The most secure scrambler in common use is the:

The vocoder is regarded as one of the most secure scramblers available because it converts speech into a series of coded signals, making it difficult for unauthorized listeners to interpret the original message without knowing the specific coding process used. This technology effectively alters vocal signals into audio frequencies, which can be altered and combined in such a way as to disguise the content of the communication.

Vocoders are commonly employed in secure voice communications due to their ability to create an intelligible yet disguised output that can enhance security in sensitive communications. Their application extends across various fields, including military use and secure communications in corporate settings, precisely because they can maintain privacy and confidentiality effectively.

In contrast, while other devices mentioned, such as frequency inverters and decoders, serve specific purposes in processing signals or altering frequencies, none provide the same level of speech security as vocoders. Laser beams may be utilized for data transmission or surveillance; however, they do not inherently offer scrambling capabilities for audio data. Thus, when considering the effective scrambling of vocal communications, the vocoder stands out as the most secure option.