The free field in acoustic terms is an acoustical area in which there are no reflections. Truly free fields are rarely encountered in reality, because there are nearly always reflections of some kind, even if at a very low level. If the reader can imagine the sensation of being suspended out of doors, way above the ground, away from any buildings or other surfaces, then he or she will have an idea of the experience of a free-field condition. The result is an acoustically ‘ dead ’environment. Acoustic experiments are sometimes performed in anechoic chambers, which are rooms specially treated so as to produce almost no reflections at any frequency –the surfaces are totally absorptive – and these attempt to create near free-field conditions.
In the free field all the sound energy from a source is radiated away from the source and none is reflected. Of course the source may be directional, in which case its directivity factor must be taken into account. A source with a directivity factor of 2 on its axis of maximum radiation radiates twice as much power in this direction as it would have if it had been radiating omnidirectionally. The directivity index of a source is measured in dB, giving the above example a directivity index of 3 dB. If calculating the intensity at a given distance from a directional source, one must take into account its directivity factor on the axis concerned by multiplying the power of the source by the directivity factor before dividing by 4 πr 2 .
In a room there is both direct and reflected sound. At a certain distance from a source contained within a room the acoustic field is said to be diffuse or reverberant, since reflected sound energy predominates over direct sound. A short time after the source has begun to generate sound a diffuse pattern of reflections will have built up throughout the room, and the reflected sound energy will become roughly constant at any point in the room. Close to the source the direct sound energy is still at quite a high level, and thus the reflected sound makes a smaller contribution to the total. This region is called the near field. (It is popular in sound recording to make use of so-called ‘ near-field monitors ’ , which are loudspeakers mounted quite close to the listener, such that the direct sound predominates over the effects of the room.)
The exact distance from a source at which a sound field becomes dominated by reverberant energy depends on the reverberation time of the room, and this in turn depends on the amount of absorption in the room, and the room’s volume Figure shows how the SPL changes as distance increases from a source in three different rooms.
Clearly, in the acoustically ‘ dead ’room, the conditions approach that of the free field (with sound intensity dropping at close to the expected 6 dB per doubling in distance), since the amount of reverberant energy is very small. The critical distance at which the contribution from direct sound equals that from reflected sound is further from the source than when the room is very reverberant. In the reverberant room the sound pressure level does not change much with distance from the source because reflected sound energy predominates after only a short distance. This is important in room design, since although a short reverberation time may be desirable in a recording control room, for example, it has the disadvantage that the change in SPL with distance from the speakers will be quite severe, requiring very highly powered amplifiers and heavy-duty speakers to provide the necessary level. A slightly longer reverberation time makes the room less disconcerting to work in, and relieves the requirement on loudspeaker power.
Absorption
When a sound wave encounters a surface some of its energy is absorbed and some reflected. The absorption coefficient of a substance describes, on a scale from 0 to 1, how much energy is absorbed. An absorption coefficient of 1 indicates total absorption, whereas 0 represents total reflection. The absorption coefficient of substances varies with frequency.
The total amount of absorption present in a room can be calculated by multiplying the absorption coefficient of each surface by its area and then adding the products together. All of the room’s surfaces must be taken into account, as must people, chairs and other furnishings. Porous materials tend to absorb high frequencies more effectively than low frequencies, whereas resonant membrane or panel-type absorbers tend to be better at low frequencies. Highly tuned artificial absorbers (Helmholtz absorbers) can be used to remove energy in a room at specific frequencies.
Helmholtz Resonator
There are two real-world ways to absorb low-frequency energy. These are time tested and proven ways used for many years by studio building professionals. One is called a Helmholtz resonator. This type of absorber uses a tube that has a certain length, depending on the design frequency we are trying to absorb. A slit is placed in the tube top and air enters through it and causes the air inside the tube “vibrating” thus producing absorption at frequencies above the tube’s design frequency. A glass coke bottle is a type of Helmholtz resonator that has a resonant frequency of around 185 Hz. Frequencies above 185 cycles are absorbed and frequencies below are not.
Diaphragmatic Absorber
The second type of low-frequency absorber that professionals have used for years is the diaphragmatic absorber. A diaphragmatic absorber does not have an opening like the Helmholtz resonator does, it is a sealed unit. It uses the length or in this case depth dimensions along with other factors to determine the unit’s resonant frequency. A diaphragmatic absorber is similar to a speaker cabinet in design and construction. The front wall moves in sympathy to sound pressure and the cabinet stays inert.
Reflection
The size of an object in relation to the wavelength of a sound is important in determining whether the sound wave will bend round it or be reflected by it. When an object is large in relation to the wavelength the object will act as a partial barrier to the sound, whereas when it is small the sound will bend or diffract around it. Since sound wavelengths in air range from approximately 18 meters at low frequencies to just over 1 cm at high frequencies, most commonly encountered objects will tend to act as barriers to sound at high frequencies but will have little effect at low frequencies.
Reverberation Time
W .C. Sabine developed a simple and fairly reliable formula for calculating the reverberation time (RT 60) of a room, assuming that absorptive material is distributed evenly around the surfaces. It relates the volume of the room ( V) and its total absorption ( A) to the time taken for the sound pressure level to decay by 60 dB after a sound source is turned off.
In a large room where a considerable volume of air is present, and where the distance between surfaces is large, the absorption of the air becomes more important, in which case an additional component must be added to the above formula:
where x is the absorption factor of air, given at various temperatures and humidities in acoustics references. The Sabine formula has been subject to modifications by such people as Eyring, in an attempt to make it more reliable in extreme cases of high absorption, and it should be realized that it can only be a guide.
Early Reflections
Early reflections are those echoes from nearby surfaces in a room which arise within the first few milliseconds (up to about 50ms) of the direct sound arriving at a listener from a source (see the diagram). It is these reflections which give the listener the greatest clue as to the size of a room, since the delay between the direct sound and the first few reflections is related to the distance of the major surfaces in the room from the listener. Artificial reverberation devices allow for the simulation of a number of early reflections before the main body of reverberant sound decays, and this gives different reverberation programs the characteristic of different room sizes.
Echoes
Echoes may be considered as discrete reflections of sound arriving at the listener after about 50ms from the direct sound. These are perceived as separate arrivals, whereas those up to around 50ms are normally integrated by the brain with the first arrival, not being perceived consciously as echoes. Such echoes are normally caused by more distant surfaces which are strongly reflective, such as a high ceiling or distant rear wall. Strong echoes are usually annoying in critical listening situations and should be suppressed by dispersion and absorption.
Flutter Echoes
A flutter echo is sometimes set up when two parallel reflective surfaces face each other in a room, whilst the other surfaces are absorbent. It is possible for a wave front to become ‘ trapped ’into bouncing back and forth between these two surfaces until it decays, and this can result in a ‘buzzing’ or ‘ringing’ effect on transients (at the starts and ends of impulsive sounds such as hand claps).
Bibliography:
Sound and Recording, Sixth Edition, Francis Rumsey and Tim McCormick.
Designing Sound, MIT
Sound Design, Maurizio Giri.
https://www.acousticfields.com/how-to-build-a-diaphragmatic-absorber/
Comments