Pressure Vs. Velocity Acoustic Absorbers
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When acoustically treating a room we have two major forms of acoustic treatment. The first method is using velocity absorbers and the second method is using pressure absorbers. To fully treat and balance a room we need to use both tools for two different purposes. In this article I will go over what each absorber is and how to use them.
1) Velocity Absorbers
Velocity absorbers are what you most likely think of when you think of acoustic panels. They use insulation and an air gap to absorb sound waves and reduce reflections back in your room. Velocity absorbers also include foam panels which absorb sound through transfer to heat much like fiberglass insulation.
2) How Are Velocity Absorbers Used
Velocity absorbers are great and affordable for treating your room, however they do have limitations. They are great at absorbing higher frequencies above 125Hz, but do very little at absorbing frequencies below 125Hz.
The key attribute all velocity absorbers have is a porous composition. The air space within the material is what turns sound energy into heat because the fibers vibrate and create frictional resistance, which is then transferred to heat energy (Everest, Pohlmann 191).
3) Effect Of Thickness With Porous Velocity Absorbers
According to The Master Handbook of Acoustics, "there is little difference above 500Hz as thickness is increased (Everest, Pohlmann 195). However, there is "considerable improvement below 500 Hz as thickness is increased" (Everest, Pohlmann 195). A four inch thick fiberglass panel of 3-lb/ft3 density has nearly perfect absorption over the 125-4k Hz region (Everest, Pohlmann 195).
4) Effect Of Airspace Behind Porous Acoustic Panels
Adding a three inch airspace behind panels will increase its absorption. It is a best practice to leave an air gap between the fiberglass and your wall. In general the farther the panel is placed from the wall the lower frequencies it should absorb. As a general rule with velocity based traps a panel placed a 1/4 wavelength from the wall is where absorption is greatest (Everest, Pohlmann 195).
For example, if we estimate the speed of sound to be 1,125 ft/second in our room then a 100 hz wavelength = 11.25ft (1,125 ft/sec / 100 Hz).
A quarter wavelength of 100 Hz is then 11.25/4= 2.8125 feet. This would mean your velocity panel would need to have a nearly three foot air gap behind it to maximally absorb 100 Hz.
Now most people cannot give up 3 feet on all their walls and ceilings to absorb down to 100 Hz. This is where pressure traps come in.
However, before I dive into pressure traps. It is important to realize that velocity traps, although not efficient at absorbing sound below 100 Hz still absorb some in that area. This is very important because our goal as acoustic designers is to get the room as good as possible with velocity based absorbers as we can and then move to pressure based traps to sniper out some of the low frequency problems in our room.
5) What Are Pressure Based Absorbers?
Pressure based absorbers also known as resonant or reactive absorbers use a different method than velocity absorbers to absorb sound. There are two main types of pressure absorbers: Panel or Diaphragmatic absorbers and Helmholtz Resonators. These two types of tools can pinpoint problem frequencies in your room below 125 Hz where our 4" thick velocity traps are not as efficient.
Notice that I said pin point. The down side to pressure traps is that they have a narrow frequency band that they absorb. They can be tuned to attenuate specific problem frequencies in your room.
For this reason, I always recommend that designers begin by treating their room with plenty of porous absorbers before going down the path of using pressure traps.
Pressure traps are great at reducing room mode issues. Every room will have low frequency resonances that can muddy how we hear the bass in our room from what is actually coming out of our monitor speakers. In order to fix these low bass mud issues we can target a specific room mode issue with a tuned trap.
6) Panel (Diaphragmatic) Absorbers
Panel traps are great at absorbing lower frequencies. They are essentially a wooden sealed box with a layer of fiberglass inside. The mass of the panel and the air spring created will absorb sound as the material is flexed and vibrates. There is additional absorption through damping by the insulation (Everest, Pohlmann 195).
Another form of panel absorption are Polycylindrical absorbers, which work the same as a flat panel absorber, but have the added benefit of creating a diffuse sound field because of their semi cylindrical bulkheads.
7) Helmholtz Resonators
Helmholtz resonators are cylindrical tubes much like a bottle. They absorb sound by using the air in the cavity as a spring that creates a resonant system. Just like blowing across the top of a bottle produces a resonant tone, helmholtz resonators also have a resonant tone and absorption is maximal at that resonant frequency.
Helmholtz resonators can be tuned to target specific problem frequencies in your room. I will say, in my research many people have warned that Helmholtz resonators are not easy to make and small errors in construction can render them useless.
8) Conclusion
Acoustically treating a room is like an onion. You start on the outside and get good results and gradually work your way towards the center getting more precise and more complicated as you go. For most DIY studios using velocity based traps will go a tremendously long way towards fixing your room issues. I also love room eq software like Sonarworks Sound ID to correct your speakers to fix issues in the low end. This can help mitigate problems when you are mixing or mastering.
However, once you get your room dialed in as best as you can with velocity porous absorbers, next you will need pressure traps to reach those lower problematic room modes. To find your room modes you will need to test your room using a measurement mic and software like Room EQ Wizard. This will tell you what is wrong at your listening position and what room modes are ringing out at your listening position.
From there you can build pressure traps to target those specific problem frequencies.
Works Cited
Everest, Frederick A., and Ken C. Pohlmann. “Absorption.” Master Handbook of Acoustics, McGraw-Hill, New York, 2015.
