**9. Metrics for sound zones**

Audio quality assessment, loudspeaker diagnostics, and active sound-field control require metrics that assess the properties of the sound-field at a specific listening point described by a probability *f*L(**r**) of the ear position. The mean sound power found in such a listening zone is a less suitable metric because the listener evaluates the local sound pressure. It is more appropriate to assess the mean and the variance of the perceptual attributes (e.g., loudness) or related physical metrics (e.g., SPL) over the listening zone [30] considering the probability of the ear positioning as a weighting function *f*L(**r**). This approach is used in IEC 60268–21 [11] for defining a mean SPL over an acoustical zone, but it can easily be applied to the nonlinear distortion metrics in Eqs. (29) and (30). The variance and the maximum deviation from the mean value are also valuable characteristics of the sound zone.

### **10. Maximum SPL output**

The maximum sound pressure output (max SPL) rated according to IEC standard 60268–21 [11] plays a primary role in adjusting the amplitude of the test stimulus in output-based testing. The max SPL can be used to calibrate any input channel (digital, analog) in passive and active systems and provides a maximum input RMS value *u*max, depending on the selected input channel, gain control, amplification, and applied signal processing. The amplitude compression *C*AC(f) from Eq. (16), the sound power distortion ratio *R*Π<sup>N</sup> from Eq. (31), and the maximum impulsive distortion ratio *R*IDR from Eq. (36) are essential criteria for rating max SPL considering the particularities of the target applications.

### **11. Conclusions**

Acoustical measurement in the near-field of the loudspeaker can provide much of the relevant information required for designing and assessing spatial sound

control applications. The spatial transfer function *H*L(*f*,**r**) expressed as a spherical wave expansion provides accurate sound pressure amplitude and phase information at any point **r** in the near and far-field. The spatial scanning effort depends on the particular loudspeaker and can be significantly minimized by considering the symmetry of the loudspeaker. In practice, the spatial transfer function *H*L(*f*,**r**) scanned on a prototype can be applied to other units of the same type as long as the loudspeaker geometry does not change much.

The time-variant transfer function *H*v(*f*|*t*) represents changes in the material caused by heating, aging, fatigue, and production variability. No scanning is required to measure the transfer function *H*v(*f*|*t*) and the equivalent input distortion *U*I(*f*), ignoring the distributed nonlinear distortion *p*D. Such an approximation is valid for most loudspeakers used in spatial sound applications and can be verified by scanning the nonlinear distortion in the near-field of the loudspeaker. All timevariant and nonlinear signal distortion can be extrapolated to any point in the 3D space using spherical wave expansions.

The multi-tone complex is a valuable artificial stimulus that can simplify the interpretation of the amplitude compression and the nonlinear distortion. The sinusoidal chirp is required to measure the impulsive distortion ratio, a sensitive characteristic for detecting loudspeaker defects and abnormal behavior degrading the audio quality.

An anechoic room is usually not required for performing the essential loudspeaker measurements at superior accuracy.

The methods for measuring loudspeaker characteristics presented in this chapter are compliant with modern international loudspeaker standards. They are the basis for simplifying the numerical simulation of sound-field control and selecting optimal hardware components offering a maximum performance-cost ratio.
