Last time, we discussed "loudness," one of the three elements of sound, and how sound pressure levels are determined by the ratio of sound pressure to a reference value.
We demonstrated how to express it as a sound. This time, we started with the sound level and then moved on to the characteristics of the sound.
I would like to explain the basic way to express information in order to grasp it. This will be a general overview.
Therefore, for technical details, please refer to the link on our website.
Now, the last two of the three elements of sound were "pitch" and "timbre." These are, in other words, sound frequency.
These can be described as numerical characteristics and temporal characteristics. Figure 1 below shows that the dimension of sound intensity is a single numerical value.
The expressed noise level (described later) has two dimensions: the height dimension (frequency characteristics) and the time dimension (time characteristics).
This shows how to represent physical information when considering gender. The following explanation follows this diagram.
I will explain.
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Figure 1
As a physical measurement of sound, measuring sound pressure level or noise level using a sound level meter is the most accurate way to measure sound.
This is also a basic measurement. A sound level meter uses a microphone, which is an acoustic sensor, to measure atmospheric pressure.
By sensing pressure fluctuations, the effective sound pressure is obtained from the instantaneous sound pressure, and then converted to a level (dB value).
The sound pressure level is displayed. [1]
This sound pressure level is the basic quantity for noise measurement, but in terms of its correspondence with human hearing, frequency weighting called A-weighting is used.
The A-weighted sound level, which uses a positive curve, is widely used. [2]
The process of reducing the diverse responses of different individuals to a single numerical value using a single frequency-weighted curve called A-weighting.
This is, in a sense, a remarkable simplification. It functions as a common "yardstick" for evaluating noise.
To achieve this, it must reflect people's perception of noise and be simple to use.
No. In that sense, the simplicity of the A characteristic is an extremely excellent simplification process.
Wouldn't that be good? The Fletcher Maestro model is the basis for the A characteristic currently in use.
More than 70 years have passed since Nson's 40-phon curve was proposed, but recent studies with a large number of subjects have shown that
High-precision experiments have yielded results that closely resemble that curve. [3]
A characteristic is one filter, but humans have more than 20 auditory filters called critical bands.
It possesses this filter, and this filter plays an important role in distinguishing sounds.
Because sounds within the frequency range can be recognized as separate sounds, humans can perceive them as different levels within a noise spectrum.
You can hear the voices buried in Iz. The deeper aspects of this story will be revealed on another device.
I would like to make this a meeting, but if you are interested in the mechanisms of hearing, please refer to [4].
Now, the sounds around us have frequency characteristics and also fluctuate over time.
Noise level evaluation is a quantity that captures the frequency and time characteristics on average (in terms of time, instantaneous).
It's fair to say that (sometimes the quantity of time is involved). Depending on the subject of the analysis, frequency and time may be involved.
It may be necessary to extract information from one or both of these sources and observe them together.
For example, there is a device that does not contribute to the overall noise level but generates noise that is dominant at a certain frequency.
If the noise affects the sound quality of the product, it should be expressed using a frequency-based representation.
It is necessary to conduct inspections and experiments. Also, transient noises that occur when the equipment is in operation are
In cases where sounds are generated that affect the noise level due to their timing,
First, it is crucial to display the changes in noise levels over time.
The level recorder, which was a standard at the time, falls into this category. As an aside, this recording paper has visual
Having a comprehensive overview, along with recordings of noise that could be analyzed later, would have been extremely useful.
In frequency-based analysis, the standard approach is to plot sound pressure level on the vertical axis and frequency on the horizontal axis. Temporal information is not necessarily sequential.
When the loss of information is minimal even after reprocessing, that is, instantaneous sounds, steady sounds with little temporal variation,
Furthermore, it is a useful expression for evaluating sound as an average value over a certain period of time. In this frequency analysis,
Depending on the purpose and target of the analysis, the coarseness of the frequency bandwidth is varied.
When the frequency characteristics of noise, such as ambient noise or factory noise, which covers a relatively wide bandwidth, become a problem,
It is common to analyze using octave bands [5]. In addition, the sound insulation performance of walls and floors is also evaluated using octave bands.
We use octave or 1/3 octave band analysis. The bandwidth of the 1/3 octave band is...
This is considered to be consistent with auditory evaluation, given that it is close to the bandwidth of the critical band mentioned above.
In cases where the operating noise of machinery has a dominant component in a narrow frequency band, or vibrations due to structural resonance,
When dealing with sounds generated from motion, FFT analysis, which allows for more detailed observation of frequency information, is also used. [6]
It is common to perform 1/N octave band analysis.
As shown in the diagram, in the case of noise level and octave analysis, the evaluation is based on the time-averaged result.
L as an indicator eq (Equivalent sound pressure level), L Aeq (Equivalent sound level) is widely used.
I'd like to explain these indicators in more detail next time.
For signals suitable for time averaging, such as stationary sounds or sounds with little variation, non-stationary sounds are...
In some cases, averaging transient signals over time may not be very meaningful.
In such cases, the frequency characteristics of the sound pressure level (or noise level), which changes moment by moment, are measured in three dimensions.
This information needs to be expressed in two main ways, as shown in the diagram.
This method uses color to display the three axes of level, frequency, and time in a three-dimensional way.
Typically, this type of analysis is performed using a PC-based application that offers flexibility in setting time intervals, axes, and colors.
It will be handled on the platform.
For sound evaluation, simplified methods that correlate highly with human perception, such as A-weighting, are widely used.
As the analytical capabilities of measuring instruments improve, the dimensionality of the analysis increases, and the amount of information may appear richer at first glance.
However, complexity that deviates from the purpose of measurement can sometimes obscure the information that should be extracted.
Yes. In the process of measurement, analysis, and result display, the original purpose (evaluation and
A key point is to simplify the evaluation quantity to match the test.
"What is a sound level meter?" 6. Structure of a sound level meter
"What is a sound level meter?" 6-5 Sound level (A-weighted sound pressure level)
AIST: International standard ISO 226 for equal-sensory curves of hearing to be completely revised.
"What is sound quality evaluation?" 5. The underlying concept of loudness calculation
"What is a sound level meter? 11. Octave analysis (frequency analysis)"
(Excerpt from the email newsletter issued on July 23, 2009)
