Relevant sound-privacy metrics for office phonebooths

ISO 23351-1:2020

When you are buying an acoustic booth, it is essential to understand how sound insulation is measured and to define the level of privacy you actually need. If you choose a booth that performs far above your real requirements, you will be over-investing. If you choose one that falls short, you will end up with booths that simply do not do their job.

For example, compare a company that needs booths for its executive team—where conversation privacy is critical and the offices are relatively quiet—with another company that needs booths for its operations team, mainly to reduce distractions during video calls in a noisier environment. These are two completely different privacy needs. Both companies would benefit from better isolation, but they do not need to invest in the same type of solution.

Investing in the right level of isolation, and in a booth designed to last for years, is a smart decision and a responsible, sustainable way to upgrade an office.

There are several industry standards used to analyze acoustic isolation. However, not all of them give a meaningful value for comparing the performance of an acoustic booth. On top of this, booth manufacturers have a natural incentive to present results that make their products look better. For that reason, Sessione is committed to explaining and educating about the different standards so that companies can choose the right product for their needs—even if that leads them to purchase another solution.

Why Sound Reduction Index (R) and Sound Transmission Class (STC) are not useful to evaluate real booth privacy?

Some of the best-known metrics among architects and building professionals are the Sound Reduction Index (R) and the Sound Transmission Class (STC). Both describe how much sound is reduced by a single construction element—for example, a door, a wall, or a window. Their usefulness, however, is essentially zero when it comes to describing the isolation performance of a complete acoustic booth, for several reasons:

• A booth is made up of many elements: doors, windows, ceiling panels, ventilation systems, joints, and so on. Once all of these are combined, the overall isolation is always lower than what any individual component’s R or STC value would suggest. For example, individual elements of the Riparo booth may show R or STC values in the 39–48 dB range. Presenting those numbers as the booth’s actual isolation performance would be misleading.

• The STC scale is based on a frequency range of 125–4,000 Hz, while the human voice is typically between 85 and 255 Hz. A booth that reports an STC 40 dB rating may be performing at that level in a frequency range that does not fully match the frequencies of human speech, giving an inaccurate picture of how well it protects spoken conversations.

For a booth, the privacy that matters is not a single R or STC number, but the actual experience of quiet and security: people outside should not be able to understand what is being said inside, and the person inside should feel confident that their conversation cannot be overheard.

ISO 23351-1:2020 is an international standard created specifically to measure how much a booth reduces the level of speech, expressed as Speech Level Reduction (Ds,A). It focuses only on the frequency range of human speech and uses a test method designed for enclosed or semi-enclosed furniture.

The result is a single value that describes the booth’s capacity—as a complete system, not as separate components—to isolate speech (Ds,A) and therefore provides a realistic metric for privacy.

Class	D S,A	Is privacy guaranteed?	Perceived isolation outside the booth	Perceived isolation inside the booth
A+	> 33 dB	Yes	Conversation inside inaudible, even at a loud voice volume and in a quiet office.	Virtually imperceptible outside noise.
TO	30–33 dB	Yes	Conversation inside is inaudible or unintelligible, even at a loud voice volume.	Virtually imperceptible outside noise.
B	25–30 dB	Yes	Conversation inside inaudible or unintelligible at normal voice volume.	Very attenuated outside noise, does not distract the user inside.
C	20–25 dB	It depends on the background noise	The conversation inside was muffled, difficult to understand in the noisy office.	Dimmed outside noise, understandable if you actively try to listen.
D	15–20 dB	No	The conversation inside was muffled, but 100% understandable.	Understandable outside noise.
N/A	< 15 dB	No	No perceived isolation.	Understandable outside noise.

ISO 23351-1:2020 class is therefore a global evaluation of a booth’s performance—not of its individual components—in reducing sound transmission at the frequencies of human speech.

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Sessione offers Class A and B cabins, how to choose?

Importance of privacy	Do your users handle sensitive information, and is it necessary to ensure privacy in any background noice scenario? If so, Class A booths are ideal for you.
Price	Manufacturing Class A solutions is complex and requires more specialized materials, making them typically more expensive. Additionally, their components are usually heavier and bulkier, increasing transportation and installation costs.
Mobility	Class B solutions are typically lighter and ideal if constant movement is required. It is recommended that Class A cabins be installed by qualified professionals, while Class B cabins can be installed by a DIYer in most cases.
Ambient sound, psychology & productivity	When ambient noise is low, the perception of sound insulation can decrease. For this reason, quiet spaces require Class A soundproofing to ensure privacy. However, for activities requiring individual concentration, 50 dB has been identified as the ideal sound level to boost productivity; if this will be the primary use of the booth, it is recommended to balance the sound insulation class with the ambient sound level to avoid creating an "overly quiet" atmosphere.

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