An Electrolarynx Control Method Using Myoelectric Signals from the Neck

Abstract

Patients who have lost vocal cord function due to laryngeal cancer or laryngeal injury are incapable of speech because it is impossible to generate the laryngeal tone from which the voice originates. For such patients, various speech production substitutes have been devised and put into practical use. The electrolarynx is one of these speech production substitutes and it can be used with relative ease. However, the sound is sometimes difficult to hear and its quality is monotonous. Therefore, focusing on the control method to improve the articulation of the electrolarynx, we have proposed an electrolarynx controlled by myoelectric signals of the neck. The sternohyoid muscle, which is located in the superficial layer of the neck, was the source of the myoelectric signals. This muscle is active during speech, and its activity increases mainly at the time of speech in a low voice. We succeeded in detecting the surface myoelectric signals of the sternohyoid muscle and performing on/off control of the electrolarynx by signal processing. This report includes the derivation of a control function for converting into a control signal of the fundamental frequency of the electrolarynx from the relationship between the myoelectric signals and the fundamental frequency of the voice. This report also includes an evaluation of the controllability of the electrolarynx by comparing the obtained control signal with the user’s intention. Regarding the control of the fundamental frequency, we have proposed a method of control in three stages – high, medium, and low – and a method of control in two stages – high and low – and compared their performances. The results of the three-stage control indicated that the use of the logarithm as a control function for converting the myoelectric signals into the fundamental frequency of the electrolarynx succeeded in the control at an accuracy of 90% or more by changing the pitch of the generated sound depending on the subjects. It was also indicated that the error rate was as low as less than 20%, while maintaining a constant sound. This makes it clear that the use of the logarithm as a control function gives the highest controllability. The two-stage control exhibits a very high control success rate exceeding 90%, regardless of the type of control function; in particular, the control function using the logarithm exhibits a control success rate exceeding 95%. These results indicate that the electrolarynx control function obtained using the logarithmic function has excellent controllability.