Best way to do real time monitoring of audio during Immunity Testing?

[leinho]

Hello,

I would need to perform EMC Testing on a device that generates audio which is played on earbuds. For radiated immunity, and magnetic field immunity, we have decided to monitor the audio in real time while the RF is being broadcasted. We want to be 100% sure the audio still runs the same while the radiation is occuring. We have considered the following options:

1- Rig up a shielded amplifier, as the signal is around 5 mV RMS and does not work well with our oscilloscope, and attach a BNC cable to it. In order to not affect the oscilloscope, we would place it outside the chamber (15 m cable is needed). The problem with this solution is that the BNC cable might pick up some noise from RF...

2- Attach a plastic tube to the earbuds to conduct the sound outside the chamber where we will have our Audio Analyzer and we can record the signal. As the Audio Analyzer is outside the chamber and we would use a non mettalic tube, the influence of RF would be minimal. We can use either a 5 m or a 12 m tube for this according to the lab. We have tried with one we have but the attenuation is too high to be able to do it even at 5 m. I actually do not know what type of tube should I look for and I could not acttually find any reference in the internet. However, there are some examples in the standard about the use of these tubes.

What would you recommend? Is this a common practice?

[PChi]

I have witnessed Mobile Radio Audio accessory immunity testing using an acoustic coupling tube. The tube was sealed to the loudspeaker using Blu-tack and the microphone outside the chamber was sealed to the tube (garden hose pipe) again using Blu-tack. I can believe that background noise could be a problem. Could you use an audio spectrum analyzer to just look for the 1 kHz AM breakthrough while rejecting the noise?

[T3sl4co1l]

Heh... does there exist a human simulant to consistently reproduce the impedance and acoustics of the human body?

That would actually be pretty important here, too bad it's not done that way...

Tim

[PChi]

There is equipment available to simulate the acoustic properties of the mouth and ear like the HATS https://www.bksv.com/en/products/transducers/ear-simulators/head-and-torso/hats-type-5128. But they may not be immune to the RF and are expensive. For the RF emissions and immunity testing I don't think that it's necessary. They are also expensive.

[leinho]

I have witnessed Mobile Radio Audio accessory immunity testing using an acoustic coupling tube. The tube was sealed to the loudspeaker using Blu-tack and the microphone outside the chamber was sealed to the tube (garden hose pipe) again using Blu-tack. I can believe that background noise could be a problem. Could you use an audio spectrum analyzer to just look for the 1 kHz AM breakthrough while rejecting the noise?

Thanks for your response. Actually, what we have are an Ear Simulator (IEC 711) and an Audio Analyzer type 2250. With the Audio Analyzer we can record the signal as a .wav file and I can do post processing to see the frequency response (using pwelch in Octave). We have obtained well results so far.

This is a possible solution but I am not sure whether we leave the Ear Simulator outside the chamber (option 2) or inside the chamber (option 3). Option 1 is the use of the oscilloscope. By the way, the distance from where the EUT will be placed and the boundaries of the chamber is of 5 meters.

I think option 2 is preferable (I have read in some of the standards that this is prescribed) but we have tried with an elastic tube from Fingerlakes (1/8th" I.D. x 3/16" OD x 1/32" Wall) but the attenuation was huge (around 25 dBS at 1kHz and more than 40 dBs for frequencies higher than 3kHz).
Do you know where I could find such tube with almost no attenuation or at least which would be its characteristics to try to find one?

Option 3 is what we are heading now based on the previous results but we would need to place the ear simulator inside the chamber. We would use a cable extender to connect the preamp (which is in the ear sim) and the Audio Analyzer.
Do you think we might pick up a lot of noise in this setup? This is something we can test there but it would be better to know in advance.

Our idea is to play a 1kHz sine wave with the earbuds and see if we get any spurious signal in the audio frequencies which can be perceived by the users.