On 10 December 2014 at 02:51, Gord Slater <[email protected]> wrote:
> > In addition, two or more radio transmitters, or a single transmitter > feeding several antennae, either by the most direct route or by adding > delays or extra distance in the transmission line or signal path between > the transmitter and receiver also cause interference patterns. They are > utilised where needed and we try to avoid them where they would be a > problem. We can even dynamically generate and manipulate these patterns in > the time domain to provide performance gains or specific effects. That is > far from a new concept. Look at a PAVE PAWS radar site for an > easily-verifiable 1980's example. > The several versions of the US "Space Fence" system also give an interesting look at beam forming going back to the 60s. Look out for the S-band system that they're going to be bringing online in the next few years. > Earlier techniques for other purposes go back decades before that. too, > not just for radar. The human head processes acoustic signal in stereo to > provide direction finding using the same techniques in receive mode. > Looking at phase differences is only one of a range of techniques the human brain uses for direction finding. Chief amongst these are differences in sound pressure, and the ways in which the shape of your ear affects the sound depending on the direction that it arrived from. This is one of the reasons why you can often tell when a sound is coming from behind you even though you only have two ears. > The self same dynamic method using interference patterns. A domestic hifi > system uses the same interference patterns ot provide a stereo image, the > perceived positioning of the sound depending on the SNR at the receiver, > which can become very complex and degraded with unwanted interactions and > reflections off nearby objects, especially ones that resonate at particular > frequencies, though the medium is somewhat different, the same interference > patterns occur for better or for worse, intentional or unintentional. > The interference patterns you find at human acoustic frequencies generally only become noticeable, read noticeable as troublesome, at the lower frequencies below 200 Hz or so. At higher frequencies the node/anti-node patterns become so tightly spaced as to become effectively immaterial. Complicating matters further is that at frequencies below 400 Hz speakers become more and more omnidirectional, even when they are not intended to me. A "hifi" is not using interference patterns. In fact, one of the reasons why there's a "sweet spot" in listening to a system with only two speakers, is because you're trying to sit in a position which is a node at as much of the spectral range as possible. You end up with parts of the room where you have good high range response, mediocre middle, and good bass response, or frequently were everything sounds good apart from bass frequencies. This is why you always use a system simulator when you're designing a line array for live use. The Line Arrays themselves have several emitters which are closely spaced, creating a system where interference patterns strongly affect the path of the sound waves vertically (this can actually be in any plane depending on how you build it) making it behave as one large directional emitter which is strongly directional. Modern line array designs use waveguide emitters to ensure good horizontal beamwidth whilst retaining the line array effect vertically. D&B go as far as having the waveguides user-alterable so that you can lay the emitters on their sides whilst retaining horizontal dispersion and not wasting sound vertically. You might remember the frequency I mentioned earlier, 400 Hz, so yes line arrays only behave as these single large emitters at high and medium frequencies. You may also remember my comments on interference patterns at and below 200 Hz. You have to use the same simulation to check that your bass bins are positioned so as to avoid interference as much as possible. To do this in a small hall, say 1000-2000 capacity, you may need only two line arrays of six to eight elements each, but you would need six to eight individual bass emitters widely spaced, to achieve the same low frequency coverage. Usually when you're using these line array systems you're carrying out your primary aiming purely by adjusting the angles of each element relative to each other, and from there relative to some fixed datum. It is possible to build phased line arrays to aim your sound purely using phase differences, but it was quickly realised that different frequencies need differing phase differences in order to produce the same interference patterns throughout the spectral range, doing otherwise lead to some very odd sounding systems. This fell out of use, preferring simply to aim the elements, and then provide fill speakers in spots that can't be covered. Of course, life is made more interesting by venues who wouldn't know acoustic treatment if you hit them with it. ExCel, I'm looking at you - "you don't have to worry about reflections" my shiny metal arse.
