In-Depth

Principles of Sound Insulation

Author

Chris Parker-Jones

January 5, 2020

These articles are inherently technical and ‘in-depth’ in nature. They are primarily intended for acoustic professionals and academics, but, this does not mean they are incomprehensible to non acousticians… so if you have a bit of time and want to understand acoustics in-depth, then please read on.

Alternatively, if you’re looking for something a little lighter, with less technical jargon, generally a bit easier to digest… take a look at some of our other articles, our A to Z, or some ‘bite-size‘ posts.

our articles are to break down acoustic terms and concepts as simply as possible, without going too far into the mathematics and every nitty gritty technicality, that acousticians usually love to get stuck into.

So please, if you’re an architect, contractor, developer, planner… or really anyone who occasionally needs to dabble in acoustic design and assessments… then read on…

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Why are the Acoustics in my office so bad... and what can I do?

In my travels as an acoustic consultant I find that many offices I go to for client and design team meetings suffer from poor levels of sound insulation. In fact, many people hearing that the acoustician has just turned up will jokingly ask me how to fix it (please, please, don’t mention eggboxes…).

The main issue with most offices is that they are designed to be flexible, supplied as shell and core, to be fit out and changed by tenants, often fitted with suspended ceilings and raised floors. There are restrictions in what we are allowed to change in the office, and hence the acoustic performance can be limited.

Of course many offices are open plan, but for this article, I’m going to focus predominantly on the sound insulation around cellular offices and meeting rooms.

There are any number of problems that may exist with the sound insulation in your office, that could apply to all building types. However, the issues in this article are those that I find to be the most common for an office, which as I outlined above, usually has to deal with a set of restrictions that other building types don’t.

What is a ‘private’ level of sound insulation?

BS8233:2014 provides guidance and recommendations regarding sound insulation in offices.

“In order to achieve unintelligible speech from another office, the minimum sound insulation between two offices needs to be approximately Dw = 38 dB. Where privacy is important the minimum sound insulation should be Dw = 48 dB. It is possible that voices can be heard, but the conversation is not usually understood.”

Note that the Dw is the weighted level difference between two rooms and is a descriptor of sound insulation (please see my other article on the difference between Dw and Rw). The targets above apply to walls without doors, we are very unlikely to achieve this level of privacy on corridor walls.

How can I achieve this?

Suspended Ceilings

A lot of offices have suspended lay in grid ceilings throughout. Whilst these are beneficial in terms of absorbing sound to reduce the reverberation time, increasing speech intelligibility and reducing the build-up of noise levels (particularly in a busy open plan office), there can be shortfalls in terms of sound insulation. Because many offices are designed as fit-outs, internal walls don’t always run to the slab, they stop at the ceiling line. Therefore, a gap exists above the separating wall that allows sound to flank through the suspended ceiling and over the wall.

Most ceiling tiles are relatively poor in terms of sound insulation, typically achieving around a 30 to 35 dB level difference across the ceiling from room to room (compared to our 48 dB Dw target for ‘privacy’). You might see manufacturers quote the sound insulation of a ceiling tile as a Dnfw, or a Dncw, essentially the same thing, the ‘weighted suspended ceiling normalised level difference’.

So how can we improve this, without building our walls above the ceiling line?

Mass is important. We could increase the mass of our ceiling by adding ‘mass loaded tile backers’ to our existing tiles (not to be confused with mineral wool backers which are for low frequency reverberation, not for sound insulation), or swap existing tiles altogether for new higher mass tiles. Still, we may fall a few dB short of the recommended 48 dB Dw from BS 8233 for ‘privacy’. We also have to consider recessed lights in the ceiling, perhaps installing ‘acoustic hoods’ to the back of them.

A cavity barrier is another option, installing a 50 mm dense mineral wool barrier above the wall line could give us a 10 dB improvement through the ceiling, with a 2nd layer adding another 3 dB.

If we are set on achieving a minimum of 48 dB Dw, we may need to do a combination of treatments, whilst also adding a layer of insulation into the void to absorb some of the build-up of sound if our ceiling tile has a mass backing.

Raised Floors

This is a very similar issue, where the wall is built off the raised floor, rather than the slab. We can again consider a cavity barrier beneath the wall, or a small infill stud ‘wall’. If our floor void is being used as a ventilation plenum, this solution may be problematic. In this case, we’re unlikely to achieve a high level of privacy, though we can still improve the performance by ducting the floor diffusers, or adding absorption around them, or by using a barrier beneath at least part of the wall.

Mullions and Transoms

Building walls against curtain walling can be tricky. Exposed hollow mullions are typically weak acoustically, and transoms that run continuously past the wall can be difficult to fix retrospectively.

Foam mullion inserts are an option for new builds, but less so for fit outs with existing mullions. The better option, is to case around the mullion with plasterboard. In some cases, if we happen to have a split mullion or two independent mullions, we may not need to case this, depending on the level of privacy desired.

