Science of acoustic absorber:
⦁ Why is it worth regulating the sound first of all?
⦁ Solution to the acoustic problem
⦁ Determination of coverage
⦁ Comparison of different bass traps
In this overview we summarise how the broadband acoustic panels operate and how to choose the most up to the job. Furthermore, we give basic guidance where to use them and how to compare them with other sound absorbing products available on the market.
This website does not want to make you an expert, we just try to provide general information on the science in question and its application.
Why should we control the sound?
Generally speaking, the control of the sound helps to improve clarity and to understand the communication.
In a simpler way: to circumvent the disorder of sound in order to hear the message clearly. In a church this can be the spoken word. In an airport this can be even the flight information.
It may be related to the safety warnings in a factory. In a hotel lobby or restaurant we just want to try to decrease the echo in order to make the communication between the staff and the customer more comfortable.
In an audio studio the control of acoustics allows us to create a predictable result in order that the recording sounds the same quality in other systems as well.
Without acoustic treatment
Without acoustic repair the sound bounces from the walls, floor and ceiling and reaches the point where the room is not able to absorb and diffuse energy.
For example, there is a big difference between a teacher talking silently in a classroom and somebody shouting in a room full of excited children. If they exceed the natural threshold of the room the conversation and communication require much more attention.
This generates an effect called “ear fatigue” when we must highly concentrate while listening and we must talk louder in order to hear and to talk down the other competing sounds.
The bouncing sounds compete:
The acoustic absorbing panels control the echo:
We call these reflected sounds reflections. These can be primary or secondary reflections which are reflected from the near surfaces or secondary reflections which generate an echoing field. Usually the sound pressure or the direction of the reflected sound must be performed by mounting acoustic panel on the wall or hanging them from the ceiling.
By adding sound panels you can easily modify the echoing sounding area which allows creating a comfortable and relatively effective communication.
The generic types of echo are as follows:
The direct or initial sound is the sound coming from the mouth, from the device by which the music is played or from the loudspeaker. This sound is usually the most important.
These reflections occur when the sound reverberates on the near walls. As they usually arrive some seconds after the direct sound, they may disturb the so-called phase interruption or comb filtering, and may make the understanding of human voice more difficult.
The control of the primary reflections is usually the first action plan. Decreasing the reverberation time is usually about the sound absorption of the room. The more broadband acoustic absorbers you mount the more energy they will absorb.
Rattling echo without broadband acoustic absorber:
If you clap your hands in an empty room you will hear fluttering sounds on the wall, the ceiling and the floor. Mainly the parallel wall surfaces cause the echo of clapping and allow the retention of the echo.
It is easy to decrease the fluttering echo if we put broadband sound sound panels opposite each other on parallel walls so that the echo cannot survive.
We mentioned broadband absorber above because the clapping is above 1000Hz therefore we can dampen this frequency more evenly only with the broadband absorber. Therefore, we cannot control the reverberation of lower frequencies.
Secondary reflections or echoes
You can hear this long echo in an old church. If we go back in time before the area of modern loudspeaker systems we can see that the church created the big and spacious surround sounding with deliberate reflections to convey their message. It is particularly effective if you listen to a choir or Gregorian songs.
Classical music makes advantage also from a long and echoing space as it enables the musical instruments to stimulate the room. The control of reverberation time is usually the question of increasing the sound absorption capability of the room.
The more sound absorbing panel you mount the more energy they will absorb.
Sound absorption with acoustic panels
When you play music loud put your hands on the loudspeaker, the floor, the near furniture or even on the window and you will feel the vibrations. The sound energy goes through the solid materials or fluids in the form of vibrations and when it sets the medium in motion it will inevitably produce heat.
The sound absorption is in fact an energy transfer function. The scientific term of this phenomenon is called thermodynamic transfer.
The use of materials is very important
When the sound penetrates the broadband absorber the acoustic rock wool fibres within the panel are vibrating. The same thermodynamic transfer is generated inside the rock wool.
