DS-2000 Frequently Asked Questions

For example, if you input a CAL signal of 94dB from a sound level meter and perform a unit calibration using the unit calibration function so that the power spectrum at that time is 94dB, then when you display the time waveform afterward, a large value will be displayed.
By using the calculation function to multiply the time waveform by 20 × ^10⁻⁶, the sound pressure can be displayed in Pa units.

The meaning and calculation method of unit calibration for DS-0221 are explained below. The same applies to the CF series.

Please read the explanation comparing the time-domain waveform (amplitude value), the effective value, and the sound pressure level (dB) and pressure (Pa). (Sound pressure level is the effective value of sound pressure expressed in dB.)
For explanatory purposes, the effective value will be indicated with a lowercase "r" as a subscript after the unit.

(1) Sound level meter CAL signal

When the sound level meter is set to the 100dB range and the frequency weighting is flat, and the CAL button is turned on, a 94dB signal is output from the AC terminal. This waveform is shown in Figure 1. Figure 1 has been calibration using the DS-0221 unit calibration function, and although the time axis of the waveform shows very large values, the power spectrum matches the 94dB value of the sound level meter. The reason for this will be explained in order.

Figure 1: Top: Time waveform, Bottom: Power spectrum (unit calibration)

(2) Sound pressure level (dB) and pressure (Pa)

Sound pressure level is defined by the following formula:

From equation (1), a sound pressure level of 94 (dBr) is 1 (Par). Note that the sound pressure levels of 94 (dBr) and 1 (Par) are generally RMS values. The ACout signal of a sound level meter can be considered as a "voltage signal proportional to the pressure change" as a sound pressure sensor. It cannot measure absolute pressure. The time waveform in Figure 1 is considered to represent the waveform of an instantaneous pressure change.

(3) Power spectrum

The power spectrum is represented by the effective amplitude value obtained from, for example, 2048 samples (depending on the settings, single-amplitude values can also be displayed).

Figure 2 shows the voltage data before unit calibration. The power spectrum is displayed in LIN so that it can be read as voltage. Since the signal is a sine wave, the effective value of the time waveform can be read from the 1kHz value of the power spectrum, but generally the overall value is looked at, so from Figure 2 this overall value can be read as "0.347Vr".

From what has been stated so far, we can conclude the following: This sound level meter shows that when the sound pressure level is 94 dBr, the pressure change is 1 Par, and at this time the effective voltage is 0.347 Vr. This indicates a relationship between sound pressure, pressure change, and voltage.

Figure 2

(4) Time-domain waveform and RMS value

In Figure 2, the amplitude (single-ended amplitude) of the time waveform can be read as 0.491V. The power spectrum displays an RMS value of 0.347Vr. For a sine wave, the RMS value and the single-ended amplitude have a relationship of √2. Since the Cal signal is a sine wave, 0.491 ÷ √2 = 0.347Vr, so we can see that 0.491V and 0.347Vr mean the same thing. Conversely, 1 Par is equal to 1.414 Pa (single-ended amplitude), so when the amplitude of the CAL signal is 0.4907V, it becomes 1.414 Pa, and 1 ÷ 0.347 = 1.414 ÷ 0.491 = 2.88 (Pa/V) is the coefficient for converting voltage to pressure.

(4) DS-0221 Unit calibration

From (3), if we find the coefficient from equation (1) so that it is displayed as 94 dBr when the voltage is 0.347 Vr, this will be the coefficient k (EU/V) used to convert the effective voltage A to the sound pressure level B (dB). Substituting this into equation (1), we obtain the following equation.

In DS-0221, only one constant (EU/V) can be set, and since Po is also a constant, we can set K = k ÷ Po;

We then determine the coefficient K. Figure 3 shows how the coefficient K is automatically set as a physical unit value.

[Calculation example]

Figure 3

In Figure 3, the power spectrum is displayed on a dB scale from the sound level meter, but the time waveform is not in Pa units. To convert it to Pa units, it needs to be converted using the coefficient k.

Since K is already set, multiplying the time waveform by 20 × ^10⁻⁶ will give you the value in Pa. This is then calculated using the calculation function (Figure 4).

◆Operation

Click the time axis waveform to activate it.

Open the [Calculation Formula Settings] page from "Analysis → Calculation Formula". Enter the calculation formula as shown in Figure 4, check "Calculate", and click the OK button. The time-axis waveform will be calculated using the calculation formula.

Change the scale using "Data Display → Y-axis Scale" to make it easier to view.

Figure 4

Figure 5 shows the signal measured by a CAL 94dB sound level meter. The time-domain waveform is displayed in Pa units, and the power spectrum is displayed in dB units (sound pressure level).

Figure 5

supplement

The explanation so far has assumed a CAL signal of 94dB. If you set the level range of the sound level meter to, for example, 90dB, the CAL will become 84dB. When calibration with this 84dB CAL signal, you can read it in exactly the same way by simply replacing 94dB with 84dB. After calibration the power spectrum to 84dB, you can display the time-domain waveform in Pa units by multiplying the time-domain waveform by 20 × ^10⁻⁶.