Basic Acoustics - A Guide To Understanding Room Acoustics

Intro - Basic Acoustics

Hello, today we are going to discuss about some basic acoustical properties. One thing that im asked quite frequently is whats the most important thing when it comes to room acoustics and i have to honestly say that the most important thing is understanding. Hearing is one thing but understanding is everything. 

What this is, is the intellegibility been able to clearly understand whats going on. Whether you are in worship service enviornment and people are able to understand the words to the song, spoken word if you are in a conference room selling million dollor business deals then they need to be clearly understood and if you watching a movie the car crash needs to happen and then be gone so when you are in a quite scene you can clearly hear whats been spoken. So we want to give basic overview of acoustic. 

We are going discuss about basic terminology words that you can learn and know how to implore into your specific situation. We also want to discuss about speech intelligibility hearing what things that can solve those problems. Products that you can use, ways that you can solve these issues and then finally we also want to look at how to make a quality decision when it comes to acoustics in your room. 

STC vs NRC - Basic Acoustics

We are first going to talk about two different worlds of Acoustics. There are two totally different realms that acoustics happen in and one of the realms is STC or relates to the way sound transmits from one room to another and the other one is NRC or Noise Reduction Coefficient and this relates to the way sound interacts within the confines of a space or within a room. 

STC - Sound Transmission Class: Lets look at STC first, again STC is Sound Transmission Class and it happens between rooms the biggest key you will learn here is sound will take the path of least resistence.

Whenever you are constructing a facility you have walls, doorways, windows and sound will take the path of least resistance between those. One of the best analogy I can give you is that we all have been in our cars and we drive upto busy intersection and you slowly roll down your window and you open the window just a little bit and all the sound from outside comes flooding in through that one little opening in the window. This is what we mean sound taking the path of least resistance. 

You can put as much material within that car try to help to solve it but the problem is the weakest link or the missing link between the outside and the inside is the opening in the window. What we talking about is everything which is made such as Doors, Windows,  Gypsum, Concrete materials all have what we call is STC ratings and it basically means that how much do they pull down, how much do they take the sound down. 

So if the door is rated of STC of 20 so it will reduce to decibel 20db and if you have a wall of STC 40 and the door at STC 20 then your doorway will become your weakest link  or the path of least resistance. So the best thing you want to do is to try to build up these two things equally.

NRC - Noise Reduction Coefficient: This deals with sound within a room, i think we all have been to gymnasium where people talk and we can’t understand somebody who is like 5 feet away from us. What all this equate to is unintelligibility you can’t understand what is being spoken and what is being communicated. The audio or the intelligible sound is unintelligible. 

There is nothing you can decide for there. Now what NRC does or product  with Noise reduction coefficient is that they help to absorb that excess energy in the room to get it down to an intelligible level. Now the NRC scale was designed many years ago where NRC at 1 was the greatest or being like an open window. 100% absorptive. 

Since that time several products that have been introduced such as panels which have diffusive characteristic as well as absorptive characteristic, great edge details that help to breakup sound and absorb energy better, so now there are few things that come out into the room and not just open window they can achieve higher NRC than just 1. 

The best way to understand is that you are able to get absorption of the product plus little bit of space around the unit if it goes over the NRC greater than one. Some common building material such as gypsum has NRC level of like 0.05 sometimes it is very low depending upon construction. Concrete wall has almost 0 absorption where as carpet has very little very thin but it will absorb more on a higher frequency range such as .3 /.35. A one inch acoustical panel like Horizon 1" Panels will absorb a .80 to .95 a two inch flat panel like the Horizon 2" Panels will absorb around .95 / 1.2.

So its depends on the type of product you want but the main thing to understand at this stage in the game is that there are two entirely different worlds of acoustics. Now in some cases they overlap but in most cases you either have a problem dealing with sound transferring from room to room or you have a problem with sound within the room.

Now lets look at the frequency range audible sound chart. On this chart you are going notice several things. First of all we see the hearing of the young and hearing of the old and how they have lost some of their higher frequencies. 

The other thing we are going to notice is the NRC scale and which frequency it focuses on to come up with its coefficient. You are also going to see here the piano range and how many frequencies it covers, the human voice and the laboratory tests. Now what we want to know about frequencies is their distinct different thing. 

You have lower frequency which are really low and then you have higher frequencies which go all the way to very higher distinct, and one thing that is important to know that every frequency has its specific length. You  look at the top of the chart you will notice way down to 31 hertz range, that range is 36ft long. It takes 36 ft for that frequency to develop. 