As we talk about a high-density fibrous structure the sound that goes through it will significantly decrease and will transform to heat due to its density. Therefore, by using broadband absorbers the echo will quickly decrease in a room.
Absorption of law frequencies with sound absorbing panel:
In the world of acoustics it is difficult to decrease the bass or the low frequencies due to their long wavelength. Without sufficient density the bass will go through almost everything.
That means that the production of bass not only requires more energy (think of the elephant or mouse) but it is also much more difficult to stop it once it has started (think of trying to stop a freight train compared to a bicycle).
The high frequencies cause much less problem as the shorter wavelength is much less strong than the low one. That is why the light specific weight foams absorb this sound energy easily but the bass go through them easily.
The simplest way to absorb low frequencies
The simplest way to absorb low frequencies is increasing the thickness and density of the panel. We can more or less estimate the necessary thickness of the Sound absorbing element by using the calculation of quarter wavelength.
But the actual acoustic testing usually shows surprising results. Unless the panel is dense enough the bass will pass through smoothly.
This is the problem of low-density foam, it is ineffective in absorbing the bass. On the other hand, if the bass traps is too dense the high frequencies will simply be reflected back into the room.
The Sound absorbing element are designed for both, they provide balanced absorption.
The bass contains more energy:
These two charts compare the low and high frequencies with the same amplitude. Take into account that the longer low frequencies contain more energy as it is shown in the yellow area. As there is more energy in the sound wave it will be more difficult to absorb or regulate the bass.
Calculation of quarter wavelength
We call “calculation of quarter wavelength” the mathematics used for prediction of the low frequency performance of element where the thickness of the panel equals to the 1/4 of the wavelength of the lowest frequency plus the factor of the angle of incidence. The thickness of the panel plays an important role.
Solution to the acoustic problem
The actual process can be simplified to four steps:
⦁ Specify the problematic range.
⦁ Chose the appropriate acoustic element to solve the problem
⦁ Estimate the amount you want to spend and the budget
⦁ Install the Sound absorbing element in the strategically most important areas for the maximum efficiency.
First you should determine the place where there are problems in the room by identifying the problematic frequency . In other words, you should consider what frequencies you try to absorb before you simply mount some bass traps the wall and expect them to operate the way you envisaged.
For example, in a studio it is very important to balance the sound absorption in the whole sound range in order that your recording sounds good also with other audio systems. In this case we wish to create a really neutral listening environment.
The desired linearity is important in the room also in case of home theatres where you should ensure that all central channels that are in dialogue be crystal clear.
In a classroom, council room or office the human voice is transmitted therefore the acoustic treatment must take into account also this sound.
The next charts show the sound measure of a typical human voice then they show how the sound energy changes depending on how loud it is. You will notice that by the increase of sound level the energy will increase in the middle band.
The chart shows the range of a typical male voice, it keeps the most of the energy in the middle range of 400Hz and 1000Hz and the harmonics extend to 3500Hz.
If we take a closer look we can see that the most energy of human voice is between 300Hz and 1500Hz.
This is why it is important that when it is time to choose the appropriate panel for the task you should chose the one which actually operates within this range such as the broadband acoustic panel.
Only for speech we do not advise to choose leather membrane which dampens the sound only under 600Hz as the upper frequencies would freely flutter in the room.
Similarly, it would be a wrong decision to choose an acoustic foam because these absorb the sound only above 800Hz.
In this case the best choice is the broadband bass traps:
In the case of absorber usually the sound absorption coefficient determines the selection of the appropriate sound absorption panel. The specification shows that if the value of sound absorption is 1,0 in a given frequency the acoustic panel will be able to absorb the sound in 100% in the given frequency. The value 0.5 means absorption of 50%.
