The recorded timbre of an electric guitar amplifier is highly dependent on the position of the microphone. Small changes in the location of the microphone can yield significant spectral differences, particularly at positions very close to the amp. This paper presents densely measured radiation pattern characteristics of an electric guitar amplifier on a 3D grid in front, beside, behind and above the amp in a hemi-anechoic space. We use this data to analyze the change in spectral differences between the numerous points on the measurement grid. Differences between acoustically measured and estimated frequency responses (predicted, using interpolation) are used to study the change in the acoustic field in order to gain insight and an understanding of the spectral directivity sensitivity factor of the electric guitar amplifier.