How to measure the anechoic response of a loudspeaker without doing the measurement in an anechoic chamber?

You can time-window the impulse response to remove unwanted reflections from surfaces. But since this time-windowing will reduce the frequency resolution, you should try to position the loudspeaker as far away as possible from reflecting surfaces and/or to damp the reflecting surfaces as much as possible.

Then set the time window so the first reflection is not included. This will work for higher frequencies, but the frequency resolution will normally not be sufficient for low frequencies.

To get the best results, it is recommended to use a short time window for high frequencies and a longer time window for low frequencies. See the High-frequency window (short) section for information on how to do this.