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Anesthesia consists of three components: unconsciousness, analgesia and neuromuscular blockade (NMB). A specific drug is administered by the anesthesiologist to control these different components. In this paper we propose a new system for monitoring the neuromuscular blockade in anesthesized patients during surgery. Neuromuscular blockade drugs are used routinely by clinicians to induce muscle relaxation in patients. However, the use of these drugs has some risks, so an adequate monitoring of the effects of these drugs is essential. This paper describes the Relaxofon, a NMB monitoring device based on phonomyography. The Relaxofon is composed of a hardware subsystem that records muscle sounds using microphones and a special circuit to filter out the noise and amplify the signal, and a software subsystem that analyses the acquired signal. We tested the ability of the system to record phonomyographic signals from the adductor pollicis and the corrugator supercilii muscles. We then performed the Bland-Altman test to compare the manual Train-of-Four ratio (a measure of the depth of muscle relaxation) calculation against the one performed by the Relaxofon. Finally, we calculated the Pearson correlation coefficient to measure the linear dependence between the two methods. Automatic Train-of-Four ratio calculations using this system showed very good agreement with manual calculations. Results from this work may ultimately lead to integration of NMB monitoring to an automated closed-loop anesthesia system.