Subwoofers

Is it true that the really low notes produced by a subwoofer are “nondirectional”? I routinely hear bass notes moving/emanating from side to side on my two 12" woofers, what is it about a sub that you need only one? I don’t get it.

I believe that most sub-woofers have a dual voice coil. They receive signals from both channels, but the magnets only move one cone. I don’t think you can tell where the low-low notes really come from. That’s why you can put the sub off in some corner of the room. I’ve had friends hide their subs and bet people to try and find them. You hear the music, but you can not tell where the low-lows are coming from. Your 12in woofers, while producing low bass, are also producing a lot of lower mid-range sounds as well. That may explain why you can hear the notes moving from side to side. You’re not just hearing the truly “sub” bass, you’re hearing the lower mid-range and the middle bass as well. The subwoofer is only reproducing the low-low notes, which, I believe, are “nondirectional”. A good system will have a sub-woofer in addition to other regular woofers, mid-ranges, and tweeters.

Although all sound waves, regardless of their pitch, travel at the same rate of speed through a particular medium, low tones mushroom out in a broad trajectory while high tones move in straight paths. A quality subwoofer will have a low-pass crossover of around 40-60Hz (or it may be adjustable). These frequencies you tend to “feel” more than hear.

I’m not sure of the limits of, say, the Bose Acoustimass systems, but I’m guessing the crossover is closer to 100-120Hz. This indicates to me that it’s not a “true” subwoofer, per se, but more like a combined (two-channel) woofer. The satellites provide for the stereo separation.

Sound becomes nondirectional when the sound waves have an amplitude that is greater than the distance between your ears.

oops. I missed part of my post. If you want to read more about this I would suggest “The History of Physics” by Isaac Asimov. I wish I could explain it better but I don’t quite remember all of the stuff.

HeadlessCow:

This doesn’t look right to me. Middle “C” (263Hz) has a wavelength of about 4.3 feet. Middle “C” is completely directional. How big is your head?

I have a 100W sub-woofer in the trunk of my car, Only on occation can I tell that it is in the back. Living in a college town, I delight to drive by campus with my mighty Kenwood blasting Beethoven’s 9th! A small revenge for the rock that they subjected me to! As a young man, I worked on pipe organs. We once instaled a rank of pipes who’s largest pipe was 64 ft. emmiting a tone of (IIRC) of 8 Hz No woofer at that time could reproduce it. Is there one now that can? Damn, I wish I figure out how to make paragraphs with this gaget!


Zymurgist

HeadlessCow is on the right track. (But if she stayed off the tracks, she wouldn’t be headless…)

You get directional hearing when each ear receives a sound at a different point in the sound wave. Turning your head until both ears are receiving the sound at the same point locates the direction of the sound. (sound levels help too - something behind you will not be as loud as something in front of you) Directional hearing stops when the sound wave becomes large enough that you can no longer detect a difference between what each ear is receiving. The pitch where this occurs is going to be quite a bit larger than your head.

Related trivia - the warning cry of most mammals is approximately the optimal pitch for determining direction, which is somewhere IIRC around the normal size of the animal’s head. Maybe this is what HeadlessCow was remembering.

Of course, this is all academic for HeadlessCow. Not having a head and all…

I don’t think I’m buying the “wavelength longer than your head” theory. Sly pretty much put that one to bed.

Zyada has it right (I think) about how we locate the source of sound, (both ears receiving the same point of the wave simultaneously) but why can’t the brain process the longer wavelengths in the same manner? Does it “forget” where the last wave was before it receives the next one for processing due to the lower frequency? I find that hard to fathom, since even a 40 Hz note is vibrating the ear drum and related structures at 40 times per second.

Remember that sound is a smooth curve change in air density. Binaural hearing is accomplished by comparing the difference in air density (effectively) being received by the two ears. Even though the ear drums are being vibrated 40 times a second, the difference in air pressure between the two is small enough that the brain can’t detect the difference.

For an illustration, go here: http://www-ece.rice.edu/~dhj/sound.html

Most super low bass notes (20-50 hz) are around 12-15 feet long. If your subs hit loud in your car, imagine the car 12 feet in front of you. I used to have 2 12 inch competition subs in my old car, and you couldnt tell where it was coming from. Most subs are put in corners of rooms because the walls tend to amplify the bass response some(by redirection). However, this can cause a muddied bass response, so you have to be careful.

