Why does a low frequency wave from a speaker or subwoofer felt and sounded different when you slightly move your position?


Why does a low frequency wave from a speaker or subwoofer felt and sounded different when you slightly move your position?

In: 4

You could be experiencing interference between two sources of the sound: either two speakers, or directly from a speaker and a reflection off a nearby surface.

At one point the two sources are constructively interfering (loud), and when you move a bit, you find a spot of destructive interference (quiet).

The reason you might notice this more with low frequencies is that the distances between constructive and destructive spots are farther apart, probably on the order of a few centimeters or more. So when moving around slightly, you notice the increase and decrease in amplitude.

High frequencies do the same thing, but the distances between the interference points are much smaller. You probably wouldn’t notice the change by just moving your head around since the distances are likely smaller than your ear.

This isn’t exactly a standalone explanation, someone else did a good explanation, but I wanted to talk about interference in a bit more depth. When the speaker cone moves outward, it increases the pressure of the air in front of it. When it pulls back, it decreases the pressure. That alternating high/low/high/low pressure wave moves out in roughly a sphere from the speaker. After a few feet, though, the sound wave has collided with some of the things in the room and bounced off. This means you may be hearing two different wavefronts hitting your eardrums at the same time without even realizing it. The speed of sound is so fast that even if it’s not exactly the same time, you won’t be able to tell that there are two voices or anything, but interference can still happen.

So imagine you and the speaker are both centered left-right in the room. As the sound propagates from the speaker, the wavefront hits both left and right walls at the same time. They reflect, and these new wavefronts both hit you at the same time. That means both are high pressure at the same time and both are low pressure at the same time. The high adds to the high and the low adds to the low, meaning even louder sound. Not a ton louder because the bounce off the wall absorbed some of the energy, but some. Now imagine taking a small step left or right. You’re now closer to one wall that the other. That wavefront hits you slightly sooner in it’s travel while the other hits you slightly later. If you moved just the right amount, the high from one hits you the same time the low from the other. These cancel out and you have no pressure difference from ambient air. Stay still and wait a tiny fraction of a second because both are oscillating, you’ll get low pressure from the first and high pressure from the other. Exactly opposite of a moment ago, but still cancelling out. You hear no sound (or rather, less sound because there are still many other reflections going on that can’t all cancel out perfectly. With 2 speakers and a very well padded room, you might get to a point where it’s inaudible.)

If you were to keep moving, you would eventually meet back up where both are high at the same time, but one is a full cycle behind the other. The lowest sound humans can perceive is usually about 20Hz. Music with deep bass probably rarely goes below 50. So one cycle out of phase means a 20ms delay. Human reaction times are typically in the hundreds of milliseconds, so like I said, probably imperceptible.

Assuming you are indoors, these are called room modes.

Bass has a long wavelength relative to the woofer diameter, this leads to the bass being essentially omnidirectional. Since it’s radiating in all directions, it thus bounces (reflects) off all the walls/surfaces. These reflections will naturally interact with other reflections, and these waveforms all have different timing. The relative difference will cause an increase/decrease in loudness depends on how they align.

These room modes differ at location, so the sound changes per location in the room.

To combat this, you need to add more subwoofers. More subwoofers cause even more interference, it becomes so chaotic that it actually averages out to be neutral and consistent.

Sound is a wave. Waves can run into each other and cancel each other out (destructive interference) or they can essentially add onto each other and result in a larger wave (constructive interference). [Here’s](https://cdn.britannica.com/07/62907-004-A3A6351E.jpg) a graphic that shows both. Realistically, the waves won’t perfectly align to completely cancel each other out or double in size.

Your speakers emit sound waves throughout your room. These waves interfere with each other (as explained above) at different spots around the room.

So maybe the waves constructively interfere on the right side of the couch but destructively interfere on the left side. When you sit on the right side, the sound will be loud and clear, whereas the left side will sound quiet and muffled.