# Correct measurement mic placement



## gaggi (Mar 19, 2010)

Hi all,

I am not sure how to place my measurement mic (ECM8000) when measuring the left or right speaker only.

My speaker placement is exactly the same as in the picture below. Should I place the mic at the focal point of the speakers (picture 1) or at the listening position (picture 2) when measuring the left and the right speaker? 

thanks,


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## JohnM (Apr 11, 2006)

Put the mic at the listening position.


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## gaggi (Mar 19, 2010)

Ok. And when I measure both speakers together? Also at the listening position?


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## SAC (Dec 3, 2009)

Agreed, put the mic where you head is located.
And measure each speaker individually. Otherwise you simply create an experiment that verifies the effect of superposition to create polar lobing that appears as comb filtering in a frequency response plot, and which is more difficult to isolate in an ETC.

While we model specular behavior as rays, they are NOT_ that_ focused.

Nor is the issue of what each ear hears an issue. Inter-aural cross correlation (IACC) - the fancy term for how what each ear hears - is well researched and understood.

The cross-correlation measurement tells us how similar two signals are. If we put a binaural dummy head (a simulated human head with microphones ‘in its ears’) in a space and connect the microphones 'in its ears' to a measurement platform that measures cross-correlation, then we are making an interaural cross-correlation (or IACC) measurement. 

At the normal width of a human head, at very low frequencies, the wavelength of the sound in air is much larger than the distance between the microphones; therefore the pressure at the two microphones will be almost identical. The lower the frequency, the more similar the two signals and thus the higher the correlation. 

In theory, if the sound source is directly in front of the dummy head, the room is exactly symmetrical, and the head is on the line of symmetry, then the sound at the two ears will be identical. Therefore, the IACC will have a high value at all frequencies. 

In reality, the signals will never be identical for a variety of reasons, not the least of which is the variation of the individuals’ pinnae (outer ear), therefore you will have a varying correlation value at different frequencies. 
Thus, while we understand the relationship and correlations between free field and at the ear measurements, the methods used to gather data are determined by the intended use of the data. This often requires more than one technique, each preserving or enhancing the information in a way that yields more insight into the particular problem being solved. 

Thus, as necessary, one can arrive equipped to acquire both accurate data and realistic data, and then let the question being pondered determine the preferred perspective.


If you are interested in measuring this, I suggest the easiest way would be to pick up a copy of ARTA or EASERA which specifically have the capability to do this using a binaural mic (via SASS, Shure M/S, assortment of dummy head :coocoo: mics, ITE, etc. mics)

A primary distinction exists between whether accuracy of the environmental physics or realism regarding how the ear-brain processes the energy is more important. Without going into depth regarding the effects of superposition on the perceived sound - including the precise phase relationships of he various arriving signals after environmental, torso, pinnae reflections and ear canal resonance. 

After a little consideration, it becomes apparent that one cannot have both. Setup parameters that provide a more accurate view of the loudspeaker’s response will require that the effects of the environment be minimized. On the other hand, if the effect of the room is to be considered, then accuracy will need to be sacrificed to include it. The question becomes, “Do I want to know what is actually happening, or do I want to know what is perceived to be happening?” The answer to this question will fundamentally affect the method used to collect the data.

But fortunately, unless you are doing research into the psycho-acoustic phenomena, there is little need to worry too much about this as we understand well the correlation between the two, and we can rather easily anticipate appropriate steps to take advantage of the knowledge and the correlations.. Thus, if you are interested, you might want to investigate the extensive existing psycho-acoustic research regarding the effect of the pinnae, ear canal resonance, and reflections off the torso which are actually very determinant in how one 'hears' the acoustic behavior.

But again, for our basic purposes, we need not worry too much about it here. Especially as we know the most advantageous orientation of a later arriving diffuse soundfield (laterally) and we are easily able to resolve all of the early and later reflections into their arrival time, gain, direction and point of boundary incidence using the ETC response.

Focus on measuring what is actually happening in terms of the speaker-room interaction at the listening position. The available acoustic response models already incorporate what is necessary to know in order to optimize the and within the room.

And in this case, you will do fine to position the mic at the center of mass of where your head is positioned.

A couple of examples illustrating the difference between 'accuracy' and 'realism':

(1 & 2) Direct ('anechoic') and with environmental interaction, and (3) as the direct signal is perceived by the interaction of the torso and pinna reflections


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## gaggi (Mar 19, 2010)

Wow, that's what I call an explanation!


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## flatfinger (Jul 27, 2011)

I'm assuming ( always dangerous ground to be on !) that given the illustrations you linked that you are doing near field monitoring and mixing audio . Just wanted to mention that Bob Katz has a Monitor LEVEL calibration method that is described at his site ( goggle Digeroo and B Katz ) he has a -20 dbfs pink noise file and pages which describe how to use a SPL meter to set a dedicated reference level .

You might want to establish your regular listening level and calibrate the spectral response ( eq) at that particular level . 
Just a suggestion ( not really a rule !)


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## SAC (Dec 3, 2009)

Thanks, but no. 

Acoustical and vibration analysis and design, primarily using Easera, TEF and EASE for LAS modeling, simulation and auralization.

Nearfield monitoring does not transcend this issue. While it attempts to remove SOME of the environmental issues, it does not remove indirect signals, nor does it negate indirect reflections from the torso and pinna. In fact, care must be taken in the nearfield as the relationship of the energie emitted from each driver varies relative to the listener's position. In other words, you perceive the drivers individually with respect to distance, rather than as a coherent soundfield whose component signals do not vary with distance as the inter-driver distance becomes small in relation to the loudspeaker-listener distance..

Nor can any form of calibration remove or reduce said effects resulting from the superposition of direct and indirect signals. 

They simply refer to several common frames of reference that either include or exclude indirect signals from environmental room boundaries or the torso/pinnae.

And while we can minimize the destructive effects of some of the variables, what our ear-brain then does with even a 'pristine' signal becomes the realm of psycho-acoustics. Thus a complete understanding of physical acoustics also includes the correlation between physical acoustics and psycho-acoustics. ( An excellent resource for this is: Blauert, J. (1996). Spatial Hearing: The Psychophysics of
Human Sound Localisation)



Also, as an additional aside, while it is quite easy to obtain 'at the ear' measurements that incorporate elements of the torso and pinnae response (initially studied by Carolyn Rodgers) are one step more psycho-acoustically accurate than simple binaural reception with various dummy heads, SASS mics or various binaural mics; none are as acoustically and psycho-acoustically accurate as those made with the Dr. Mead Killion developed In the Ear (ITE) micing system (and since HTS freaks when trying to link this URL, here it is : http://www.aes.org/e-lib/browse.cfm?elib=5820 - or if anyone is interested in much more information on this system, PM me ) that records sound in the pressure zone of the ear drum and maintains the complete phase relationships of the arriving sound - which also results in recordings that retain all of the phase information as processed by the ear, and after simple compensation of the ear canal resonance retains a 360 degree spherical soundfield accurate to that of the human head - Meaning that you have the most accurate 3D recording method available with complete and accurate 3 space acoustical cues - and no encoding or decoding is required.

Once you have heard the ITE system, no amount of manipulated quasi-surround sound even comes close - as X.1 remains a system whereby if you even try to make it quasi- realistic would have people violating the cardinal rule of video as, if it were indeed accurate, people would respond to accurate acoustic cues by turning to look to the side or behind them in the direction of the stimulus - and hence looking away from the screen. So the surround most imagine to be so accurate is limited to artificially created ambiance and psycho-acoustical special FX. :bigsmile:


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