# Constraint-Based Time-Scale Modification of Music Recordings for Noise Beautification
## 2 Background  
In this section, we give an overview on existing approaches that are related to our MRI-based noise beautification scenario and give pointers to relevant literature. Further related work is also discussed in the subsequent sections. Methods to reduce the acoustic noise reaching the patient ears in an MRI environment are largely divided into passive methods and active methods. Passive methods reduce the noise by acoustic noise barriers to prevent vibration of the MRI equipment itself or by protecting the patient using ear muffs or ear plugs. However, the noise reduction performance achieved by passive methods is rather poor; it is good enough for reducing the sound pressure below a safety level that guarantees a required protection of the auditory system, but it is insufficient to avoid the patient's discomfort during MRI. Active methods usually complement passive methods by further attenuating the acoustic noise through counteracting acoustic pressure waves. Most of these methods use an external speaker (positioned far away from the MRI magnet) and a plastic tube, which transmits the counteracting sound waves from the speaker to the patient's ears. However, due to long acoustic paths, this solution is subject to sound field disturbances and signal delays. As a result, particularly high-frequency noise components (above 1 kHz) cannot be canceled effectively, thereby not providing satisfactory noise reduction. In the past, various alternative methods have been suggested that neither eliminate nor attenuate the MRI acoustic noise. Instead, these methods try to circumvent the problem by trying to better \"accommodate\" the patient by inducing a subjectively pleasant noise perception. For example, as suggested in [2], it is possible to make use of the sound generation proprieties of the MRI gradient coils to generate additional harmonic, music-like passages at the end of each iteration of an MRI sequence during the scanning process. This idea is to break the monotony of the MRI acoustic noise by adding a succession of harmonic chords that combine into a melody, thus providing a more relaxing and calming acoustic ambient for the patients. However, this method increases the scan time as well as interferes with the evolution of nuclear spins, which may impair the image quality. Furthermore, it also does not guarantee an effective protection of the hearing organs against loud MRI noise peaks. Recognizing this problem, [PERSON] [3] suggests using a power audio amplifier and loudspeakers to generate so-called protective sounds, which rise slowly in amplitude, thus increasing the impedance in the middle ear directly before the onsets of the loud MRI sounds. This invention makes use of the _middle ear reflex_--a mechanism the hearing of vertebrates possesses, where at high sound pressure the impedance of the auditory is adjusted to attenuate the vibration amplitudes of the small bones and  
Figure 1: (a) Schematic overview of the noise beautification application; (b) MRI noise signal that serves as reference signal. The vertical lines indicate periodically spaced positions of knocking sounds, which are interrupted by periods of silence; (c) Music recording that serves as source signal. The vertical lines indicate beat positions of the music; (d) Reference signal and modified source signal using TSM. The beats of the modified music signal are in sync with the MRI scanner’s knocking sounds. tympanic membrane in the middle ear. Even though the patent [3] mentions that music may be used as a protective sound, it does not disclose a method to avoid an erratic and thereby unpleasant mixing of music and the regular noise emerging out of an MRI scanner. In [4], [PERSON] and [PERSON] propose a method to select a suitable music piece that rhythmically matches the periodic MRI noise. The goal is to rhythmically and harmonically synchronize the complementary music piece to a specific MRI gradient sequence. Thereby, this invention coined the term of _noise beautification_.