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Ever been in an office and suddenly been able to follow the ongoing discussions in the conference room next door, this despite closed doors and seemingly robust walls? Or, on the contrary, ever held a meeting when the mindless chatter from the hallway distracts and pulls the attendees’ attention away from the agenda? In the industry of heating, ventilation and air conditioning (HVAC) this overhearing phenomenon is called crosstalk, which our expert, Tony Olsson, Product Manager Acoustics, will explain further.
As mentioned, the overhearing phenomenon in buildings is called crosstalk and it is recognised in all sorts of buildings. As a starting point, it is essential to note that the sound generally doesn’t pass directly through the wall, but rather through the ventilation’s duct system or the cut-outs needed to get an indoor climate solution in place.
In professional terms, crosstalk is a measure of how well a building reduces airborne noise between two rooms, or between separate spaces without direct openings.
It is all about the wall rating
The HVAC industry has a lot of competence, and also excellent tools, for calculating noise from fans, dampers and air terminals. Various software help assess insertion loss in ductwork and it is typically only dB(A) and dB(C) that needs to be thoroughly considered, which is sometimes complex enough.
It gets complicated when it comes to sound between rooms because then, wall’s acoustic rating (Rw) and other weighted, normalised values, such as Dn,ew, adds to the equation. This is an area of sound and acoustics where many can feel a pinch of uncertainty and start to linger on definite answers.
A wall often have a decent Rw value in laboratory testing, but at the site of installation there may all of a sudden be a door along the wall or maybe an air diffuser with duct connections creating an “opening” to the room next door. That completely changes the conditions. In practice, the lab-tested Rw value is exchanged for the actual R’w, which needs to be inserted in the calculations and requirement compliance has to be ensured.
A short note on Rw
Rw is expressed in decibel and is a weighted measure summarising sound insulation across multiple frequencies (100–3150 Hz). The greater the Rw value, the better the sound insulation in terms of airborne noise.
Here is an example, a lightweight interior wall has an Rw of about 30, meaning conversations can be overheard between rooms. A solid concrete wall on the other hand, may reach an Rw value of 55 and offer an excellent acoustic performance, leaving any conversation private and quiet for others. Under laboratory conditions, Rw is determined according to defined standards. For on-site installations, R’w is used instead, it is the real-world dimension which typically measures a few decibel lower because of leakage and cut-outs,
Common difficulties
So, despite the fact that the above is known, buildings repeatedly end up with crosstalk issues that is because there are a few things that impact on the end-result:
- Assuming a robust wall solves any crosstalk issues.
- Underestimating how well ducts can turn out to be speakers between rooms.
- Overlooking the negative impact from seemingly simple components, air diffusers or shorter ducts.
- Ignoring airflow, pressure drops and pressure differences, which certainly affect sound.
One part of the problem is obviously that not all discussions are to be heard outside the closed room, but the people affected by crosstalk may not be interested, only disturbed and negatively affected in terms of concentration and productivity. Ensuring a comfortable sound environment is even more important when a room is intended for sleep or rest, a quiet room in a school for instance. On the other hand, some spaces are used for making sound and it is then essential to keep disrobing noise on the outside. A music or podcast studio exemplifies this well.
How can we prevent this and get it right from the start?
The good news is that we now have software tools that help us calculate crosstalk. By entering wall details, Rw values and ventilation system components we can simulate how that will affect the total Rw of the wall and by that know if we have a sufficient attenuation to fulfil the purpose of the area/room/environment.
The new software features allow for comparison of different solutions which helps the design work forward to a final set-up that can meet the set project requirements:
- If the transfer grille is sufficient?
- If it would be better to run separate ducts?
- If there is a need for attenuators?
- If a better insulated air terminal should be selected.
This will significantly progress the end-result, and hence improve tenant satisfaction. In the long run, it will also reduce cost as a better designed solution will need less adjustments and retro-fitting.
In sum, when planning correctly from the beginning, the result is not only buildings that meet set requirements, it is indoor spaces where the walls no longer “talk”. Go straight to our Acoustic Design software or read our expert's other blog posts on sound and acoustics.
