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R/E/P => R/E/P Archives => Acoustics in Motion => Topic started by: Oldfart on August 14, 2010, 08:05:55 am

Title: Nearfield / midfield distances
Post by: Oldfart on August 14, 2010, 08:05:55 am
Hello,
     not sure if this is the right place for my question, but here I go.

Triangle size wise, what distances are considered nearfield and midfield?

Thank you in advance for your replies,

Denis
Title: Re: Nearfield / midfield distances
Post by: Constantin on August 14, 2010, 07:06:07 pm
this question is related to room size.
In a 1000m
Title: Re: Nearfield / midfield distances
Post by: Barry Hufker on August 14, 2010, 10:22:12 pm
I need a clarification.  Is there such a thing as "Mid-Field"?  "Near Field" is where the majority of the sound a listener hears is "direct" from the source.  "Far Field" is where the listener hears mostly reflected sound.  The "Critical Distance" is when both fields seem to the listener to be equal in level.  I would be surprised if there were anything but Near Field audio in a control room.  But this is not my area of expertise.

Barry

Title: Re: Nearfield / midfield distances
Post by: Oldfart on August 15, 2010, 12:38:51 am
My room is 22 feet deep by 14 feet wide, with a ceiling at 8 feet

I always assumed that distance would be the determining factor!

Oldfart
Title: Re: Nearfield / midfield distances
Post by: bruno putzeys on August 15, 2010, 03:29:36 pm
As technical concepts near-field & far field are strictly defined in function of the ratio distance / wavelength. In the far field, several wavelengths away, acoustical (or in electromagnetic theory radiative) impedance is real and the inverse-square law works. In the near field the imaginary component becomes significant and the inverse square law no longer holds.

The terms near and far field as applied to loudspeakers are not technical concepts but simply a description of application. With "near field" most people mean "sitting on the meter bridge". Far field are the mains and mid-field is a free-standing pair typical of a home hi-fi listening setup.
Title: Re: Nearfield / midfield distances
Post by: Thomas Jouanjean on August 17, 2010, 06:31:15 am
bruno putzeys wrote on Sun, 15 August 2010 14:29

As technical concepts near-field & far field are strictly defined in function of the ratio distance / wavelength. In the far field, several wavelengths away, acoustical (or in electromagnetic theory radiative) impedance is real and the inverse-square law works. In the near field the imaginary component becomes significant and the inverse square law no longer holds.

The terms near and far field as applied to loudspeakers are not technical concepts but simply a description of application. With "near field" most people mean "sitting on the meter bridge". Far field are the mains and mid-field is a free-standing pair typical of a home hi-fi listening setup.


(I'm know you know about what I'm going to say, but for the other readers and just to clarify Smile )

In a room the inverse square law doesn't fully apply because reflections will contribute to the soundfield. There is a massive difference between Studios and a 'normal' room though. Good studios can approach invert square law behaviour.

As usual, grey area.
Title: Re: Nearfield / midfield distances
Post by: jimmyjazz on August 21, 2010, 12:43:11 am
My acoustics prof (David Blackstock) always said "nearfield is when wavefronts are approximately planar, and farfield is when wavefronts are approximately spherical".  There is an impedance derivation for farfield (re: Bruno above) pointing to the Rayleigh distance (0.5*ka^2), but I'm not at all clear how room effects come into play.  I'd venture a guess that farfield is pretty much not going to happen in the average room, but I'll leave it up to the others to tell me whether that's right or wrong.