For most speakers today, doppler is not a real problem. It does exist and can be measured. It would most accurately be described as FREQUENCY Intermodulation distortion. When drivers are used and ran over a very large frequency the drivers move at very different rates and that causes the doppler effect. As drivers have improved, natural crossover points in 2 way designs has lowered overall and most 3 way plus designs have never had real issues. Of course, there are always exceptions, but overall, Frequency intermodulation distortion is not an issue in todays speakers.
