Technical Report: What is a sound level meter?

The microphone outputs an electrical signal proportional to the fluctuations in air pressure captured (i.e., instantaneous sound pressure). This electrical signal is alternating current (AC) due to the nature of sound, which has the same characteristics as the 100 VAC (AC voltage, frequency 50/60 Hz) used in household power, where the direction of the current alternates.

This AC output is a signal from the microphone that has passed through a frequency correction circuit (A, C, Z characteristics), as shown in the previous Figure 8-1. It is mainly used in combination with analysis equipment (such as FFT analyzers and octave analyzers) for frequency analysis of sound, or in combination with level recorders to record noise levels.

Please note that the AC output is unrelated to dynamic characteristics, but it is a signal affected by frequency correction.

(Note)
Even if the frequency-weighted sound pressure level is displayed, there are sound level meters that have the capability to output unweighted (Z characteristic) AC output (e.g., our LA-1401/4400A series sound level meters).

The AC output is detected and processed through dynamic characteristic circuits and logarithmic operation circuits, resulting in a voltage signal that is proportional to the dB display value of the sound level meter.

In electrical terminology, this refers to the effective value of AC output (instantaneous sound pressure) converted to a logarithmic scale (levelized value) (see Figure 8-1).

When the time waveform of amplitude V _A is passed through the effective value detection and dynamic characteristic circuit, a constant height V ₀ can be obtained as shown in (B). The V ₀ obtained at this time is referred to as the effective value of the waveform in (A), and is expressed with the notation rms.

If waveform (A) is a sine wave, the following relationship holds.

When the effective value V ₀ rms obtained from (B) is passed through a logarithmic operation circuit, a voltage output DC out proportional to 10 log ₁₀ (V ₀ rms) ² is obtained.

If the amplitude of the waveform (V _A) is constant, the DC output will also be a constant value, but if it varies over time, the DC output will correspondingly vary to a time-varying DC value (depending on the time weighting).

The AC and DC output voltages of the sound level meter vary by model.

for example

AC output: 0.707 Vrms / full scale
DC output: 2.5 V / full scale, 0.5 V / 10 dB

If it is stated as such, each value has the following meanings.

0.707 Vrms corresponds to an amplitude of ±1 V. Pressing the CAL button on the sound level meter outputs a signal equivalent to -6 dB at 1 kHz full scale. -6 dB (= 20 log 1/2) represents 1/2 of the full scale, so monitoring the CAL AC output results in a ±0.5 V sine wave at 1 kHz.

At 30 dB, it becomes 2.5 - 0.5 × (70 - 30) ÷ 10 = 0.5 (V).

In sound level meters with a wide linearity range, the output voltage can become negative at extremely low sound levels. Please note that values exceeding the linearity range for both DC and AC outputs are not accurate.

The AC output signal corresponds to fluctuations in sound pressure and has a complex waveform synthesized from various frequency components. Understanding the frequency components that make up the noise targeted in noise control measures allows for effective countermeasures, which is why AC output is used as input signals for FFT analyzers and real-time (octave) analyzers.

FFT analyzers and real-time (octave) analyzers convert signals into sound pressure levels through internal signal processing and display the analyzed sound pressure levels for each frequency.

Furthermore, since the AC output is a signal that is not leveled in terms of instantaneous sound pressure, a level recorder with an RMS leveling circuit is necessary to record it as a sound pressure level. To record data in combination with a standard pen recorder, it must be connected to the DC output. Additionally, in an FFT analyzer, the time constant for RMS conversion corresponds (approximately) to the length of the FFT time window and the averaging time.

The display section shows the sound pressure level as a numerical value based on the DC output signal adjusted to the measurement range set on the sound level meter. There are analog displays that indicate the level through needle movement, and digital displays that show the level either numerically or with a numerical value and bar indicator. Currently, digital displays have become mainstream. Figure 8-11 shows an example of the digital display screen of our sound level meter.

Additionally, a simplified illustration of the conversion of acoustic signals until they are displayed digitally is provided in Figure 8-12 for your reference.

The digital display shows the level as a numerical value every 1 second, while the bar indicator shows the level in an analog manner every 100 milliseconds.