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:
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 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).
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 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.
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.
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).
Figure 5: Illustrations of the free field (zero reflections) and diffuse field (only reflections).
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).
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.
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.
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.
In a diffuse field, like a reverberant chamber, the sound level is the same, no matter where the microphone measurement recording is made.
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.