Sound volume, level
In the previous lesson, we began explaining the properties of sound as a 'wave'. We showed that sound is a longitudinal wave, how sound propagates through air, and the most basic relationship between wavelength, speed of sound, and frequency. Now, this time, we will explain the loudness of sound, which is the most familiar aspect of dealing with sound, both physically and intuitively.
"Loudness" is the most easily understood quantity and sensation among the various characteristics of sound.
It is also one of the three elements of sound. These three elements are "loudness," "pitch," and "timbre." Of these, frequency, which is the physical measure of "pitch," is the number of times per second that the vibrations of the air that transmit sound (sound pressure fluctuations of sound waves) repeat. This was also explained last time.
Many researchers have proposed various definitions of "timbre." Below are two definitions of timbre, which refers to the characteristics that give rise to the identification and impression of a sound.
- Characteristics that serve as clues to recognize (identify) what the sound source is.
- Emotional color is a collective term for the various aspects (multidimensional attributes) of the impression that a listener receives from a sound.
To become colorful. This tonal impression can be described using various tonal descriptive terms.
Source: Seiichiro Nanba, Methods for Measuring and Evaluating Timbre and Their Application Examples (Applied Technology Publishing, 1992)
The concept of "timbre" is somewhat difficult to define. Timbre involves complex sensory perception, and there is still room for further development in the field of measurement. I would like to address this topic in more detail later. Now, let's return to the simplest concept: "loudness."
Sound is a vibration of air centered around atmospheric pressure, also known as static pressure. Atmospheric pressure is a large number, 1013 hPa (hectopascals) under standard conditions, or approximately 100,000 Pascals. Sound pressure also uses the same unit of pressure, Pascals. The sound pressure of the quietest sound an average person can hear and the loudest sound around us range from 20 μPa (micropascals) to 20 Pa, respectively, a ratio of 10⁶. This shows that its magnitude is considerably smaller than atmospheric pressure. Furthermore, because of this large range between minimum and maximum, it is very inconvenient for humans to perceive the loudness of sounds solely in terms of sound pressure.
Furthermore, due to the Weber-Fechner law, which states that "the magnitude of a sensation is proportional to the logarithm of the intensity of the stimulus," sound pressure is typically expressed using a logarithmic scale in engineering. The denominator is a reference value of 20 μ[Pa], and the numerator is the measured sound pressure. The sound pressure level is calculated by multiplying the common logarithm of the ratio obtained by squaring each value by 10, and the unit is decibels [dB]. This logarithmic scale relative to a certain reference value is generally called a "level."
(dB)
p0: RMS value of the reference sound pressure 20μ[Pa] = 2 × 10⁻⁵ (N/ m²)
p: Measured RMS value of sound pressure [Pa]
The concepts of Pascals, Decibels, and levels are explained in detail on our website under "What is a Sound Level Meter?", specifically in sections [2. What is Noise] and [4. Units of Noise Measurement]. This page not only explains sound level meters but also comprehensively covers everything from the basics of sound to noise regulations, so please refer to it.
Technical Report What is a sound level meter?
[2. What is noise?]
[4. Units of Noise Measurement]
Now, there's not much point in duplicating information from the website, and since this article is a column and not a technical document, I'd like to proceed with a slightly different approach.
Decibels are something that sound professionals deal with all the time, but for those who have had little experience with sound or who are just starting to work on sound-related issues, it's necessary to become familiar with this logarithmic scale. First, I've included samples (wave files) of relative sound reduced in 3dB steps and then in 6dB steps, so please take a look.
(Clicking each button above will play the corresponding sample sound.)
The sound you just heard is called white noise, which is a sound whose loudness does not change with frequency. The perception will vary depending on the type of sound, but with white noise, which is a basic signal, it is enough if you can get a sense of what a 3dB difference is like. The number 3dB is actually a familiar number to sound engineers. When there are two sound sources of the same loudness, the total sound pressure increases by 3dB. We will not go into the formula for this here, so if you are interested, please refer back to [12. Calculation about Decibels (dB)] in "What is a Sound Level Meter?".
[12. Calculations about Decibels (dB)]
If two identical sounds increase the noise level by 3dB, then four sounds would increase it by 6dB, and eight sounds by 9dB. If there are 10 sound sources of the same magnitude, the noise level will increase by 10dB compared to a single sound source. Conversely, this shows how difficult it is to reduce noise by 10dB.
Over the past 30 years, regulations on acceleration noise from automobiles have been gradually strengthened, increasing by 11 dB for large trucks, 8 dB for passenger cars, and 13 dB for motorcycles. As regulations have been strengthened, automobile manufacturers have strived to comply with noise reduction measures, resulting in a dramatic reduction in noise, equivalent to reducing the noise level of 10 vehicles to just 1. In my first column, I wrote about the shift from individual noise countermeasures to a balanced approach, and the opposite of the above example shows that if the noise levels of each component are all at the same level, countermeasures will have little effect. In other words, if there are 10 sound sources at roughly the same level, countermeasures against one or two of them will have almost no effect on the overall noise level.
(Excerpt from the email newsletter issued on June 18, 2009)
