non-directionality of sound below 80hz (1 Viewer)

[John Doran]

could someone provide me with specific web-sites that have descriptions of the tests that demonstrate the (invariable?) non-directionality of sub-80hz frequencies?

i keep hearing about this as a particular sonic property of bass the existence of which has received "scientific proof".

however, i have seen claims to the contrary - namely that bass can be localized down to 50hz. while i recognize this could be due to audible harmonics or other secondary audible cues (speaker-enclosure noise, rattling, etc.), the claims of this sort haven't received any comprehensive support either...

so i'd be very interested to see the methodology and the results of the various tests i keep seeing mentioned, or any relevantly similar experimentation...

thanks.

 

[John Doran]

man. tough crowd.

i bet if i put "SVS" in the title of my post, i'd have about 1500 hits by now...

someone's gotta have a line on the info i'm looking for.

i'm going to start thinking non-directional bass below 80hz is an urban legend pretty soon....

 

[jeff lam]

How sharp is your crossover? If you use a 12dB/octave slope at 80Hz, you still get significant output at 160Hz, as well as 200Hz and above. These frequencies are directional. I beleive true non-directional material is 50Hz and below. Just my experience and ears though.

 

[Brian Bunge]

John,
I imagine 50-60Hz with at least a 24dB/octave slope would be a good starting point. From your original post, you've basically answered your own question (harmonics, etc.).
Brian

 

[John Doran]

thanks for the replies, but i think maybe i haven't been clear enough.
what i'm looking for are links to sites that document tests that either demonstrate that (fundamental) frequencies below 80hz are non-directional, or, if it's not 80hz, determine what actually is the point below which bass frequencies are non-localizable.
i wasn't implying that i am suffering from localizable bass frequencies in my set-up, and looking for a solution.
although i appreciate the suggestions i received.
can anyone help me?
tom or ron from svs? you guys have to have the skinny on this sort of information...

 

[Richard Greene]

Sound is directional down to 40Hz. for some people.

THX and others have done research on this but I don't have a link.

The research is not relevant for home theater because it used test tones playing through a single bass speaker.

The key question for home audio is how low does your crossover frequency have to be so that the location of the subwoofer is sonically invisible when ALL the speakers are playing.

THX came to the conclusion that an 80Hz. 24dB/octave low pass filter (down 6dB at 80Hz.) would work for almost all listeners.

I suggest experimenting in your own room.

Whether a subwoofer is non-directional also depends on:

(1) the level of harmonic distortion (extra bass output) above 80Hz.,

(2) whether or not the walls/floors/room objects near the

subwoofer rattle, and

(3) how much louder the subwoofer is set relative to the main speakers (+3 to +6dB is common)

(4) the distance between the subwoofer and the main speakers

In my own room, all of these factors combined required me

to use a 50Hz. 24dB/octave low-pass filter to keep my own

subwoofer sonically invisible. My main problem was the large distance between my corner subwoofer and the nearest main speaker. In the past when my subwoofer was located closer to my main speakers, a 70Hz. 24dB/octave low-pass filter was satisfactory.

 

[EricHaas]

There are dozens of websites that talk about this 80 hz cutoff for non-directional sound. Here are some examples:
http://www.geocities.com/kierancoghlan/SPEAKERS1.html
http://www.audiovideo101.com/learn/h...uyspeakers.asp
http://www.audiovideo101.com/learn/a...speakers17.asp
http://www.widescreenmuseum.com/sound/colortek11b.htm
None of these discusses it in detail however. Some use the term "relatively non-directional" and some say "about 80 hz." I think it just isn't something that has been established with scientific exactitude. Like most commonly held beliefs, there seems to be an authority behind it, that being the folks at Lucasfilm THX. I can't find anything on their website about it though. None of the links documents tests. Sorry, but I can't find any.

 

[John Doran]

thanks, richard - you have given me some focus and made me consider some things i hadn't before.

- jd

 

[Tom Vodhanel]

I have never heard of any THX tests showing SOURCE material is directional down to 40hz. Test tones with the absolute absence of higher shadowing freqs...yes.In fact, didn't the last round by Holman show that 120hz was also non-directional in most situations?

