What is it which makes a subwoofer "omnidirectional"?

[mikehende]

Hey guys, I am thinking of getting this sub for my garage:

https://av.loyola.com/products/audio/ev ... or-kw.html

Can you guys tell me exactly what it is which makes a sub omnidirectional please?

I have this one:

https://av.loyola.com/products/audio/ev ... i-sub.html

which is omnidirectional too but instead of trying to find another one, I am thinking might best to get the dual 18" version but these 2 subs look totally different in design which is why I am asking the question.

[ef37a]

One of those times Mike when Size Matters. Say a sub takes over at 60Hz and below? The wavelength of sound is around 18 feet at that frequency so even an 18" woofer is virtually a 'point source' at 60Hz and below and generates a spherical wavefront.

Of course, woofers generally sit on a floor and that will give some 'gain' , it will be even more efficient in a corner (but almost certainly give the most 'lumpy' response) You can modify the polar response of course, there was a review of a 'cardioid' speaker system in SoS a while ago but, costly to implement.

Dave.

[Drew Stephenson]

It's not so much the sub that is omni-directional, it's the behaviour of low frequency sound waves.
Low frequencies have long wavelengths and will diffuse around an object in a way that high frequency won't.
Mount a driver on a flat board and put some HF sound through it and walk around the board. It will get much quieter the more you move off axis until its minimum in the plane of the board, then back up to near its maximum directly behind the driver.
Play some midrange through it and this directionality will still be there but much reduced.
Get onto the low frequency stuff and you'll find it even less directional.
HF frequencies behave almost like rays, point a speaker in a direction and that's which way the sound will propagate. LF is more like pushing your hand through water, the wave spreads out all around your hand.

Also, the usual mechanisms we have as humans to determine location are easily confused when the wavelength of a sound is greater than the distance to the source of the sound; proportional distances between the distance from the source to our left and right ears are negligible compared to the source wavelength, and the masking effect of our heads is bypassed by the sound diffusing around our heads.

So basically: physics and biology.

[mikehende]

So what does this all mean guys, that these speakers in question is not omnidirectional as it's description shows?

[Hugh Robjohns]

They are both notionally omnidirectional. The brochures state that clearly.

This is normal for subwoofers because, as Dave says, the wavelength of sound produced by subs is pretty huge compared to the size of the cabinet -- certainly for frequencies below 100Hz or so. So the cabinet effectively behaves like a point source, and point sources inherently radiate omnidirectionally.

This is basic acoustics 101.

There are ways of making subwoofers that are not omnidirectional, but its complex, expensive, and not relevant to you here.

[Wonks]

Neither are actively designed as an 'omnidirectional' speaker. It's just the way that the physics of sound reproduction and propagation works.

Above roughly 125Hz, the sound will start to be slightly directional, and get more directional as the frequency increases. But you need to get to quite a high frequency before an 18" speaker starts to become truly directional and not spread the sound out over a very wide angle.

There's a decent description of speaker size vs directivity here.

http://www.mcsquared.com/speakers1.htm

Omnidirectional sound for a sub isn't what you want, as you are wasting a lot of sound energy backwards, when you want it to go forwards towards the audience. In an indoor situation, most of that energy will end up being reflected back forwards, but it will be delayed in comparison to the direct sound and make the bass muddy.

You can get subs that can produce a cardioid bass pattern when used in pairs (primarily through phase cancellation), which will give a better quality of bass sound, and much less risk of bass feedback through stage mics (very little bass sound now going backwards to fill the stage), but you are wasting energy through the phase cancellation, so you don't get the same volume that two omni subs would provide. But the bass sound is less muddy as a result and most is going towards the audience, which is where you want it to go.

[Folderol]

Just to add my 2d
At 1kHz the wavelength is approximately 1ft.
At 100Hz, that's 10ft.
At 50Hz, it's 20ft.
See the point?

[Hugh Robjohns]

Wonks makes good general points, but as this application is a static garage DJ rig considerations for stages and large venues don't apply here.

[mikehende]

Well here's my update and train of thought with this omnidirectional issue, guys.

One day last couple weeks I had turned on my system and no sound was coming from the EAW SB1000 Sub which is always at my back wall facing forward. So I took all the speakers off of it and placed them in the middle of the room. I then as first test plugged the speaker cable from the EAW into my EV's full range to test and was totally surprised the I was hearing the bass more evenly anywhere in the room and even at the one trouble spot which is right in front of the garage's side door.

So I moved the EV's to the back wall facing forward to see if I would get the same result but no, it was the same as the EAW. Then thinking well, since the EV's were facing the side walls when I had plugged the cable in to them let me try facing them towards the side wall and this gave me the bass all around the room just as when they were in the middle of the room.

I then decided to add an EV Sub in the middle of the side wall to "flank" it and this doubled the bass all around the room. So right now with one full range EV acting as a sub at the back wall facing the side wall and the addition of the EV Sub on the side wall, I am now getting what I have been fighting for all these many years.

I am just wondering now instead of having the one EV Sub at the middle of the side wall if that Dual 18" Omnidirectional Sub can be placed at the back wall facing forward might give the same result I am now getting with what's mentioned. So, think this might work or should I leave the the 2 speakers where they are and get another EV Sub to replace the full range EV at the back wall and be satisfied with that?

[resistorman]

Well, if this configuration works for you, just add another EV sub.

[Drew Stephenson]

What you're experiencing probably has very little to do with the subs and almost everything to do with the room.

[resistorman]

What you're experiencing probably has very little to do with the subs and almost everything to do with the room.

Yep

[AlecSp]

What you're experiencing probably has very little to do with the subs and almost everything to do with the room.

That's the one!

[Hugh Robjohns]

We've been round this loop before. The garage is untreated and standing waves are uncontrolled. Therefore placement of the bass speakers will have a profound effect on the balance at different points in the room.

[mikehende]

Yes guys, I see that now. This will explain why the full range and the sub both giving the same results, thanks for helping me narrow it down!