In open-plan offices, sound masking is often used to lower speech intelligibility and raise cognitive performance of the employees by reducing the irrelevant speech effect. Classic sound masking methods use speakers built into the ceiling of the office to increase the overall background noise level in the office and reduce speech intelligibility. However, the emergence of activity based offices is increasing the need for personalized sound masking methods that are no longer used globally in the office, but can be controlled by each employee individually depending on their activity and, for example, played back through headphones during activities that require particularly intense concentration. The playback of a classical sound-masking noise (e.g. a simple pink noise filtered by -5 dB per octave) via headphones is effective, but not pleasant. For this reason, a new sound-designed masking signal was developed in the present study, which consists of slowly fluctuating binaural harmonic components, as well as atmospheric sounds like water sounds and masking noise. A listening test with a cognitive task and a survey after each test condition showed that the developed signal had a similar positive effect on cognitive performance as a classical masking noise, but was rated as significantly more pleasant.