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I've recently purchased 40 kHz transceivers and I tested its frequency response in anechoic chamber using a B&K microphone located 90cm away from the emitter. The response provided in the datasheet is

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whereas my finding is

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What can be causing this discrepancy? The center frequency is clearly not 40 kHz as it was claimed to be.

Is a standing wave causing the notch that is at ~38 kHz?

I am pretty sure the experiment setup is functioning well.

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  • \$\begingroup\$ The scale on the first graph is suspect. Why is the 2 kHz from 38 to 40 kHz narrower than 2 kHz from 43 to 45 kHz? \$\endgroup\$ – The Photon Oct 31 '18 at 5:43
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    \$\begingroup\$ It would also be easier to compare your result if you plotted it on the same scales: 35 to 48 kHz on the x-axis, and dB scale on the y-axis. \$\endgroup\$ – The Photon Oct 31 '18 at 5:47
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    \$\begingroup\$ It appears that the first graph is in dB. The second graph is labelled "amplitude." If you convert the second graph to dB, maybe it will match the first one? \$\endgroup\$ – mkeith Oct 31 '18 at 5:47
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    \$\begingroup\$ what impedance were you driving the transducer with. What impedance does tyeh data sheet say to drive it with? \$\endgroup\$ – Neil_UK Oct 31 '18 at 5:51
  • \$\begingroup\$ Normally, in order to have a standing wave, you would have to be in a room that can produce echoes. Also, the wavelength at 38 kHz is less than 1cm. Are you calibrating out the frequency response of the microphone? Could it have a null at 38 kHz? \$\endgroup\$ – mkeith Oct 31 '18 at 5:53
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The linear Vs Db scale thing accounts for much of this, and these are not exactly precision devices so the centre frequency being slightly off is not a huge surprise.

The notch is only about 3dB, and may well be two transducer modes being excited, you may also be seeing a null due to a reflection from the housing or something, try rotating the transducer very slightly off axis and see if the notch moves.

If reflections causing standing waves are a concern, you can try setting up a time gate on your measurement (Emit a pulse, gate the measurement for when you expect the signal to arrive).

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