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Find the dB SPL value

ls_ns.pngThe provided graphs show the raw output of my acoustic sensor in response to different sound levels. First graph represents a "loud sound" which is using sound calibrator with 114 dB SPL at 1kHz . The second graph represents "no sound". 

Question:

  • How to calculate the acoustic sensor sensitivity from that given information above?
  • How to calculate dB SPL after i found the sensor sensitivity?
  • Do i need to use signal amplitude for this calculation or frequency

Plss guide me 😉

 

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Message 1 of 10
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Hi madzngo,

 


@madzngo25 wrote:

Question:

  • How to calculate the acoustic sensor sensitivity from that given information above?
  • How to calculate dB SPL after i found the sensor sensitivity?
  • Do i need to use signal amplitude for this calculation or frequency

Questions:

  • Which kind of sensor?
  • Can you provide a manual for this sensor?
  • Why do you get ~6.2V in "no sound" case and a range of 1.7V…7V for "loud sound"?
  • Which DAQ device do you use?
Best regards,
GerdW


using LV2016/2019/2021 on Win10/11+cRIO, TestStand2016/2019
Message 2 of 10
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  • The sensor i used. (im not sure if >25 mv/Pa mean the sensitivity can be higher)

acoustic image.png

 

 

  • Now im using PCI-6034E card

 

Please let me know if u need any necessary information to solve this problem 🙂

 

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Hi madzngo,

 

how do you measure (AC) current with your PCI6034?

Best regards,
GerdW


using LV2016/2019/2021 on Win10/11+cRIO, TestStand2016/2019
Message 4 of 10
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Hi GerdW,

 

Do you mean the sensor's current output?

 

Actually, i cant measure it because there is no built-in function inside PCI-6034E when using DAQ-Assistant. Is there any other way to measure it instead of using DAQ-Assistant ? Do i really need the sensor's current output to determine dB SPL value? 

 

daqmx error.png

(I got this error when i try to measure the sensor's current output)

 

BR,

madzngo

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The 6034 does not have current inputs but instead measures voltage. But thanks to Ohms law you can actually convert a current fairly easily into a voltage by simply letting it flow through a resistor and measure the voltage across the resistor.

 

The 6034 does not have internal current measurement resistors that you can switch on, so you can not select that when setting up a current measurement. Instead you have to specify that you use an external current measurement resistor and provide the according value.

 

Since you have 0 - 10mA range you want most likely to use a resistor of:

 

R = U / I = 10 V max range / 10 mA max range = 1k Ohm

 

And you have to check that its power rating is ok as you will have a maximum power dissipation of:

 

P = U * I = 10 V * 10 mA = 100mW

 

So your resistor should have at least 100mW power rating and generally it is best to have it at least double that value for safety margin.

Rolf Kalbermatter
My Blog
Message 6 of 10
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Hi madzngo,

 


@madzngo25 wrote:

Do i really need the sensor's current output to determine dB SPL value? 


Yes, you need to measure the current as it is the output signal of your sensor (according to the documentation provided by you)!

Without measuring the sensor signal you will not be able to determine values like "dbSPL"…

 


@madzngo25 wrote:

Actually, i cant measure it because there is no built-in function inside PCI-6034E when using DAQ-Assistant. Is there any other way to measure it instead of using DAQ-Assistant?


Rolf already explained the (very basic) Ohm's law to you - you should have known that on your own when you want to measure electrical signals…

 

How did you even create the graphs in your first message when you aren't able to measure the sensor output signal (yet)?

Best regards,
GerdW


using LV2016/2019/2021 on Win10/11+cRIO, TestStand2016/2019
Message 7 of 10
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Aside from the matter of how to correctly measure the output from the acoustic sensor, which is being dealt with by Gerd and Rolf, there is also the calculation of the dB SPL value from the signal to deal with.  The definition of sound pressure level (SPL) in dB is

 

SPL = 20 * log10(p / p_ref)

 

where p is the measured sound pressure and p_ref is the reference pressure which for sound waves in air is 2e-5 Pa (20 micropascals).  The inverse of this to get from a known SPL (such as from a calibrator) to pressure is

 

p = p_ref * 10 ^ (SPL / 20)

 

In general the pressure value used for p is an average (often the RMS) but sometimes just the peak amplitude is used.

 

I'm slightly confused by the sensor spec.  As you note, the sensitivity is given as ">25mV/Pa" which is not specific, and the sensor provides a current output with no obvious information about how that relates to the sensitivity.  Since the voltage output will depend on the value of the current measurement resistor, you should select your resistor then measure the voltage output with the calibrator applied to the sensor and assume that the system remains linear across your frequency and amplitude range of interest.

 

PsyenceFact

Message 8 of 10
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Thanks to rolfk and Psyence,

Im gonna try with formula you given after i got the answer from GerdW "why do i need current to measure db SPL"

 

here is the Vi

simple.png 

Im literally just taking voltage value from Dev1/ai1(analog input) and put it into a waveform to make graph. As you can see the graph has Y-axis label named "Volt(raw)". The graph also oscilated from ~7,6 to ~1,7 when sound is detected. So i can just use voltage to measure dB SPL right? (unless im dealing with watts too, just way too complex i guess :v).

 

Please explain me GerdW why i need current to measure dB SPL?

Is there any dB SPL formula that using specified signal?

 

 

BR,

madzngo

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@madzngo25 wrote:

 

Please explain me GerdW why i need current to measure dB SPL.


The data sheet you showed says: Output current: 0 ~ 10 mA (AC)

 

But I would admit that that page is not really a full datasheet but just a very limited feature list in my opinion. So that output current specification might or might not be anything about the signal output range. It could also be the maximum source current while the ouput signal is actually voltage. But we can't really say for sure from that very limited information.

 

It also nowhere specifies anything about what the relationship is between SPL and voltage/current. As such it is absolutely impossible to do SPL measurements except by doing a measurement with a well known sound source in a very well defined distance and then calculate the calibration values that you will need to calculate SPL from the measured voltage/current. 

Rolf Kalbermatter
My Blog
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