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
whereas my finding is
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.