"Linguistic sounds, considered as external, physical phenomena have two aspects, the motor and the acoustic"

Roman Jakobson's quote is referring to the 2 aspects of linguistic noises, which are the motor and the acoustic. The motor aspect of linguistic sounds is the physical action of producing the sound, such as the motion of the tongue, lips, and vocal cords. The acoustic element of linguistic noises is the sound itself, which is the outcome of the motor aspect. Jakobson is highlighting that linguistic noises are physical phenomena, and that they have two unique elements.

The motor element of linguistic noises is the physical action of producing the sound, and it is the basis for the acoustic aspect. Without the motor element, the acoustic aspect would not exist. The acoustic aspect is the noise that is produced, and it is the result of the motor aspect. The acoustic element is what is heard by the listener, and it is the basis for comprehending the language.

Jakobson's quote is emphasizing the importance of both the motor and the acoustic elements of linguistic sounds. Without the motor aspect, the acoustic aspect would not exist, and without the acoustic element, the language would not be comprehended. Both aspects are important for language to be comprehended, and Jakobson is stressing the importance of both elements.