STC - when do you need it and how do you get it

STC – Sound Transmission Class

This measures the sound that transfers through a wall, from space to space.  The scale of measure for this is not as simple as NRC, an STC <35 is considered low while an STC >55 is high.  STC is the result of an assembly and not based on a single component.

When deciding if you need a high STC wall look at the design, the highest STC rated wall in the world will not help if you have an open plenum where sound goes over the top of the wall.

Things such as off-set stud walls and resilient channel have been around a long time and certainly do work when installed correctly.  Sound board has been around for a long time as well.  It is basically mineral fiber in a large sheet, think 4’x8’ ceiling tile, and installed behind the drywall.  See manufacturer’s literature for ratings by assembly.

Of course an extra layer of drywall often times works; don’t let the seams line up. 

Another product that has been around awhile and is very effective is vinyl loaded noise barrier.  The material is approximately 3/16” thick and comes in rolls and it weighs one pound per square foot.  This can be applied to the studs behind one side of the drywall or for very severe cases woven between the studs in an off-set stud construction.

One of the newer products on the market these days are drywalls that have a layer of material in the middle that will help stop sound transfer.  These products are more expensive than regular drywall of course but do a good job.  These can be added to an existing wall, the board can be finished just like standard drywall.  See the manufacturer’s literature for assemblies

Important things to know when designing a wall to stop sound: back to back electrical boxes are a super highway for sound – if you have them in an existing wall there are pads that can be put between them to help stop the transmission.  Sound caulk should be used where the wall meets the floor, ceiling and corners.  A room designed with all the best sound control in the world won’t make up for a door that doesn’t stop sound, or an open window.  Sound through a small opening fans out into the opposite space.

One of the things I have just learned a bit about is something called the Triple Leaf Effect.  This tends to go against our idea of just adding a bit of air space to a wall to help with sound transmission.  It is a complicated subject but I will try to give you a brief idea of what it is.

A triple leaf wall is a wall with two air cavities rather than the typical one air cavity. Similarly, a quadruple leaf wall would be a wall with three air cavities. This applies to both walls and ceilings alike and is also referred to as a retrofit installation when discussing the installation of resilient sound clips in an assembly that will create a triple leaf. A leaf in a wall is a solid layer; like drywall or plywood, and a triple leaf wall has three leafs. 

It is not illogical to presume that the quadruple leaf wall would have the best sound isolation. After all, the sound has to go through drywall, air space, drywall, air space, drywall, air space, and then again drywall often with insulation within each air space. In a double leaf wall it has to make it through only one air space, and in a single leaf wall there is no air space at all. While it makes sense that the quadruple or triple leaf assemblies are better than double leaf assemblies, it is unfortunately not the case. To understand why this is, we have to take a look at how decoupling works.

So I would suggest you look up triple leaf effect and see what it is all about.

Evaluating a project for STC needs in the design phase is certainly the best way to go, too often acoustical issues get cut back to save money.  As with all these things it will cost so much more to fix the problem after the fact.

As always – please feel free to contact me for more information or product samples.

Here is a copy of a presentation I made recently to our local AIA chapter, when it says refer to handout it's the ABC's of acoustics I used as a handout for this meeting and can be found on this blog as "Alphabet Soup of Acoustics".  Once again - my opinion only based on experience.

NRC refer to handout

Ceilings:

The most common method of adding acoustical absorption to a space is through the use of acoustical ceiling tile but remember – just because it’s in the book doesn’t mean it absorbs sound.  When picking a tile look for the NRC, 55 is a middle of the road rating however you can get up to a 70 with mineral fiber tile.  If you want to get a higher rating you need to look at fiberglass tiles, you can get 90-100 with them.  Be aware of some tiles that have a very smooth, hard face – often they have no NRC.  Tiles suitable for kitchen areas don’t have acoustics.

Metal tiles can have acoustical value if they are perforated and backloaded with fiberglass.  The same is true with wood.  No perforation – no acoustical absorption

With the trend of leaving ceilings open to the metal deck acoustics can get overlooked until the space is occupied, then it’s really costly to fix.  There are a variety of products available to add acoustical absorption into spaces like this; clouds for spot absorption, adding material to the bottom of the deck between the joists where it’s not so noticeable and can be done in black.  There are sprays that can offer acoustical properties sometimes fairly low so read the literature. 

