## 2 The Room Response and Its Perception
The resolution of the ear is nonlinear and nonuniform with frequency, with an almost logarithmic dependency on frequency [36]. This aspect has led to the introduction of psychoacoustic frequency scales in the equalizer development with the aim of modifying the spectral content according to human perception. The mel scale [37], the Bark (critical band rate) scale [38], and the ERB (equivalent rectangular bandwidth) scale [39] are examples of psychoacoustic frequency scales that usually build on a filterbank model of hearing. The mel scale is a perceptual scale of pitches judged by listeners to be equal in distance from one another. The Bark scale is based on the critical bands (i.e., the bandwidth of the auditory filters modeling hearing and frequency masking at different frequencies). The ERB is also related to the Bark scale and to auditory filters, since the ERB filters pass the same amount of energy as the auditory filters they correspond to. It can be concluded that the logarithmic frequency scale of human hearing largely explains the low sensitivity to peaks and notches at high frequencies, and this aspect should be considered in the equalizer design. The temporal integration and masking properties of the human auditory system also affect the perception of reflections. The ear perceives sounds by integrating them with a window of around 60 ms duration, having an equivalent rectangular duration of 5 ms [40]. The window is asymmetrical, with a slower rise and faster decay. The ear is insensitive to temporal events shorter than about 2 ms [41]. Masking indicates a condition where sounds which presented isolated would be audible are hidden by the presence of a higher level sound (the masker). We can have both simultaneous and non-simultaneous masking. Simultaneous masking depends on the frequency of the masker and the masked signal. It has its maximum effect when the two differ by less than a critical bandwidth. It diminishes quickly when the frequency of the masker is greater than the masked signal, while it diminishes more slowly when the frequency of masker is lower than the masked signal [42; 43]. Non-simultaneous masking refers to situations where the masker and masked sound are separated in  
Figure 2: Real RIR behaviour (**a**) in the time domain and (**b**) in the frequency domain. time. It is divided into backward masking, with the masked sound preceding the masker, and forward masking, with the masked sound following the masker. Backward masking is quite limited in time [43]: its effect disappears after 15-20 ms [44; 45], with the most significant portion fading out after 5 ms [46]. Forward masking has a longer extension of 100-200 ms. Its behavior is similar to simultaneous masking, and it depends on the frequency relationship between masker and masked sound. According to [39], its effect starts as simultaneous masking and then fades out over time with a straight line in a graph representing the masking reduction in dB versus time [43]. An average forward masking curve has been introduced in [43; 47]. For the first 10 ms, the curve has a constant value equal to \(-9\) dB, which is the maximum level of masking in [19], and then it decays over time. This phenomenon can be exploited in the equalization procedure as discussed in the next sections. The audibility of room reflections also depends on the direction of arrival of the direct sound and reflections with respect to the listener [48], on the loudness of the direct sound (reflections can be more easily perceived with louder direct sound), on the kind of signal [19], and on the spectral content of direct sound and reflections (masking has a stronger effect if the spectral content of direct sound and reflections coincides) [48]. In the following sections, different RRE techniques are discussed highlighting the problems following from the physical properties of the room response and how the characteristics of the human auditory system can be included.