# Re: Near field, technical definition

```Hi Kevin-

The nearfield of an acoustic source is more complicated because it
contains a reactive (energy storing) component which does not
propagate.  Most people stay away from measurements in the nearfield
because the acoustic pressure is an incomplete description of how the
source is behaving.  The electrical analogy is that we all walk around
with voltmeters (pressure microphones) and most everyone likes to
ignore the need to measure current (particle velocity).

We can get away with voltmeters because we're usually in the farfield,
where the acoustic waves look resistive (pressure and particle
velocity are in-phase).  When we move to the nearfield, suddenly there
are inductors and capacitors in the circuit and simply measuring the
output voltage no longer describes the output power.  We need to use a
voltmeter and a current meter (particle velocity sensor*), to measure
the amount of power transfered from circuit to the output terminals.

The challenge of listening in the nearfield is that the pressure can
change quickly as you move around the studio, resulting in unintended
changes in loudness perception of the program material.

I hope that this is useful.
-Tony

*Particle velocity sensors do exist, although they remain quite
expensive (http://www.microflown.com).  Some like to estimate the
particle velocity using two pressure microphones that are closely
spaced and subtracting the signals.

--
Tony Miller
Cirrus Logic, Acoustic Systems Group
Austin, TX 78736

On Mon, Oct 8, 2012 at 12:28 PM, Kevin Austin <kevin.austin@xxxxxxxxxxxx> wrote:
>> A recent discussion on loudspeakers as monitors or in other applications provoked these definitions of near field:
>>
>>>> The near field is less than half of the wavelength of the lowest frequency emitted from the sound source or less than twice the largest dimensions of the sound source, whichever is less.
>>
>> and
>>
>> http://www.who.int/occupational_health/publications/noise1.pdf
>>
>>>> The near field of a source is the region close to a source where the sound pressure and acoustic particle velocity are not in phase. In this region the sound field does not decrease by 6 dB each time the distance from the source is increased (as it does in the far field). The near field is limited to a distance from the source equal to about a wavelength of sound or equal to three times the largest dimension of the sound source (whichever is the larger).
>>
>>
>> and the excellent general introduction,
>>
>> http://cafefoundation.org/v2/pdf_tech/Noise.Technologies/PAV.Sound.B&K.Primer.pdf
>>
>>>> The near field is the area very close to the machine where the sound pressure level may vary significantly with a small change in position. The area extends to a distance less than the wavelength of the lowest frequency emitted from the machine, or at less than twice the greatest dimension of the machine, whichever distance is the greater. Sound pressure measurements in this region should be avoided.
>>
>> and
>>
>> http://www.acoustic-glossary.co.uk/definitions-n.htm
>>
>>>> that part of a sound field, usually within about two wavelengths of a noise source, where there is no simple relationship between sound level and distance, where the sound pressure does not obey the inverse square law and the particle velocity is not in phase with the sound pressure
>>
>>
>>
>> Some parts of the definitions coincide, some disagree, some omit certain elements. Because of B&K's eminence in the real world of acoustics, acoustical measurement, and design and manufacture of critical equipment, I am inclined to take this as [more] definitive.
>>
>> Any thoughts / comments / wisdom?
>>
>>
>>
>> Thanks
>>
>> Kevin

```