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Why point the mic at the ceiling?
Old 22nd November 2011
  #1
Gear Guru
Why point the mic at the ceiling?

One sees recommendations to point the measuring mic at the ceiling. Others recommend pointing the mic directly at the tweeter. This must be confusing, so here's the why.
The following graphs were made using one speaker running with the mic about a metre from it.

The sound field between speaker and listener in a typical CR is predominantly Direct or Free.

Most measurement mics sold for CR analysis purposes are Free Field mics.
So point the mic directly at the tweeter and we get this. All good.

Why point the mic at the ceiling?-free-field-mic.jpg

A Diffuse field mic is intended for use as the name suggests. When confronted with a Direct Field it will show rising HF like this.
This is a false reading due to incorrect use. Wrong mic wrong place.

Why point the mic at the ceiling?-diffusse-field-mic.jpg

There is a fix, a way around this.
The correct way to use a Diffuse Field Mic in a Free or Direct field (or one predominantly so) is to point the mic away from the Directly radiating source. So we point 90 degrees up at the ceiling. DOH! Bruel and Kjaer say 70-80 degrees, it depends on the mic design really.

So, this is the same Diffuse Mic pointed up 75 degrees.
It now delivers the true result, fully agreeing with the Direct Field mic.

Why point the mic at the ceiling?-diff-mic-75-degrees.jpg


Most mics sold for CR or other acoustically small room measurement are Direct Field. The mics in U.S. ANSI standard Sound Level Meters are Diffuse Field.


DD
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Old 22nd November 2011
  #2
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avare's Avatar
 

thank you Dan for a great explanation with real life examples!

Andre
Old 22nd November 2011
  #3
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hmmm ... guess i've been doing this wrong
at least in WHERE the mic is pointed.

If I may ask ...

Using ECM 8000 cal mic with 'REW'.

I've [been] positioning the mic at the 'apex', which is 16" further back into the room from my ~ 38% [non-rule] seating position.

The mic was then pointed straight ahead aimed at the center between monitors.

Measurements taken one speaker at a time.


I've just completed installing all the prescribed room treatment [traps, wall absorbers, clouds] ... and now I need to do an accurate measurement to see 'what the heck' i done did'.

side note ... I've been taking measurements through various stages of the build, and, now, most likely ... the data is bogus

where's my Tull's 'Thick as a Brick' record .....
Old 22nd November 2011
  #4
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Jens Eklund's Avatar
The type of mic best suited for control room measurements depends on the type of design. If NE or other near anechoic designs, free field is the right choice. If “normal” control rooms (that actually features reflections, not anechoic in other words), at least according to B&K, a diffuse field microphone (naterally used correctly) is preferred, or slight roll off in the upper frequency range might show up in the meassurements.

EDIT:
If before treatment measurements, all rooms suffers from reflections.
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Old 22nd November 2011
  #5
Gear Guru
Correct

Cheers Andre.
RJ it reads to me that you have been doing the right thing. Mic pointed directly at tweeter or half way between the two is a subtle difference.
Pointed up at the ceiling would be daft but you didn't make that mistake unless I am reading you wrong.

DD
Old 23rd November 2011
  #6
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capoeira's Avatar
 

is a Panasonic Electret Capsule WM-61A free field? I aimed to the tweeter and my "HF-hill" looks very close two the second graph here.
How do I know if it's reflection or mic?
Old 23rd November 2011
  #7
Gear Guru
Good Question

Interesting question cap. I reckon it is a normal unaltered mic response. i.e. because it's very size it is increasingly similar to increasingly short wavelengths. Thus they get reflected. The longer wavelengths just pass by. At these short, comparable wavelengths we get a pressure zone doubling of energy at the diaphragm. Try it at 70 degrees and see if you get a 'flat' response.
DD
Old 23rd November 2011
  #8
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Quote:
Originally Posted by DanDan View Post
Jens:-
please delete your unhelpful and unwelcome post. Start your own thread if you really believe you have something constructive to contribute.
I will happily delete this no longer necessary response if you chose to do the right thing here.
DD
Dan, I just thought this would be the place for you, Jens (and others) talk about measurement mics, "field", "etc" (etcetera...heh) since it has been debated in other threads.Even if there´s a "right or wrong", what´s wrong having a different opinion, this is not a normal thread?

