[TRADERXFAN]

MediumJim
My conjecture is that this happens when your subwoofer is giving acoustic output higher than crossover point would indicate. This is part of why Geddes prefers bandpass topology, I believe. 

[AJinFLA]

MediumJim
My conjecture is that this happens when your subwoofer is giving acoustic output higher than crossover point would indicate. This is part of why Geddes prefers bandpass topology, I believe.

That's not conjecture. When a (sub) driver is pushed hard, it will generate non-linearities (distortion) in it's acoustic output (from the motor, etc) after the electrical filter. What Geddes does with the bandpass is create an acoustic filter to counter this, though it is entirely possible that some could "leak" through the vent (depending), so it is not entirely eliminated, just greatly attenuated (especially if also internally damped).

cheers,

AJ

[*Scotty*]

The pathetic thing here is that for a very minimal increase in cost every sub-woofer could have a 24dB/oct or 48dB/oct Bessel derived filter for it's crossover and "leakage" of upper harmonics into the output of the sub could be greatly attenuated.
Scotty

[medium jim]

If you remove the upper harmonic structure from the fundamental bass frequencies produced by the aforementioned instruments, say everything above 50Hz, I would bet that the bass would cease to be directional. It's the presence of the upper harmonics accompanying the fundamental tone that allow for the localization of the instruments.
 The crossover slope used in many subs is frequently 12dB/oct. this allows for reproduction of a considerable amount of the upper harmonics that are related to the fundamental tone to be reproduced by the sub rather than the main speakers. Even a 24dB/oct. crossover slope is not a brick wall which will prevent the sub from seeing some upper harmonics.
Scotty

Scotty:

That is true in a perfect world, but we are humans and our hearing is biased.   Science is wonderful in theory, shit, Einstein was a scratch golfer in theory, gosh darned golf course didn't know that   Sometimes the science needs to go out the window and let the good times roll!

Jim

[Davey]

Fortunately, science can't be thrown out the window. 

Anyways, even if bass frequencies were propagated from a completely omni-directional source, we humans could still locate the source.  In either a reverberant or a free-field environment.

That's science!

Cheers,

Dave.

[medium jim]

Fortunately, science can't be thrown out the window. 

Anyways, even if bass frequencies were propagated from a completely omni-directional source, we humans could still locate the source.  In either a reverberant or a free-field environment.

That's science!

Cheers,

Dave.

Dave:

You realize that your statement is contra to what others say the science indicates?  I agree that bass for the most part (to the human ear) is identifiable by location within the soundstage, both in height and width.

In one side of science it says that under the Schroeder curve that it is just energy, but in other arena's of science we are told that the brain doesn't know this

Jim

[JohnR]

Gee, I can't agree that we should "throw science out the window" when optimizing bass and system performance. The distortion generated by a (not so good) sub driver could well be the reason for an anecdotal report that you can hear where bass is coming from. My own (anecdotal) experience is that with music playing, it's hard to localize a sub, the ear "goes for" the harmonics. Whether that is a psycho-acoustic effect or whether it's because you can't actually hear the direction of a (pure) low frequency tone, I wouldn't know. I'd be interested to know more though.

Dave, you are saying that direction of low frequencies can be detected?

[medium jim]

JohnR:

I was being a bit flippant about tossing out the science.  Subs should be invisible and not detected as a directional thing if well integrated.  As Neekomax correctly pointed out, it is the transients and bass above the Schroeder curve that creates the bass within the soundfield.  Then again, some in the science world seem to opine that low frequency distortion is not audible. 

For me stereo subs makes more sense as if one can detect the location of bass above certain frequencies, that the corresponding lower frequencies still need to be connected, e.g., left to left and right to right, to maintain the continuity.

Jim

[sts9fan]

This is audio not science!!
Keep your science off my ears!!
I don't think the propagation if low frequencies in small rooms is a mystery.
Just because you think using "stereo" subs below 80hz is right does not make it so.

[medium jim]

This is audio not science!!
Keep your science off my ears!!
I don't think the propagation if low frequencies in small rooms is a mystery.
Just because you think using "stereo" subs below 80hz is right does not make it so.

So much of what we call audio is subjective, yes, stereo subs are right (for me).

Jim

[Davey]

Dave, you are saying that direction of low frequencies can be detected?

Sure.  Turn your head back/forth and let that computer between your ears analyze the time delay.  Maybe move around a little bit to judge amplitude changes.
We certainly don't have the ability that some other animal species have, but we could localize any sound source given enough time.

Audio is not science?  Audio reproduction is certainly a science, no?

Cheers,

Dave.

[sts9fan]

So much of what we call audio is subjective, yes, stereo subs are right (for me).

Jim

Remember, just because you THINK something does not make it so.

[medium jim]

Remember, just because you THINK something does not make it so.

Got to love the negativity....there is life outside of the test tube

Jim

[Davey]

Some things are true whether you believe them or not.

It's pesky a lot of the times, but that's the way it is. 

