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Blir något betydligt grottigare istället..

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My bass drivers are housed in enclosures which are very near to square in terms of width and height. This translated into a rather serious internal resonance (standing wave) corresponding to the 1/2 wavelength of the dimension in question. This resonance manifested itself as a low Q bump of around +3 dB centered around 420Hz when measuring what's happening inside the cabinet, behind the driver. The effect of this resonance is a bit of a ring on certain vocals and especially electric guitar notes.

The easiest way to resolve these types of issues is by adding damping material to the enclsoure. I subsequently added damping material, and it definitely damped the resonance to much more acceptable levels, but the result was a less lively sound with a slightly nasal quality which I didn't care for too much.

The next step was to remove the excess damping material in the enclosure, and I added a sheet of wood inside the cabinet which runs the depth of the enclosure at a 45 degree angle such that the top triangle created by this piece of wood is smaller than the bottom triangle. This then breaks up the one large standing wave, and creates numerous smaller (in dB) standing waves at different frequencies, which the damping material is much more effective at absorbing.

With that done, the speakers had a much more lively sound to them - more open and interestingly better defined bass. It also seems as if the bass extends a bit deeper, though I am sure that is purely a perception rather than fact.

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Ian, I'm trying to reverse engineer, but not winning what is the dimension causing the resonance. Also why would the resonance be 1/2 wavelength, would this be predictable, or just a charactaristic of that specific enclosure? (learning things here)..


Interesting question!

There are many standing waves inside an enclosure, and all have to do with pathlengths between reflective surfaces. The more surface area a specific path length has, the stronger the resonance, and the more intrusive it becomes. Even a triangle or curved speaker has internal standing waves - the trick is to minimize surface area along a single path length in order to reduce the magnitude of the resonance. Where that can not be done, you try to break up the path length and / or damp it to death. The smaller the path length, the higher in frequency the standing wave. The higher in frequency the standing wave, the easier it is damped.

For a distance of 406mm between two parallel surfaces (internal height of my bass enclosure), and a distance of 376mm (internal width of my bass enclosure), take a look at the following table showing the modes:

Resonance 406 mm 376 mm
1 422 Hz 456 Hz
2 845 Hz 912 Hz
3 1.27 kHz 1.37 kHz
4 1.69 kHz 1.82 kHz
5 2.11 kHz 2.28 kHz
6 2.53 kHz 2.74 kHz
7 2.96 kHz 3.19 kHz
8 3.38 kHz 3.65 kHz

You will notice how the 2nd resonance is the first resonance x 2, and the 5th resonance is the first resonance x 5, and so on. You will also notice how closely the 406mm and 376mm resonances track each other.

Now, my bass drivers roll off (crossover) around 450Hz or so. Because of this, the first resonance is the only one which is really an issue. The drivers just don't operate to any significant SPL at higher frequencies, therefore the effect of those resonances up there is minimal (although still there to be sure).

Secondly, the higher the frequency of a resonance, the easier it is to damp such a resonance using good old dog blanket and isotherm, etc which is why we tend to focus more on the lower frequency resonances as they are the most problematic and difficult to deal with.

Out of interest, this is exactly the same theory which applies to your room modes, etc as well. When it comes to standing waves, your enclosure behaves like a very small room in it's own right :thinking:

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kapar man mycket MDF blir man nog ett vackert lik iaf..

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