Bass localization - directivity - why mono bass
How well can we hear the direction of a bass source, how does directivity work in the bass range, and why is mono bass the best solution in small rooms.
A mono bass-system with 120hz crossover is the best solution for small rooms - but how can that be true. Is, say, a bass guitar is panned to left, will this guitar now move to center, with the mono source? And can we be sure it is not possible to hear the location of a subwoofer at 120Hz?
This thread will attempt to answer those questions.
What we can hear:
I once did a controlled experiment to determine ability to locate bass sound direction. I found it becomes quite difficult to detect direction below around 100hz, but it was still possible to hear sound coming from the side down to around 60hz.
The experiment used headphones and signals with delay on one ear. The delay is determined by the distance around the head, such that the worst case will be sound coming from one side. Level is the same, due to the wavelengths at low frequencies it is not possible to have sound at different amplitude on one ear compared to the other, the ears are too close and the acoustic shadow of the head body is too small to be effective.
Placing a loudspeaker/subwoofer at different locations inside a room will not work for this experiment, because the resulting sound field direction is determined by the combination of the sound source location and room reflections. Room reflections causes this direction to be all over the place.
How does subwoofer location affect the direction of the sound field - is it determined only by properties of the room, or can direction change when subwoofers are moved?
In the "1x-2x-4x" article I describe different subwoofer bass-system configurations. Comparing the 1x to the 2x - which is 1 front left placed vs. 2 placed symmetrically front left and right, we sure do see a difference - the 1x left-only has a very significant increase in left-right direction velocity:
Green: 90 degrees sound field particle velocity 1x subwoofer front left.
Red, blue: 90 degrees sound field particle velocity 2x front, 4x front subwoofer units.
This increase is caused by the placement of the sound source now creating a left-right wave moving between the left and right side walls, which in the case of a symmetrical left-right source is cancelled.
So, it is possible that the sound source can create a sound field with direction that to some degree correlates to the placement of the sound source, even at bass frequencies. If a front-left subwoofer unit plays a bass instrument, the direction of the sound field will be from the front and from the left, while the same instrument played form both front-left and front-right subwoofer units will have a direction from front only.
But this does not mean we can hear the location, or direction, of the instrument. Hearing works on the leading edge of the wavefront, and measures the time difference between the ears, to find the direction. At low frequencies the change in amplitude per time interval is too small, it gets easily masked by all other simultaneous sound at higher frequencies. On transient instruments like drums, it is the first leading edge that is used to find the location, and this leading edge is spectral dominated by higher frequency energy, so that the direction perceived is determined by the direction of the sound field at higher frequencies.
Just adding a quick note here:
When I investigated perception of low frequency sound field properties (p and velocity vector), I found direction of sound p can be heard starting around 60Hz, I found direction of velocity for tactile perception to be undetectable (feels the same regardless of direction), BUT: level of velocity relative to pressure is indeed perceivable, and makes a huge difference in how low bass is experienced.
Having 2 channels, and feeding those individually to different sound radiators in the bass-system, it is also possible to change the properties of the sound field, so that a out-of-phase signal can create low frequency sound with a higher velocity and low pressure. This induces some quite special requirements for the system, to make it work. It can be done. Measurements from one of the tests shows a very significant difference in horizontal velocity comparing 1 radiator in one front corner to 2 radiators l and r (See above posts).
Stereo bass does nothing for placement of low freq instruments, but there may still be a difference in perception of bass and thus "space", due to differences in sound field properties.