3/20/23, 5:16 PM
`
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`
`SIMCENTER
`
`SIMCENTER - TESTING
`
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`
`Jul 29, 2020 • Knowledge
`
`DETAILS
`
`Direct YouTube link: https://youtu.be/PcwFjB6z17A
`
`In the world of acoustics, there are many terms that are used to describe the acoustic
`field around a sound emitting object. Four of the most important are listed below:
`
`Near Field
`Far Field
`Free Field
`Diffuse Field
`
`This article explains the differences and usage of these acoustic sound field terms.
`Near Field versus Far Field
`
`As one may suspect, the acoustic terms “near field” and “far field” have to do with the
`physical distance from the sound source (Figure 1). Depending on how far away an
`observer is from a sound emitting object, the acoustic energy produced by the sound
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 1/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 001
`
`

`

`3/20/23, 5:16 PM
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`source will behave quite differently. It is therefore important to understand these
`differences, and design measurements carefully.
`
`Figure 1: Sound waves behave differently in the near field (A) and far field (B).
`
`Far field
`
`The acoustic far field begins approximately at a distance of 1 wavelength away from the
`sound source, and extends outward to infinity (Figure 2). As wavelength is a function of
`frequency, the start of the far field is also a function of frequency. The far field is
`defined as the region where the sound pressure and acoustic particle velocity are in
`phase, and where the sound pressure level decreases by 6 dB for each doubling of the
`distance from the source.
`
`Figure 2: The far field begins at approximately 1 wavelength away from the source.
`
`In the far field, the source is far enough away to essentially appear as a point in the
`distance, with no discernable dimension or size. At this distance, the spherical shape of
`the sound waves have grown to a large enough radius that one can reasonably
`approximate the wave front as a plane-wave, with no curvature (Point B in Figure 1). At
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 2/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 002
`
`

`

`3/20/23, 5:16 PM
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`this distance, sound pressure level is governed by the inverse square law, and a single
`microphone sound recording will give reliable & predictable results. For each doubling
`of distance away from the source, the sound pressure will drop 6 dB in the far field,
`assuming no reflections (see "free field" below).
`
`In many acoustic standards, measurements are often specified at a distance of at
`least one meter from the sound emitting object to ensure that the measurement is taken
`in the far field for the most critical frequencies.
`
`Near Field
`
`When close to a sound emitting object, the sound waves behave in a much more
`complex fashion, and there is no fixed relationship between pressure and distance. Very
`close to the source, the sound energy circulates back and forth with the vibrating surface
`of the source, never escaping or propagating away. These are sometimes called
`“evanescent” waves. As we move out away from the source, some of the sound field
`continues to circulate, and some propagates away from the object (Figure 3).
`
`Figure 3: The near field is complex, with sound energy both circulating and
`propagating.
`
`This transition from circulating to propagating continues in an unpredictable fashion
`until we reach the threshold distance of roughly a wavelength, or three times the largest
`dimension of the sound source, whichever is greater. This complex region is known as
`the acoustic "near field". This mix of circulating and propagating waves means that there
`is no fixed relationship between distance and sound pressure in the near field, and
`making measurements with a single microphone can be troublesome and unrepeatable.
`Typically, measuring in the near field requires the use of more than one
`microphone (Figure 4) in order to accurately capture the energy borne by the circulating
`and propagating waves.
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 3/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 003
`
`

`

`3/20/23, 5:16 PM
`
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`
`Figure 4: Acoustic arrays featuring many microphones can be used close to a source to
`accurately capture sound energy in the near field.
`
`Free Field versus Diffuse Field
`
`When sound radiates from an object, it can reach an observer directly by traveling in a
`straight line, or indirectly via reflections. Reflected sound waves can bounce off surfaces
`such as walls, the floor, ceiling, as well as other objects in the area. Often when we
`experience sound, we are receiving both direct and reflected sound waves. Under
`carefully controlled circumstances, however, we can experience the extreme ends of this
`continuum: 1) an acoustic field where zero reflections are present, and only the direct
`sound is observed, and 2) the opposite acoustic field, where zero direct sound is
`observed, and only reflected sound is present. The names given to these two extreme
`acoustic environments are FREE FIELD and DIFFUSE FIELD respectively (Figure 5).
`
`
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 4/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 004
`
`

