When choosing a telephone booth, a soundproofed workspace or a meeting room for an office, the topic of acoustics is often discussed in general terms. A product may be described as quiet, sound-absorbing or promoting privacy. Such descriptions can be useful, but they do not explain how the product’s acoustic performance has been measured.
That is why ISO 23351-1 certification is an important benchmark. It provides a common method of measurement for assessing telephone booths, sound-insulated workspaces and meeting rooms. Once a product has been tested in accordance with the ISO 23351-1 standard, its acoustic performance can be described more precisely: not merely on the basis of subjective impressions, but on the basis of the measured reduction in speech levels.
What is ISO 23351-1?
ISO 23351-1 is a standard that specifies a laboratory method for measuring the reduction in speech level within furniture-like assemblies and enclosed spaces. The standard applies, for example, to telephone booths, workspaces, meeting rooms and other products designed to improve the privacy of speech for people within the space.
In other words, ISO 23351-1 does not measure just any sound or the acoustics of an entire office. Its focus is on speech: how much the voice of a person speaking inside the product is attenuated before it reaches the surrounding space.
This makes the standard particularly important in modern offices, where remote meetings, phone calls and confidential discussions are part of everyday life. When the noise of an open-plan office disrupts the working day, a soundproof telephone booth or workspace can divert conversation away from the open-plan work area.
What does ISO 23351-1 measure?
ISO 23351-1 measures the reduction in speech level. The measurement result is denoted by the symbol DS,A and is expressed in decibels, i.e. as a dB value.
In practice, the measurement involves comparing two situations. First, the sound source generates a test signal in the room without the product under test. The same measurement is then carried out with the sound source located inside the product, at a position corresponding to the user’s speaking position. The difference between these measurements indicates the extent to which the product reduces the transmission of speech sound energy into the surrounding space.
The higher the DS,A value, the more the product reduces the transmission of speech to the outside.
Why is speech, in particular, important?
In office acoustics, speech is often the most distracting sound. Background speech easily captures one’s attention, as people naturally tend to recognise words and their meanings. That is why suppressing speech is different from reducing general background noise.
A telephone booth in the office is therefore not just a nice extra. It is a spatial solution that can reduce noise levels in an open-plan office whilst providing users with a quieter space for phone calls, video meetings and tasks requiring concentration.
ISO 23351-1 classes in practice
The measurement result for the ISO 23351-1 standard is reported as a DS,A value, i.e. the reduction in speech level in decibels. The higher the DS,A value, the more effectively a telephone booth, workspace, meeting room or other sound-insulated space reduces the transmission of speech spoken inside to the outside of the space.
| Class | Decrease in speech level DS,A | What does that mean in practice? | Typical use case |
|---|---|---|---|
| A+ | more than 33 dB | Excellent speech attenuation. A rare category used to describe the very best acoustic performance. | Highly demanding confidential discussions and environments where the privacy of conversations is particularly important. |
| A | 30–33 dB | Excellent speech attenuation. A Class A soundproof telephone booth or workspace provides good protection of speech privacy and reduces the spread of speech into open-plan areas. | Customer calls, video meetings, HR discussions, recruitment interviews and work that requires concentration. |
| B | 25–30 dB | Basic speech attenuation. Suitable for certain office environments where a quiet space is needed for phone calls and remote meetings. | Ordinary phone calls, online meetings and brief moments of concentration in a multi-purpose office. |
| C | 20–25 dB | Moderate speech attenuation. Speech may be attenuated, but speech intelligibility depends more on the background noise in the surrounding office and the layout of the space. | Short phone calls and less confidential conversations in areas where background noise helps to mask the conversation. |
| D | 15–20 dB | Minimal speech attenuation. This solution may reduce interference, but is not usually sufficient on its own to ensure the required level of speech privacy. | Lightweight acoustic screening, short-term use and spaces where complete privacy is not a priority. |
| E / not classified | less than 15 dB | Very little speech attenuation. The product may enhance the sense of space or provide visual privacy, but its effect on speech privacy is limited. | Open-plan or semi-open-plan furniture layouts where acoustic privacy is not the main objective. |
What does ‘Class A’ mean in an office?
Class A means that the product is highly effective at reducing the transmission of speech from inside to outside, in accordance with the ISO 23351-1 measurement method. In practice, a Class A soundproof telephone booth or workspace is well suited to situations where you do not want the content of a conversation to be heard in the surrounding office.
Examples of such situations include customer calls, remote meetings, HR discussions, recruitment interviews, financial matters and other confidential discussions. Class A acoustics also aid concentration, as they reduce the distraction caused by ambient office noise from the user’s perspective.
