Impulse response (IR) measurement requires that sufficient samples be accumulated before the actual computation is ready, depending on the values of the Max Length and Time averaging settings. The user interface displays a message indicating the remaining time before the display is ready, whenever the related settings are changed or the reset button is pressed.
Because the software cannot detect whenever you make changes to the analyzed system, you need to press the Reset button in the setup or wait for the display to stabilize before reading the display.
Once your test setup is ready, press the ‘Reset’ button and wait for the display showing remaining time to disappear, at which point the IR display is ready. When a sufficient amount of samples have been accumulated, IR computation goes on as long as the ‘Run’ button is active, and is updated with new incoming samples.
Make sure the actual impulse response is shorter than the maximum specified time, otherwise mild to severe time-aliasing will occur, and the measurement will not be reliable. A good rule of thumb is to set the Max length parameter to twice that of the estimated RT60 of the room.
If in doubt, raise the Max length setting until the impulse response curve does not change, and check the tail of the curve does indeed fall to zero.