Technical Report: Sound Quality Evaluation (Part 4)

Here, we will look at two interesting examples: "The sound of an excavator and a symphony" and "The sounds of a car engine before and after warming up."

In the following analysis example, we compare the sound of an excavator with a symphony (Beethoven, Symphony No. 5, 4th movement). These two sounds are completely different, but when we perform the 1/3 octave analysis that is commonly used to evaluate sound, they end up with almost identical spectra, as shown in the data below.

There are various parameters for evaluating sound quality, but here we will use roughness, a parameter that represents the roughness of the sound, to compare and evaluate these two sounds. The following graph shows the spectrum of roughness. A higher value indicates greater "roughness" of the sound, and the difference between the sound of the symphony and the sound of the excavator can be clearly captured by the difference in "roughness".

Next, I will introduce an example of sound quality evaluation of automobile engine noise. The sound quality of an automobile engine differs depending on whether the engine temperature is low or high. Before warming up, a high-pitched sound is noticeable, and after warming up, it becomes quieter. When these sounds are analyzed using the conventionally used 1/3 octave spectrum, the spectrum is almost the same both before and after warming up, as shown below. In other words, conventional methods cannot analyze or evaluate the difference in sound quality before and after warming up.

Fluctuation intensity is a quantity that expresses fluctuations in sound volume in a way that is similar to how humans perceive them. A significant difference can be seen in the area circled in red on the graph. This frequency is around 4 kHz. From these results, it can be seen that the high-pitched, unpleasant fluctuations in engine noise before warming up are due to fluctuations in the 4 kHz component. In this way, by using sound quality evaluation quantities, effective analysis of abnormal noises and other issues can be performed.