Home » Velocity vs. Pressure – Acoustic Sound Absorbers

Velocity vs. Pressure – Acoustic Sound Absorbers

When first starting out, a lot of home studio owners don’t use acoustic treatment. Sooner or later, though, there comes the time where we want to improve our recordings. One of the best ways to do that is by putting in sound absorption. This article is about comparing two main types of acoustic sound absorbers – velocity vs. pressure – and which type you actually need.

The main difference between these two types of absorbers is how they work. Velocity absorbers with with friction and pressure absorbers work with pressure. Velocity absorbers tend to be broadband and work over a large range of frequencies whereas pressure absorbers usually work with a more narrow range. If you’re building a home studio, you need a lot of broadband velocity absorbers and probably no pressure, also known as resonant, absorbers. Let me go into more detail so that, by the end of this article, you can understand why.

Velocity Absorbers

Velocity absorbers have many names. They’re also called ‘frictional’ and ‘broadband’ bass traps. These are the most common type and what you should be using in your studio.

They work by turning the movement energy of a sound into heat. Think of a group of soldiers running through a field. The best way to slow them down is to put a forest in the way. This is how to visualize what’s happening with velocity absorbers.

The reason you don’t just put a wall in the way is because the sound will just bounce right back. Frictional absorbers are made up of millions of tiny fibers with space between them. They allow just enough air to pass through that it slows down and doesn’t bounce.

These acoustic absorbers work over a wide range of frequencies. Home studios have problems all through the audible range so these are perfect for that. Their weakness is that they need to be very thick to totally absorb very low frequencies.

Pressure Absorbers

Pressure absorbers also come in many flavors. Some of the most popular are diaphragmatic absorbers and Helmholz resonators. These are special types of absorbers and not as common in home studios.

Diaphragmatic absorbers work by having a skin called a membrane which moves along with a sound wave’s pressure. Since a sound wave has both high and low pressure, the membrane moves along with it. Inside the sealed box this membrane is attached to is our old friend the velocity absorber. By having the membrane, we can use the pressure changes from the sound, turn that into moving energy then let the frictional absorber do it’s work.

Another kind of pressure absorber is the Helmholz resonator. This is actually a common thing and you experience it when you blow into a glass bottle. Whatever note that bottle rings at will cause a spring effect to take place whenever that sound happens in the area.

So imagine you have a very big coke bottle which rings at F1 when you blow into it. There’s air around the opening and there is air inside the bottle. If a bass player plays F1, the sound pressure will go up and down and will push and pull the air near the opening in and out of the bottle. That air acts almost like a slippery cork and the air deeper inside acts like a trampoline. Push the air in and the trampoline will bounce it back as the wave’s pressure decreases.

One Kahuna, Many Faces

You might get how a wave can have pressure and movement – we’ve all seen surfers. The thing is with sound, if you were so slow it down, the parts of the wave that are fast aren’t the parts that have the most pressure. Let’s take a look at a picture so you can see.

velocity and pressure are 90 degrees offset from one another which gives rise to the so-called "quarter wavelength" rule for using frictional (velocity) absorbers.
Acoustic Waveform – Pressure vs. Velocity

As you can see, the two parts aren’t totally lined up. In fact, if a wave’s cycle is 360 degrees then the difference between the pressure peaks and the velocity peaks are just one quarter of that, or 90 degrees out of phase. This is why some people say that you shouldn’t put broadband bass traps against a wall.

While that might be partly true, and using a gap between frictional absorbers and a wall is helpful, broadband bass traps work at nearly all frequencies. Even a 4″ rigid velocity absorber can have some effect in the sub bass range. While it won’t be as much as a tuned resonant absorber, it will be more effective in the ranges higher up.

Because the boundaries of your room line up with certain frequencies, you have waves which fit perfectly inside them. These are called standing waves. Normally, these are the frequencies where you’d tune your resonant absorbers. However, many of your longer dimensions line up with very low frequencies that don’t need to be treated anyway.

Velocity vs. Pressure: How Low To Go?

There are several sources saying that the most important frequencies to treat are above 50-60Hz. Sadly, many beginners don’t know this so they go down the rabbit hole of resonant absorbers to tame a 40Hz room mode. Let me show you the European Broadcasting Union recommendation for sound decay in a critical listening space.

decay times of EBU Tech 3276-1998
Sound Decay Times – EBU Tech 3276-1998

Clearly, the decay times in the bass range below 63Hz are not specified. One can imagine that they can be as long as they like. Since broadband treatment has to be very thick, up to 6′ in some cases to totally absorb this range, we can forget about using it for that.

But like the image shows, we don’t need to treat that range as much. Broadband absorption works very well over a wide range of higher frequencies. These are the frequencies that matter most to us as listeners and audio professionals alike.

Velocity vs. Pressure: -60dB

By now, I hope you understand the difference between pressure and velocity acoustic absorbers and which kind you need for your home studio. Unless it’s a pet project, you can forget about pressure absorbers and just focus on broadband velocity types. If you have any questions, feel free to contact me on social @michael

Also remember that I give my music away to Hexie Dose Newsletter subscribers. I just put out my third EP, Eight Bar Embryos Volume 1, at the end of 2018. It’s a collection of six tracks featuring synths, drums & my own brand of songwriting. Check it out, I think you’ll like it!


Special thanks to everyone at the Audio Expert Forum for years of acoustics chat!