# Effect of Transducer Burn-In on Subjective and Objective Parameters of Loudspeakers
## 4 Objective Measurement
### Frequency Response  
Figure 1 presents the frequency response plots of the tested loudspeaker systems. The legend indicates the serial numbers of the loudspeaker units included in the study. The letters U, W, and N denote, respectively, used, burned-in, and new devices. The frequency responses are shown with 1/12 octave smoothing applied.  
As can be observed, there are significant discrepancies in the frequency characteristics of the tested units, particularly in the low-frequency range. This variation is evident even among loudspeakers from the same series with consecutive serial numbers. Figure 2 provides a more detailed view of these differences. The most pronounced discrepancies were observed among non-burned-in devices from the same production series, despite having sequential serial numbers.  
the loudspeaker system, the impedance magnitude at each frequency point was calculated using the following relationship:  
\[Z=\frac{U_{f}}{I_{f}}\]  
where \(Z\) is the impedance magnitude, \(U_{f}\) is the voltage, and \(I_{f}\) is the current at a given frequency. This method allowed for the precise determination of the impedance characteristics of the tested loudspeaker systems.  
Figure 3 presents the impedance magnitude plots for all tested loudspeaker units. As observed, the impedance peaks associated with the interaction between the diaphragm and the bass reflex port occur at the same frequency across all tested devices. However, the peaks resulting from the parameters of the transducer's vibratory system appear to be shifted by several hertz relative to one another. This variation is attributed to manufacturing tolerances in the transducers and the aging processes affecting the components of the vibratory system.  
The most significant differences in impedance magnitude were observed among loudspeakers from the same production series, which confirms that manufacturing inconsistencies have a greater impact than the burn-in process.  
By analyzing Figures 4 and 5, it can be observed that there are notable differences in the impedance magnitude plots between loudspeaker systems from the same production series that differ by as little as one serial number. These discrepancies appear despite identical design, materials, and nominal parameters, suggesting that small variations introduced during the manufacturing process can result in measurable differences in electroacoustic behavior. Such variability may stem from tolerances in material properties, adhesives, mechanical alignment, or suspension geometry introduced during assembly.  
Furthermore, the burn-in process caused a slight but consistent shift in the impedance peak frequency of the transducers. This shift typically indicates an increase in the mechanical compliance of the suspension system, particularly in the surround and spider components. The most likely explanation involves physical relaxation or softening of the elastomeric materials used in the upper suspension (usually rubber or foam) due to repeated mechanical stress and localized heat buildup during prolonged operation. Additionally, early-stage microstructural degradation or plastic deformation of the phenolic resin used in the spider (lower suspension) may reduce stiffness and alter resonance characteristics.  
Figure 3: Impedance magnitude characteristics of the tested loudspeaker systems.  
These changes, although small in magnitude, reflect real physical phenomena that can influence transducer performance in the early phase of use. However, it is important to note that the observed variations remained within acceptable industry tolerances and did not result in perceptible changes in subjective evaluations, as confirmed by blind listening tests. This suggests that while burn-in may slightly alter mechanical properties, the resulting acoustic impact is minimal and largely inaudible under normal listening conditions.