Posts Tagged broadband absorbers
For anyone new to the world of acoustics, there is a multitude of terms, coefficients and numbers that are thrown around. This flood of information can seem intimidating, especially to beginners. In this series, acoustician Cameron Girard of Acoustics First® hopes to help you distinguish between what’s useful and what’s not.
Part 1: Acoustic Terminology – Sound Absorption vs Sound Isolation
In order to make informed decisions about acoustical treatment, it is vital to know the difference between materials that are meant to absorb sound within a room and materials that are meant to block sound from leaving or entering it. In an overly reverberant auditorium, absorptive treatment is needed to reduce echoes and improve speech intelligibility. If the problem is sound passing in between spaces, like offices or apartments, then isolation treatment is required. These are two separate acoustic issues which require separate solutions.
In both scenarios, it is important to know which data is relevant and helpful. Also, given sheer volume of information available on the internet, it is perhaps unavoidable that some info will be incomplete or simply incorrect. It should not be assumed that something which sounds technical is, in fact, backed up by proper testing.
Terms for Sound Absorption
We recently encountered an acoustical ceiling tile which was said to “absorb 50% of sound”. On the surface this sounds like an extremely efficient product. However, let’s delve in closer and decipher what is actually usable information, and what is just marketing.
When sound waves meet a room surface such as a wall, ceiling or floor, some of the sound energy is reflected back into the room and the rest is considered to be “absorbed”. The absorbed sound energy has not vanished, it’s actually been converted into kinetic (vibration of a solid material) and thermal energy (heat due to friction within a porous material) or has simply passed right through the material (transmission). The more surface area a certain material has the better absorber it will likely be. “Soft” materials, like heavy blankets, fabric and fiberglass, have loads of nooks and crannies, which sound tries to “fill”. These porous materials are great for reducing reverberation within a room, but will only marginally reduce the sound that leaves it (but more on that later).
When comparing sound absorbing products, there is a particular set of terms you should look for: The Sound Absorption Coefficient (SAC) and Noise Reduction Coefficient (NRC). These are used to specify the fraction of incident sound that a material absorbs per 1’x1’ area. An NRC of 1.0 indicates perfect absorption (an open 1’x1’ window) and a value of 0.0 represents perfect reflection (polished concrete has an NRC of .02).
To measure sound absorption, a large sample of the material is placed in a reverberation room with all other surfaces being hard and reflective. The time it takes a test sound signal to decay by 60dB (rough point of inaudibility) after the source of sound is stopped is measured first with the sample in the room and again with the room empty. The difference in decay time defines the efficiency of the absorbing material and thus the absorption coefficients. Large spaces with low-NRC materials (tile, drywall, etc.) have longer reverberation times, while small rooms furnished with high-NRC materials sound much more “dead”.
Clearly, a single 2’x2’ ceiling tile is not going to reduce the reverberation in a real-world space by 50%. So is the above claim false? Not exactly… The ceiling panels do have an NRC rating of .50, so the tile does absorb 50% of incident sound. However, one might assume a much more drastic improvement based on the “50%” claim. In reality you’d need a large square footage of these ceiling tiles to cut the amount of total reflected sound in half. Always be sure to check the NRC number!
Terms for Sound Isolation
Our customers often call with issues related to neighbor noise or office-to-office privacy and are looking for “sound proofing” treatment. Unfortunately, many do not realize that simply installing acoustic foam or fiberglass panels will not appreciably reduce the level noise entering and leaving their space. These absorptive materials are great at reducing unwanted reflections within a room because they are porous and air/sound energy can flow through them. That being said, they are generally poor sound barriers for this exact reason. They will help to reduce noise buildup in a room and improve the ‘acoustics’, but will do very little to “block” sound coming in or out.
Sound is like water; it will “flow” into an adjacent space if everything isn’t sealed up. Materials that are air tight and heavy, like our BlockAid® sound barrier, provide the most relief of air-born sound transmission. Continuous coverage of floors/ceilings or walls is necessary to ensure that sound doesn’t ‘flank’ around these barriers. Multiple layers of varying materials, the use of resilient clips or channels, and additional walls will provide even more control. For a demonstration of how different materials affect sound isolation, check out our video http://acousticsfirst.com/educational-videos-the-barrier-and-the-bell.htm
Like NRC for sound absorption, there is also a laboratory tested figure that can be used to compare the sound “blocking” properties of acoustic barriers and wall constructions: Transmission Loss (TL) and Sound Transmission Class (STC). These describe how much air-born sound is attenuated through a given material.
In the lab, the material to be tested is mounted over an opening between two completely separated rooms, one with a speaker (source) and the other with a microphone (receiver). Save for the open “window”, these rooms are completely isolated with thick and massive walls, so virtually all the sound energy transmitted between rooms will be through the test specimen. The difference between sound levels in the source room and the receiving room is the transmission loss (TL). The TL is measured at multiple frequencies, which is fitted to a Sound Transmission Class (STC) “curve” at speech frequencies (125Hz-4kHz). The STC of the material is the TL value of the fitted curve at 500 Hz. For example, a material with an STC of 27 typically “blocks” 27dB of sound. Keep in mind though, the STC’s of materials do not add up linearly; in other words, adding a material with an STC of 27 to an existing wall with an STC 45 will not result in an STC of 72.
As always, Acoustics First is here answer questions and help you find the best solutions.
Modern AVR’s include all kinds of wizardry for speaker setup, positioning and room equalization. Anyone in the know will tell you that room EQ hardware and software should be a last resort. Correct acoustical treatments will pay much bigger dividends.
