## 2 The Room Response and Its Perception  
The characteristics of the room response in the time and frequency domain are related to the acoustic properties of the environment that influence human perception. Due to this aspect, it is sensible to shape the impulse response analysis in order to handle important issues that should be considered in the RRE procedure to reach a sound listening improvement. This includes knowledge on human perception and psychoacoustics to be exploited explicitly in the equalization procedure. Figure 1: A general classification of room response equalization (RRE) systems. Possible approaches: 1� short filters, 2� complex smoothing, 3� frequency warping, 4� Kautz filters, 5� multirate approaches, 5� room impulse response (RIR) reshaping, 2� homomorphic filtering, 8� linear predictive coding analysis, 5� least-squares optimization techniques, 6� frequency domain deconvolution, 1� multiple-input/multiple-output inverse theorem (MINT) solutions, 1� average and weighted average methods, 1� clustering methods, 1� prototype approach, 1� common acoustical poles compensation, 1� modal equalization, 2� plane wave approach, 3� quasi-anechoic approach, 3� wave domain adaptive filtering, 3� transform domain approaches, 2� room geometry aware methods. MIMO: multiple-input/multiple-output; MISO: multiple-input/single-output; SIMO: single-input/multiple-output; SISO: single-input/single-output. An impulse response, obtained from a sound source in a specific position of a real environment, can be divided into three parts [13]: (i) direct sound; (ii) early reflections, and (iii) late reflections, as reported in Figure 2a. The transition from early reflections to late reflections is given by the mixing time, estimating the time elapsed from early to late reflections. It can be estimated in several manners [14,15]. Direct sound and early reflections are fundamental for the localization of the sound source and perception of its timbre [16, 17, 18], while the late reverberation provides cues on the spaciousness of the room [19]. Studies on the perception of reflections and their influence on the timbre can be found in [19, 20, 21, 22, 23, 24, 25]. The spectral content of direct and reflected sound is different. Walls, drapes, and upholstery typically absorb the high frequencies of reflections. The effect is boosted by multiple reflections, with the late reverberation typically having a much lower energy in the high frequencies. At low frequencies, the wavelength is comparable to typical room dimensions: standing waves may appear in a room for steady-state signals, resulting in well defined position-dependent maxima and minima of the magnitude response. At these frequencies, the room response has a smooth behavior characterized by well separated resonances and notches, as illustrated in Figure 2b. The resonances and notches are determined by interference patterns caused by the direct sound and reflections, with notches appearing when the path-length difference is an odd number of half wavelengths. The notches become increasingly dense with increasing frequency. For frequencies greater than the Schroeder frequency [13], the frequency response becomes extremely irregular. Spectral peaks are more audible than notches [20], but wide-bandwidth notches are also audible [26]. At high frequencies, the peaks and notches strongly depend on the position in the room and on factors like the room humidity and temperature [27, 28, 29] or obstacle movements [30, 31, 32, 33, 34]. It must be pointed out that these large variations in the response have little influence on the subjective impression of the listener [18]. It has been suggested [19, 22, 24] that the ear is more sensitive to signal onsets (i.e., to the full spectrum of the initial part of the RIR) and that it largely ignores the high-frequency components of the late reverb [35]. This aspect should be considered in the equalizer design. The perception of high frequencies is particularly affected by the frequency resolution of the human auditory system.