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

The Brief

The Atrium space was being under used due to the uncomfortably high reverberance within.

A good deal of absorption would be required to tackle the problem but the many fixing points required were extremely limited. Aesthetics was a key factor along with introducing an element of diffraction and installing for maximum absorption. Treating the booths within the Atrium also assisted with the performance.

CAD modelling helped to identify problem areas and prove our design and calculations


The design was two-fold; firstly deciding the best acoustic materials to use in order to achieve the required results and secondly the most efficient way to install them. We created CAD drawings of each wall and panel to enable the framework and panels to be designed with the highest accuracy.

We would require 560 metres of aluminium extrusion to build a secure frame on which we could mount the absorption panels.

Fixing the frame to the upright aluminium extrusions was a precise exercise with every fixing point requiring a tolerance of less than 0.25mm. Any tiny error would lead to failure and so precise jigs were made to ensure we were on the mark every time.

The absorption panels were arranged in a number of ways to both absorb and diffract unwanted sound and therefore a number of frame/panel arrangements were created around the atrium.

Provided by CMS Danskin at very short notice, the absorption panels arrived in good time and perfectly cut to our specification.

 “We engaged with BL acoustics to come up with the solution to our sound issues. From the start they impressed us with their technical understanding of the issue, and the solution proposed made sense. Now that the installation has been completed we can honestly say that has made a huge difference to the staff experience. We are now able to host large company events without concerns over the audio quality, and we are sure that our savings on external venue hire will pay back our investment in a very short space of time.”
Earl Peacock
on behalf of Amadeus


The brilliant white panels needed to be installed up to a height of 9 metres and above some very large bleacher steps.

Specialist access equipment was going to be required. Access to the Atrium was restricted via a series of four doorways, the narrowest being 850mm wide. This meant that access for both the 9m Scissor Lift and the 20m Spider required for the installation was going to be extremely tricky. Some excellent driving skills and much patience we eventually arrived in situ. All floors protected by boarding throughout to avoid any damage.


In addition to the Atrium the adjacent Refectory was a high noise area which was uncomfortable for all those taking lunch.

The area was too active, destroying communication and resulting in raised noise levels.

Two levels of treatment were offered; Foam in the soffit areas and strategically positioned suspended absorption panels at ceiling height

Installation was developed in two ways.  Firstly, the use of the area at soffit level and the ‘I’ beams by filling the cavity with acoustically absorbent foam.  Secondly, we installed a series of vertical and horizontally mounted panels that were distributed to give a general coverage of absorption. The objective was to provide a comfortable environment with easy communication for people sitting and talking across the dinner tables.


The Atrium reverberation time was measured at 3.4 seconds and our target was to reduce this to 2.1 seconds. On completion we measured 1.1 Seconds (at worst point at 250Hz). This resulted in a significant improvement in comfort and intelligibility.

Our target was to reduce RT from 1.9 seconds to 1.25 seconds. On completion we measured 0.45 Seconds (at worst point) again resulting in a significant improvement in comfort, communication and a reduction in background noise when occupied.

“I would say this was a very successful project. From a detailed and innovative design through to the installation and final test results, it was managed very professionally. The initial scope objective to reduce reverberation of noise and improve the audio quality and experience in the Atrium and Refectory was bettered and has exceeded expectations.  A job well done by all”
James Newman
on behalf of CBRE