Introduction
Hello everyone! In this blog, we will learn about bass absorbers, also known as "bass traps".
We will explore how they work, how to use them, and even demonstrate a live demo where we can see an increase in the sound levels of the absorbed frequencies instead of a decrease.
We at Consonance Acoustics specialize in designing spaces and acoustic panels to enhance sound quality. We have designed patented absorbers that improve speech intelligibility and address room modes. If you need assistance in improving acoustics in your space, feel free to reach out to us.
Room Modes
When considering the acoustics of a space, it is important to take into account the concept of "room modes". In our previous blogs, we explained how air particles vibrate to create sound.
The distance that an air particle travels is called the wavelength, and the number of times it travels that distance per second is called the frequency.
What happens when the distance between the extreme points of the air particle vibration and the distance between the walls of a space are the same?
The air particles will stop near one wall and then move toward the other wall. At the midpoint between these walls, the air particles will be moving at their highest speed. Now, let's think about what happens when all the air particles begin to accumulate in one plane. The pressure on that plane will be at its maximum.
Conversely, when all the air particles are moving at their highest speed away from one plane, the pressure on that plane will be at its minimum. This pressure is known as the sound pressure! Its level is known as the sound pressure level.
To reiterate, when the distance between two parallel surfaces equals half of the wavelength, the pressure (and hence the level) near the walls is at its maximum, while at the midpoint it is at its minimum. This type of wave is referred to as a standing wave.
The effect of room modes
Now, let's watch how room modes act in real-world scenarios. This is not just a theoretical concept; it occurs in every room. We have Shubham from Sync Equips helping us in the demo today.
The distance between these two walls is approximately 3.7 meters. We are playing a frequency that has a wavelength approximately twice the distance between the walls. This frequency is roughly 47 Hz.
Using an omnidirectional microphone, we measured the sound pressure level, which you can see on the screen of the laptop placed on the table. Pay attention to the change in sound pressure level as Shubham moves from one wall to another.
Notice the drop in the sound pressure level for this frequency as we approach the middle,
and the increase in the sound pressure level as we move closer to a wall. This phenomenon occurs because the wall surface is hard, causing the air particles to reflect nearly perfectly and create a standing wave. This will happen for the harmonics of the fundamental mode too. You can observe a drop occur at a different position for 94 Hz. These positions are different for varying frequencies.
The effect of adding a bass trap
Now, let's consider what happens when a bass absorber is introduced. Shubham will move a version of our patented bass absorber across the room. Because the particles reflect with less energy, they won't travel at their highest speed at the midpoint of the walls. This may seem counterintuitive, but notice the increase in sound level at the same position!
Without the formation of room modes, you will notice a decrease in pressure near the walls but an increase in pressure (and thus an increase in sound level) for that frequency across the room.
As you can see, the sound levels do not behave as they did in the previous case. This demonstration highlights a key observation: no matter how good the speakers are, room acoustics will always be influenced by the room's geometry. There are often claims from speaker sellers that their speakers are exceptional and do not require consideration of room acoustics. Such claims are simply not true. What's worse is that even accomplished sound engineers have made similar claims. Without proper acoustical absorption or diffusion, it is impossible to control room modes. It's basic physics! Speakers cannot defy the laws of physics, they are built using them.
This demonstration also emphasizes another important point. Even if you increase the level of certain frequencies, such as through equalization (EQ) or adding another subwoofer, you cannot eliminate the imbalance caused by room modes. If you find that the bass lacks punch or sounds muddy, investing in an additional subwoofer may not solve the problem.
This demonstration highlights another key aspect. It is often believed that bass traps should be installed in the corners of a room. While this is roughly true in many cases, you could see that Shubham was not exactly at the corner of the room. The placement of the bass traps requires careful acoustical modelling often done by simulation software.
Real-world application
One of the best applications of understanding room modes is demonstrated in the tracking room of Epiphanies Media Studio.
Take a look at the response of the room to pink noise. It is extremely flat, even at as low as 20 Hz! This result has been achieved using our patented low-frequency absorber, which absorbs frequencies below 500 Hz only.
This approach ensures a balanced absorption without affecting frequencies above 500 Hz.
Another important application of this knowledge is in the design of home theaters. System integrators often add a bigger subwoofer or two subwoofers in the hope that it will add bass. It often doesn’t help at the listening positions.
This knowledge of acoustics can now empower you to make informed decisions when it comes to these systems. With that, I conclude. Please share this blog with anyone who can benefit from this knowledge, and don't forget to subscribe to our blog to stay updated with more such posts.
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