Cheers,

Dave.

[ricardojoa]

When you figure the length of waves in the bottom octave are in the 10's of feet, the baffle would in most rooms have to be the entire room.  Now an Infinite Baffle type of arrangement does address at least the rearward interaction of the waves but you're still radiating down, up, left, right, forward, etc.

Bryan

Well in this case, what is left is the woofer and the baffle. The enclosure no longer is part of the contributing factor. In an open baffle, when you get in 90 of axis, there will be a cancelation of front and rear waves, so how does that constitute a omni?

[ricardojoa]

how about this, in terms of speakers?
-All sound frequency emitted by a driver are omnidirectional if the wavelength is larger than the baffle...

Illustration of monopole source
http://www.acs.psu.edu/drussell/demos/rad2/mdq.html

Salks explanation for baffle step
http://www.salksound.com/wp/?p=42

If you do measurements of frequency along the speaker response from measuring on the rear, there will be a big gap in response. The high will be dminish over the lows. Im not sure if that would still qualify as omni if you get a dimish response from a different angle.

[ricardojoa]

Ricardo,

You are either misunderstanding/conflating what is said, or "they" saying it are confused. An enclosed piston source/cone (like a woofer in a box) radiates essentially hemispherically and when the wavelengths radiated are large relative to the baffle or enclosure, these waves simply wrap around, creating essentially omni-directional radiation. "Sub" frequencies wavelengths of say 100hz-20hz, will vary from around 11 to 56 feet, i.e much larger than the piston/enclosure. If a closed sub was lifted up in the air outside, say 20' and the dustcap was made the center point, then a mic at any point around say, a 2m sphere, would measure near exact same frequency response. It radiates omni-directionally at "sub" frequencies. This is a completely separate issue from localization.
The reason why it is said to be difficult <80hz, is because we use a "difference" signal (intensity and time) between our L and R ears to detect direction of a source. Because wavelengths have become so large <80hz, the "difference" between a signal coming from say the left of you, is relatively (very) small compared to the length of the wave itself, so there is no (or minimal) difference component to detect.
Now a tweeter, especially one mounted on the top of an enclosure ala B&W, is also radiating essentially "omni-directionally" at the lower end of its range (say 3k), but it would be quite easy to localize, as those wavelengths (around 4") are relatively small compared to the distance between your ears (and the "shading" provided by your head).

cheers,

AJ

Yes AJ, i undertand that in a enclosed sub, most of the frequencies will wrap around the sub itself. But this is also dependent to the baffle length. Though, no one will make a sub with 20 feet long baffle, the example is to show that the frequency it self is dependent to the baffle length. If you do measurement of a sub with 20 fett baffle, from front and back, you will end up will different response, specially to the 20 feet wave length frequency. In this case, at least an enclosed subwoofer ( the whole unit box and the woofer) is not a ominirectional source. The woofer becomes  the radiating source, right? Because thats what the vibrations starts and create that low frequency. The frequencies will wrap around the sub from the front and not from behind.

If we use an open baffle sub, there will be a pahse cancelation at 90 degree, and how would that be considered omni?

[ricardojoa]

Just put on any philharmonic symphony orchestra recording where you know where the double basses, tympani, kettle drums, contrabassoon, pipe organ, and you will be able to define said instruments within the soundstage, either live or when using stereo subs.

Psychoacoustic's are how we hear and represent things.  While low bass is a form of energy unto itself, it is directional with relation to a proper soundstage.  Call it a form of trump loy.

Jim

Sure Jim, but thats taking the matter into a new level .
I was specifically refering to low frequencies and subwoofer in general. Is frequencies or a subwoofer omnidirectional? That seems like the general will say that low frequecies is omni. It is like saying a omni ligtning, that there lumen is omni, but in fact the lightning source that is omni.

[ricardojoa]

Dave:

You realize that your statement is contra to what others say the science indicates?  I agree that bass for the most part (to the human ear) is identifiable by location within the soundstage, both in height and width.

In one side of science it says that under the Schroeder curve that it is just energy, but in other arena's of science we are told that the brain doesn't know this

Jim

I think what Dave says hold true. Just use the same example of omni lightining. Wouldnt you know where is brighter in a room or out in a filed, even if it is immiting its lumen in all the direction?

[ricardojoa]

Gee, I can't agree that we should "throw science out the window" when optimizing bass and system performance. The distortion generated by a (not so good) sub driver could well be the reason for an anecdotal report that you can hear where bass is coming from. My own (anecdotal) experience is that with music playing, it's hard to localize a sub, the ear "goes for" the harmonics. Whether that is a psycho-acoustic effect or whether it's because you can't actually hear the direction of a (pure) low frequency tone, I wouldn't know. I'd be interested to know more though.

Dave, you are saying that direction of low frequencies can be detected?

I think, the lower the frequecies that harder is to located as what AJ said. In a real world, we dont listen to test tone, we listen to musics with a range of low frequencies, and that helps us locate where is comming from.