The importance of room acoustics
No matter how much quality equipment you’ve acquired for your studio, all that technology will do you no good in a space that isn’t acoustically treated. There seems to be a common misconception that soundproofing and acoustic treatment are interchangeable terms. However, there is a clear difference. Soundproofing deals with blocking out sounds from entering or leaving the room, where treatment focuses on controlling the sound waves within the studio. There are very technical and precise ways of measuring your room’s acoustic qualities. However, no math is necessary to improve your listening space. This article will give you a general understanding of the three key elements involved in treating your room. Besides, you can learn how to use them: bass traps, acoustic panels, and diffusers.
With all the fuss surrounding acoustic treatment, you inevitably arrive at the question – “Why should I treat my space?”
In project and home studios, treatment is a necessity because of the nature of sound waves in small rooms. They propagate and reflect in ways that cause interference.
Room Modes and Standing Waves
Room Modes and Standing Waves will affect the formation of low frequencies in your listening area and, ultimately, how you perceive them. The wavelength of a frequency is the speed of sound (344 meters per second) divided by that frequency in Hertz. This means the lowest audible frequency (20 Hz) has a wavelength of about 17 meters! In certain conditions, the length of the wave matches a dimension of the room it is in exactly. This can create a standing wave if no obstructions or absorptive materials are introduced. This means the reflections of the wave will bounce back and forth between the two fixed boundaries (two parallel walls is most problematic).
This periodic overlapping of similar sound waves causes both destructive and constructive phase interference (See Blog: Phase and Phasing). Thereby, introducing peaks or nulls in the amplitude of that particular frequency that is not in the source. Rather, they are a result of the room. The calculated frequencies that will cause standing waves in a given space are called Room Modes. They can occur along all three-dimensional planes and can involve more than just two surfaces.
Speaker Boundary Interference Response (SBIR) and Comb Filtering
When we listen to music in our studio, we receive the signal originating directly from the speakers as well as the reflected signal coming off the walls. SBIR is the result of the reflected and direct signal arriving at our ears simultaneously. The slight difference in arrival times often creates constructive, destructive, and comb filtering effects. Comb filtering happens when the timing differences cause alternating constructive and destructive interference. It introduces peaks and nulls in the spectrum of a complex sound that was not there before. This is what comb-filtering sounds like on a triangle wave and a sine wave of the same frequency. You may have experienced or created this effect before. Notice there are obvious level changes – yet, I did not modulate level at all.
Both room modes and SBIR present some valid pro-treatment arguments. Let’s look more specifically at three elements: bass traps, acoustic panels and diffusers; and, see how we could integrate them into our space.
1. BASS TRAPS
Bass traps are a great starting block for two reasons. First of all, they deal with low frequencies; and – as I stated in this article’s introduction – the nature of low-frequency sound waves makes them highly problematic in small rooms. Secondly, (although bass traps are great for controlling low-frequency content) they are considered effective broadband absorbers. Meaning, they will also clean up artifacts in mid and high-frequency ranges.
The internet is full of DIY formulas for acoustic treatment which tell you how to make your bass traps. The alternative method, of course, is to buy one. Although this may be more expensive than making your own, the advantage of manufactured parts is that they have often been proven to satisfy certain expectations over years of practice.
These absorbers will act on the pressure of a sound wave. This property is highest at a boundary, which is the wall in this case. That is where you should place them. If the room already has sufficient treatment for higher frequencies, resonant bass traps are the appropriate choice. This is because they only affect low frequencies.
If your room is completely untreated and you are only allowed one single piece of treatment, you should probably get a porous bass trap. These are broadband absorbers that will greatly minimize the effect of room modes and standing waves in your listening environment. The key is to place them at a calculated distance from the wall, leaving an air gap. This is because porous absorbers act on the velocity of the wave, which is the highest a quarter of a wavelength away from the wall. Once you have identified the most problematic frequencies below 150 Hz in your room, you can determine the optimal placement of the absorber. You can identify these by measuring the dimensions of your room and dividing 344 by them or plugging them into a free room mode calculator online.
Again, due to the nature of low-frequency sound waves, bass builds up in the corners of your room. This is often precisely where bass traps should be placed – ¼ of the most problematic wavelength away from each cornering wall. With porous absorbers, you can even use low-density materials such as mineral wool in the space created between your wall and the absorber to combine the benefits of porous and resonant absorbers.
2. ACOUSTIC PANELS
Acoustic panels are secondary to bass traps because they have little to no impact on the low-frequency sound. Despite this, treating with acoustic panels has its benefits. They are relatively cheap, and they can attenuate mid-frequency standing waves between parallel walls rather than just in corners of the room. There are hundreds of ways to make your acoustic panels and just as many available to buy online or in stores. Keep in mind; effective placement is more important than the specifications of the device.
On a budget, the most effective placement of acoustic panels is on the walls behind and next to the monitors. This will greatly reduce SBIR – as it will attenuate reflections. The remaining reverberant signal will not noticeably interfere with the direct signal because it will have lost some energy and will take significantly longer to reach your ears.
A thorough application of acoustic panels will have a greater effect on your space. As a matter of fact, the conventional opinion is that between 10% and 25% of your wall space should be covered in acoustic panels. This won’t affect SBIR so much as the general reverberance of your room. Having a dry room will enable you to hear your mix “objectively.” However, having a space that is TOO dry and absorptive is unrealistic. So, you’ll probably end up adding too much reverb to the mix. The majority of your fans will not be listening to your music in this type of environment. This topic leads me to diffusion.
Diffusers should only be applied as a method of fine-tuning after the other two have been implemented. The previously mentioned absorbers all help to dampen the sound and reduce the effect of reverberation on the listener. The issue that can arise from this is a listening environment that is too dry. A certain amount of reverberation is necessary for the sake of realism. Music will always be heard in a reverberant space. If we mix a song to the standards of excessively dry space, it will inevitably sound too reverberant anywhere else; this is where diffusers come in.
Contrary to acoustic panels, diffusers don’t absorb sound by converting it to heat. Instead, they help scatter reflected sound waves throughout the room such that they are non-coherent. This reduces the effect of SBIR whilst conserving the reverberant qualities of the space. Their placement in the room greatly depends on its shape and where sound waves are most likely to reflect. Like acoustic panels, they should be used on walls that are parallel to each other, in an orientation that is symmetrical to the optimal listening position (wherever you sit).
I hope this article has solidified your understanding of basic acoustic treatment and how it can be implemented in your listening space. Remember, the internet is full of resources for making your own absorbers – and, most of them are very good!