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11 Aug 2020

How sound works in your room - small room acoustics - part 2

producer How sound works in your room - small room acoustics - part 2

This article is written in collaboration with GIK Acoustics, together with GIK we're going to release a series of 7 articles where we help you to understand more about acoustics, how it works and how to treat a room. GIK Acoustics has it headquarters in the USA and UK, they develop an own line with state-of-the-art sound absorbers, diffusers and much more! GIK sells directly world-wide though their own website. Their website is super extensive and covers so much information for studios with advice. So have a look on their website and discover your studio solutions, any questions about this subject? Let us know below this article.

Read part 1 here.

By GIK Acoustics:

For our second article we are going to look at low end issues that you can (and will) experience in your room. We are starting here rather than say first reflection points or getting into the nitty gritty of diffusion and the art of scattering because this is where all your problems really do start, get to grips with the lowest common denominator and you are well on your way to getting a great sounding room and secondly because it is the hardest part of the room to get right. You need exactly the right products placed in the correct position to get the maximum absorption. It may be the trickiest part of the room to get right but it can be a fascinating journey learning about your room and achieving the desired results.

Last time we talked about how the shape and size of the room itself can affect how you hear the sound in the room and this is certainly one of the biggest influences on low end problems in a room but it is not the only factor that creates low end issues but we will look in those in more detail in later articles. 

How Does it Work?

What do we even mean by absorption? To understand it we have to take a step back and look at the Law of Conservation of Energy. This law basically states that energy can neither be destroyed nor created but only transformed or transferred from one form to another. So even though the energy can change from from a liquid to gas and back again the amount of energy actually remains constant.  The energy itself has not changed only the way it is perceived. 

When it comes to room acoustics this principle is being applied to sound. Here the motion of gas at a certain temperature is being caused to slow down via friction. This friction gives off heat thus conserving the total amount of energy and reducing the strength and intensity of the waves released. This is known as acoustic attenuation. 

Types of Bass Traps

Bass traps work by providing resistance to the sound waves bouncing around the room, so our bass traps need to provide a good resistance to these soundwaves often referred to as the gas flow resistivity of the material. In general, most bass traps fall into 2 different categories

Velocity Based Absorbers

These are the most common type of absorbers as it based on porous absorptive material because of this you will often see them referred to as porous bass traps.  Basically, a trap like this will work simply by the conversion of that energy into heat as previously explained. In general, this type of trap will absorb across a broadband of frequencies. The density, thickness and type of material will define which frequencies that they absorb and how much. Generally speaking this type of absorber is applied when a broad range of frequency absorption is required (which is most small rooms).

Pressure Based Absorption

This category actually includes a number of different types of absorbers, but the commonality is that they are pressure based. They are also known as resonant absorbers, tuned traps, pressure-based traps or narrow band traps. These are generally much more complicated and use factors such as depth, mass, size of air cavities materials all into account to deal with the absorption of specific very low frequencies. The key with pressure-based adsorption is to understand the purpose and scope of when, how and where to use them. So, understanding your room and how it effects the sound is essential. These traps deal with peak pressure points and can be used in conjunction with broadband treatment. 

 

GIK Acoustics Scopus Tuned Traps. They look very similar to the porous absorber but are constructed completely differently using pressure to tune the traps in a narrow bandwidth. They are designed to be as compact as possible.

Let’s get technical

The best way to understand how this the pressure-based absorption trap works is to break it down into two further categories; the tuned membrane bass trap and the Helmholtz Resonators

Tuned Membrane Bass Trap – One of the advantages of these narrow band bass traps is that they are very compact. These work by creating a depth and sound pressure to a quarter wavelength of the frequency you are trying to absorb. Depth of the trap, the density and type of membrane used all effect the pressure within the trap to tune it to the right frequency. 

Helmholtz Resonators – Most people are familiar with the concept here and many of you will have seen the demonstration of how this works in principle by blowing across the neck of a bottle. This works because of the way air works and that it has a natural ‘springiness’. By this we mean that when you compress air its pressure increases and then it more or less goes back to its original form. The frequency of resonance is determined by the volume of air cavity, the length and diameter of the neck and this then absorbs the frequency you require.

