human ear, in terms of frequency content as well as intensity fluctuations it is a signal with speech-like characteristics. Speech can be described as noise that is intensity-modulated by low-frequency signals. The STIPA signal contains such intensity modulations at 14 different modulation frequencies, spread across 7 octave bands. At the receiving end of the communication system, the depth of modulation of the received signal is measured and compared with that of the test signal in each of a number of frequency bands. Reductions in the modulation depth are associated with loss of intelligibility. Indirect method An alternative Impulse response method, also known as the "indirect method," assumes that the channel is linear and requires stricter synchronization of the sound source to the measurement instrument. The main benefit of the indirect method over the direct method (based on modulated test signals) is that the full MTF matrix is measured, covering all relevant modulation frequencies in all octave bands. In very large spaces (such as cathedrals), where echoes are likely to occur, the indirect method is usually preferred over direct method (e.g. using modulated STIPA signals). In general, the indirect method is often the best option when studying speech intelligibility based on "pure room acoustics," when no electro-acoustic components are present within the transmission path. However, the requirement that the channel must be linear implies that the indirect method cannot be used reliably in many real-life applications: whenever the transmission chain features components that might exhibit non-linear behaviour (such as loudspeakers), indirect measurements may yield incorrect results. Also, depending on the type of impulse response measurement that is used, the influence of background noise present during measurements may not be dealt with correctly. This means that the indirect method should only be used with great care when measuring Public Address systems and Voice Evacuation systems. IEC-60268-16 rev. 4 does not disallow the indirect method for such applications, but issues the following words of warning: "Critical analysis is therefore required of how the impulse response is obtained and potentially influenced by non-linearities in the transmission system, particularly as in practice, system components can be operated at the limits of their performance range." In practice, verification of the validity of the linearity assumption is often too complex for everyday use, making the (direct) STIPA method the preferred method whenever loudspeakers are involved. Although many measuring tools based on the indirect method offer STIPA as