For most common applications, bass is omnidirectional. Both the tone and the
volume must be low enough.

Subs are directional at high volumes. This phenomena is sometimes refered to as the “power alley.” Home/car audio gear (aficionados opinions to the contrary) is simply not powerful enough to observe this effect. It would be a mistake to confuse home/car audio with pro audio.

ServoDrive ContraBass subs have a flat response from 14-125Hz. They do not have a voice coil. I don’t know off-hand of a sub that can go down flat to 8Hz.

If you take a sub suspended in the air as 0dB, then each side of the sub that is backed by a wall represents ~3dB increase in volume. This is called “loading.” You will probably not gain 21dB if you cover six sides.

If you have more than one sub and the distance between the center of the cones is less than 1/2 the wavelength, you will gain ~3dB. This is called “coupling.”

One fun thing to do if you have 2 subs is to run one at the opposite polarity – just switch the + and - somewhere between the amp and the speaker leads. Place the subs side by side, so they would couple, and turn on one, then the other. The bass will vanish. Some people are amused by this. Turn it way up and your speakers will blow. Everyone is amused by this, except you.

As a young man, I worked on pipe organs. We once instaled a rank of pipes who’s largest pipe was 64 ft. emmiting a tone of (IIRC) of 8 Hz No woofer at that time could reproduce it. Is there one now that can?

Ummm, did you hear, or rather, get to feel this tone?

And my other question for you Heinlein fans, is did this coagulate blood, or just cause all the rats to run from the building?

OK, mostly WAG here, but I would say that because subs produce sound near the limit of human hearing, that is the source of its nondirectionality. If you’ve ever heard a piercingly high note, it is hard to locate as well. It may be nothing more complex than that our brains cannot determine directionality of sounds that our ears have a hard time distinguishing in the first place.

Oh, and a few more notes on subs. Its often claimed that you cannot hear a true sub, you can only feel it. This may be true for the sub played alone, but as part of a full set of speakers, you can definately “hear” a properly tuned sub (and in a full pipe organ, even hear the effect of the 8 Hz pipe) since the sub has an effect on the “overtones” of the “hearable” notes, giving a fuller sound. Now, the problem is that lots of people don’t know how to tune a stereo for optimum sound quality these days. People who overdrive the bass are missing out on the great sound that a sub was meant to give you.

Oh, and since subs receive sound from all channels, they are accenting the sound from whatever speaker is producing it, giving the sound the directionality of whatever speaker it is coming from. For a neat effect, turn off one of the channels to your sub. You can really hear the effect it has on the whole system.


Jason R Remy

“Open mindedness is not the same thing as empty mindedness.”
– John Dewey Democracy and Education (1916)

I visited that site Zyada provided, and it seems humans locate frequncies below 1k Hz by the time difference between arrival at each ear and notes above 5k Hz by amplitude differences. (I assume there is overlap between the two techniques for sounds in the 1-5k range).

So. Humans have a built-in mechanism for detecting the source of low notes, which would lead me to believe the “non” or “omni” directional terminology is bullshit. But reading the rest of the posts here, I guess humans have more difficulty locating tones near the upper and lower limits of the audible frequencies, (I work around some very loud complicated machinery, and high-pitched compressed air leaks can drive you nuts looking for their source) so the net effect is, you just can’t tell where the really good pants legs-flapping stuff is coming from.

To Nickrz’s: “…so the net effect is, you just can’t tell where the really good pants legs-flapping stuff is coming from.” I say:

 ...or you need to get some Bean-O.

“There will always be somebody who’s never read a book who’ll know twice what you know.” - D.Duchovny

Do humans actually determine sound direction by phase variance? If so, wouldnt you get aliasing when the wavelength or some whole number of wavelengths was the exact size of your head.

Falcon - No. Visit the link in Zyada’s post.
It’s VERY helpful.
(Thanks, Z).

Speaking of the pipe organ, 8Hz thing:

As a youngster, I was addicted to the “Alfred Hitchcock’s Three Investigators” series of books… Cheesy, but I was about 8 or 9 at the time.

I recall in one that a pipe organ in an old mansion was used to “create feelings of terror” in people by playing notes “lower than the range of human hearing”…

Anyone have any idea about that? Will subsonic sounds create a feeling of upset, edginess, etc.?

My dad gets real upset when a car blasting heavy bass drives by his house.

Are you doubting Jupiter Jones?