How to check the background noise spectrum of an impulse response measurement?

If you have measured an impulse response, the noise spectrum can be found by time-windowing the last part of the impulse response.

Go to Setup->Load... and load the setup named General_Freq. and Time Response. Open the measurement using File->Open... or perform a measurement. The result is typically as shown below. In the upper plot we see a flat frequency response. This is a measurement of a sound card.

Now in the lower plot, click on the vertical yellow line to the very left in the plot. Drag this line with the mouse to the right. When releasing the line, the result is shown below.

From the upper plot (frequency response) we see that there is a noise floor with clear components at 1 kHz, 2 kHz, 3 kHz, 4 kHz and 5 kHz.