Nondirectional & Inaudible Bass: What Frequencies?

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.

The fallacy of the whole non directional bass arguments is that harmonic distortion generated by cone movement is, by definition, at higher harmonics of the fundamental. I would argue that these harmonics are at frequencies that are directionally audible even if the evidence is out on the fundamental frequencies.

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.

Hope that helps.