Over the past five installments, we have discussed "noise evaluation," covering equivalent noise level, loudness, and the evaluation of noise that fluctuates over short time periods. We have also delved into sound quality evaluation, particularly focusing on mechanical noise, and presented analytical examples.
These are all evaluations of sound at the location where noise is received, or more precisely, at the position where a person hears the sound. If we consider the scope of sound evaluation more broadly, we can think about the overall evaluation that includes how to evaluate the sound generated by the sound source itself, such as urban transportation infrastructure and various types of machinery, and how much attenuation to expect during the propagation process to the listening position.
The process before a person hears sound involves the "propagation" of sound to the ear, and before that, the "generation" of sound. In the first installment of this measurement column series, "Fundamentals of Sound Measurement," I stated the following:
Fundamentals of Sound Measurement - Part 1: "Sounds Around Us and Their Attributes"
"...Measures against prominent noise sources (emission) have largely been completed, and recent discussions in related academic societies have focused on the need for a comprehensive approach that includes the transmission path of noise, the environment in which the noise is received (inmission), and the relationship with other noises present in that environment (masking)..."
Ideally, I should have given a more general overview of this noise problem before going into the specifics, but I ended up discussing it in the order I mentioned at the beginning. This time, I'd like to shift our perspective back to a macro level and organize my thoughts, hoping to provide a kind of map of what we've discussed so far and what we'll continue to discuss in the future.
We will refer to this emission → transmission → immission process as the "ETI process" and proceed with our discussion accordingly. While "immission" is generally translated as "exposure," here we will use the term in a broader sense, interpreting it as "a place where people are exposed to noise."
This eti process is commonly used as a model for ambient noise, but as shown in Table 1, it is a common process for ambient noise, for noise within buildings (architectural acoustics), and for sounds generated by machinery.
Table 1 ETI Process by Target Noise Level
| classification | Emission (occurrence) | Transmission | Immission (sound reception/influence) |
| Environmental noise (e.g., road traffic noise) | • Driving conditions (steady, transient, acceleration, deceleration, stopping) | Distance attenuation | - Total transmission loss of the building's exterior wall (House Filter) |
| Driving speed by vehicle type | • Diffraction effect of sound barriers | - Background noise in living spaces (other than the target noise) | |
| • Correction conditions (pavement type, longitudinal gradient, directionality, etc.) | • Ground surface effect | Noise evaluation index | |
| - The sound sources mainly consist of engine noise, tire noise, and aerodynamic noise (treated as total acoustic power in calculations). | ・Multiple reflection sound | ・Room arrangement, living hours | |
| - Noise from elevated structures | - Sleep impact, sensitivity to traffic noise | ||
| - Building density, wind effects | |||
| Equivalent noise level in front of the building | |||
| Architectural acoustics (e.g., sound insulation inside buildings) | • Target sound source (e.g., audio speaker sound) | - Sound insulation performance of partition walls | - Intrinsic modes and reverberation time of the receiving room |
| ・Device settings (Vol, F special) | Side-path propagation sounds other than sound transmitted through the boundary wall | - Background noise other than the target sound | |
| - Installation location and contact relationship with walls and floors | Noise evaluation index | ||
| - Intrinsic modes and reverberation time of the sound source room | • Receiving position (e.g., head position during sleep) | ||
| • Time and duration of use | • Daily routine, impact on sleep, sensitivity to neighbor noise | ||
| Mechanical noise (e.g., evaluation of engine noise inside a vehicle) | Engine noise, intake and exhaust noise | - Structure-borne sound (various transmission paths) | - In-car specific mode |
| - Muffled sound due to vibration transmission | - Sound transmission from the engine compartment (sound insulation and vibration damping performance of the partition wall) | - Sound radiation characteristics of interior materials | |
| - Noise from auxiliary equipment and drivetrain | Time-frequency characteristics of acceleration sound | ||
| - Unusual noises (injector noise, gear clicking noise, etc.) | - Unique evaluation metrics | ||
| • Ambient noise | |||
| - Evaluator's preferences and sensitivities |
- Physically operable
- Physiological/psychological/social factors
Figure 1 illustrates the eti process of noise propagation within a building.
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Figure 1: Building ETI Process
As shown in Table 1, it may be necessary to define the process from sound generation to reception before beginning to consider problems and challenges. Of course, when addressing individual problems for each item, it is important to further subdivide and clarify the issues. However, with sound, reducing one noise may make other noises more prominent, so it is also crucial to explicitly depict the eti process for each target noise, as shown in Table 1, and to understand the issues as parts of the overall picture.
As also clearly stated in the table, the aspects of emission transmission can be dealt with physically. Similarly, the spatial characteristics of the listening environment, such as the receiving room, can be physically grasped. However, once sound enters the listener's ears and becomes subjective, social influences and psychological and physiological factors come into play. As I have mentioned several times in this column, sound exists as a physical quantity that can be dealt with physically, as a sound wave, and also as a sensation that has meaning and context. This is why, ultimately, it becomes necessary to clarify the relationship between the target physical quantity and sensory evaluation.
Table 1 does not fully explain many important points. For example, in the field of emission, the extremely important indicator of acoustic power, which serves as a standard for the energy generated by the sound source, has not yet been discussed in this column. Also, in the field of transmission, acoustic technologies such as sound insulation, sound absorption, vibration damping, vibration isolation, and soundproofing are usually employed, and distance attenuation, diffraction, reflection, absorption, and refraction are considered phenomena in real space. I believe that explanations of each of these are also necessary. As I have mentioned many times, the field of input is one that contains many aspects that await further clarification. I would like to introduce what we know so far.
This measurement column, like our email newsletter, has now reached 100 installments, including the 90 from the previous series and this one. Going forward, we intend to continue writing with the goal of providing topics that offer helpful hints for solving our customers' problems.
We would be very grateful for any comments or suggestions you may have regarding this measurement column.
We look forward to your continued support.
(Excerpt from the email newsletter issued on January 21, 2010)
