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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.
Posted by Acoustics First in Absorption, Articles, Broadcast Facilities, Customer Feedback, Diffusion, DIY, Home Entertainment, Home Theater, HOW TO, Media Room, Music Rehearsal Spaces, Music Tracking Room, Product Applications, Recording Facilities, Recording Studio, Studio Control Room, Teleconferencing, Theater, Vocal Booth, Voice Over on July 14, 2016
This month we thought we’d share a few Real-Life pictures of an idea we first introduced back in summer of 2013: The “Back Wall Diffuser Array/Bass Trap”.
This is the DIY project which incorporates our Art Diffusors®, Cutting Wedge® foam and a couple of isolation hangers into one large free-floating unit, which is acoustically decoupled from the wall.
This particular array was put together by a music producer/bass player for his home. As you can tell from the pics, the construction of this unit was executed beautifully and it’s very close to the original concept drawings.
It’s never too late to get started on your own DIY project.
Ever wonder what gives us a sense of space? Obviously, our eyes visually tell us what’s going on, but there are other senses that contribute. Peak your head into a dark front hall closet, and even without seeing much, you can “feel” the close proximity of the walls and perhaps even the presence of the coats. Walk in to New York’s Grand Central Station, and you are confronted by a completely different sensation. Close your eyes, and the raucous environment tells you are in a large room with a lofty ceiling. Often times we take for granted the relationship that sound has to our spatial perception.
This sonic “sense of space” can be generally attributed to the room’s reverberation qualities. In simple terms, reverberation is the sound energy that remains in the listening environment as a result of lingering reflections. Reverberation time (RT or RT60) quantifies how quickly an impulse sound decays in a space. RT60 is how quickly the amplitude (volume) of short exciting signal decreases by 60dB in a large room. Reverberation time is dependent upon the volume and surface materials of a given room. Large spaces with hard materials (tile, drywall, etc.) like Grand Central Station have longer reverberation times, while small rooms furnished with “softer” materials, like the coat closet, sound much more “dead”.
Excessive reverberation is one of the most common acoustic issues that we encounter on a daily basis. As you may have experienced at some point, it’s difficult to understand what is being said when reflections from old information cover up what is newly spoken. In spaces where speech intelligibility is paramount, like classrooms or conference rooms, a short reverberation time (under 1 second) should be targeted.
That said, sometimes a long reverberation time is desirable. In spaces like cathedrals and orchestral halls, reverberation helps create ambience for the audience by sustaining musical notes, while allowing choirs and orchestras to blend more easily. These spaces may lack a sound system, and instead utilize the room to propagate sound. Rock venues, on the other hand, have amplified instruments, so a medium-short reverb time is needed to ensure that the music won’t become “muddy” and difficult to perform and enjoy.
There are a number of questions that an acoustician must ask when recommending appropriate treatment. These questions include, but are not limited to: Is there live music in this room? What kind of music is being performed? Is speech intelligibly important? What’s the audience size and where are they in relation to the sound source? So, the ideal amount of reverberation in a space is wholly dependent on the use of the space.
Listed below are the ranges of “ideal” reverberation times at mid-frequency (average of 500 and 1000 Hz) for a variety of rooms. The numbers are derived from David Eagan’s Architectural Acoustics (New York: McGraw-Hill, 1988), in which he breaks down rooms into Speech, Music and Speech/Music spaces. We hope you find this helpful.
Optimum Reverberation Times (T60)
Recording and Broadcasting Studio – .3 to .7 seconds
Classroom (elementary size) – .6 to .8 seconds
Conference/Lecture Room – .6 to 1.1 seconds
Intimate Drama – .9 to 1.1s
“Speech & Music” Rooms
Cinema – .8 to 1.2 seconds
Small Theaters – 1.2 to 1.4 seconds
Multi-Purpose Auditoriums – 1.5 to 1.8 seconds
Worship Spaces – 1.4 (Churches) to 2+ seconds (Cathedrals)
Dance Clubs and Rock Venues (w/ Sound System) – 1 to 1.2 seconds
Semi classical Concerts/Chorus (w/ Sound System) – 1.2 to 1.6 seconds
Symphonic Concerts (Classical) – 1.6 to 2.3 seconds
Liturgical (Organ/Chorus) – 2+ seconds
Contact Acoustics First to have our acousticians help you find the ideal reverb time for your space.
With current trends leaning toward improving the quality of life for all creatures living on our great planet, we at Acoustics First® feel that we can contribute to this in our own small way.
So we proudly introduce, the ArtDiffusor® – Micro Model D!
Birds are very musical creatures, in the past, while Beaker was listening to the Bee Gees, he was bombarded with harsh specular reflections and flutter echos, it was a tragedy – but no longer.
Harsh acoustic environments are not for the birds… The ArtDiffusor®- Micro Model D… is.
Sometimes you enter a space that sounds amazing and wonder what treatments are being used. Many times the treatments are behind the curtain… both figuratively and in reality.
Learn about some of the ways acoustic treatments are hidden in plain sight, obscured from view, camouflaged, or just displayed proudly – Nick Colleran points out the not so obvious.
You just might find – you get what you need.
(Special thanks to the Great Philosopher Jagger)
– Acoustics First