Ciro
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Old 23rd November 2011
  #9
Gear Guru
Debate

It would indeed be nice Ciro. All but one in this thread are doing exactly that.

It is suggested, indeed persisted, that we are all doing this wrong. It is claimed that it is incorrect to use a free field mic for anything other than a NE room.

We are all doing it every day. The Bruel and Kjaer Test of multiple rooms used a DF mic. I have not seen a single text anywhere recommending the use of DF mics in Control Rooms. Genelec did a test of 164 Control Rooms. Lets ask them what type of mic they used.

Apart from debating the absurd, lets consider the absolutely pointless.

The results obtained by using either type of mic correctly in a Control Room are identical.

So how could anyone blessed with even a hint of logic or sanity claim that one of the two is more correct.

DD
Old 23rd November 2011
  #10
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Dange's Avatar
 

Quote:
Originally Posted by DanDan View Post
Genelec did a test of 164 Control Rooms. Lets ask them what type of mic they used.
Free-field, it was a Neutrik 3382.

A minor point for continuing on. Keep to one name for each mic type (and no abbreviations) I've re-read things a lot of times because diffuse looks like direct etc.

So can we have free-field mics and diffuse field mics only please.....
Old 23rd November 2011
  #11
Gear Guru
Good

Thank you Dange. Sorry about the Diffuse Direct confusion, I have a thing about Double Dees. heh

I threw the phrase Direct Field in for subtlety to point at the fact that none of these terms really works for our little rooms, but let's fix that.

The definitions we have to work with, Free Field, Diffuse Field, don't apply to Control Rooms.

A CR does not support a Diffuse field. Jens is definitionally correct when he says T60 is not a valid term in such spaces. However since the field is not Diffuse why use a Diffuse Field mic? I mean is it possible to be wrongly wrong here?

A CR does not have a pure Free Field. There are boundaries. However the boundaries are effectively eliminated at HF. Plus either type of mic is Omni, i.e. the same, at LF.

Dange, you are well versed in measurement. How would you decsribe the Sound Field in a typical CR? And how would you go about measuring the Speaker Room response at the listener spot?

Bruel and Kjaer did a study of end user listening rooms. Free field mic. http://www.bksv.com/doc/17-197.pdf
They came up with a 'common' listening curve which is tilted down towards HF.
Genelec did a test of 164 Control Rooms. Free field mic.
They came up with a different curve, basically flat.

That is a startling difference IMHO and I welcome debate on it over at the thread started for that purpose.
Flat Frequency Response- Not

DD
Old 23rd November 2011
  #12
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Mctwins's Avatar
 

Hi,

Intresting topic and good initiative of measurments, DanDan.

I do have a question...

How does one know if the mic is diffuse or free field. My two mics is just saying omnidirectional mics in specs. Thus it mean that it is diffuse field or free field mics??

I have dbx RTA-M and Behringer EMC8000.

Thanks
Old 23rd November 2011
  #13
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Dange's Avatar
 

Quote:
Originally Posted by DanDan View Post
Dange, you are well versed in measurement. How would you decsribe the Sound Field in a typical CR? And how would you go about measuring the Speaker Room response at the listener spot?
I don't actually know the right answer, hence my interest in the thread! The extremes are easy, if it's a diffuse field then a diffuse mic, for example RT60 measurement in a suitable room or noise levels. A free field mic, if you're measuring a loudspeaker in an anechoic chamber.

Good link here - What is the difference between microphone types? | Prosig Noise & Vibration Blog
Old 23rd November 2011
  #14
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Jens Eklund's Avatar
As stated previously, a control room is (unless NE or other anechoic designs) not free field and not a diffuse field. If you use a free field microphone, the direct sound will be rendered correctly by the mic but all reflections (apart from those arriving head on, not that many hopefully) that arrive after the direct sound will be “equalized”). If you instead use a diffuse field microphone but naturally angled so that the on-axis incidence is about 70 degrees, the direct sound will be correct and also the majority of the reflections (if a control room features reflections, they normally arrives laterally) so when analyzing the total frequency response (direct + room contribution due to all the reflections) the measurement will be as correct as possible.

