Technical Report: What is a sound level meter (sound level meter) 5

High-end stereo systems faithfully reproduce sound from low to high volumes and from deep bass to high frequencies. To reproduce sound accurately, the original sound must be captured precisely; otherwise, the reproduced sound will differ from the original. The microphone captures this sound accurately and converts it into electrical signals, while the preamp amplifies these delicate electrical signals to a certain level and performs impedance conversion.

Sound is a wave phenomenon in which alternating densities of air are transmitted, and the microphone and preamplifier of a sound level meter act as sensors that convert these changes in air density (instantaneous sound pressure) into electrical signals, requiring high sensitivity and good frequency characteristics.

There are three main types of microphones: condenser, dynamic, and ceramic. Condenser microphones are used in sound level meters because they can be made in a small shape that is acoustically advantageous, have a flat frequency response over a wide frequency range, and offer higher stability compared to other types. The structure of a condenser microphone is illustrated above. Additionally, there are two types of condenser microphones: bias type and back electret type, with the main difference being whether a direct current voltage is applied to the diaphragm or a permanently polarized polymer film is used instead of applying voltage. Generally, the bias type is characterized by being more sensitive. Furthermore, the microphones used in sound level meters also need to consider self-noise and temperature stability as important factors.

Measuring noise requires absolute measurements of sound pressure levels within the audible frequency range; however, the sensitivity of the ear varies by frequency, and merely taking the effective value of sound pressure does not represent the perceived loudness of sound.

There are equal loudness contours (equal-loudness curves, ISO 226) represented as curves showing sound pressure levels that are perceived to be of the same loudness at different frequencies. The A characteristic in Figure 8-3 approximates the perception of quiet sounds, while the C characteristic approximates loud sounds. In the past, these characteristics were used to measure sound levels based on loudness. However, subsequent research has revealed that while this characteristic represents auditory loudness, it is not suitable for indicating the loudness of noise, and it has become clear that using the A characteristic curve is preferable even for loud sounds.

Currently, the sound level meter is equipped with A characteristics, C characteristics, and additionally, Z (or FLAT) characteristics that have a flatter frequency response. Measurements of noise levels are always conducted using A characteristics. Since fluctuating noise cannot be accurately measured in a single measurement, equivalent noise level L _Aeq is obtained by taking a long-term energy average, as described later, and time-weighted noise level L _X is derived from the cumulative frequency distribution. Additionally, C characteristics have a relatively flat frequency response, so they are used when recording the AC output of the sound level meter or measuring impact noise (which has a wider frequency range). Z (or FLAT) characteristics have an even wider frequency range with a flat response, allowing the sound level meter to be used as a general-purpose acoustic sensor with guaranteed frequency response microphones and amplifiers.

In traditional sound level meters, the so-called flat frequency response, referred to as "FLAT characteristics," did not have a frequency weighting, but the specific specifications, such as frequency range, were left to the manufacturers. Therefore, the FLAT characteristics of commercially available sound level meters were not uniformly standardized. In the new standard, a new "Z characteristic" has been established, defining its frequency range as "flat from 10 Hz to 20 kHz." However, considering tolerances and other factors, it seems reasonable to continue to regard the FLAT characteristic as equivalent to the Z characteristic.

Time weighting characteristics refer to the regulations concerning the movement of indicating meters (including digital displays). Specifically, it corresponds to the time constant for averaging in the effective value detection circuit shown in Figure 8-4. This dynamic characteristic (time constant) is detailed in JIS C 1509. There are fast dynamic characteristics (F[Fast]: 125 ms) and slow dynamic characteristics (S[Slow]: 1 s), where the Fast characteristic approximates the ear's time response, and the Slow characteristic indicates the average level of fluctuating noise. Typically, fast dynamic characteristics (Fast) are used for noise measurement. Furthermore, even with Fast, the magnitude of impulsive sounds cannot be accurately measured; therefore, time weighting characteristic I (Impulse = 35 ms <rise>, 1.5 s <decay>) is provided. However, recent studies have shown that this weighting characteristic I (Impulse) is "not very suitable for evaluating impulsive sounds," and it is not included in the standards by IEC (and JIS). Instead, there is a tendency to adopt the peak value of instantaneous sound pressure as a parameter for evaluating impulsive noise.

The effective value detection in Figure 8-4 is equivalent to passing the squared signal of the instantaneous sound pressure through a first-order RC low-pass filter shown in Figure 8-6. The time constants of the above dynamic characteristics (Fast and Slow) correspond to the time constant of this low-pass filter (τ = RC). Figure 8-7 shows the rise and fall response waveforms when a tone burst signal is input to this equivalent circuit, similar to Figure 8-5. As shown in this figure, if the effective value of the steady input sine wave is e _i, then the time constant τ is;