Ken: Dad, the printer you bought is quiet compared to the one before.

Dad: Yes. I thought so, too. It's not noisy even if I'm on the phone. Printer, hard drive of computers, and other electronic device's noise are becoming low-noise like home appliances, such as washing machine and refrigerator.

Ken: Companies that make home appliances or electronic devices must be taking measures to make the noises from their products quieter.

Dad: Yes. In order to lower the noise, they minimize the vibration source like motors and other mechanisms. Also, making the vibration transmission route difficult to transmit, restraining the vibration of the parts that emit noise, and many other measures are being taken.

Ken: There are many methods in lowering the noise.

Dad: Yes. By decreasing the noise they are obtaining the quality of "silent-performance". This means that they are trying to pursue products goals other than multifunctional and high performance.

Ken: I see, even if it is multifunctional and high performance, if the product is loud, the quality decreases.

Dad: Yes. In other words, the quietness of the product is becoming a necessary requirement. But, on the contrary, differentiation by quietness is becoming difficult.

Ken: Does differentiation by quietness become difficult when noise reduction continues?

 Dad: A silent product can be produced by simply decreasing the level of noise. But if every company aims for this, difference of the product's silentness will become minimal and it would not appeal to customers. So the manufacturers are trying to differentiate by the quality of sound.

Ken: Quality? Does that mean to make a tone?

 Dad: Yes. It's called "sound design". They are attempting to make the tone of the noise more pleasant. But the problem is the numeric expression of it.

Ken: Numeric expression of sound quality? How can you do that?

Dad: Previously when we talked about the insect noise, we compared the cricket with rough sound and the bell cricket with less of a rough sound.

Ken: Yes. There was no difference in the octave-band analysis, but with the fluctuating sound analysis, the fluctuating sound data of the cricket was bigger.

Dad: Yes. The fluctuation amount within the fluctuating sound cycle produces a difference in sound quality.

Ken: By using fluctuating sound analysis, you can extract the sound quality difference?

Dad: Cricket and bell cricket produce sound by rubbing the serrated veins on their wings. They are moving their wings at the same cycle, so a sound characterized by that cycle is produced.

Ken: Difference in time cycle caused the noise to be unstable or rough?

Dad: Yes. Unstableness index is fluctuating strength, index for rough sense is called roughness and it is possible to express them numerically. They are sounds fluctuating periodically, so it is possible to numerically express the audible sense that is caused by such physical characteristic. The same could be said with other sound quality evaluation index such as sharpness (high-pitched sound) and tonality (sound including pure tone component).

Ken: On the other hand, are there situations where physical characteristic can't explain the auditory sense?

Dad: Well, all the sound quality evaluation indexes mentioned before - sound volume and pitch, fluctuations and roughness - are defined as primary sense and cause no major differences in evaluations between people who listen to the sound. The sense you questioned including integrated higher-level senses, for example, high-quality sound, charming sound, and rich sound are difficult to express with simple values.

Ken: Then how are higher-level senses evaluated or how they are used in sound design?

Dad: The story is becoming complicated so I'll continue another time. In short, for some major primary senses of sound, their physical characteristic have been extracted, and they have already been used as the sound quality evaluation indexes for sound evaluation and sound production. On the other hand, for the higher-level senses, feeling or evaluation of them differs depending on the individual or the situation. Therefore, it is important to have a subjective evaluation experiment with the conditions clarified. Then the relationships between the result of subjective evaluation experiment and the physical quantities which highly correlates with the result - in most cases, they are function forms of many physical quantities - must be clarified.

Ken: It's a little hard. Please teach me in simple terms next time.