Acoustics on the ceiling doesn’t always have to be tile and grid.  There are stretch ceiling systems that can add acoustics while giving you a fairly monolithic look, depending on the size of the room.   Then there are acoustical plasters, these products usually get applied to fiberglass and result in a clean, monolithic look while offering the function you need.  Always asses the need for access before deciding on a ceiling.

Walls:

·       Fiberglass is the best for reverberation NRC

·       Durability for gyms – Tectum, fiberglass behind it doubles the NRC

·       Mineral fiber has lower NRC but higher tackability

·       Adding 1/8” layer of 20 lb. density fiberglass improves the impact resistance and increases the tackability

·       Copolymer facing can be very expensive and be more difficult to handle than fabric

           

Picking the right fabric:

·       Please don’t leave it TBD unless you have no choice – it makes a huge difference when bidding, price can be $10.00 yd - $80.00 yd

·       Decide up front if you are using multiple colors and communicate that clearly,  it makes a difference in the amount of fabric needed

·       know the lead time – if it’s 12 weeks there may be a problem

·        know the width the fabric comes in and don’t design panels larger than the fabric can cover, allow for thickness of the panel and figure 4-5” for wrapping the back

·       Be careful of fabric with backing, a quick test to see if a fabric is acoustically transparent is to blow through it – if you feel the air on the other side then sound can get through it as well

·       Does the fabric have a directional pattern, if so these are likely to cost more because there will be more waste when laying out panels.  Fabric can be “railroaded” or run the “long” way on the panels.  Just be sure you understand the pattern and how it is laid out

How design of sound panels affects the schedule:

If you design panels floor to ceiling, corner to corner the wall cannot be measured until it is actually built.  This is very likely to cause a delay.  The order has to be placed with accurate measurements, the manufacturer does a set of drawings and returns them for approval, the drawings are checked over and returned approved – and then production is scheduled.  If there is a long lead time on fabric things get worse.  Ways to deal with the situation: fabric can be ordered ahead of time but someone (not me) has to take the responsibility for the quantity of fabric since there aren’t final measurements.  By designing “island” panels, sizes independent of the built wall, the panels can be ordered at any time so they can be delivered when they are needed

Stretch panel systems can help with the schedule in some cases, still have the fabric lead time.  If you want a complex design then perhaps stretch systems are the way to go.  Keep in mind you are not likely to get as many bids since it is not something just everyone installs but it can be a problem solver and offers some additional design flexibility.

When trying to figure out how much absorption to put in a space there are some on-line tools that can help.  Some of it is common sense to a degree, how much will the design allow.  If you are limited by space and/or budget put absorption on two adjacent walls rather than opposite walls.  The results will be better.  Feel free to ask me about a tool I use frequently to help determine how much and where to put panels.

STC refer to handout

Sound Transission Class – sound that goes through the wall, from space to space.  It is important to remember STC is the result of an assembly, not an individual product.

Things such as off-set stud walls and resilient channel have been around a long time and certainly do work when installed correctly.  Sound board has been around for a long time as well.  It is basically mineral fiber in a large sheet, think 4’x8’ ceiling tile, and installed behind the drywall.  See manufacturer’s literature for ratings by assembly.

Of course an extra layer of drywall often times works, don’t let the seams line up.  Another product that has been around awhile and is very effective is vinyl loaded noise barrier.  The material is approximately 3/16” thick and comes in rolls and it weighs one pound per square foot.  This can be applied to the studs behind one side of the drywall or for very severe cases woven between the studs in an off-set stud construction.

One of the newer products on the market these days are drywalls that have a layer of material in the middle that will help stop sound transfer.  These products are more expensive than regular drywall of course but do a good job.  These can be added to an existing wall, the board can be finished just like standard drywall.  See the manufacturer’s literature for assemblies

Important things to know when designing a wall to stop sound: back to back electrical boxes are a super highway for sound – if you have them in an existing wall there are pads that can be put between them to help stop the transmission.  Sound caulk should be used where the wall meets the floor, ceiling and corners.  A room designed with all the best sound control in the world won’t make up for a door that doesn’t stop sound, or an open window.  Sound through a small opening fans out into the opposite space.

