In the field of room acoustics, the modulation transfer function (MTF) can be used to predict speech intelligibility in stationary noise and reverberation and can be expressed in one single value: the Speech Transmission Index (STI). One drawback of the classical STI measurement method is that it is not validated for fluctuating background noise. As opposed to the classical measurement method, the MTF due to reverberation can also be calculated using an impulse response measurement. This indirect method presents an opportunity for STI measurements in fluctuating noise, and a first prerequisite is a reliable impulse response measurement. The conditions under which the impulse response can be measured with sufficient precision were investigated in the current study. Impulse response measurements were conducted using a sweep stimulus. Two experiments are discussed with variable absorption, different levels of stationary and fluctuating background noise, and different sweep levels. Additionally, simulations with different types of fluctuating noise were conducted in an attempt to extrapolate the experimental findings to other acoustical conditions. The experiments and simulations showed that a minimum impulse-to-noise ratio of +25 dB in fluctuating noise was needed.