Nondirectional & Inaudible Bass: What Frequencies?
Hello. A few quick technical questions: I am curious as to what frequency bass goes "nondirectional." I saw a reference in print to it being around 100-125Hz, but that doesn't seem right to me. Does the room have an effect or is it purely psychoacoustic?
Also, wondering at what frequency bass becomes inaudible. Thanks much!
Well, there are two perspectives on the issue of "directionality" with bass frequencies. Some folks will say that bass frequencies become essentially non-directional below about 40 Hz, while others will argue that we are much aware of low-frequency directionality than we realize.
I was of the school that thought bass was non-directional in the bottom octave (20-40 Hz), until I bought a really good subwoofer (a Vandersteen 2Wq). I had the sub in one corner of my listening room, and when I played recordings of pipe organ music, I sure could tell where the lowest frequency notes were coming from. This led me to buy a second 2Wq sub, and it made a vast difference in the quality of bass in my room.
Based on this experience, I now advise people who want good bass reproduction for MUSIC to buy a stereo pair of subwoofers. I believe strongly that a pair of subs is necessary to achieve realistic musical presentation.
Reproducing low-frequency effects (LFE) for home theater is a somewhat different matter. Low frequenices on DVD are usually sound effects such as explosions, deep rumbles, etc., and they are normally of short duration. The short duration of the sound gives us less time to become aware of directionality, unlike music where the low tones may be sustained for 5-10 seconds or more.
As for the audibility level of low frequencies, that will depend on: 1. your listening room; 2. the roll-off curve of your speakers and subwoofer(s); 3. the amount of amplifier power available for your speakers; 4. the nature of the recording (there is very little audio signal below 25 Hz in home theater).
Most home listening environments, if they have not been acoustically treated, show pronounced roll-off below 30 Hz, and "bad rooms" may even cut off the bass frequencies around 35 Hz.
Depends: non-directional as you hear it or when the low frequency wavelength becomes long enough to bend around objects?
The frequency that you perceive to be non-directional depends on the distance between your ears. This happens when the distance between your ears is less than 1/2 the wavelength - so take twice the distance between your ears (in feet) and multiply by the speed of sound (1080 fps) and that's the frequency where you start to lose directionality of the source (about 1000 to 1500 Hz for most people).
Where the frequency starts to bend is about 300 Hz.
All of the above is independent of the source. Where the source comes into play is based on the size of the driver. So the point of non-d depends on a specific ratio of woofer size to wavelength (for most drivers anywhere from 80 to 125 Hz).
Bass may go non-directional at 100-125 Hz (equal power in all directions), but that does not mean we CAN'T tell where it is coming from. In that case, the answer to your question is that the room does have some effect and that psycoacoustics are taking over at that point.
The audible cut off may vary from person to person, just as it does at higher frequency's. However, sound presure on the skin, the movement of hairs all over the body and the vibrations felt through the floor, chair etc... all take over at even lower frequencies. Your brain combines all of this audible and inaudible information to figure out what is going on. With that said, the lowest frequency in which you can DETECT it's source is probably lower than you think.
Sdcampbell make some good points, however I disagree with the time duration comment. Longer may be more difficult to detect, as the sound waves and their respective decays are colliding and overlaping each other all over the room. A short duration sound is much less confusing to the brain. We hear the initial sound and it's decay with nothing else to fool us.
I also disagree with the lack of low frequency info on home theater software. The LFE might not have much below 25Hz, but the amount of bass in classical music on a movie soundtrack is the same as what is found on 2ch. If you think about it, 80% of most movie soundtracks is actually music.
There are two issues about this. One is directionality, such as hearing a distinct sound coming from a source, vs perceived energy source. Thus GS has a point about the distance between the ears (except it should be 1/8 of a wavelength and your head can't move for that to really be true). My own listening says we lose directionality at a fairly high level, perhaps as high as 100 to 150 Hz, but we lose energy source perception somewhere between 40 and 60 Hz. Now there are many things that change these subjectively perceived values. For example, distortion in the subwoofer will translate into harmonics at higher frequencies, thus we may perceive directionality of a subwoofer that is crossed over at 60 Hz.
