Last time, we referred to Emission, Transmission, and Immission as the eti process, and explained that most sound problems can be understood within the processes of generation, propagation, and exposure (hearing). Starting this time, we will explain sound generation in accordance with this process.
Sound problems can arise in any space where people hear sound, whether indoors, outdoors, or in a mobile environment. These problems can occur when there is noise that disrupts the expected sound environment, or when the sound environment itself is unsuitable. The first step in addressing these issues is to understand the nature of the sound present. This involves identifying the source of the problematic sound, as well as understanding the background noise. Identifying the source of the sound can be extremely difficult depending on the situation, such as its relationship to background noise. I will discuss techniques for identifying sound sources on another occasion.
This time, we will explain acoustic power as a basic measure of the sound emitted from a sound source during the sound generation process.
Sound propagating through the air can be thought of as a flow of energy, and this energy is called acoustic energy. The quantity that represents this acoustic energy is the amount of acoustic energy passing through a certain surface per unit time, and this is called "acoustic power" P (W). "Acoustic intensity" I (W/ m²), which represents the strength of sound, can be said to be power per unit cross-sectional area.
Therefore, since acoustic power is the product of the area (S) through which the energy passes and the acoustic intensity (I), it can be expressed by the following equation.
................................................................（1）
Now, let's look at the relationship between acoustic intensity, sound pressure, and particle velocity.
By analogy with electrical circuits, where (sound pressure) → (voltage) and (particle velocity) → (current), we have (power) = (voltage) * (current), so the acoustic intensity (I) is:
.................................................................（2）
Furthermore, if the sound wave is a plane wave, then, with the density of the medium being ρ (kg/ m³) and the speed of sound in the medium being c (m/s):
.................................................................（3）
This is the result. Equation (3) makes sense if we consider it as follows.
Assuming a single tube with a unit area in the medium, its length is the distance sound waves travel in one second.
If we assume that it is equal to ρ, then if the density of the medium is ρ, the mass of the medium in the tube will be ρ•c.
If a pressure p acts on one end of the tube for 1 second, producing a velocity v, then the change in momentum...
Since this is equal to the impulse, we get p = ρ•c •v, which gives us equation (3).
Now, let's return to the topic of sound power.
This is the third installment in this series, "The Nature of Sound as a 'Wave'" (2) Sound pressure, which is the loudness and level of sound.
When expressing sound pressure level, the process of dividing by a reference value, taking the logarithm, and multiplying by 10 is called using dB units.
I explained it as "leveling to the required level." Similarly, the sound power level is the level relative to the reference value P0.
This is expressed by the following formula.
Fundamentals of Sound Measurement - Part 3: "The Wave-like Properties of Sound" (Part 2)
................................................................（4）
The reference value P0 is 10⁻¹² W, which is calculated as follows from the reference value of the sound pressure level (5) and the following equation (6) derived from equations (2) and (3).
.......................................................（5）
.................................................................（6）
Here, the speed of sound in air (1 atmosphere, 20°C) is c = 343 (m/s), and its density is ρ = 1.205 (kg/ m³), so;
Multiplying this by the unit area gives P0 = 10⁻¹² (W), which is the reference value for sound power level.
Now, the sound power level is defined as sound power P and reference value P. 0 We were able to define it, but how do we find this
What kind of measurements should be taken to achieve this? Also, regarding the problem mentioned at the beginning...
So, what kind of relationship should be established between the sound power level and the sound pressure level in the room?
I will explain this in more detail next time.
(Excerpt from the email newsletter issued on February 25, 2010)
