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 a 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.
By GIK Acoustics:
Normally when I give a lecture, I will begin with answering why we even need to treat a room at all. I demonstrate this by using two sound files of the same recording, using the same equipment, in the same room. The first sound file was recorded with the room empty (untreated) and the second was recorded using acoustic panels and bass traps. Even on a simple demonstration like this the audience is clearly able to hear the difference between the two recordings. In the first example where the room is untreated, vocals are echoey and the bass is muffled. Once room treatment has been applied the echo is removed from the vocals and the bass is cleaner and punchier. It’s a simple demonstration but what it does illustrate is: the room you are in effects how you hear sound.
A little math goes a long way
So, starting at the beginning we need to look at what the sound coming from your speakers actually is. What makes the music you are hearing? The notes in music all equate to a frequency. Technically a frequency is the number of soundwaves that pass a particular point in a fixed amount of time. We are all more familiar with Hertz, however which is more specifically the number of waves that pass by in a second. So, 220 hertz is 220 cycles which is also known as the note A3. A soundwave isn’t static, so we are also interested in the amplitude of the wave. This is how far the wave moves from its average position, i.e. how far the particles are displaced. The reason the amplitude is important is it comes across to the listener as the loudness, or volume. So, we have the musical note (the frequency) and then we have how loud you hear it (the amplitude).
As you can see in the drawing above: the top of a soundwave is referred to as a peak, which represents the loudest part of the soundwave. Then we have the low part of the wave, which is called a null, which is the quietest part of the soundwave.
A wavelength is measured by the distance between two peaks (or nulls).
You can work out a wavelength from the frequency and speed of sound (343m/s)
Wavelength = Wave Velocity divided by Frequency
So, if we take 220 hertz
It would be 343ms / 220 = 1.56
So, the wave length of 220 hertz is 156cm’s.
A very low wave like 40 hertz would be
343 / 40 = 8.56 or 856cm’s (8.56metres)
Now that you have a basic understanding of the physics of a soundwave, why is this relevant to your room? In basic terms: room modes are pre-existing resonances created by the room’s dimensions. There’s no way around it. The size and shape of your room dictate how soundwaves behave and react. Especially in the low end of 200hetrz and below.
Room modes are activated when you play music and the soundwaves from your speakers hit the boundaries in the room. If the boundary is the same length as (or half or one quarter of) a soundwave, then they create what is known as standing waves. A standing wave occurs as a result of two different waves moving in two opposite directions. As they pass, they create interference which then make our nulls or peaks. This is because the boundaries are stopping the soundwave from fully decaying as they would if there were no boundaries and the energy remained in the room.
There are three types of modes in a room
- Axial modes are created between two opposite surfaces. Best to think of this in terms of length, width and height of the room
- Tangential modes are created between four surfaces in the room, most commonly see problem with this in square room
- Oblique modes are created by six surfaces, less common but I have seen this type of mode created in Bay windows for example
- Axial modes are the strongest and many times, the only ones that are considered. Tangential and oblique room modes have less impact per mode but are also more prevalent. A combination of tangential and oblique modes can cause just as many issues as axial modes can.
Peaks and nulls and what to do
Room modes can cause both peaks and nulls (dips) in frequency response. When two or more waves meet and are in phase with each other at a specific frequency, you will have a peak in response. When they meet and are out of phase with each other, they cancel, and you end up with a dip or null in response.
Dealing with modes is accomplished by placing sound absorbers at a room boundary to minimize the reflections off it so there is nothing to combine or cancel. These boundaries include, but are not limited to: floor, ceiling, front wall, side walls, back wall, and all twelve corners. While treating corners is not a complete solution, placing treatments in corners offers the advantage of being at the end of 2 or even 3 of the room boundaries so there is a lot of benefit to treating that area. However, in some situations there are modal issues which require treatment of the rear wall or even the ceiling over your head that treating corners would not solve.
In conversation with GIK Acoustics President Glenn Kuras he mentioned the following analogy which can really help to understand how sound works in a room. He likened the room modes to as the shape making a ‘tone’ like a hollow block of wood. The shape and size dictate the sound and the lowest frequency it supports. Also, as you dampen the sound you are changing the frequency, but really the major effect is stopping it from ringing/sustaining. If you filled a small hollow box full of rags it would dampen/absorb the ringing and change the frequency, but more damping of the ringing then changing of the frequency which you want.