However, Class A does not mean complete silence. A telephone booth does not eliminate all sounds, nor does it, on its own, solve the entire office’s acoustic issues. The final result is also influenced by the layout of the space, ambient background noise, surface materials, the frequency of use and the nature of the work carried out in the space.
What is the difference between Class A and Class B?
The difference between Class A and Class B may seem small in numerical terms, but the decibel scale is logarithmic. This means that a difference of just a few decibels can be significant in practice.
Class A offers stronger speech attenuation and is particularly suitable for confidential conversations. Class B may nevertheless be a viable solution for some everyday office needs, such as phone calls, video meetings and brief periods of concentration.
Why aren’t lower grades automatically a bad thing?
Class C and D solutions can be useful when the aim is to reduce noise, organise the space or provide lighter acoustic screening. However, they are not usually the best choice for situations where a high level of speech privacy is required.
If the office deals with confidential matters, handles a large number of customer calls, or wishes to significantly reduce noise levels in an open-plan office, it is advisable to choose a solution whose acoustic performance has been measured and which meets the requirements of the intended use.
What does the ISO 23351-1 certificate not mean?
ISO 23351-1 certification is a useful and important benchmark, but it should not be interpreted too broadly. It does not mean that all office acoustics problems can be solved with a single telephone booth. Nor does it provide information on the overall noise level in the working environment, reverberation, ventilation noise or how the space functions in that particular office.
The standard measures a product’s ability to reduce the transmission of speech in a laboratory setting. The actual user experience also depends on the surrounding environment, the layout, background noise in the office, surface materials, the number of users and how the telephone booth or POD space is used in everyday practice.
A certificate is no substitute for good spatial planning
Even if a soundproofed workspace is of a high standard acoustically, its effectiveness also depends on its location. If a telephone booth is placed too far away from users, it will not be used. If it is placed in the middle of a busy thoroughfare, using it may feel disruptive.
Good office acoustics are the result of a holistic approach: soundproofed workspaces, acoustic materials, clearly defined zones, sufficient meeting rooms and a layout tailored to users’ needs.
Why is ISO 23351-1 important when choosing a telephone booth?
It can be difficult to compare telephone booths and PODs if their acoustics are described using only general adjectives. ISO 23351-1 facilitates comparison by providing a common framework for manufacturers, designers, procurement officers and end users.
When looking for a soundproof telephone booth, workspace or meeting room for the office, it’s worth asking:
- Has the product been tested in accordance with the ISO 23351-1 standard?
- What is the product’s acoustic class?
- What is the intended use of the space?
- How will the solution be integrated into the overall office environment?
Answering these questions will help you choose a solution that is based not only on appearance or size, but also on measured acoustic performance.
ISO 23351-1 and noise in open-plan offices
Noise in an open-plan office is often caused by talking. One employee is holding a video conference at their desk, another is on the phone with a customer, and a third is chatting to a colleague in the team area. Individual sounds may not necessarily be loud, but together they create a stressful soundscape.
A soundproof telephone booth helps to isolate conversations from the open-plan office. When calls and remote meetings are held in a space designed for that purpose, the workspace can remain quieter. At the same time, the person on the call enjoys greater privacy and can concentrate on the conversation without feeling as though the whole office is listening in.
ISO 23351-1 helps to assess how well a telephone booth or workspace performs this task.
How should the certificate be interpreted in the procurement decision?
ISO 23351-1 certification is a strong indication that a product’s acoustic performance has been tested in a comparable manner. When making a procurement decision, it is worth considering this alongside other characteristics.
In addition to acoustics, other important factors include, for example:
- ease of use
- ventilation
- lighting
- ergonomics
- size of the property
- furnishability
- aesthetic compatibility
- portability and adaptability
- serviceability
- office layout
The best solution is one that meets actual needs and integrates seamlessly into the working environment.
ISO 23351-1 in plain language
Put simply, ISO 23351-1 answers the question: how effectively does this product reduce the transmission of speech from inside to outside?
If your office needs a space for short phone calls, a soundproof telephone booth for one person could be the right solution. If, on the other hand, the need relates to team meetings, client meetings or longer video conferences, a larger soundproofed workspace or meeting room may be more suitable.
In both cases, ISO 23351-1 helps to make acoustics easier to understand and more comparable.
In conclusion
The ISO 23351-1 certificate makes it easier to assess the acoustics of telephone booths and soundproofed workspaces. It indicates that the product’s ability to reduce speech transmission has been measured using a standardised laboratory method.
In practice, this helps you choose a telephone booth for the office, a soundproofed workspace or a meeting room that better supports privacy, concentration and the office’s acoustics.
However, a certificate alone does not determine how well a working environment functions. The best results are achieved when measured acoustic performance is combined with good spatial design, appropriate layout and an understanding of users’ actual needs.
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