Technical Report: What is a Sound Level Meter? (Part 6)

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

As shown in Figure 8-1 above, this AC output is the signal from the microphone after it has passed through a frequency correction circuit (A, C, Z characteristics). It is mainly used in combination with analytical instruments (FFT analyzer, octave analyzer, etc.) to perform frequency analysis of sound, or in combination with a level recorder to record noise levels.

Note that the AC output is unrelated to the dynamic characteristics, but it is a signal affected by frequency weighting.

(Note)
Even if a frequency-weighted sound pressure level is displayed, some sound level meters also have a function to output an AC output without frequency weighting (Z-characteristic) (e.g., our LA-1401/4400A series sound level meters).

The AC output is passed through a detection/dynamic characteristics circuit and a logarithmic calculation circuit to convert the signal to dB units, and then outputs a voltage signal proportional to the dB display value of the sound level meter.

In electrical terms, this represents the effective value of the AC output (instantaneous sound pressure) converted to a logarithmic value (level value) (see Figure 8-1).

Amplitude V _A When the time waveform (A) is passed through the RMS detection/dynamic characteristics circuit, a constant height V ₀ is obtained as shown in (B). We can obtain V ₀ at this time. This is called the RMS value of waveform (A), and is represented by the letters rms.

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

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

(A) If the amplitude of the waveform (_VA) is constant, the DC output will also be a constant value. However, if it fluctuates over time, the DC output will also fluctuate over time (it depends on time weighting).

The AC output and DC output voltage of sound level meters vary depending on the model.

for example;

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

When a semicolon (;) is used, each value has the following meaning:

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 on a 1 kHz full scale. Since -6 dB (= 20 log 1/2) is half of the full scale, monitoring the AC output of CAL will show a sine wave of ±0.5 V at 1 kHz.

At 30 dB, the calculation is 2.5 − 0.5 × (70 − 30) ÷ 10 = 0.5 (V).

With sound level meters that have a wide linearity range, the output voltage may become negative when the sound is extremely quiet. Please note that for both DC and AC outputs, values exceeding the linearity range are not accurate.

The AC output signal is a signal that corresponds to fluctuations in sound pressure and has a complex waveform composed of various frequency components. Knowing the frequency components that make up the noise being targeted in noise reduction measures allows for more effective countermeasures, so the AC output is used as an input signal 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.

Note that the AC output is an instantaneous sound pressure signal that has not been converted to a level; therefore, to record the sound pressure level using the AC output, a level recorder with an RMS level conversion circuit is required. To record data in combination with a general pen recorder, it must be connected to the DC output. Also, in an FFT analyzer, the time constant for RMS conversion corresponds (approximately) to the FFT time window length and averaging time.

The display unit numerically displays the sound pressure level based on the DC output signal, adjusted to the measurement range set by the sound level meter. There are two types of displays: analog, which indicates the level by the movement of a needle, and digital, which displays the level numerically or with a numerical value and a bar indicator. Currently, digital displays are becoming the mainstream. Figure 8-11 shows an example of the digital display screen of our sound level meter.

Furthermore, please refer to Figure 8-12, which provides a simplified illustration of the conversion process of the audio signal up to the digital display stage.

The digital display shows the level numerically every 1 second, while the bar indicator shows the level analogously every 100 milliseconds.