CAC refer to handout

Ceiling Attenuation Class

This typically comes into play when you have an open plenum and sound can carry from office to office. Acoustical ceilings with good NRC ratings will not automatically result in a good CAC rating.  There are ceiling tiles with CAC backing, often times foil.  Many times with tile and grid batt insulation will be put along the edge of the dividing wall to help stop sound transfer.  This is one of those common practices that isn’t officially endorsed by tile manufacturers.  When doing this it is important to remember to put the weight of the batts on the mains, not on the cross tees.

Drywall lids don’t usually cause a problem in this area.

AC refer to handout

 Articulation class, this refers to open office areas and the sound that carries over the partitions.  As the issue of daylighting becomes more prevalent partitions are getting lower and/or have glass at the top.  These things will have an effect on the acoustical function of the space.  When designing an open plan office using a high NRC tile will help.  Sound masking can also come into play but that is an area I know nothing about so I won’t go there. 

SABIN refer to handout 

This term often comes up when using things such as hanging baffles or cantenary banners – product that can absorb noise from both sides.  Still comes down to NRC, how much sound is kept from being reflected back into the space.

So when it comes to acoustics, please always consider it in any design, you may find you don’t need to do anything extra but you may also find you do need to do something.  It is always more expensive to do it later.

1.    Figure out the specific problem you need to solve – NRC, STC

2.    How bad is the problem likely to be

3.    Look for the right solution, remember often times it’s a combination of solutions

4.    Don’t waste money on overkill but don’t leave it out either

THE WONDERFUL WORLD OF SOUND PANELS

 

Some tips on what works and what causes problems:

The very best product to reduce reverberation in a space is fiberglass – period.  For those that say what about Tectum; 1” of Tectum type A mounting (flat to the wall) has an NRC .40 but with furring and fiberglass (Finale panel) behind the 1” panel the NRC is .80.  Tectum has the high durability that other panels may not offer which is why you see it in school gyms across the country. 

Let’s look at the traditional fabric wrapped fiberglass sound panel.  There are ways to make this go smoother and there are things that will make everyone crazy.  I would like to avoid crazy as much as possible so I thought I would share some things I’ve learned.

A 1” thickness is usually enough, 1 ½” is more expensive than 2” – has something to do with the way the fiberglass comes

Picking fabric – this is important for several reasons. 

·       The fabric has to be acoustically transparent or it is a waste of time.  For a good article on fabric selection check out: http://www.soundseal.com/soundquality/2011-fabric-document.pdf.  

·       When deciding on the size of the panels keep in mind the width of the fabric, don’t draw panels wider than the fabric or they will have to be split.  To help understand the size, look at the width of fabric.  Let’s say it’s 54” wide if you are using a 1” panel then you need to deduct 2” for that then another 6” to allow for wrapping the back of the panel.  Now the 54” wide fabric is down to approximately 46” to 48”.

·       Fabric availability is critical as well, some are readily available and some will have a 12-15 week lead time.  This can cause havoc with schedules.  If picking a fabric with a long lead time you can elect to give the manufacturer approval to purchase a set quantity before the walls are built.  The risk with this approach is that if it’s too much you are stuck paying for it, if it’s not enough then it will take forever to get the rest and it may not be the same lot.

·       Keep in mind an equal number and sizes of panels in one fabric will be less expensive than if they are five different colors.  Colors are fine; just make sure everyone knows about them up front.  Changing that after the bid will cost.

Panel layout and why it matters:

·       If you design sound panels to go floor to ceiling, wall to wall, the order cannot be placed until the wall is built and measured.  The fabric isn’t ordered until these dimensions are measured.  This can result in a schedule crunch.  The simplest way to avoid that is to design island panels – a set size independent of the exact wall construction.

·        Using a stretch type of system will also help with the schedule on full wall panels but may not be as readily available depending on installers in your market.

Edges and attachment:

·       I always recommend hardened edges; think they hold up better during the installation process.  Square edge panels are the least noticeable joint but also can call attention if the alignment is off slightly.  One of the most effective designs I’ve seen is using 2” panels with a 1” gap between them – looks very sharp.

·       Two part clips versus impaling clips – I often hear the reasoning for two part clips as “we want to be able to take them off”.  Here is the drawback to that thought – first of all no one ever actually takes them off anyway, and you can take a panel off an impaling clip as easily as a two part clip.  Just reposition the impaling clip and put the panel back up.  If you do use two part clips a couple of things to be aware of – you can’t use them on panels that meet a bulkhead.  You have to have room to lift the panel up at least ¾” to engage the clip on the wall.  Two part clips will add slightly to the overall thickness so don’t mix the two methods if you don’t have to.  Then there is the fact that glue is used on impaling clips where they meet the panel, but often glue is also used between the two-part clips to keep them solid – those won’t come off.