How Do I know which bass traps to use?

Once you start looking into bass trapping and visit several different manufacturers websites you will possibly be more lost than ever. Which bass traps do I need? Although we have described just the two main types of bass trapping there are various different designs available using a combination of these theories, products like wood faced diffusion panels actually use a form of resonation in their design caused by vibrations leading to frictionless losses. 

The simplest answer to the above question is to take acoustic advice from a respected company and they can talk you through all the aspects of a room and how to treat it. 

For now, let’s jump forward a step and presume we do know which traps we want to buy, how to decide which one would be best for your room?

Reputable brands of acoustic treatments will have had their products tested in an independent laboratory and this is how we choose the correct treatment. Tests can come in different forms depending upon the type of product, where it was tested etc. but ultimately, we are interested in the Absorption co-efficient. This measures how well a material absorbs different frequencies. A perfect sound absorber would absorb 100% of a frequency and if it was to perfectly reflect a frequency then it would be 0%. These figures are sometimes written as a figure between 0.0 and 1.0


The above data is taken from an official ISO test conducted by University of Salford on GIK Acoustics 6A Alpha Panels. The Alpha panel is a porous broadband absorber with an 8mm wooden resonator in front. Official tests like seen above are conducted in a certified reverb room and need to cover 11 square meters, flat to the ground. This is known as an A mount test. You will also often see J Mount tests. This is where the same panels are tested in the position they would be used (i.e. Manufacturers recommendation) rather than lying flat on the floor. All the numbers above 1 are 100% efficient. So, in the above example, the panel is 63% efficient at 100hz and 100% efficient at 125hz. This is a great broadband absorber that doesn’t over deaden the highs and can go very low for a panel style absorber. Another factor to consider is the absorption area, the more surface area you have the more that it can absorb. 

A simple demonstration for this is to consider 2 panels

  1. 100mm thick absorbs down to 125Hz
  2. 200mm thick absorbs 63% at 100 hz and 100% at 125Hz

If you had a problem with 100hz it would not matter how many Number 1 panels you placed in the room. They would never absorb 100hz. However, we can calculate (based on your room dimensions) how many panels of number 2 we would need. Above 125hz the panels may then perform very similarly. 

I always recommend that you work with companies that have these independent test results for two reasons 

A) It is a certification to say that the product you are buying performs to a certain pre-defined criterion

B) Without this information these companies would not be able to advise you on how to deal with your room. It is essential to know how each product performs to be able to recommend the most effective acoustic solution for your needs. 

Where to put them

Again, this isn’t a straight forward question or answer. It never seems to be, does it? This isn’t us making the subject complex instead our articles are designed to try and bring the complex physics of sound as it applies into our small listening rooms into something we can practically use in our own spaces.

Bass trap positioning can broadly be placed into three positions or categories

Corners

In an ideal scenario we would recommend placing bass traps floor to ceiling in all available corners but why is this? Firstly, we need to understand what we mean by a corner. A corner is anywhere where a wall meets a floor or a ceiling, so there are 12 corners in a room but only 4 tri corners. Bass builds up in corners as this is where two room modes meet. Why corners have such a build up of bass is more straight forward it is here that all room modes end. You can actually hear this in action yourself by just playing some music and going and listening in the corner. You will hear an increase in the low end. 

Walls

Bass traps can also be placed on the walls and specifically the back walls. This is top deal with non-modal standing waves. These are basically standing waves created by your position within the room. As you move forwards and backwards though the room you will hear peaks and nulls of different frequencies so treating the back wall will prevent this. As the back wall is often the longest mode in the room then thick treatment is aloes required to deal with this. 

Places of high pressure

When we are concerned about individual low frequencies, we are trying to absorb then it is essential they are placed in the correct position, specifically in the areas of high pressure for the frequency the trap is tuned to. 

 

An example of a 2 Channel listening room set up. Note the floor to ceiling traps in the corners and the thick traps on the back wall.

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