I don't know of any links on line, but You can try doing a search for Nousaine,Toole,Tomlinson Holman,and the NRC...you might find some links to the testing like that.

Richard, my bad...I missed this..

"The research is not relevant for home theater because it used test tones playing through a single bass speaker."

TV

 

[Bob_A]

Some good information here.

Richard, with a crossover set at 50Hz, would you recommend that one use main channel speakers which extend effectively down to about 25-30Hz?

 

[Bob McElfresh]

I was under the impression that bass WAS directional. But in most rooms, the very efficient wall reflections gave you the same sound from 5 locations (front, back, left, right, & direct), and "hid" the actual sub position.

 

[RichardH]

They way we tell directionality has to do with the delay between what the left ear hears and what the right ear hears. Also, the head itself casts an acoustical shadow, affecting the sound arriving at the ear on the other side. These issues of delay and acoustical shadows play less and less of a role as you go deeper in the frequency spectrum. This is why low bass is non-directional.

 

[Mifr44]

"Richard, with a crossover set at 50Hz, would you recommend that one use main channel speakers which extend effectively down to about 25-30Hz?"

More self-serving drivel. I wonder what kind of speakers those might be?

Michael

 

[Roger Kint]

 

[Steve Zimmerman]

Bob wrote:

...with a crossover set at 50Hz, would you recommend that one use main channel speakers which extend effectively down to about 25-30Hz?

That's correct, Bob! You are absolutely right. For best results, a person would not arbitrarily choose 50Hz as the crossover point but would need to make sure that his main channel speakers were able to adequately reproduce frequencies up to an octave lower. This would (obviously) require the use of a sound level meter and test tone playback material.

Whether or not a person chose to go down to 50Hz on the crossover would be determined by more than just the speakers. The position of the subwoofer(s) would be crucial to smooth frequency response as well, with the best results being more likely achievable if the person has some flexibility as to where they are placed in the room.

--Steve

 

[Ryan Schnacke]

Here's an excerpt from one of my previous posts:

Now lets talk about why low bass might be non-directional. Sound travels at, what, 700mi/hr or so? I'll use that number since I don't know exactly. So that's 1026.7ft/second. Now lets say that your home theater room is 12 feet long. So it takes 11.7ms for the sound to travel 12ft. 11.7ms is also the time it takes for one complete cycle of 85.6Hz sound. If you have a lower frequency sound then it takes even longer for a complete cycle and the sound travels farther than 12ft before it completes that cycle. So any sound with a frequency lower than 85Hz has very likely bounced off a wall or two before you hear a complete cycle.

I believe this is why low bass is non-directional when you are enclosed in a small room (Can someone verify or disprove this?). By the time you've heard a full cycle, the bass has bounced off walls and is coming at you from several directions. So you don't know where its really coming from.

This would mean that with a bigger room the bass must be lower before it is non-directional.

In that thread no one ever said whether I was on the right track or not.

 

[Dustin B]

The only issue I see with your theory is I've talked to a person in the military who has heard tanks coming before seeing them. The individual said when the tank was a ways away and all you could hear from it was bass, he couldn't tell what direction it was coming from. This was in the open with nothing to bounce the sound off of.

I think RichardH's post needs to be read again:

They way we tell directionality has to do with the delay between what the left ear hears and what the right ear hears. Also, the head itself casts an acoustical shadow, affecting the sound arriving at the ear on the other side. These issues of delay and acoustical shadows play less and less of a role as you go deeper in the frequency spectrum. This is why low bass is non-directional.

I would think another way of putting it is as you go lower in frequency the wave lengths get longer. As the wave lengths get longer it will become harder and harder to differentiate a delay. I'm not gonna work out some exact numbers, but in my mind this is what I'm seeing. Say you need a 5ms delay to the sound between your ears to detect a direction. If the wave length is long enough that it takes significantly longer than 5ms to propagate past you, say 10ms, you won't be able to tell the direction.

Sort of the same thing as off axis lobing with horizontally arrayed MTM center channels (just in reverse). If you crossover between the midwoofers and tweeter at to high of a frequency, the delay between the arrival of the sound from each midwoofer to an off axis listener will be detectable. But if you lower the crossover point so that the highest frequency the midwoofers produce has a wave length longer than the distance between the two midwoofers you won't be able to detect the delay.