About acoustic foams:
The broadband absorbers is made of high-density acoustic wool of 90kg/m3, while the most of foam panels are made of low-density polyethylene of 10-15kg/m3. As people cut the most part of foams to create an artistic design the density of the foam will further decrease due to the huge air gaps.
The final density of the majority of foams is no more than slightly above 8kg/m3. As the density of broadband acoustic panel is more than 10 times of that no wander that the polyethylene foam cannot absorb the bass.
Therefore, it is not true if anybody states that the acoustic foam or xps can absorb the bass even with results measured as a bass trap. Physics may not be belied whatever publicity text is behind it.
Let’s move on
If we go further and compare the sound energy with that of the most acoustic panel we can clearly see that the broadband elements provides absorption of 95% up to 100 Hz while the foam panel can absorb 95% at not lower than 1000Hz.
This emphasizes the importance of dampening our target frequency in the appropriate frequency range. (Most of the popular manufacturers offer a wide range of products. Before taking a decision check the technical description, the use of material and the weight of the product!)
Depending on what kind of frequency or noise you try to absorb you have to choose an bass traps which operates within a given frequency range. The thickness of acoustic panels is usually of 6-11-13-20cm.
People usually use the thinner acoustic panels for absorbing the middle and high frequencies while the thicker panels from 11cm absorb the low frequencies as well.
Selection criteria for acoustic absorber:
1. 6cm for higher sound range in offices and restaurants or for rattling echoes in studios
2. 11cm in musical locations and for controlling the primary reflections where we want to dampen in the broadband range.
3. 13cm 20-30-40cm for absorbing the bass, where the broadband sound-proofdo not dampen the low frequencies sufficiently.
Suppose a dot matrix printer causes a problem in an office where the high frequency echo is reflected from the wall and causes annoying reverberation. As this grating problem is mainly in the higher frequencies the thinner broadband sound-proof of 6cm would be the correct option.
Multifunctional living room or hall used sometimes for dance classes and also for community meetings. You have a limited budget. Broadband sound absorbing element in thickness of 11cm which provides excellent sound absorption in the whole bandwidth of the sound range, thus these panels may be the right choice and are very efficient for the music.
Determining the coverage:
Put a small acoustic panel in a high-school and you will probably will not hear any change. Put one million panels on the wall and ceiling and the sound will be completely dead. Most of the room treatments is somewhere between these two. That is why it is advisable to follow the so-called reverberation curve.
The reverberation time indicates the coverage of the wall surface and the sound absorption. The efficiency will slowly increase by increasing the coverage of the wall or by increasing the number of panels in the room.
After a point the room slowly turns from a cave-like echoing chamber into a comfortable environment. Thereafter the effect will decrease by adding further panels and adding panels does not mean a significant advantage anymore. You have reached over-dampening.
There are no absolute rules regarding “how many sound panels will perform the required work”. In case of “speech” where the maximum understandability is required the acoustic engineers usually talk about a reverberation time shorter than 1 second. In bigger rooms it can be longer.
In case of concert halls for classical music the long reverberation is usually preferred because the instruments combine the atmosphere by stimulating the room and the audience.
A reverberation time curve:
Reverberation time in halls of different sizes
The application, the common sense and the preferences determine the extent of coverage. For example, if you work in a studio you can mix rather in an enjoyable environment.
On the other hand, if you treat a church where the spoken word and rock bands alternate the bigger sound absorption can be more useful. Start from 10% with a coverage of 20%. If you are unsatisfied simply add some more. It is really so easy.
Comparison of different acoustic element:
When dealing with the acoustics of a room we usually try to control the sound reflections by mounting diffusers on the wall and ceiling where they are the most effective. This is usually performed with absorber. You can choose from several types of acoustic panels on the market. Each of them has its strength and weakness.
In order to facilitate the choice of products we present each product and offer them with our opinion and the comparison of the products.
Written by Róbert Polgár
If you have any comments, please contact the contact. Written by Róbert Polgár
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