Bass - Directional or omni-directional?

[stevemac]

I know the easy answer, its omni-directional, but why is it then if the speakers are too low on the ground or if I am beside my amp rather than in front of it it can be so hard to hear? Shouldnt the sound come at me from any angle?

I've read a lot on this lately, on these forums also, and most seem to debunk the idea of bass frequencies needing space to form properly. I can accept that there is a lot going on which may not be apparent at first such as cancellations happening due to reflections from back wall etc. So I am good with the idea that it is probably something else going on.

The reason I am looking into this is we are re-examining stage setup since we have a new guitarist and he wants to point both guitar amps across stage and put bass in middle facing out, since it is "omni-directional and doesnt matter where you put it".

I agree in theory, but in practice, like at this open air festival I did last weekend, if I wasnt standing directly in front of the ampeg 810 it was pretty much gone to me. So whats going on? Any good web articles I could use?

 

[PaulNYC]

The low frequencies are omnidirectional, but each note played also contains higher frequency harmonics. These higher frequencies could beam and then be hard to hear. Your 810 has ~ half the dispersion of a vertically stacked 410. Because you cannot hear the higher frequencies, you cannot hear yourself as well.

 

[Jimbob Jones]

The 'mud' is omnidirectional, (<200Hz) but all your clarity (~400-5000Hz) isn't.

 

[stevemac]

Thanks guys, that answer makes a lot of sense! I thought most bass frequencies were under 200hz however.

 

[MuzikMan]

A sting vibrates at the fundamental frequency and at integer harmonics. The sound of your bass is a combination of all of the frequencies, not just the fundamental.

For example, let's say the low E fundamental is exactly 40hz (it isn't exactly but this makes the math easy). When you hit the open E, you will here the following frequencies (at different amplitude).
---------------f1: 40hz
---------------f2: 80hz
---------------f3: 120hz
---------------f4: 160hz
---------------f5: 200hz
---------------f6: 240hz
---------------f7: 280hz
---------------f8: 320hz
and so on. The higher harmonics are important for intelligibility. If you don't have the cab pointed at least near the listener you will only hear the lower harminics and it will sound boomy.

 

[stevemac]

Thanks muzikman, I didnt realise harmonics played such an important role. So really saying bass is omni-directional, at least with regards to the bass amp in a band, is probably not the best way to look at it. Probably should be careful about using a low pass filter if I ever wanted to also.

I've also read that bass is not omni-directional, its just the brain cannot localise it, but this might be myth.

 

[JimmyM]

Thanks muzikman, I didnt realise harmonics played such an important role. So really saying bass is omni-directional, at least with regards to the bass amp in a band, is probably not the best way to look at it. Probably should be careful about using a low pass filter if I ever wanted to also.

I've also read that bass is not omni-directional, its just the brain cannot localise it, but this might be myth.

If the brain can't localize it, it's omni-directional

I would probably stop using the term "bass" to describe the omni-directional low end frequencies. As you've found, there is way more to bass than just those frequencies, and it causes confusion when talking to others.

 

[walterw]

in fact, of the sound put out by a typical bass amp in that chart, there isn't even all that much 40Hz; you're hearing more of those harmonics than anything else, and they are plenty directional, getting more so as you go higher in frequency.

goofy cross-blasting of stage amps can sometimes just make overall stage volume louder.

 

[DukeLeJeune]

How directional a sound source is depends on a) the physical dimensions of the source and b) the wavelength(s) of the sound being reproduced.

When the source (woofer cone or cones) is much much smaller than the wavelengths being reproduced (for example the wavelength of the 82 Hz first overtone of low E is nearly 14 feet long), the source is effectively omnidirectional. When the wavelengths are about 4 times longer than the source, it starts to get a little bit directional. When the wavelengths are about equal to the width of the source, it's fairly directional.

The bigger the woofer or array of woofers, the lower the frequency at which it starts to become directional. A really big horn or woofer array might well be directional at 82 Hz.

