The Science of the ThunderTiles
When attempting to control sound, there are in fact two aspects one must consider: The noise or echo inside the room and the noise that we are trying to contain from either entering or escaping the room. Both are inter-dependent and are addressed using a different solution.
Controlling sound inside the room
Excessive reverb or echo in an overly ‘live’ room makes communication difficult. This is particularly acute in boardrooms, classrooms and conference rooms where understanding each word is critical to ensuring the message being transmitted is clearly understood. This is referred as intelligibility.
A problem occurs when the direct sound from the orator’s voice is interfered with echoes reflecting off of hard surfaces such as windows, walls, ceiling and table top. The following image shows the direct sound (green) and the reflected sound (red) coming off a hard surface that will arrive a few milliseconds later. When the intensity of the echo approaches the amplitude of the direct sound, the two sounds combine, introducing ‘peaks and valleys’ known as comb-filtering whereby various frequencies amplify each other when in phase, or cancel each other out when out of phase.
Why is it easier to understand when you are there in person?
People often comment that when you listen back to a recorded event, the sound is often less clear than when you are there in person. This is because the brain is much smarter than a microphone.
When you are in the room, the ‘brain’ is able to discern what you want to hear and what you do not. It does so by automatically ‘ignoring’ the echo from the walls and ceilings and chatter from other folks in the room. This is often referred to as the cocktail effect whereby in a noisy room with hundreds of people talking at once, you can still have a conversation with a person and somehow ignore the ambiance.
A microphone is not so smart. It simply records the whole event and plays it back without judgment. When the recording is played back, all of the sounds are jumbled together coming from a single pair of speakers and the brain is no longer able to extract the information from a given direction.
Folks often complain about headaches after being in very noisy environments. This is because the brain has to work extremely hard – double time if you will – at both listening to what you want to hear and ignoring what you do not.
This is one of the most important reasons that schools are beginning to take action by acoustically treating classrooms. If it is difficult to understand the teacher due to excessive echo, noisy air ventilation systems and disruptive discussions from around the room, some kids simply turn their brains off.
This image takes a more detailed look at the phenomenon whereby the initial sound (green) arrives first, then a series of powerful first order reflections from the walls, windows and ceiling arrive. This is followed by a trailing series of primary reflections that create a reverberant field. When the room reverb exceeds one second, voice localization becomes difficult, reducing our ability to comprehend what is being said.
The following graph shows the relative energy of a typical male voice at two different amplitudes. The blue line depicts a normal speaking voice while the red line depicts a raised voice. As the voice level increases, the energy tends to gather in the mid range, between 300Hz and 1500Hz.
To further illustrate the problem, one merely needs to superimpose the frequency range of the human voice on top of the absorption coefficient of glass and gypsum board. The graph below does this. These hard surfaces absorb less than 5% in the voice range, resulting in almost 100% of the energy being reflected back into the room.
The usual approach to solving the problem is mounting absorptive acoustic panels to the walls. But sometimes, treating the walls may be impractical. For instance, a classroom may have windows and blackboards that obviously cannot be covered while a boardroom may be surrounded by windows, bookcases and wall ornaments. In these cases, limited space often makes treating the ceiling the only option. Ideally, one would employ a mixture of both.
Controlling sound in the voice range
Before we look at ceiling tiles, let’s quickly look at the absorption of Primacoustic acoustic wall panels.
The graph shows the same voice range (300Hz ~ 1500Hz) superimposed over the absorption characteristics of a Broadway 2" thick wall panel. Broadway panels are made from high density 6lb glass wool. This yields excellent absorption across the audio range and is particularly well suited to absorb the frequency range of voices.
Comparing Ceiling Tiles
Ceiling tiles work differently. When they are tested without an air gap behind them, the absorption yields excellent performance at the upper end while low frequencies appear to suffer. This will in fact change based on the amount of air space above the panel in the plenum. The greater the air space, the greater the low frequency absorption, as long as the panel is sufficiently dense.
Combining Different Treatments
Space permitting, the combination of Broadway acoustic wall panels along with either StratoTiles or ThunderTiles will provide optimum performance.
Wall-mounted Broadway panels are used to control reflections of the walls, eliminate chatter echo (ricochet) and break-up standing waves. Primacoustic ceiling tiles are used to control floor-to-ceiling standing waves plus reflections off of hard surfaces such as the table.
Stopping Noise From Escaping
Sound blocked by ThunderTiles
Sound passes between offices through plenum
To stop sound you need mass. This is why recording studios and five star hotels build their walls with multiple layers of gypsum board and skin coat their floors with concrete. For sound to pass through the wall, it must set the wall into vibration. So the heavier the wall - the better. Walls made from gypsum board are generally quite effective. In fact a gypsum board wall will usually attenuate the sound by 25dB. But if these walls are so good, why do people often complain that they can hear conversations from adjacent offices?
The problem is not so much the walls, but sound traveling up through the plenum, bouncing off a hard surface, and reflecting right into the office next door. Common mineral wool ceiling tiles are very light; they have no mass. They are therefore ineffective at stopping sound. This is precisely why we developed the ThunderTiles. Primacoustic ThunderTiles combine the sound absorption characteristics of high density glass wool with the benefits of added mass with a gypsum backing board. The two are bonded together into a single easy to manage tile that retrofits in all standard T-bar systems. The minute wool fibers are set into vibration, converting excessive energy inside the room into heat (a thermo-dynamic transfer) while the mass of the gypsum board blocks sound from either entering or escaping through the plenum. This delivers excellent attenuation while effectively absorbing frequencies in the voice range, making it ideal for all types of applications.
As ceiling tiles are often used in high rises, schools and all types of commercial buildings, ThunderTiles have been tested for flame spread and smoke density to exceed the stringent class-A specification. This makes ThunderTiles an ideal choice for use in public places.
Primacoustic is a division of Radial Engineering Ltd. Copyright 2009. All rights reserved.