This is why in some cases you can be in your car ten rows back and you can hear sumone bumping in the trunk about ten cars up and its just blowing you away and they can hardly hear.  They feel the energy passing passed them but it really takes that frequency 36 ft to develop before it booms in your house or cars. Now look at the NRC scale again. It covers a 250 range, 500 range, 1000 and 2000 range . Whats important to note here  is the low tones that you may be experiencing in your facility. 

You may want to look at the product  that’s going to address those lower frequency ranges. As well as notice that it stops at 2000. If you are having some problem with diction frequency in your room we may need the product that’s going to cover little bit higher in the scale with these frequencies. This is where the two world overlap just a little bit is it that sometimes lower frequencies can pass thru walls and STC rating where higher frequencies are stopped and in the NRC world where frequencies kind off collide and cause problems within the room. But overall these are the two Basic worlds of Acoustics.

Terminology - Basic Acoustics

Now we are going to look at acoustic terminologies, these terminologies are still going to deal with in the realm of room acoustics “acoustics within a room”. The first two I’d like to call the bad boys of acoustics, these two are the ones that cause intelligibility problems, communication issues, makes the room too loud, too problematic.

1). Reverberation: Reverberation can be easily defined by the overall noise that bounces around in a room and clutters up and causes problems. It’s the time it takes sound to decay 60 decibels within a room.

Its also been referred to as the decay time of the room, this is the reverb time and it can be measured by a test called RT60, again its the time it takes for sound energy to decay 60 decibels. 

If in a room you put a burst of energy that may have 100 dB, it’s the time it takes to get down to 40 dB or if it’s a 120 dB burst of sound then it’s the time it takes to go down to 60 dB, it’s a 60 dB reduction. Most big areas untreated are somewhere in the range of 5 to 7 seconds of decay time, this gives you an overall idea if you were to clap and create a burst of energy at 100 dB and could stop hearing it at usually about 40 dB, you can sometimes go into a room make a loud burst of energy and then count the time it takes the room to an overall decay.

2). Echo: Echo is the other bad boy that sometimes creates a lot of problems, echo is easily defined as a simple reflection of energy that comes right back on to your face. Echo is a distinctive energy that comes back to you at a slightly later time interval. There are different kinds of echoes present in a room facility like the flutter echo, slap, focused echo and the ghost echo..

In a room facility you can face different problem with different kinds of echo. Its not just a distinctive time interval, sometimes there maybe 2 parallel surface areas and the sound bounces back and forth in between, creating a different kind of echo, known as flutter echo.

There is a distinct slap echo, where you have a large surface area in a large room where you make a burst of energy or you speak and it goes out hits that wall and bounces back to you.

The other one that can be really problematic is a focused echo and it occurs when you have a circular or spherical surface area and the sound energy goes out and hits that and then all the points in the room redirect back to the focal point. 

This is a great problem in a lot of auditorium spaces because in focused echo situations most of the time the buildings can be constructed based on site lines. The problem with site lines is its usually all circular spherical space faced all directly on that central focal point so wherever that central focal point is that’s where all the energy likes to unload in the room.

The final one is just different type of ghost echoes or flutters, where it hits of the surface and gets a glancing reflection.  These guys can cause some serious havoc in a room, they can wreck intelligibility and communication.

Absorption: Absorption can be used in a way where you take a porous material and it absorbs the energy and stops it from reflection of, it will also absorb other noises around. There are also products being manufactured that act both as absorbers and diffusers simultaneously like the Ultracoustic’s Ultra Wood Panels or the AbsoFuser - Combination Absorber Diffusor.

Diffusion: Diffusion helps eliminate flutter echo & standing waves like the Z-Edge QRD Diffusor. One of the best examples I can give you is like an echo is a laser pointed towards a mirror, the sound energy source or in this case the light energy source will intersect the mirror and will come out at the opposite or glancing blow. Diffusors, what they do is that they take the sound energy and diffuse or scatter it in many particles and directions.

So if you like to maintain the air or the ambience of the room, you can use diffusers to do the job. To absorb the sound energy at primary and secondary reflection points you can use absorption products such as the Horizon Acoustic Panels series.

Solutions - Basic Acoustics

1    Fire Protection: All products have acceptable fire ratings that need to be within certain ranges.  You need it to be Class A as a unit.

2    Durability: You want the panel that you put in your facility to last for a while. Sometimes you need to look at space issues, where you put up the product, how you are using the products but mainly there are durability standards that go in to products that are manufactured.

    Absorption: What you are really purchasing is something that is going to take care of the problems in the room such as early reflections, echo or reverberation. Pick a product that has a high NRC values also the one that is going to take care of the frequency issues in your room. If you have low frequency issue then you pick a product such as a bass trap like the UltraTrap that does well down the 80 to 125Hz range.

For more info : http://www.ultracousticin.com