So it's not so much that "bass is omnidirectional"... rather, it's that the sources we're using to generate low frequencies are small enough that they are omnidirectional at those frequencies.

To give an example of this principle higher up the scale, note that at 2 kHz a 12" woofer is certainly not omnidirectional... but a 1" dome tweeter is! Or would be, if not for the baffle it's mounted on.

 

[MuzikMan]

The Grateful Dead's "Wall of Sound" had sixteen 15" speakers in a vertical array dedicated to the kick drum. I wonder how omnidirectional that low end was!

 

[uhdinator]

Putting your amps facing across stage and using amp stands when possible allows you to hear better and at lower volumes. Think of your amps as your instrument monitors.
You want the sweet spot to be where you are standing center stage.
1. It also makes it easier to hear the PA over the stage volume and you will get a better mix.
2. Drummer can hear better
3. Less bleed of stage volume in vocal mic's
4. If you can't hear well take a step left or right towards your amp and further from the guitar amp on other side of stage.

here's what I see most often doing sound:
Guitar amp on floor on back wall behind guitarist. The guy thinks it's great tone because his ears are above off axis facing the other way which attenuates the HF. The mic on the cab and the audience however are not hearing what he is hearing. They are getting their hair parted and the sound guy is turning the guitar off in the mix and pushing everything else up in volume to balance it out. Now the club owner is pissed cause your too loud but need to be to hear the vocals. Not to mention feedback issues due to needing the vocals on top and now the amp is coming thru the vocal mic's too.
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[DukeLeJeune]

The Grateful Dead's "Wall of Sound" had sixteen 15" speakers in a vertical array dedicated to the kick drum. I wonder how omnidirectional that low end was!

If the 15's were in a single vertical line sitting on the floor or ground, the array would be directional above 24 Hz in the vertical plane (you get an effective doubling of the array's height from the ground plane reflection), and directional above 1 kHz or so in the horizontal plane.

 

[seamonkey]

Any good web articles I could use?

Here's some:

Good FAQ:
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Pro FOH will make it directional with some tricks:
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[MuzikMan]

If the 15's were in a single vertical line sitting on the floor or ground, the array would be directional above 24 Hz in the vertical plane (you get an effective doubling of the array's height from the ground plane reflection), and directional above 1 kHz or so in the horizontal plane.

That's insane! The Wall of SOUND

 

[walterw]

That's insane! The Wall of SOUND

from that link:

The bass guitar system has two columns of fifteen inch tranducers stacked 18 high. Four power amplifiers are used as the bass requires more power for equal loudness. Since teh instrument has the capability to operate with individual outputs on each of the four strings, the array can be fed in this manner which makes it possible to play chords on the bass without intermodulation.

a bass rig for each string!

 

[Bong Bong]

Putting your amps facing across stage and using amp stands when possible allows you to hear better and at lower volumes. Think of your amps as your instrument monitors.
You want the sweet spot to be where you are standing center stage.
1. It also makes it easier to hear the PA over the stage volume and you will get a better mix.
2. Drummer can hear better
3. Less bleed of stage volume in vocal mic's
4. If you can't hear well take a step left or right towards your amp and further from the guitar amp on other side of stage.

here's what I see most often doing sound:
Guitar amp on floor on back wall behind guitarist. The guy thinks it's great tone because his ears are above off axis facing the other way which attenuates the HF. The mic on the cab and the audience however are not hearing what he is hearing. They are getting their hair parted and the sound guy is turning the guitar off in the mix and pushing everything else up in volume to balance it out. Now the club owner is pissed cause your too loud but need to be to hear the vocals. Not to mention feedback issues due to needing the vocals on top and now the amp is coming thru the vocal mic's too.
[Invalid or Expired Link Removed]

That guitarist story rings a bell

 

[Mike Caldwell]

The Grateful Dead's "Wall of Sound" had sixteen 15" speakers in a vertical array dedicated to the kick drum. I wonder how omnidirectional that low end was!

Actually it was becoming directional due to the length of the vertical "line array"
Same concept/math used today, the longer the line array the more control and directivity of lower frequencies.