at the beginning of a long journey... building my first true (fully self-developed) DIY speaker project. I have built several kits before and have also developed a very small 2.1 system for my PC workspace several years ago. These projects were cheap, with little amount of labor and cost involved. This will change now: 3-way active with Hypex Fusion amps, professional neodymium drivers and quite a lot of sawdust is to be expected.

Since I have several promising drivers for each position to compare, a lot of measurements will have to be conducted. These will be fully polar in order to obtain directivity patterns for each of the options. Now, I am tinkering with an automated turn-table approach.

For gated IR measurements I can use a large garden. Putting the prototype cabinet on a speaker-stand like this:

K&M 21435 Boxenstander – Musikhaus Thomann

will yield roughly 2m distance from the lawn. There is also no other obstacle (buildings, trees etc.) in this 2m radius around the pole. This will lead to about 9ms before the first reflections will hit the microphone (assuming 1m meter distance between baffle and mic). Please correct me if I am mistaken!

Now, instead of rotating the cabinet in two meters height (quite a mechanical challange...) I propose rotating the microphone around the center pole with a rather lightweight turntable mounted underneath the cabinet-plattform.

Is this (acoustically) sensible approach? Any ideas and comments are highly welcome!

kind regards
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Old 5th July 2020, 01:36 PM   #2
AllenB is offline AllenB  Australia
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New Method for Automated Polar Measurements... Feasability Question!
They say to rotate the speaker not the mic. I involve walls in my speaker design so I've had to learn to move the mic instead.

There are a few basic issues. One is that you should discover an appropriate axis around which to rotate. You should ensure the reflection free time remains useable, and in some cases it becomes appropriate to ensure accuracy of mic placement.
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Old 5th July 2020, 03:00 PM   #3
BlacK_Chicken is online now BlacK_Chicken  Germany
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Quote:
Originally Posted by AllenB View Post
They say to rotate the speaker not the mic. (...)
What is the exact reasoning behind this? This is the point i wanted to discuss in depth

I can imagine: if the acoustic conditions vary (a little bit) with every mic position it will be harder to define a common gating for all measurements.

Quote:
Originally Posted by AllenB View Post
There are a few basic issues. One is that you should discover an appropriate axis around which to rotate. You should ensure the reflection free time remains useable, and in some cases it becomes appropriate to ensure accuracy of mic placement.
Of course, the acoustic centres or the baffles have to aling with the center-pole just like with any other turntable. The Mic-fixture mounted to the aforementioned turntable ensures equal distance to the pole and therefore also to the acoustic source.
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Old 6th July 2020, 05:53 AM   #4
AllenB is offline AllenB  Australia
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New Method for Automated Polar Measurements... Feasability Question!
To the best of my knowledge the main reasons given have been to optimise the reflection free environment and to greatly simplify the process of calculating and positioning the mic. It seems as though you have considered each of these.
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Old 6th July 2020, 08:02 AM   #5
jan.didden is online now jan.didden  Europe
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I've done an-echoic measurements at a university and always rotated the mic. Everything is relative, at least since 1916 ;-) and I cannot think of any reason why it would make a difference one way or the other.

Jan
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Old 6th July 2020, 02:31 PM   #6
mark100 is offline mark100  United States
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New Method for Automated Polar Measurements... Feasability Question!
I have a rotating mic boom off of an outdoors deck, and a couple of rotating turntables.

I used the boom exclusively for a few years, but kept realizing how hard it is to keep mic to speaker distances the exact same, and perhaps the bigger issue was even outdoors the different mic position had different reflections getting in.
I could remove the reflections with gating, but over time i've come to distrust gating...it invariably throws away good data imo/ime. But i digress.....

Lately, I've been using a turntable off the deck, after i position the boom in a single spot. I feel this has been giving more consistent results as i try to make sense of polars.

But by far and away, I'd say the quality of the setup dominates which will work better. In a lab setting, like an anechoic chamber, I'd expect equal results.
With my homemade rigs, I expect different results lol...and who knows which is more valid
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Old 6th July 2020, 02:52 PM   #7
gdan is offline gdan  Greece
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New Method for Automated Polar Measurements... Feasability Question!
I think it will be easier if you build a simple rotating table (e.g. from wood), and put the speaker stand on this.
You could also calibrate this rotating table every let's say 15 degrees.
In this way the acoustic measurement environment will be the same, the distance between mic and speaker will be the same etc.

A nice example can be found here (page 1):
http://www.artalabs.hr/AppNotes/AN6-...EngRev1.01.pdf

I have build a similar device and my measurement results are very good.

Another advantage is that it will be much easier to make vertical polar measurements by simply rotating 90 degrees the speaker.

regards
George

Last edited by gdan; 6th July 2020 at 03:01 PM.
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Old 7th July 2020, 04:04 PM   #8
BlacK_Chicken is online now BlacK_Chicken  Germany
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Thank you very much for all the valuable comments.

I think my question has been covered thoroughly, and your answers give me the necessary background on potential pittfalls to wath out for.

I'll report back, when i've gathered some practical experience :-)

Last edited by BlacK_Chicken; 7th July 2020 at 04:04 PM. Reason: fixed typo
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Old 9th July 2020, 04:22 PM   #9
KJ_Knowles is offline KJ_Knowles  United States
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Definitely not DIY, but I thought members might find it of interest. Even at a commercial level with a substantial budget, automated acoustical measurements can be pretty daunting.

The photo below shows a recently added manipulation system for automated measurements in an anechoic chamber. It is used for development and documentation of commercial and pro audio loudspeakers. Everything from ceiling speakers to touring and install line array components. The chamber is in use pretty much 24 hours a day. The previous system had been pieced together over the years and required much "hand-holding" during measurements. This one along with some custom software can operate unattended. For balloon measurement formats (used in acoustical modeling) it can take hundreds of measurements for one device.

The person in the photo is not me. It is Mike Kasten, the young and very talented head of Engineering at Electro-Voice showing his delight in the new rig.

The chamber is the largest owned by a speaker manufacturer in the US (unless something has changed in the last few years). It is flat into the 50 Hz range and can be reliably correlated into the 30's. When EV moved from Buchanan, MI to Burnsville, MN around 1999 the chamber was dismantled, moved and reassembled.

Click the image to open in full size.
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