Room Acoustics and Speech Intelligibility

In July, we were asked to complete an acoustic survey in the offices of a large telecommunications company. The client had recently had a few of the floors refurbished and the new layouts were causing communication problems, especially in the meeting rooms where speech intelligibility was poor. Our work was to involve assessment of the rooms, recommendations and possible installation of dependent on findings.

The client reported that the intelligibility within their board room and the refurbished meeting rooms was poor and especially noticeable since Covid had reduced the number of people physically able to attend meetings. This made sense as the Covid measures had resulted in both the reduction of numbers permissible in the room but also the removal of soft furnishing in order to provide more space. Both the furniture and the bodies would have helped absorb sound within the room and by removing them they had inadvertently reduced the sound absorption within the room. Sound absorbance is key to managing a rooms reverberation time, and reverberation time is key to speech intelligibility.

Intelligibility within rooms is largely dependent on the reverberation time of a space. If an area is very reverberant (>0.8s) then the intelligibility will be adversely effected. The reverberation time of a room is dependent largely on the quantity of absorbent surfaces within a space.
With both the board room and meeting room there was a limited amount of absorbent surfaces. This, combined with an increased use of conferencing systems, quickly led to complaints.

B L Acoustics measured the dimensions of the rooms and completed subjective observations of each. Background noise levels were recorded with the air conditioning active and found them to be very quiet. Reverberation time measurements were completed in both rooms in order to quantify the extent of the issue.

Results showed that reverberation in the spaces was often greater than the recommended 0.8s and peaked at 1.15s between measurements of 125Hz and 1000Hz. Mid-frequency sound (500Hz to 2000Hz) is generally the most critical because it’s where the human ear is most sensitive and in the range where speech is produced.

Our engineers applied these results and calculated the current absorption of the space, taking into account the Board Room and Meeting Rooms’ glass wall(s), plaster walls, ceiling, and carpeted floors.
B L Acoustics recommended the installation of 8 off 1m² 40mm ceiling absorption panels to cover the board room and 4 for the meeting room to be suspended from the ceilings in order to reduce the reverberation times; the more evenly spaced out, the better for the overall effect, and so we suggested installation in the form of a uniform grid.

The CLOUDSORBA “suspended acoustic rafts” proved to be a suitable solution; they’re an innovative method of absorbing reverberant noise in rooms without the visual appearance of being just another suspended ceiling. In turn, “the stunning visual effect of acoustic ‘rafts’ on a ceiling space leaves an occupant or visitor with an impression of flair and forward-thinking on behalf of the designer of the room or hall”. Source: CLOUDSORBA Brochure 2019 Two engineers were required to complete the works over a two-day period.
We completed the installation of the 8m² and 4m² CLOUDSORBA panels in the board room and meeting rooms respectively. These were suspended by steel wires from the soffit by the means of special spirals hooks at a depth of 50mm. The engineers reported that this subjectively had a good effect on the acoustics of the rooms.
Following completion of the install, the reverberation time (RT60) of the 2 rooms was measured again. The results showed the times had dropped in both spaces, with some banding such as 1KHz seeing a dramatic difference. These results are compared to the previous readings in the table below.

As a result, these absorption panels will greatly improve the intelligibility of speech within the spaces and the client have reported no further issues.

The project was successful and the measures are now being rolled out on the remaining meeting rooms across the campus.


CLOUDSORBA Brochure 2019

B L Acoustics Ltd