By Joe Begin
In comparison to electronic audio tests, the electroacoustic measurement of loudspeakers presents a number of challenges. In addition to the need for precision microphones, the interactions between the DUT (device under test) and the environment creates unique measurement issues. This three-part article discusses relevant international standards for loudspeaker measurements and describes different test environments, providing some basic requirements to achieve good, repeatable loudspeaker measurements.
IEC60268-5 is an international standard intended to ensure loudspeakers are tested in a meaningful and repeatable way. It applies to passive loudspeaker drive units and passive loudspeaker systems. Standards for powered loudspeakers are in development, and so will not be addressed here. The IEC60268-5 standard specifies mounting, acoustical environment, loudspeaker and microphone position, and the test signal and rated conditions for conducting measurements.
The performance of a loudspeaker drive unit (or driver) depends on the properties of the unit itself and its acoustic loading, which in turn depends on its mounting arrangement. Drive units may be mounted in one of three configurations, with the selected configuration clearly described in the test results:
• A standard baffle or in one of two specified standard measur-ing enclosures.
• In free air without a baffle or enclosure.
• In a half-space free field, flush with the reflecting plane.
Loudspeaker systems are usually measured without any additional baffle. The manufacturer can specify that a baffle be used, in which case a description of the mounting arrangement should be included with the test results.
IEC 60268-5 requires that measurements are made in one of five specified acoustic fields: free-field, half-space free-field, diffuse, simulated free-field, and simulated half-space free-field conditions.
Generally speaking, acoustic loudspeaker measurements should be conducted in a free-field. To achieve free-field conditions, testing can be done outdoors, where sound may propagate freely in all directions, or in an anechoic chamber, which approximates a free-field; however, both have their challenges. For free-field conditions outdoors, the loudspeaker and microphone would have to be placed high above the ground to minimize the influence of ground reflections, and ambient noise must be mitigated. Conversely, controlling the environment using an anechoic chamber can be an expensive proposition.
In an anechoic chamber, the minimum requirement for a free-field is that sound propagation from the source follows the “6dB/dd” rule within ±10% on the axis between the reference point and the measurement microphone. The 6dB/dd rule states that the SPL radiating from a source decreases by 6dB per doubling of the distance from the source.
An inexpensive alternative to an anechoic chamber (to approximate the free-field response of a loudspeaker) is ground plane measurement (see Fig. 1), where the loudspeaker and microphone are placed on a hard surface in an open area. When set up correctly, the direct and reflected sound waves will be in phase and will have a combined level 6dB higher than the level of the direct sound. There are some issues to bear in mind, however, such as the baffle appears to be twice as high as it really is, due to the reflected image of the loudspeaker, causing a different diffraction response along the edge that is in contact with the ground. It also has the same issues as the standard free-field outdoor measurements in that it is affected by ambient outdoor noise from vehicles, machinery, aircraft, and wind, especially for low-frequency measurements.
A half-space free-field condition is specified for testing a loudspeaker driver alone. In a half-space free-field, the three-dimensional free-field space is split in half, usually by a hard, reflecting plane. For example, a sound source located outside on hard ground, away from any other reflective surfaces constitutes a half-space free field. A hemi-anechoic chamber (an anechoic chamber with one of its six interior surfaces being a hard, reflective plane) can also be used. A hemi-anechoic chamber should meet the 6dB/dd rule within ±10% between the surface and the measurement microphone.
An example of a diffuse sound field is a reverberation chamber in which all of the interior surfaces are made of hard, reflective material. Here, measurements should be conducted with 1/3-octave band limited noise.
Joe Begin, PE (Acoustical Engineering), a graduate of McGill University (B.Sc.) and the University of Canterbury (M.Sc.), has over 35 years’ experience in test and measurement. As Director of Applications and Technical Support at Audio Precision, he is involved in product management, audio and electroacoustic test applications engineering, and technical support.