Again, if the room is anechoic or near anechoic, it doesn’t matter if you use a free field microphone as long as you don’t point it at the ceiling …


EDIT:
And regarding the mic used in the Genelec study; se post 59.
Old 23rd November 2011
  #15
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Dange's Avatar
 

That does make sense Jens. A diffuse mic should give you the net response at that point
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Old 23rd November 2011
  #16
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Jens Eklund's Avatar
Quote:
Originally Posted by Dange View Post
That does make sense Jens. A diffuse mic should give you the net response at that point


/Jens
Old 23rd November 2011
  #17
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Mctwins's Avatar
 

Yes, but it doesen't answer my question.

Is the mics I have for diffuse field then?



Thanks
Old 23rd November 2011
  #18
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Jens Eklund's Avatar
Quote:
Originally Posted by Mctwins View Post
Yes, but it doesen't answer my question.

Is the mics I have for diffuse field then?
As far as I know, the Behringer EMC8000 is free field.
Old 23rd November 2011
  #19
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Mctwins's Avatar
 

Quote:
Originally Posted by Jens Eklund View Post
As far as I know, the Behringer EMC8000 is free field.
Thanks...

It is strange that it is not mentioned what type of mic it is.
Old 23rd November 2011
  #20
Gear Guru
Fields of Green

McTwins. There are quite a few 'Measurement Microphones' sold for the CR/Home Theatre, Room Eq, purposes.
Here's a bunch RealTraps - Measuring Microphones

All of the stand alone microphones are Free Field. I have not seen any product sold in this market which is Diffuse Field. The Behringer looks very similar to all the others, the Diffuse mic elements in the RS SLMs have a very different curve. You can rest assured your Behringer is Free Field.

The reason for this thread is to shine a light on the dichotomy which has caused grief for a few GS now and again. The Microphone elements in USA ANSI standard Sound Level Meters (and possibly some older stand alone Measurement Mics) are Diffuse Field devices.
They will read correctly, exactly the same as a Free Field mic, when orientated 70 degrees away from the axis of the direct radiator.

The results would not be the same in a Diffuse field, which is further evidence of the predominantly Free nature of the macro field between speaker and listener in a CR. In this Zone Without Early Reflections, ZWER if you will, levels are typically 15-25dB below the direct signal. That is why many of us chose Free Field as the lesser of the two 'incorrect' mic choices.

It might be worth musing on, that all of these mics are subject to the laws of physics, a trade off between sensitivity/noise and pure omniness. Most are deliberately corrected physically and/or electonically for use in a Free Field.
The DPA 4060/90 is kind of a step away from these issues. Very omni with a decent S/N ratio achieved by double diaphragms.

Capoeira, please do compare the graphs with your Panasonic at 0 and 70 degrees. There are quite a few DIY Panasonics out there which assume that is flat on axis.

DD
Old 23rd November 2011
  #21
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DanDan...

I think that my 8000 is for free field and that goes for dbx as well.

But should there not be a mic just for, when measure the acoustics of the room. Is mics that is for free field, up for the task?

Thanks
Old 23rd November 2011
  #22
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JLiRD808's Avatar
Quote:
Originally Posted by RJHollins View Post
...I've [been] positioning the mic at the 'apex', which is 16" further back into the room from my ~ 38% [non-rule] seating position.

The mic was then pointed straight ahead aimed at the center between monitors.
Just wondering....why 16" behind the 38%? I know the 38% rule....is the 16" to compensate for "aiming the monitors behind ur head"?

THX!
Old 23rd November 2011
  #23
Gear Guru
Physics

Quote:
But should there not be a mic just for, when measure the acoustics of the room. Is mics that is for free field, up for the task?
All of the readily available Free Field mics do the job perfectly well.
IF (big IF) one did have a competent affordable Diffuse Field mic that would deliver exactly the same result as my graphs show.
It would probably be called The Hen's Tooth.

I recommend NOT using the mics in cheap SLM's, irrespective of ISO or ANSI standard. That is because they are crap for audio purposes, and have caused many a GS trouble.

DD
Old 23rd November 2011
  #24
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Mctwins's Avatar
 

DanDan...

Thanks,

but when mesure with a free field mic one should point in 0 angle pointing towards the source, is this still valid for acoustical measurment?
Old 23rd November 2011
  #25
Gear Guru
Direct

Cheers McT. Yes point the mic directly at the speaker.

Quote:
That does make sense Jens. A diffuse mic should give you the net response at that point
Dange these Free Field microphones are Omnidirectional. They too tell the net response. They just don't have a problem on axis which the Diffuse ones do.

