In this introductory column, we will introduce an analytical method for quantifying "the impression a person has when they hear a sound." When you hear the term "sound quality," what comes to mind? Some may think of the timbre of instruments such as violins or pianos. Others may think of a cell phone ringtone. As you can see, "sound quality" refers to the impression a sound makes, which cannot be expressed simply by its volume or pitch. This time, we would like to talk about the basics of this analytical method.
The background behind the focus on sound quality
We live surrounded by a variety of sounds every day. In the office or at home, we hear the sounds of air conditioners, photocopiers, and vacuum cleaners. On the other hand, the moment we step out of the office or our homes, we hear the noise of cars and motorcycles, factory noise, and music from shops in busy areas—some kind of sound is always around us, regardless of our will.
Sounds can be broadly classified into three categories: noise (unwanted sounds) that we don't want to hear, sounds that aren't unpleasant but aren't pleasant either (normal sounds), and pleasant sounds that are enjoyable. It's easy to imagine that it's desirable to eliminate noise (unwanted sounds) as much as possible, and to make normal sounds more pleasant.
Recently, this concept has been applied to the sounds of various industrial products and environmental sounds, shifting from the traditional approach of simply reducing loud noises (noise reduction) to a more pleasant approach of changing the "sound quality" to create more enjoyable sounds (sound enhancement).
Parameters for evaluating sound quality
As an approach to "improving sound quality," sound quality evaluation analysis is used. As the name suggests, sound quality evaluation analysis is an "analysis" method for "evaluating" "sound quality," and it can quantitatively evaluate the basic sound impressions shown below.
- Loudness refers to the human perception of the perceived loudness of a sound.
- Sharpness: Describes the "impression of the high pitch of a sound."
- Roughness: Describes the impression of "a muddiness or imperfection in sound."
- Fluctuation Strength: Represents the "impression of the sound's fluctuation."
- Tonality: Describes the "impression of the pure tone of a sound."
Examples of sound quality evaluation and analysis
Here's an example of an analysis of abnormal noises contained in the idling sound of a car.
Before the engine warms up, a high-pitched rattling noise can be heard at idle, which fluctuates over time. This noise is not heard once the engine is fully warmed up.
Figure 1 shows the results of a 1/3 octave analysis performed to identify the frequency of this abnormal noise. It sounds unpleasant because it has a "shhh-shhh" sound with temporal fluctuations, but because the level of the abnormal noise itself is low, there is no clear difference when comparing the loudness of each frequency.
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Figure 1 1/3 Comparison of idling sounds using octave analysis
Therefore, we will compare the two using a fluctuation intensity that allows us to evaluate the perceived fluctuation of sound (Figure 2). Sound before warm-up
(Left) shows a large value around 4kHz. This is because the sound at this time contains a 4kHz component.
This indicates a large degree of inter-frequency variation. This matches the impression of a "shhh-shhh" sound. On the other hand, in the graph after warm-up (right), the value of the 4kHz component is not so large.
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Figure 2 Comparison of idling sounds based on sound quality evaluation analysis (variability intensity).
As described above, sound quality evaluation analysis is often used to quantify the impression of sound that cannot be expressed solely by its loudness or pitch. In this example, we introduced it as an example of finding the characteristic quantities of sounds that are perceived as unpleasant (unpleasant sounds), but this analysis method is also often used to improve sounds that are not unpleasant but are not pleasant either (ordinary sounds) into more pleasant or attractive sounds.
For those who have read this column and become interested in sound quality evaluation and analysis, or who are interested in each parameter...
If you would like to learn more, please read our technical report.
【reference】
Ono Sokki Technical Report: "What is Sound Quality Evaluation?"
(Excerpt from the email newsletter issued on May 18, 2022)
