Note: you need a modern browser that supports WebGL (I recommend Chrome, as the simulation works best on Chrome) to read this post. This post also assumes you're on a desktop or laptop. Mobile devices (iPad etc) have poor support for WebGL at the moment.
Why are room modes bad?
Room modes accentuate specific frequencies. Here are some examples of when you might have stumbled upon them:
- When listening to music using your high quality audio equipment, some specific bass notes always tend to sound much louder than the others.
- The sound level on low frequencies seems to vary a lot depending on where in the room you are located.
- When the neighbor is listening to music, and you always hear some bass notes louder than the rest of the music, it might be caused by room modes in your or your neighbors apartment.
- A large vehicle drives by your apartment, and you can hear how the sound resonates at a specific frequency. This is also often caused by room modes.
- The low frequency sounds from your washing machine gets amplified at certain rotation speeds.
The easiest to understand, and perhaps most obvious, disadvantage of room modes is in sound reproduction. It should be noted that room modes can cause numerous other problems, not directly related to high fidelity audio, in residential apartments. They might amplify sounds caused by traffic. They might sometimes amplify the sounds caused by HVAC equipment (ventilation, pumps, compressors). They might also cause some low frequencies to travel very efficiently from the neighbor's apartment to your apartment in a residential building (due to coupling), even if the structures in themselves have good sound insulating properties.
What are room modes?
A sound wave can be visualized, literally, as a wave. In the simulation above, you will see what happens when a sound source emits an impulse in a room with two walls (the sound is allowed to freely escape in the free directions). The plane represents a cut plane, i.e. the sound pressure at a certain height in the room. The deflection of the plane represents sound pressure. You can specify how many times the sound reflects from the walls using the controls ("open controls - reflections").
Try moving the source around a bit, to get a feeling of how the simulation works. You can do this by adjusting the "position" slider in the control panel. Press "reset" to restart the simulation.
In this post, I will explain to you what room modes (standing waves) are. Just follow the steps below. If you want to, you can open the simulation in a new window.
- Try setting the reflection count down to 1, to get a clear picture of what happens when the sound reflects from the walls.
- Restart the simulation ("reset").
- Enable "show reflections". This shows virtual sound sources, which is another way to think of reflections. It might be a bit confusing at first, but you'll see that it makes some things clearer later on. Take a while to see how virtual sources are formed to form a single reflection (remember to reset the view!).
- Change the signal type to "SIN", which represents a pure sound at a specific frequency.
- Set the reflection count to 0, to get a clearer view of what's happening. The sound this type of curve represents is very close to what you hear when you whistle. A sine wave with a long wavelength is perceived as a low note, while a short wavelength is perceived as a high note.
- Set the sound position to -10 for the next step. Remember to keep the reflection count at 0.
- Try playing around with the "frequency scale" setting (still without the reflections!). When the scale is set to 1, the length of the wave (the distance between two "peaks") will be the same as the distance between the walls. When the scale is set to 2, two wavelengths will fit into the room. When the scale is set to 3, three wavelengths, and so on.
- Set the frequency scale to 2.
- Set the reflection count to 1.
- Reset to get a feeling of what is happening. Remember that you can also close the controls.
- If you're confused at this point, try setting the signal type to PULSE, and then change it back to SIN. This should make things clearer.
- At this point, what you're seeing is constructive and destructive interference.
- Try adding more reflections, this will make the effect even clearer.
- This is what a room mode is. It's exactly this, but with more complicated rooms with additional walls and details. Note that the mode can be heard clearly in positions where the sound pressure varies the most.
- When you now change the frequency scale slider to something else than a multiple of 0.5, you'll see that the room modes disappear (completely, if you're far away from a multiple of 0.5). They only happen close to specific frequencies. At these frequencies, you might sometimes hear a distinct ringing sound in the room.
The good news is that annoying room modes can be attenuated. There are multiple ways to do it. In the case of hifi equipment, some modern amplifiers attempt to correct room modes using digital signal processing. But these digital methods won't sound nearly as good as the room would sound if you would fix the acoustics of the room itself.