Room and placement of a subwoofer absolutely have an effect on this. There are multiple ways the room plays a significant role, from reinforcing bass frequencies (thus reducing total distortion) to positions at certain (say 1/4 wavelength) places that can create nulls and quickly give us clues as to where something is (at least generally).
One thing that makes a huge difference is multiple subs (as has been stated). This allows a flatter frequency response through the room, less work for each sub (thus less distortion), and two sources that make picking out where the sound is coming from very difficult.
From personal experience my REL sub is crossed at 27Hz (so probably outputting up to 35Hz). It was directional enough that I ended up placing it between the speakers, next to the left hand speaker (since I listen to a lot of orchestral music, and that's where most of the bass comes from). The sub now blends very well with my main speakers, to the point where it is very difficult to pinpoint the subs location with your eyes closed. I know nothing about psychoacoustics, room resonances etc, but my experience would lead me to believe that saying bass is non-directional below 100Hz is something of an over-simplification.
Sean: Please keep in mind the differences between directionality and perceived source of energy. They are distinctly different, but often confused. It does seem by your comments (left hand speaker since that's where most of the bass comes from in an orchestra) that you did experience directionality as opposed to energy source. Therefore, I would suggest that there is something other than pure bass coming from the REL. Either distortion or harmonics at much higher frequencies than 27Hz. Having heard some of the larger RELs I am surprised, but not knowing the model, it could be one of the smaller ones and could be the cause for having this occur. You really have me very curious about this, because in theory it should not exist--but we all know that "in theory" is just that--and often does happen. Could I pursuade you to use a test disc, or better sweep tones and turn off your main speakers and find out what the actual response of your sub is. I'll bet you are getting information out of it much higher than you expected.
Viridian hit it on the head. Moncrieff's research shows that bass becomes non-directional or "loads evenly" at appr 100 Hz. What allows us to precisely place the woofer or subwoofer at frequencies below that are either high levels of harmonic overtones or plain old distortion. This is one of the reasons why downloaded subs sound very different from front loaded subs. The harmonic overtones and distortion are masked to a great degree when downloading, making them harder to localize AND affecting their tonal balance or "attack" characteristics. Try listening to a sub that is downfiring and then turn it on the side so that the driver is now front firing. Other than the difference in the type of footers used, i think that you'll hear a MAJOR difference. Sean >
I think the theory might hold up better in an anchoenic chamber than in real listening rooms. Bass frequencies can excite all kinds of resonances in real rooms the que the listener back to the source of the added vibrations, the bass.
I think that theoretical calculations may be helpful, but eventually, each particular setup will have its own characteristics, depending on room size, the components, placement, and even the furniture (and so on). In my experience, the importance of bass directionality increases proportionally with room size. Using test tones, in my smallish living room, I could not identify the source below 70 Hz or so. Again, with test tones, the very lowest limit where I could hear a signal was 20 Hz, but this can vary from person to person and the equipment also makes a difference. At 16 Hz, I could clearly see the woofer's cone moving, and feel some physical air pressure, but no sound (at least for me). I think with real music these low frequencies are seldom "audible" per se, what they most often do is to "round up" or to "fill" the sound of the instruments (complementary harmonics? I'm not sure). Anyway, this is just MY experience from the period when I was concerned with these questions spending hours with measurements and tinkering. Lately I just try to enjoy the music :).
Different theory. My room is 15 x 25 feet with high ceilings. Instead of stereo subs, I am crossing everything over at 100hz and have 1 sub in the front and 1 in the rear. With a lot of furnishings in the room, the bass just wasn't even throughout the room. What subs: pair of Nelson-Reeds with Janis 1/a sub amp-crossover and an energy.
I agree Unsound, Viridian did bring up a VERY good point. The fact that the mass majority of powered subs that are out there are not properly calibrated in terms of crossover frequency does not help any either. Most all subs are actually crossing over at a higher frequency than what the controls state. Top that off with most of them having a less than "ultra-steep" crossover slope and you have a LOT of "blending" between the sub and the woofers. Too much in most cases. That is why so many people have a hard time trying to get a good blend between the mains and the subs.