If you are like many people, you have recently upgraded your home theater, or you’re getting ready to do so. During this process you’ve likely learned that the speakers in the TV are sub-par and you should upgrade your audio experience to include an external sound system. This has become common practice and common knowledge. What you haven’t learned, and which is probably even more important, is how to upgrade the room itself.
One of the things people overlook when trying to make a home theater, is that a real theater pays a lot of attention to the design and treatment of the theater environment. They have good left-right design symmetry (see the diagram for a sample home theater layout). A good theater also has acoustically treated the space so that people can hear everything that is happening, from anywhere in the theater. This article will teach you some of the tricks of getting that great big theater soundstage into your home theater.
Bass Traps – the low-down solution.
Low frequency problems are common to almost any room, regardless of size. The good news is that, most of the time, the solution is simple: put bass traps in the corners of the room. This is one place where it pays to put a little extra in the budget. Corners are defined as the intersection of two or more surfaces. There are not just corners at the end of each wall, but also along the floor and ceiling where the walls intersect them. The more corner you cover with a good trap, the better bass response you get – it’s that simple. Bass loves the corners, and by putting bass traps there, you keep the bass crisp and natural. If bass frequencies are allowed to build in the corners, it causes the bass frequencies to become muddy and undefined – trap them.
Some bass traps work double duty as broadband absorbers as well, which can keep your costs down when considering covering a bunch of square footage with absorbers. Bass traps alone can solve many problems in your room, and due to the simplicity of the implementation, I recommend you start with these first – at least fill the four main corners.
Foam works. But this is a theater, not a studio. Fabric wrapped absorbers look as good as they sound, and Geometrix™ by Acoustics First, fit the corners like a glove. Pick a fabric and get some quarter rounds to fill your corners – that’s it.
Broadband Absorption – tame the ring.
Another common problem in home environments can be easily verified with a clap test. Go into your room and clap your hands. Most likely you will hear more than just the initial clap. Depending on the severity of the problem, you will hear a flutter, ring, or echo after the initial *pop*. This is caused by the sound waves bouncing around off the hard surfaces of the room and returning to your ears after a delay – the more times they bounce without losing energy, the longer the delay. The best way to remove energy from these waves is to use broadband absorption, but where do you put them?
More than likely, your TV and sound system are going to be in a fixed position, and your listening position will also be fixed – so the early reflection surfaces should be easy to locate. You will need a friend for this activity, a mirror, and a pencil. Have your friend place the mirror flat against the side wall and move it around until you can see the speakers in the reflection from your seat – then mark the wall. This is where you are going to place the absorber. The diagram shows a good place to start looking for these reflection paths. Repeat this for all the walls from all the seats.
What you are looking to create is a reflection free zone, which basically means, wherever the sound could bounce off a surface and get to your ears, we are going to absorb energy from it. You can spend a good deal of time on this, but this is the only step that requires this time and effort, so make it count. Sound travels in all directions from the speaker, including behind it, so put absorbers behind it on the wall. Don’t forget the floors, ceiling, and the wall behind you – sound will bounce off those as well.
This simple process will show you where you need to treat. Hang broadband absorbers over all the early reflection points – left, right, front and back. Absorber clouds should be hung on the ceiling, and place a nice thick carpet on the floor. Placement is the first key to getting this reflection free zone. The second is the right choice of absorber.
To match your fabric wrapped bass traps, the simple choice is get some more panels wrapped in fabric. The Sonora® line of broadband absorbing panels coordinate with the bass traps, and come in a plethora of sizes and mounting options to work in your space. Need 2’x4’ behind the speakers, 4’x4’ on the sidewalls, 2’x6’ on the back wall, and a 2’x6’ ceiling cloud – all in material that match those bass traps? Done.
Finally, use broadband absorption with caution, specifically using too much. If you plan on covering more than 50% of your walls with this stuff, you’re going to notice a muffled almost claustrophobia inducing deadness. We are not trying to suck the life out of the room. We are just trying to take enough energy away from those early reflections to keep the focus on the initial sound produced by the speakers.
Diffusers – put life back into your space.
The steps we have taken up to this point have been using absorption to control excess energy that can have an adverse effect on the listening environment. We have removed the unwanted direct reflections and we have tamed the bass, but there is something more we can do to give life to this room – diffusion.
Diffusion will give us something we couldn’t attain through absorption – a sense of open space. Even after treating with absorbers, there are still areas of the room where sound waves will sit, because your room is a fixed box with fixed speakers. Diffusers scatter the energy, creating ambiance with residual energy, like sitting quietly in a forest – the energy around you being directionless, omni-present, and spacious. This simple step does not remove energy from your room, but redistributes it into a soundscape that can make you forget you are in a room at all.
There are many ways to diffuse the sound and coordinate with your room, from the fabric covered HiPer™ Panel and Double Duty Diffusers™, to the striking line of Art Diffusors® like the Model C, which can be painted to match your décor.
Advances in acoustic treatments are being made all the time, bridging form with function, creating products as visually stunning as they sound – and helping your theater, or any theater, be the best that it can be. So use these tips to set up your home theater like a real theater, and experience the difference a soundstage upgrade makes.
Acoustics First Corporation supplies acoustical panels and soundproofing materials to control sound and eliminate noise in commercial, residential, government, and institutional applications worldwide. Products include the patented Art Diffusor®, sound absorbers, noise barriers, acoustical fabrics and accessories. Acoustics First® products are sold for O.E.M applications, direct, and through dealers. For more information on acoustical materials and their application, please visit www.AcousticsFirst.com or call Toll Free 1-888-765-2900 (US & Canada).
The complete article is posted at the following link: http://acousticsfirst.com/article-setting-the-sound-stage-how-to-treat-your-home-theater-like-a-real-theater.htm