Evaluating Loudspeaker Performance
In comparison to electronic (analog or digital) audio test, evaluating the acoustic performance of loudspeakers presents several complexities. First off, measuring sound levels accurately requires precision measurement microphones and supporting electronics, which have a known, stable sensitivity and flat frequency response over the frequency range of interest. Testing is further complicated by the interaction of the loudspeaker under test with the test environment. Ideally, we want to measure the direct sound radiated from the device under test (DUT) without any contamination caused by reflections from walls, floors, or ceilings, etc. Special rooms called anechoic chambers are available for this purpose, but they are very expensive and are rarely fully anechoic at the lowest frequencies of interest.
Adding to the above complexities are the role physical dimensions play in acoustics. The range of human hearing is commonly accepted to be 20 Hz to 20 kHz, and the corresponding range of wavelengths of sound in air at room temperature is 17.2 m to 17.2 mm (56 ft to 0.68 in). So, at 20 Hz, a typical loudspeaker is tiny compared to the wavelength of sound, and it behaves like a point source, radiating uniformly in all directions. [Note: this low-frequency wavelength points to why most anechoic chambers aren’t fully anechoic.] At 20 kHz, the opposite is true; a typical loudspeaker is large compared to the wavelength and its radiation pattern is radically different in all directions. Furthermore, the wavelength at 20 kHz is close to the diameter of a typical measurement microphone (12.7 mm or ½ in), making measurements highly sensitive to small changes in microphone position.
Despite the challenges involved, when recommended procedures are followed using quality test equipment, it is possible to make good, repeatable loudspeaker measurements. Within this section of AP.com are a variety of resources discussing these challenges, key objective measurements, references to relevant measurement standards, discussion of sound field types, and various practical aspects of loudspeaker testing.
In general, measurement standards represent consensus among industry experts concerning test conditions, practices, etc., that will help to ensure that devices are tested in a meaningful, consistent, and repeatable way. The key international standard covering loudspeaker measurements is IEC 60268-5, Sound system equipment, Part 5: Loudspeakers. It should be noted that this standard applies to passive loudspeaker drive units and passive loudspeaker systems only (i.e., excludes loudspeakers with built-in amplifiers). Owing to the widespread use of powered speakers today in professional audio and consumer applications, work to revise IEC loudspeaker measurement standards is in process.
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