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Author Topic: Neumann Anechoic Chamber Mic Measuring Setup?  (Read 481 times)

Scott_Smith

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Neumann Anechoic Chamber Mic Measuring Setup?
« on: February 13, 2019, 02:25:29 pm »

Klaus:
I've searched for this information elsewhere, bu am not finding anything definitive. Can you enlighten me as to whether Neumann used a standard distance from speaker source to mic when performing response plots and polar plots? I have seen the distance of 1 meter bandied about, but don't know if that is in fact correct. Has the distance varied over time? Is it the same for all microphone types?

Also, are the polar plots typically conducted with the diaphragm located at the 0 degree axis point, or with the front of the mic grille located at 0 degrees?

I would also be interesting to know what the termination impedance is of the mic preamps typically used. (I am assuming about 1.2K ohm).

S. Smith   
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Scott D. Smith
Chicago Audio Works, Inc.

klaus

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Re: Neumann Anechoic Chamber Mic Measuring Setup?
« Reply #1 on: February 13, 2019, 03:56:19 pm »

I asked David Josephson to chime in. He is the expert on measuring protocols for condenser microphones.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Kai

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Re: Neumann Anechoic Chamber Mic Measuring Setup?
« Reply #2 on: February 13, 2019, 05:52:12 pm »

I can't comment on what Neumann currently does, but I have first-hand information from Schoeps, being located in my hometown.
They do not use an anechoic chamber, but a long tube to generate a clearly defined sound field.

Unechoic chambers have certain disadvantages:
- There is a lower limiting frequency connected to the chambers size and kind of damping.
- They are not perfectly free from reflections, for example you need something to walk on which is usually done with steel net, and the damping materials cannot be made 100% unreflective too.

There are other techniques for microphone measurements that can exclude reflected sound.
One of the most popular is named Time Domain Spectrometry, TDS. 
The idea behind these types of measurements is, that reflected sound takes longer to reach the device under test than the direct sound stimulus.
So reflections can be excluded by using a very narrow band sweeping filter that has swept out of their way at the time the reflection arrives.
The stimulus would be a swept sine wave in this case, as usual.

I have used TDS with great success in absolutely normal rooms, and the other advantage is, the room does not even need to be very quiet, as noise is filtered out by some amount too.

Cardiod mic's frequency response measurements are usually referenced to 1m, except for special types that are made for close up use only.
These measurements only show a small window into the real situation, as going closer to the source will boost the lower frequencies up to 20dB due to the proximity effect, and going further away will leave a roll off in the range below ca. 100Hz.
The rolloff is partly compensated by room reflections in real recording situations.

BTW, LDC (Large Diaphrag Condenser) cardioid mics are not better in this regard, just different.
Usually their proximity effect is less pronounced, as they tend to go into omni characteristics for lower frequencies.

This all is purely academic, finally your ears needs to judge the result.
I would go so far to say that studio microphones all have quite similar published measurements and sound much more different than the measurements would suggest, on the other hand.
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David Satz

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Re: Neumann Anechoic Chamber Mic Measuring Setup?
« Reply #3 on: February 13, 2019, 07:30:24 pm »

I've been in the anechoic chamber at Neumann in Berlin--but that isn't where they build their mikes any more.

At any rate, while a 1-meter effective measuring distance is a somewhat common practice, it isn't specified by any established standard. As I understand the IEC standard, frequency response curves shouldn't include any proximity effect at all. (At one meter there is some proximity effect for cardioids, more for super- and hypercardioids, and even more for figure-8 microphones--see the attached excerpt of an old Gotham Audio publication on the U 67, for example.)

If an anechoic chamber is used--as Kai mentions, there are alternatives--measurements are generally carried out at a somewhat greater distance, then "corrected" so that the published curves will show the low-frequency response that would be expected at the chosen distance, whatever it is. Even that is a generalization, since the geometry of the sound source--point versus plane or in between--is a major variable as well.

That "chosen distance" has been 1 meter for several of the most respected manufacturers, including Schoeps and Neumann, for a long time. For other manufacturers (DPA among them) it is less, e.g. 30 cm. There also are manufacturers who choose the measurement distance on a product-by-product basis according to the intended use, e.g. if a microphone is designed for close speech pickup it will be measured at (or "corrected to") a smaller distance than if it is intended for general studio use.

Of course when you "correct" to (or actually measure from) a point that's closer than another manufacturer uses, you raise the apparent bass response in your curves relative to theirs. Most manufacturers don't say what their measurement distance is--and even if they did, it's not exactly obvious how to use that information when interpreting the low end of two published response curves if they were made at different effective distances. This makes it difficult to compare the low-frequency response of directional microphones between different manufacturers, which I find unfortunate.

--best regards
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Scott_Smith

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Re: Neumann Anechoic Chamber Mic Measuring Setup?
« Reply #4 on: February 14, 2019, 02:02:12 pm »

Thank you all for your speedy responses!

Kai/David: yes, I am acquainted with the pitfalls of anechoic measurements.  Mostly, I was just trying to get a handle on how various manufacturers conducted their tests, so I can compare apples with apples (although, as David notes, there can be significant differences in the sound source). I have also used TEF systems in the past to make TDS measurements as well, which can be very useful, but I find it easier to do polars in a chamber equipped with a turntable.

When it comes to actually determining what mic to use for a specific application, I of course always depend on my ears, but I find measurements useful to corroborate what I hear with what the measured performance is, as well as to weed out mics that may have issues.

Regards,
--S
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Scott D. Smith
Chicago Audio Works, Inc.

Kai

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Re: Neumann Anechoic Chamber Mic Measuring Setup?
« Reply #5 on: February 14, 2019, 05:40:10 pm »

I am a measurement guy, but I have given up doing measurements on microphones long time ago.

Even lesser so comparing graphs made by the manufacturer, for example comparing a Sennheiser MKH40 to a Schoeps MK4 looks like they should sound almost the same, but the MKH is much duller.

Taking the two in my hand and listening with headphones to my own voice is the best comparison I can do covering everything at once.

Something you almost never find in factory measurements is the proximity effect's variance.
This is, in practice, one of the most important factors when recording musical instruments or voices.

One has to keep in mind that no instrument or voice produces a homogeneous soundfield like in measurements.
Out of this specially shaped soundfield results a strong interaction between the microphone and the instrument.

This interaction very much affects the audible "impact" or "punch" a microphone produces.
Some do boost the upper bass, some the lower and some are especially constructed to reduce the boost like the Electrovoice RE20 with it's "Variable D" system.
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Timtape

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Re: Neumann Anechoic Chamber Mic Measuring Setup?
« Reply #6 on: February 20, 2019, 05:31:03 am »

Something you almost never find in factory measurements is the proximity effect's variance.
This is, in practice, one of the most important factors when recording musical instruments or voices.

Yes it seems increasingly rare. Even a single plot published often has no reference to distance.

On another forum a fellow bought the latest Shure stage vocal mic, the expensive KSM8. It is ingeniously designed for minimum proximity effect in a cardioid mic. He  tried it and gave it to his performer friends to try it also. All of them rejected it. It seems that over time they had become so used to proximity effect  that  they couldn't cope without it and went back to the much cheaper mic.


Some do boost the upper bass, some the lower and some are especially constructed to reduce the boost like the Electrovoice RE20 with it's "Variable D" system.

Yes and the EV principle goes back to the 1950's.

https://www.prosoundweb.com/topics/education/variable_d_and_beyond_classic_ev_microphone_design_evolution/


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