Doors

Doors are an inherent weak point, considering that they need to be light enough and with spacing around them in order to move. A typical acoustic door is rated at around 30 to 35 dB Rw. We can achieve up to around 50 dB Rw with a single, but at this point our doors are thick, heavy, and made of steel, not aesthetically pleasing for an office…

Doors need to be installed correctly. Spending money on an acoustic door but leaving an air gap beneath defeats the point. All acoustic doors need to have seals around the perimeter and threshold, whilst keyholes should also have covers.

Of course a door is unlikely to be placed between two private offices, so this isn’t a problem. But that doesn’t mean we’ll make do with colleagues in the corridor hearing our private meetings! Realistically, we’re not going to achieve a ‘private’ level of 48 dB Dw in this case, with a door. We could therefore consider the layout of the office, placing sensitive rooms away from the open plan areas, or lobbying them.

Glazed Partitions

The most obvious sound transmission between two rooms, is directly through the wall. Usually this is relatively easy to design, we just need to choose a wall construction with a sufficient sound insulation rating, and avoid compromising this with poorly sealed service penetrations and back to back plug sockets and light fittings.

However, many open plan offices utilise glazed partitions around cellular areas to maintain the open plan feel. Achieving the equivalent level of sound insulation from a stud wall with a glazed partition is costly. In context, standard double glazing achieves around 30 – 33 dB Rw. Our stud wall to achieve a ‘private’ level may be around 55 dB Rw. This is at the very upper limit of glazing; you might expect this level of sound insulation from a vision panel in a professional music recording studio. Realistically, we’re likely to achieve around 40 dB Rw with a laminated, glazed partition.

Therefore, achieving a ‘private’ level of sound insulation (48 dB Dw) through glazing is unlikely, so we might want to consider a stud wall around our most sensitive spaces.

Of course when we can see someone through glazing, our expectations for sound insulation are likely to be lower than through an opaque stud wall. Therefore, many users may find this acceptable, and be prepared to keep their most private conversations for elsewhere.

Ventilation Systems

In our mechanically air conditioned office we might have ducts that run from air handling units through several spaces. Usually noise attenuators are installed in duct work to mitigate noise levels from the mechanical units themselves. But we must also remember to install attenuators along the duct path between two rooms. If two adjacent meeting rooms are serviced by the same duct, without an attenuator between them, we’re likely to notice speech from each room, what your acoustician will refer to as ‘crosstalk’.

Reverberation and Indoor Ambient Noise Levels

The points considered so far are related to increasing privacy by reducing the transmission of sound through the separating elements and flanking paths between rooms. But we can also look at the acoustic environment of the rooms themselves.

If we look at the reverberation within a room, a higher level of reverberation (you might describe this as being ‘live’ or ‘echoey’) leads to higher noise levels, due to a lack of sound absorption from the rooms surfaces. Earlier I referred to lay in grid ceilings. These are typically efficient absorbers of sound and produce a low reverberation time, but many offices with acoustically reflective exposed soffits or plasterboard ceilings have high levels of reverberation. Particularly in an open plan office with many people speaking, noise levels can build to a significantly higher level in a reverberant room. So if noise levels are high in the room next door, they’re probably going to disturb us if the wall between us is lacking in acoustic performance. So if we can’t do much with our constructions, we can look at adding more ‘soft’ acoustically absorbent finishes into our rooms.

The indoor ambient noise level is also important. This refers to the noise in a fully operational but unoccupied building, so you would expect this noise to arise from external noise ingress through the façade and internal building services. You might think we would want to minimise these noise levels as much as possible. After all we don’t want to be disturbed by noise whilst we work, and whilst this might be the case in many building types, this is not always the case in an office. In fact, many offices artificially inject noise through ceiling mounted speakers!

Some noise can be a good thing; it helps to mask the sounds produced by our neighbours. If we can hear a pin drop in the room, we’re more likely to hear speech in the neighbouring room, than if the air con is emitting a steady hum above our heads. As long as our background noise isn’t too loud, irregular in tone, intermittent or impulsive, it shouldn’t disturb our work and should aid in providing speech privacy.

To demonstrate this, the images below show the ‘speech transmission index’, a measure of speech intelligibility, in an open plan office with different background noise levels. It can be seen that as the background noise level increases, the distance at which speech is clearly understood decreases. The same effect can be described for cellular spaces.

Open Plan

So lastly and briefly, the open plan office. In a space without walls, sound insulation is far less, hence background noise as shown above is one of the ways of increasing privacy. ‘Screens’ are another method. We can’t build a full height wall, but we can at least put objects in the way that sound has to travel around, losing energy along the way. Desk dividers can achieve this, and furniture such as bookcases, lockers and high backed furniture are all ways of creating small zones of more private working areas in an open plan office. Plenty of acoustic absorption is also important, the build-up of noise in a highly reverberant office does not make it easy to take a phone call. Reflective surfaces help to spread sound across a room, hence by suppressing these reflections we can contain our conversations into smaller areas.

I hope you enjoyed this short article, and keep an eye out for more articles on the common questions that I get asked by clients in my job as an acoustic consultant. Feel free to connect and message me through LinkedIn, send me an email at chris@parkerjonesacoustics.com, or through our Contact Us page.

I’d also like to credit Ecophon for the featured image of this article.

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