Discrepancy in my dB SPL calculation - Page 1
Forum section: electronics / projects

author: Merida HU
author_rank: Newbie
timestamp: February 15, 2019, 01:55:08 am
content: Having read through thishttps://electronics.stackexchange.com/questions/96205/how-to-convert-volts-in-db-splI tried an experiment.I connected a 407744 Extech sound level calibrator* to a Type 4189 microphone**. The output sensitivity of the microphone is50 mV/Pa,-26 dB ± 1.5 dB re 1 V/PaThis microphone is connected to a Type 2671 preamp***. Using a BNC 50 ohm cable the preamp is connected to a signal conditioning amplifier**** through a 50 Ohm BNC-BNC cable. The output from the amplifier is connected to a standard oscilloscope.The calibrator generates a consistent 1 kHz, 94dB SPL sound. The calibrator was recently calibrated, so the output is assumed to be the standard.The gain at the signal conditioning amplifier is 100mV/unit.The Vpp measured at the oscilloscope was 16.2 Vpp. Converting that to Vrms,Vrms = 16.2/2.828 = 5.72843Calculating for dBrms (with the microphone sensitivity as the reference) = 20*log10(5.72843/0.05012) = 41.1605Since the gain at the signal conditioner is 100 mV/unit. the dB value is -20 dB.For dB SPL = 41.1606+94-26-20 = 89.1605.Would I be wrong in expecting this to have amounted to 94 dB SPL instead? If so, why the discrepancy ? what am I missing?*http://www.extech.com/resources/407744_407766_DS-en.pdf**https://www.bksv.com/-/media/literature/Product-Data/bp2210.ashx***https://www.bksv.com/-/media/literature/Product-Data/bp1446.ashx****https://www.te.com/commerce/DocumentDelivery/DDEController?Action=showdoc&DocId=Data%20Sheet%7F161_Signal_Conditioner%7FA1%7Fpdf%7FEnglish%7FENG_DS_161_Signal_Conditioner_A1.pdf

author: bson
author_rank: Unknown
timestamp: February 16, 2019, 07:47:40 pm
content: Quote from: Merida HU on February 15, 2019, 01:55:08 amWhat's "100mV/unit"? Per unit of what? Figure out the actual gain of the amplifier, in V/V or dB.One way is to feed it a 1kHz, 1mV signal from a function generator on the input and measure the output.Repeat at 10mV to check for linearity (fixed gain).

author: Merida HU
author_rank: Newbie
timestamp: February 18, 2019, 06:39:11 am
content: i did what you suggestedbased on the Func Gen --> Sig cond --> Osc setup.10 mV at func gen = 100 mV/unit at Sig Cond = 1.02V at Osc.Input of 10 mV = 0.01 V -- > AMplification factor of 100 -- > Output is 1.02 V.There is an amplification factor of 100.For the sake of linearity, i also tried an input of 0.02 V to receive an output of 2.02 V. Therefore the amplification factor is consistent at 100x.With the same setup, I replaced the function generator with the Extech 1 kHz Signal generatorInput . 50 mVrmsSame amplification factor of 100 .Output at Osc is 16.2 Vpp or 5.7 VrmsThe amplification factor is evident with a discrepancy of 0.7 V. Do you think this discrepancy can be attributed to the connections ?

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 18, 2019, 03:11:39 pm
content: Not sure about your setup. Your "signal conditioner" is actually a charge amplifier. Are you using it as intended?Also, are you sure of the output of both the func gen you used and the Extech signal gen? Have you measured both with a good voltmeter?In your first test, you get an amplification of x102 actually, which is +2% error. Way above the +/-0.5% noted in the signal conditioner's specs. So either your signal generators are the culprit, or you are actually using the signal conditioner incorrectly, which I suspect. As far as I've understood, it has two modes (charge input or IEPE), which are you using? Is it adjustable? In IEPE mode, I think the inputs will be sourced or sunk with a 4mA current (meant for sensors), so that could very well disturb the output stage of your signal generators. This may be adjustable, perhaps you have it set all well, but I don't know. This signal conditioner is not just a voltage amplifier.

author: Merida HU
author_rank: Newbie
timestamp: February 19, 2019, 04:42:01 am
content: I am certain I am using it as intended.I am using it in IEPE mode. (http://www.modalshop.com/calibration.asp?ID=1045)the main menu of the Sig COn has two optionsa. Amplifierb. TransducerI have set it up as an Amplifier. The one doubt I had was the 'units' to be used.In the amplifier mode, the Unit can be set to m/s-1 or mm or left in the default mode which is just 'unit'. THe m/s-1 & mm correspond to Velocity (V) and displacement (D) measurements. Since I am doing neither, I have set it to 100mV/unit.However I realized that I had not 'grounded' the Sig Conditioner. It has an additional ground line. After I grounded the conditioner, I tried the Function Gen - Sig COnd - OScillscope . I now get a clean signal with 0% error rate.As for the output with the Extech Signal generator,The mic is specified at 50mV/Pa +/- 1.5dB so the o/p can be between 42mV/pA and 59mV/Pa and I'm getting a 58.7 mV at the o/p.So i suppose the o/p is acceptable after all. Thank you for your input.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 19, 2019, 03:57:29 pm
content: Glad you finally got it right. I suppose in "amplifier mode" it doesn't actually activate the 4mA excitation source.The IEPE inputs are stated "single-ended" in the datasheet so I guess you would have expected that the BNC shield would have served as grounding.