How many panels do I need?

That really is the toughest question but there are some basic rules you can follow.  If the budget is tight and you can only do a limited amount put them on adjacent walls, not opposite walls.  Getting something on the ceiling may be key; we’ll talk about options next time.  I’m sure there are many terrific programs out there as well as knowledgeable reps that can help.  One of the free programs I use is by Armstrong ceilings: http://www.armstrong.com/commceilingsna/article21088.html?intcid=homepagehttp://www.armstrong.com/commceilingsna/article21088.html?intcid=homepage 

In case the link doesn’t work go to Armstrong commercial ceilings and look under Selection Tools, Improve Speech Intelligibility.  The thing I like about this is that it lets you allow for more than one product on the wall and you can go back and forth to see what effect the product selection has on the reverberation time.  It will give you a square footage to shoot for at least.

For the best outcome with acoustical wall panels – keep it simple please.

THE ALPHABET SOUP OF ACOUSTICS

As promised this is the first of several articles and links to information regarding acoustical solutions for commercial spaces.  First of all, and this is very important, I AM NOT an acoustician nor do I claim to be.  However over the course of the last eleven years I have learned a great deal about acoustics and one of the biggest things I have learned is that it often gets overlooked in the design process for one reason or another.  I have also learned it is much more difficult and expensive to fix the problem after the fact.

The best place to start with anything is with the basics, so for those of you that are very familiar with this I apologize.  After having conversations with young interior designers that have no idea what NRC means I felt it was best to start at the beginning. 

Terms:

NRC – noise reduction coefficient

This measures the amount of sound absorbed by a surface and the amount reflected back, this is “echo” within the room.  The scale is an easy one: 0% – 100% absorption

·       drywall NRC .06

·       standard ceiling tile NRC .55

·       1” fiberglass wall panels with the right fabric NRC .80 -.85 

STC – sound transmission class

This measures the sound that transfers through a wall, from space to space.  The scale of measure for this is not as simple as NRC, an STC <35 is considered low while an STC >55 is high.

The STC is the result of an assembly and not based on a single component.

CAC – ceiling attenuation class

This is the rating that measures sound going through a ceiling in an open plenum design, sound from office to office.  A ceiling system with a CAC <25 is low while > 38 is high performance.

AC – articulation class

For those spaces with cubicles; measure for rating the attenuation of reflected speech passing over the top of wall partitions or furniture.  A ceiling system with an AC<150 is considered low whereas an AC> 180 is considered to be high performance.

SABIN – okay, it’s a whole word but there are always exceptions

This is always a confusing one for me, here is the official definition.  A measure of sound absorption provided by a material when installed within an architectural space.  The number of Sabin per unit is approximately equal to the total surface area of the unit (in sq.ft.) that is exposed to sound, multiplied by the absorption coefficient of the material.  This is typically used in reference to hanging baffles.  Asked a supplier for an explanation:  I guess the first thing to remember is that NRC (Noise Reduction Coefficient) is defined as "sabines of absorption per unit area". In other words, if a material has an NRC of 0.85 it means there are 0.85 sabines of absorption per square foot. If there's 10 square feet of material then there would be 8.5 sabines of absorption. So essentially sabines is NRC.

PI – privacy index

This is for rating the speech privacy, or lack of speech intelligibility of a space.  A privacy level of PI>95% represents confidential speech privacy, a PI between 95-86% represents normal or non-intrusive privacy and PI <80 is poor.

IIC – impact insulation class

It is a measure of the floor-ceiling assembly to isolate impact noise (footfall) and other impact noises such as furniture being drug across the floor.  The IIC rating typically ranges between a low of around 20 to a high of 80, with 20 being very poor and 80 being excellent.  This typically applies to multiple family housing.

Next article I’ll get into a couple of the basic issues and the best way to solve them.  If there is some aspect of acoustics you would like to see addressed please let me know and I’ll see about getting input from those that know more than I do.

One sales reps views on products primarily in Division 9.  Things I've learned over the years that may help you.