Booths are increasingly used in offices and schools to improve noise control and speech privacy. Their acoustic performance (speech level reduction in decibels) shall be measured by ISO 23351-1 standard. The results are classified to speech level reduction classes AD to facilitate design and trade. We investigated the benefits that an office worker might get from booths with different classes against unnecessary speech. We conducted two psychological laboratory experiments. Experiment 1 investigated how speech affects a worker when the speaker is not in the booth (DS,A = 0 dB) or when the speaker is inside class A booth (DS,A = 30 dB). In experiment 2, class A and C (DS,A = 23 dB) booths were compared. If the class is A, the worker’s benefits are large and versatile compared to a condition without the booth: improved cognitive performance, improved subjective experiences, and reduced physiological stress. Comparison of Class A and Class B booths indicated that Class A booth is better with respect of cognitive performance and subjective perception. The results support the choice of Class A office booths where the aim is to minimize the disadvantages of irrelevant speech.