`

`3/20/23, 5:16 PM
`
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`
`Figure 5: Illustrations of the free field (zero reflections) and diffuse field (only
`reflections).
`
`Free Field
`
`In an acoustic free field there are no reflections; sound waves reach an observer directly
`from a sound emitting object. The sound wave passes the observer exactly once, and
`never returns.
`
`Two common examples of acoustic free fields are:
`
`The sound source is far enough away that it appears as a single point source, far in
`the distance. Visualize an airplane flying high overhead on a clear day.
`An anechoic chamber is a special facility constructed to approximate an acoustic free
`field by using materials to absorb sound waves before they can be reflected (Figure
`6).
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 5/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 005
`
`

`

`3/20/23, 5:16 PM
`
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`
`Figure 6: An anechoic chamber is used to approximate a free field.
`
`In an anechoic chamber, specially designed fiberglass wedges cover the walls, floor and
`ceiling to absorb sound so it is not reflected. In order to be effective (especially at low
`frequencies,) these rooms need to be very large, with long wedges, and often feature
`mechanical isolation from the surrounding building and foundation so no vibration is
`transmitted to the chamber.
`
`Diffuse Field
`
`A diffuse field describes an acoustic field where sound waves reach the observer from all
`directions. The reflected sound is of similar magnitude to the direct sound when it
`reaches the observer, and as a result, does not appear to have a single source. A
`microphone in a diffuse field measures the same magnitude regardless of orientation or
`location; the sound level is the same everywhere. A reverberant chamber for acoustic
`material testing is shown in Figure 7.
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 6/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 006
`
`

`

`3/20/23, 5:16 PM
`
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`
`Figure 7: A reverberant chamber has highly reflective walls to create a diffuse sound
`field.
`
`A reverberation room is designed to have reflective walls built at oblique angles so no
`walls are parallel to each other. This causes the sound waves to be reflected a maximum
`number of times around the room to help create a diffuse field. Often, hemi-spherical
`features are added to large walls to increase wave diffraction (spreading out), adding to
`the diffusivity of the chamber. One of these hemispheres can be seen in Figure 7.
`
`How to Know
`
`It is difficult to determine by visual inspection what type of acoustic field is present.
`
`Using acoustic measurements, the following can be observed:
`
`If in a free field, far field acoustic environment, there is a 6 decibel decrease in the
`measured sound pressure level when doubling the distance from a sound emitting
`object. This behavior is explained by the inverse square law.
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 7/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 007
`
`

`

`3/20/23, 5:16 PM
`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`In a diffuse field, like a reverberant chamber, the sound level is the same, no matter
`where the microphone measurement recording is made.
`In a perfectly diffuse sound field the sound intensity is zero.
`
`Conclusion
`Acoustic field behavior is an important consideration when measuring sound. It is
`important to know the field type so sound measurement levels can be properly
`interpreted.
`
`In some circumstances, it is possible to apply diffuse or free field corrections to adjust
`the measurement levels. Diffuse and free field corrections are often provided for
`microphones, headsets, and binaural measurement devices.
`
`
`Questions?
`
`Contact Scott MacDonald (macdonald@siemens.com) or check on the free on-demand
`webinar: Fundamental of Acoustics.
`
`Related Links
`
`Index of Testing Knowledge Articles
`History of Acoustics
`Sound Pressure
`What is a decibel?
`What is A-weighting?
`Octaves and Human Hearing
`What is Sound Power?
`Decibel Math
`What is the Acoustic Quantity called Q?
`Sound Intensity
`Sound Absorption
`Sound Transmission Loss
`Noise level certification, how to select the right standard?
`Sound pressure, sound power, and sound intensity: What is the difference?
`Sound fields: Free and diffuse field, near and far field
`Loudness and Sones
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 8/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 008
`
`

`

`Sound Fields: Free versus Diffuse Field, Near versus Far Field
`
`3/20/23, 5:16 PM
`Auditory Masking
`Tone-to-Noise Ratio and Prominence Ratio
`Fluctuation Strength and Roughness
`Critical Bands in Human Hearing
`Kurtosis
`Sound Quality Jury Testing
`Describing Sounds with Words
`Fundamentals of Sound Seminar
`
`Simcenter - Testing
`
`Acoustic Testing
`
`Nothing here yet?
`
`Log in to post to this feed.
`
`https://community.sw.siemens.com/s/article/sound-fields-free-versus-diffuse-field-near-versus-far-field#:~:text=Near Field versus Far Field,source will b… 9/9
`
`Jawbone's Exhibit No. 2012, IPR2022-01124
`Page 009
`
`