Internal standing waves – How to eliminate them from speaker boxes
How to deal with unwanted enclosure resonances?
Dealing with internal standing waves can be a pain. These are most often encountered when building floorstanding speakers. Since they have a more noticeable height, they have enough room for some standing waves to develop. Speaker placement also plays a role, but after you already made the enclosure and discover that the box has problems, what are you going to do? Fear not, there are solutions.
Device under test
In my recent build of floorstanding speakers, I encountered an internal standing wave problem. As a result, I decided to write this article on how I managed to mitigate it. Here is a picture with the actual build, so you can form an image :
The midrange and the tweeter are in one chamber, and the 2 bass speakers + port are in another chamber. The port is 70 mm in diameter and tuned to 25 Hz, and the bass speakers are Dayton Audio SD215A-88. (Amazon affiliate paid link)
How to detect internal standing waves
Depending on how bad your case of internal standing waves is, the resonance might be audible only on some parts of an audio track. Other times, it can overpower other frequencies and ring like a bell. But let’s not rely on our ears and do some measurements. The standing wave appears on my chart when I did a nearfield measurement of the port. This is more easy to follow, because otherwise you would have to use a CSD plot.
Normally, you would have just “the hump” around 25 Hz, which is the resonant frequency of the port. Then, it should slowly roll off to 300 Hz. Anything above 500 Hz doesn’t interest us. Also, the measurement is not accurate that high up in frequency.
You can see in the graph that there is a huge spike at 220 Hz. The output level is the same as the port at resonance frequency. This means that it’s audible even at the port’s peak performance. And you actually do hear it easily, even without this measurement telling me that there is an issue.
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Why does this happen? Standing waves usually happen if there is a correlation between the internal dimensions of the box and the half wavelength of any given frequency. In our case, the whole speaker is 1080 mm tall, but we are interested only in the internal height of the bass chamber, which is 750 mm. The wavelength of 220 Hz is 1.54 m. So if we divide that by half, we get 770 mm, which is roughly the same as our 750 mm internal height. As a result, there is your explanation of why we got an internal standing wave at 220 Hz.
How to deal with internal standing waves?
Considering that the enclosure is already made, your last resort is to use damping material. A lot of times people are tempted to use those egg-crate looking sponges. Those are only good in absorbing high frequencies. You see them mostly on music studio walls. For 220 Hz we need a thick pad of dense material.
For this project I used 5 cm thick mineral wool (Amazon affiliate paid link). Make sure it’s not fiber glass wool. While it’s very effective, it’s very harmful if you breathe it. Since we are dealing with a bass reflex enclosure, you cannot place a large quantity of damping material. However, if you have a sealed enclosure, I highly encourage you to fill it up with damping material. Maybe a different material, like polyfill. But in the case of bass reflex, apply material in the following order until you get a satisfactory result :
- On the bottom. Thinker layer as space usually permits.
- On the top.
- On the sides.
- Try to avoid placing damping material on the back panel in a bass reflex enclosure.
Be warned that working with mineral wool will cause some itching. It’s normal.
The first measurements I took with only the top speaker connected (it was more convenient for me). As a result, the port response look a bit off, because the second speaker acts like a passive radiator. However, this is not of concern because we are interested only in the standing wave. In the end I hooked up the second speaker as well, to make a comparison without damping material and with damping material on all walls except the back.
As you can see, by placing a 15 cm thick pad of mineral wool (3 pads of 5 cm stacked) and 5 cm on the top, the standing wave reduced it’s amplitude by 10 dB. It’s very effective. Now let’s place it on the sides as well.
Now the response looks like there is no standing wave. It’s completely absorbed.
Now you know that if you run into internal standing waves problems, you don’t have to scrap your enclosure. Using thick damping material yields great results. In conclusion, applying sufficient material can kill the standing wave all together.
- Image source : link.