THE TEST
One number from the lab
ISO 23351-1 measures a booth in a reverberation room with a sound source where the speaker would sit. The result is the speech level reduction, DS,A, in decibels.
See the classes
WHAT THE TESTS SAY
A quiet box
in a busy room.
Not every soundproof pod is equally quiet.
Office pods and phone booths are tested in a laboratory and given a single number: how many decibels they take off the sound of a voice. That number puts each one in a class from A+ to D. Knowing the scale helps you tell a pod that keeps a call private from one that only looks the part.
dB speech level reduction
Check a sound class- One number per pod
- Five classes, A+ to D
- Every 5 dB counts
- A plain guide to booth ratings
THE SHORT ANSWER
Booths vary
more than they look.
Same shape, very different results.
The decibel class tells you which is which.
THE SPREAD
Fifteen to thirty decibels
When researchers tested 11 commercial phone booths, results ranged from 15 to 30 dB, a range they called surprisingly large.
See the numbers
IN AN OFFICE
Pods made quiet places easier to find
After seven pods were added to a university office, staff agreed more strongly that they could move somewhere quieter when they could not concentrate.
Read the field study
FOUR THINGS TO ASK
Before you
pick a pod.
You do not need acoustic training to compare booths. You need the right number and a sense of how the booth will be used.
Ask for the ISO 23351-1 result
The speech level reduction, DS,A, in decibels and its class are the figures designed for comparing booths. A booth without one is hard to judge.
Check it was measured in a lab
The standard calls for a reverberation room test. Researchers note that very large booths can be hard to test there, so field methods are being developed.
Match the class to the job
For confidential calls or concentrated work, the Turku researchers' findings support choosing class A where the aim is to minimise the effect of nearby speech.
Think about the whole office
Pods help most where people need privacy or a quiet place. In the Oulu study, average office sound levels barely changed after pods arrived, but staff found quiet space easier to get.
TWO NUMBERS WORTH KNOWING
How much
a booth can cut.
One figure on how far booths differ,
one on what a good one does to a voice.
15–30 dB
Range of speech level reduction
Eleven typical commercial phone booths were tested to ISO 23351-1. The weakest cut speech by about 15 dB and the best by about 30 dB. Ordinary rooms with full walls and a door can insulate 5 to 10 dB more than any commercial booth.
11 PHONE BOOTHS · HONGISTO AND KERÄNEN 2020
54 → 24 dB
Speech 2 m away, with a class A booth
With no booth, speech reached the listener at 54 dB. With the speaker in a class A booth it fell to 24 dB, under the 34 dB ventilation noise, and became unintelligible.
LAB TEST · HONGISTO ET AL. 2025
Decibels are logarithmic: a 30 dB drop is a very large reduction in sound energy, not a small step. Results are from laboratory conditions.
A QUICK CLASS CHECK
What does
the class mean?
Pick a class to see the speech level reduction it stands for. The bar shows how much a voice is cut compared with the other classes.
Classes are for laboratory results. A booth's real performance also depends on the room around it and on keeping the door shut.
WHAT HAPPENED IN A REAL OFFICE
Not quieter,
but calmer.
At a University of Oulu office of 58 employees, staff had complained about speech privacy. Four single-person pods and three four-person meeting pods were added. Measured sound levels stayed almost the same, about 35 dB on average, before and after.
Yet satisfaction with speech privacy, visual privacy and workspace rose. Agreement that there were enough meeting rooms went from 2.2 to 3.1 on a five-point scale, and agreement that staff could easily move somewhere quieter went from 2.7 to 3.6.
Questions
Questions people ask
How is an office pod's soundproofing measured?
Under ISO 23351-1, the booth is tested in a reverberation room with a sound source at the speaker's position, measured with and without the booth. The result is a single number, the speech level reduction DS,A, in decibels.
What are the booth sound classes?
Class A+ is over 33 dB, A is 30 to 33 dB, B is 25 to 30 dB, C is 20 to 25 dB and D is 15 to 20 dB. Booths below 15 dB are unclassified.
Are all phone booths about the same?
No. Eleven commercial phone booths tested to ISO 23351-1 ranged from about 15 to 30 dB, which the researchers called surprisingly large.
Is a pod as quiet as a proper meeting room?
Usually not. The researchers note that rooms with full walls and a door can insulate 5 to 10 dB more than any commercial booth they had measured.
Does the class make a difference to people outside?
In lab experiments, speech from a class A booth was not intelligible 2 m away, and people did better on a memory task and found speech less annoying than with a class C booth.
Sources
- Hongisto and Keränen (2020), Acoustic performance of eleven commercial phone booths according to ISO 23351-1, Turku University of Applied Sciences
- Hongisto et al. (2025), Class A booth reduces stress due to office noise, Forum Acusticum
- Radun et al. (2025), How did the addition of soundproof pods influence office employees' experience?, Forum Acusticum
- Keränen et al. (2023), Alternative measurement methods for speech level reduction of booths in field conditions, Forum Acusticum
Updated