To achieve free field conditions, two options are available: testing outdoors or in an anechoic chamber.

Outdoors

To achieve free field conditions outdoors, a loudspeaker and microphone must be elevated high above the ground to minimize the influence of ground reflections. For example, the loudspeaker could be mounted on a tall tower, suspended from a crane, or on a boom projecting from the corner of a building’s roof. The required height depends on the size of the loudspeaker which in turn determines the required distance for the microphone to be in the far field.

Consider a bookshelf-sized loudspeaker cabinet with a largest dimension of 13 inches (0.33 meters). Using the lowest rule of thumb value for the far field distance of three times the largest dimension of the source would require the microphone to be 3.28 feet (1 meter) away from the loudspeaker.

Based on the 6 dB per doubling of distance rule, to reduce the level of reflected sound to 20 dB less than the direct sound would require a height of 16.4 feet (5 meters) above the ground. Doubling the size of this bookshelf speaker would double the height required to get the same 20 dB reduction of reflected to direct sound.

Obviously, conducting measurements with a loudspeaker and microphone located at 16 to 33 feet (5 to 10 meters) above ground is challenging! In addition to the inconvenience, inclement weather and ambient noise from wind, traffic, etc. can be problematic.

Half-space measurements can be conducted outdoors by using the ground as the reflective plane. In this case, the loudspeaker would be mounted in a baffle that is flush with the ground surface.

Anechoic Chambers

An anechoic chamber is a special room lined with highly absorptive material on its interior surfaces – walls, ceiling, floor and doors (such as the anechoic chamber shown at the outset of this article). To improve absorption at lower frequencies, the absorptive material (fiberglas or open cell foam) is formed into wedges, with the wedge tips facing into the room. In fully anechoic (4π) chambers, an acoustically transparent working surface above the absorptive floor wedges is usually provided by means of an open, steel wire mesh suspended from the walls.

An anechoic chamber approximates a free field above a lower limiting frequency determined mainly by the length of its absorptive wedges. This lower frequency limit can be estimated from another rule of thumb: to absorb sound of a given frequency, the significant dimension of an absorptive material must be at least 1/4 of the wavelength. Based on the 1/4-wavelength rule and the speed of sound in air at room temperature (1,129 feet per second or 344 meters per second), 3.28 feet (1 meter) long wedges can effectively absorb sound above about 86 Hz in frequency.

For practical reasons (and cost, of course), it’s rare to find an anechoic chamber with a lower limiting frequency below 60 to 80 Hz. Chambers can be calibrated to lower frequencies using a loudspeaker system with sufficient bass (e.g., a subwoofer) with known frequency response. The challenge then becomes finding the frequency response of this reference subwoofer.

It’s interesting to note that anechoic chambers are qualified in terms of how closely they adhere to the inverse square law or 6 dB/dd rule. The ISO standard covering sound power measurements in an anechoic chamber [7] specifies that measurements along traverse lines within the chamber must not deviate from the inverse square law by more than 1.0 to 1.5 dB, depending on frequency. And IEC 60268-5 [1] specifies that a chamber must be within ±10 percent of the inverse square law in the region between the loudspeaker and the microphone.

A low level of ambient noise is also desirable in an anechoic chamber, to enable measurement of low-level harmonic distortion from loudspeakers as well as the sound emitted from low-level noise sources like display screens and electronic components.

To minimize ambient noise, the best anechoic chambers are designed as an inner anechoic room inside an outer room. In addition, the inner room is often mounted on flexible vibration isolators to minimize structure-borne noise transmission from the outer room to the chamber. The chamber shown in the opening image recently set the new world record for the quietest location on earth, with an ambient noise level of -20.3 dBSPL.