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03-07-2022 03:56 PM
I am a researcher who presents acoustic sounds to adults and infants. For adults, a system in my lab is used. For infants, a system on a cart with wheels is used, so that I can move this cart around to the infants.
For the system in my lab, the setup is as follows:
For the system on the cart, its setup is as follows:
As I am calibrating the sound pressure levels of the acoustic outputs generated by these two systems, I connected the output of the insert earphone to a 2 c.c. coupler and measured its dB SPL by using a sound level meter (Extech 407732, dB A, fast, Hi). Surprisingly, the sound coming from PCI 6221 reads 76.7 dB SPL, while the sound coming from USB-6216 BNC reads 68.3 dB SPL.
To make sure both the PCI 6221 ND USB-6216 produced the same voltage, I connected the Ao0 channel of each system to an oscilloscope. Their waveforms looked identical and both had a baseline-to-peak amplitude of 5 volts.
The thing that confuses me is that, with the same baseline-to-peak amplitude and the same waveform morphology on an oscilloscope, how come the sound pressure level is 8.4 dB softer when delivered through USB-6216 BNC (compared to when delivered through PCI 6221)?
I do not have a background in engineering, and maybe this is the reason that I am confused.
Any comments or suggestions will be highly appreciated!!!
Solved! Go to Solution.
03-07-2022 04:19 PM
The reason is the difference in the output drive current of each of the cards.
PCI 6221 has a drive strength of 5mA whereas the USB 6216 has a drive strength of just 2mA.
USB-6216 output specification
PCI-6221 output specification
03-07-2022 04:42 PM
BTW, as per Ohms laws, your amplitude cannot go beyond 0.25Vpk for 6221 and 0.1Vpk for 6216 (V = I*R), if you try to measure the 5Vpk amplitude with the speaker connected, you will not be able to see the 5Vpk as 50ohm is a quite large load for the instrument to effective drive.
03-07-2022 08:42 PM
Hi Santhosh,
Thank you SO MUCH for pointing out the facts. Greatly appreciated!!!
I have to admit that this is a new setup that I am current trying. I am glad that I have tested and asked for clarifications before using the new setup in actual experiments. Please allow me to reiterate, just to make sure that I have understood the concepts correctly.
03-07-2022 09:50 PM
03-08-2022 06:55 AM
If the headphone ER-3A has the same power than then the ER-3C , mean
102.5 dB SPL in HA-2 coupler at 0.2 Vrms (50 Ohms)
and you build your own buffer, please think of an attenuator 😄 (just like the HB6/7)
(increase the signal to noise too 😉 )
03-08-2022 03:07 PM - edited 03-08-2022 03:09 PM
To confirm what I just learned, I went ahead and did the following three measurements in my lab:
I must have done something wrong...
Any comments and suggestions will be greatly appreciated!!!
03-08-2022 05:21 PM
It is rather overperforming by driving 7Vpk into a 50-ohm earphone (~140mA), please verify the earphone impedance using a DMM.
BNC 2090A in noway provides additional drive strength because it is a straight pass-through for AO signals as described in its manual
Please share the scope waveforms you're observing.
03-09-2022 01:53 AM - edited 03-09-2022 02:01 AM
the earphone has a complex impedance (valid assumption: inductive) so with increasing frequency the impedance migth increase 🙂
Your amplifier has a input impedance of about 10 kOhm , and a output impedance of 4 Ohms ( if I remember the spec rigth )
if you connect a load to the output, the voltage level will drop a bit. (all Ohms law :D)
03-09-2022 12:26 PM - edited 03-09-2022 12:36 PM
Thanks a lot for the comments. Greatly appreciated!!!
This afternoon, I did the following measurements:
First, I measured the impedance of the ER-3A insert earphone. The DMM reads 52.8 ohms. Please note that, in order to reduce potential electromagnetic interferences, I have used some alloy metal to wrap around the transducer box and taped it with some red tapes. I have also replaced its cable with a shielded BNC cable. This may be the reason that the impedance is slightly higher than 50 ohms.
Second, I measured the maximal amplitude of the Ao0 output voltage from PCI 6221. For clarity, I presented a series of electric pulses (pulse width = 100 microseconds), by using a custom program written in LabVIEW (which generates a series of pulses to the Ao0 channel). When the 50-ohm insert earphone is not connected. The oscilloscope reads exactly 10 Vpk and the pulse width is exactly 100 microseconds. So far, everything is good.
Third, I connected the 50-ohm insert earphone, through a BNC T connector, to the Ao0 channel of PCI 6221. The oscilloscope now reads an amplitude of about 7 Vpk. The waveform is bit distorted, but the pulse width is still exactly 100 microseconds. This is where I get confused... The waveform is not clamped to 0.25 Vpk. Why?
My lack of engineering background is really a problem here, and I apologize for that. However, I really want to get this figured out and get it to work in my lab.
Any comments and suggestions will be greatly appreciated!!!