Any mic directly confronting a directional flow of sound will have a pressure increase, rising with frequency, at its front/diaphragm. This is due to the the wavelength getting closer to the size of the mic. A Free Field mic has a built in correction to show the true picture, i.e. the actual pressure spectrum at that spot if there was no lump of metal there disturbing the flow.
Tiny microphones obviously don't suffer this effect until much higher frequencies. But the diaphragm is small so sensitivity suffers.


DD
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Old 23rd November 2011
  #26
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
McTwins. There are quite a few 'Measurement Microphones' sold for the CR/Home Theatre, Room Eq, purposes.
Here's a bunch RealTraps - Measuring Microphones

All of the stand alone microphones are Free Field. I have not seen any product sold in this market which is Diffuse Field. The Behringer looks very similar to all the others, the Diffuse mic elements in the RS SLMs have a very different curve. You can rest assured your Behringer is Free Field.

The reason for this thread is to shine a light on the dichotomy which has caused grief for a few GS now and again. The Microphone elements in USA ANSI standard Sound Level Meters (and possibly some older stand alone Measurement Mics) are Diffuse Field devices.
They will read correctly, exactly the same as a Free Field mic, when orientated 70 degrees away from the axis of the direct radiator.

The results would not be the same in a Diffuse field, which is further evidence of the predominantly Free nature of the macro field between speaker and listener in a CR. In this Zone Without Early Reflections, ZWER if you will, levels are typically 15-25dB below the direct signal. That is why many of us chose Free Field as the lesser of the two 'incorrect' mic choices.

It might be worth musing on, that all of these mics are subject to the laws of physics, a trade off between sensitivity/noise and pure omniness. Most are deliberately corrected physically and/or electonically for use in a Free Field.
The DPA 4060/90 is kind of a step away from these issues. Very omni with a decent S/N ratio achieved by double diaphragms.

Capoeira, please do compare the graphs with your Panasonic at 0 and 70 degrees. There are quite a few DIY Panasonics out there which assume that is flat on axis.

DD
Dan,


Could you (try to) explain why you think that a free field microphone, linear only for sounds hitting it at 0 degree incident, would be a better choice for anything but anechoic conditions? No matter how low the reflections from the room are (approximately about -12 dB or higher for LEDE/RFZ for instance), they will contribute more or less to the complete response of the room and if you use a diffuse field microphone (and use it in a correct way), the direct sound will be linear AND the majority of the reflections as well. If you use a free field mic (pointing at the speaker), only the direct sound and reflections hitting the capsule at 0 degrees incidence (not the majority of reflection in other words) will be recorded correctly and all other reflections (hitting the microphone at various angels of incidence apart from 0 degrees) will be equalized and thus not recorded in a linear fashion. Most reflections do not arrive at 0 degree incidence …

Also, if you use a free field microphone, do you re-aim it for L & R speaker so that the angle of incidence is exactly 0 degrees? Do you use a diffuse field microphone in an untreated room and then switch to a free field after treatment?


I believe the market for acoustic measurements is extremely small compared to the “speaker measurements” market and for this application, a free field mic is naturally preferred since the people doing these kind of measurements are not interested in reflection from the room but only the direct sound from the speaker.
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Old 23rd November 2011
  #27
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Thanks, DanDan..
Old 23rd November 2011
  #28
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Jens Eklund's Avatar
Quote:
Originally Posted by DanDan View Post
Dange these Free Field microphones are Omnidirectional. They too tell the net response. They just don't have a problem on axis which the Diffuse ones do.
?

So they are free field and diffuse field at the same time then or what? ...
Old 23rd November 2011
  #29
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Mctwins's Avatar
 

Jens, but they do are omnidirectional. If putting a free field mic pointing up then one will get all the reflections in the room, just thinking out loud here.

Thanks
Old 23rd November 2011
  #30
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JLiRD808's Avatar
Quote:
Originally Posted by Jens Eklund View Post
[COLOR=black]I believe the market for acoustic measurements is extremely small compared to the “speaker measurements” market and for this application, a free field mic is naturally preferred since the people doing these kind of measurements are not interested in reflection from the room but only the direct sound from the speaker.
Wow so if you ARE measuring the characteristics of your room (ie a mixing room) would u still want a free field? With the mic pointed straight at the speakers, are you picking up enough of the room to make accurate measurements?

I'm not doing speaker design...knowing the peaks/lulls of the room is what I'm trying to get also .

THX!
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