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If you want to get the frequency response of your box or your loudspeaker, you have to follow the different steps below :
1 Generate your MLS signal
2 Record the response of the loudspeaker
3 Compute the impulse response

Open the GUI : Analyze->Frequency response. The screen looks like this :

1 Select the impulse file
2 Select the impulse file channel
3 Type the name of the output file
4 Option : if you want, you can select an option such as smoothing the response, applying the microphone calibration or adjusting the level, but they are all available as stand alone option in the 'Analyze' menu.
5 Print the Impulse on the screen by clicking the 'Show WAV file" button'
6 Use the toolbar in order to select the impulse correctly :

From the left to the right, you can :
Set selection start
Set selection end
Fit to selected start and end
Reset Selection

There is a very important thing to care about when selecting the start and the end of the impulse response : you have to select the end of the impulse just before the first reflection in order to get rid of the influence of all the reflections. This way, the result is the same as if you had measured your box in an anechoic chamber, execpt that you only have 4 ms signal available to compute the frequency response. For this reason, it is intersting to use a higher sampling rate such as 96000 Hz.

In order to get a better result, you have to put your box far away from walls and ground. In my example, the first reflection is due to the ground. Let say your box is 1 meter high, the distance to the microphone following the first reflection is : 2xroot(1+0.5*0.5)=2x1.23=2.46m. The decay between impulsion and first reflection is also 2.46-1=1.46m or 1.46/330(sound speed)=3.7ms

Note that the phase response will be generated automatically with the extension '_phase'.
Try to design your own "Do It Yourself" loudspeakers