Omni Directional Bass

[jaxwired]

I've always read the deep bass is omni directional. This is why you can point a sub-woofer down at the floor. This is also why you can put a sub-woofer in a corner rather than centered beween the 2 main speakers. The theory being that the listener cannot tell where the bass is eminating from and will integrate into the main sepeakers regardless of placement.

But, my experience with the bass on my system does not comport with this theory. Now, I'm not using a sub-woofer, but I would expect the lowest bass registers hit by my main speakers to comply with the omni-directional theory. However, when I stand up or move significantly off center from my main speakers, there is significant audible change in bass tone. Primarily there is less bass, than when seated.

Now, I suspect that when seated (its a pretty low seat), my proximity to the room boundary (the floor), is changing the audible bass level. But when I move left or right of center, I am not changing my proximity to the floor, yet bass still is reduced.

So, is this just because bass is only really omni-directional in the very lowest bass frequencies that sub-woofers produce?

 

[Anonymous]

jaxwired - when you say move left & right - are you talking about moving from the central position in your listening seat?

You are probably experiencing moving away from the 'sweet spot'. Theres an excellent video from the WhatHifi team about system speaker toe in & how to create the best sweet spot for your system. I think its in the System Setup Hifi video....

 

[Anonymous]

What you're describing here are the effects of constructive/destructive interference. There is indeed a 'sweet spot' - this is a point where the sound waves coming from each speaker reinforce each other. That is to say that if you could see the sound waves at this point they would both be at the peak of the wave or both be at the trough of the wave, and so they complement each other and the sound is optimised. If you move around the room you should actually find more than just one sweet spot.

Conversely, the areas where the two waves are contradicting each other are your dead spots. This is where the peak of one wave meets the trough of the other and they sort of cancel each other out (I believe this is how noise cancelling headphones work). Of course, the reason you don't have perfectly defined spots of 'perfect' sound and spots of no sound at all is that the sound is also being reflected off other surfaces.

What you've said about low notes being "omin", or non-directional is true, but I think the phenomenon you're experiencing is the interference I've described. Don't take anything I say too seriously, though; after all I did quit my A-level in physics (where I learned this) to take economics instead.

 

[SteveR750]

It is well known that the human ear loses its sense of directionality at lower frequencies (part of the logic behind ambulances now having sirens that sound like asphyxiating crows), so in theory a sub can sit anywhere. The problem is indeed the room acoustics, as the sound waves are reflected off surfaces at different angles and speeds, and refracted around corners etc. These all lead to slight phase shifts that in certain places lead to reinforcement (boominess) or cancellation (no bass at all).

On top of this, most of the room (floor, plasterboard walls, furniture cabinets etc) will be resonating at different frequencies adding or subtracting from the overall sound. Since bass is omnidirectional aurally then you can move your sub around till the sweet spot is where you sit! This is where the skill of domestic negotiation comes into play...