As i mentioned earlier though, downloading will help to reduce the amount of apparent upper bass output. This has to do with the "tuning" of the space between the driver / baffle board and the flooring underneath it. This type of system has a narrow band of efficiency with everything else acoustically sloping off outside of that band. By playing with the distance of the driver from the floor, one can alter the loading / coupling of the driver via the pressure front and "fine tune" the bandpass that you want highlighted most. This also changes the "Q" of the speaker system, which will affect the low frequency extension, output and transient response of the sub. If you are looking for anything BUT the lowest bass though, downloaded designs do not have as much apparent "attack" and / or defintion due to the lack of upper harmonic structure. As such, they do not work well when being used as a woofer and are best suited for sub-woofer only use. Sean >
Sean, I'm sure your aware of the downloaded woofers on Dunlavy's Athenas, Catenas, SC IIIA. John Dunlavy told me as a string bass player he liked the results of this approach, but he somewhat avoided the issue when I asked him if it was as accurate as his direct firing (to listener) designs. I have to admit I'm a little dubious of this approach as everyones flooring is different and so would the reflected sound be. Also I suppose that it would have a greater chance of exiciting the the rest of the room and surrounding rooms structure and would probably be limited to ground floor rooms. I really don't think I'd like to live below someone with such an arrangement. Further more as a believer in time coherent designs (I do try to keep an open mind about this, though) it would appear to be a contradiction in that regard. I suspect that it might really work best with omnidirectional "satelite" designs on a somewhat detached ground floor.
In my listening room 65hz seems to be a point where I can no longer notice it's origin.80hz is very locatable.Wish i knew why.A single sub exites the room at certain locations and not others.Two subs in opposite corners exite the room at different locations,mostly "spreading" the sound out more.While putting two in one corner only increases bass out put in the same location as the first.I have seen my subs peaking front to back at freq below 18hz and for what ever the reason the room never reenforces it.No matter where i put the sub.Confused?so are the pro's
Unsound: I had forgotton all about those models. Personally, i think that downloaded subs lack "attack". This probably has to do with the longer signal path to our ears as compared to a direct front radiating design and the damping of upper harmonics that typically takes place in such a design. I only find such designs suitable for use as SUB-woofers if you know what i mean. I don't think it works well for anything but the bottom octave or two at the very most. The downloaded subs in my HT system are actively crossed at 30 Hz and the downloaded subs ( as used with small two way monitors on stands ) in my bedroom system are actively crossed at 65 Hz.
As far as exciting the floor and the building structure itself, yes, you get a lot more of that. It is especially noticeable if you are on a suspended floor aka over a basement or on the second floor, etc...
Your observations about various loading conditions from installation to installation are also quite valid. There are two different approaches that one can take to this. One can allow adjustable loading via moving the box up and down via threaded spiked feet or cones. This allows one to adjust the amount of loading and vary the tuning to best accomodate individual rooms and tastes.
The other approach is to have a permanent "sound board" ( like a piano uses ) or "loading plate" built into the design. This approach maintains consistent loading characteristics due to the consistent space between the board and the driver regardless of what type of flooring ( carpet, tile, etc... ) you have underneath it.
Both are valid approaches and up to the designer and end user as to what they like best. Personally, i like the adaptability of adjustable feet but some people would prefer relying on the design expertise of a professional as they don't trust themselves to "get it right". Sean >
Sean , I think Roy Allison designed a system such as you described with a canted down firing woofer reflecting it's sound off a fixed platform some years ago.
I don't doubt that Allison did something like that as he was always playing with / taking into account room loading and gain. There have been other designs that fire the woofer upwards into a "loading plate", which achieves the same effect without causing the spider ( part of a speakers' suspension ) to sag quite as much. Obviously, it is just another variation on a somewhat common design. As such, it has trade-offs of its' own to deal with. Sean >
You might be right. I think that the first commercially available "loaded" woofer system was the Janus made by Jan Marovskis. That is, as far as i can remember.
Design Acoustics, a division of Audio Technica, also did this on some of their models. These were probably the most commercially succesful down-firing or "loaded" speakers that i recall. I personally think that DA took this approach primarily for space saving reasons, but they really weren't bad sounding speakers for the price and era. Sean >
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