Many of us spend a great deal of time and money constantly upgrading our hardware in order to achieve the peak of audio nirvana. Some of the audiophiles among us spend much time worrying about our hardware distortion figures and very little time worrying about something that can (and does) have the same if not worse derogatory effect on the overall sound quality of their systems. "Acoustic Distortion" is a much overlooked and misunderstood subject. Some see it as a black art when in fact it is not. A basic understanding of the types of acoustic distortion will go a long way into opening the door to this subject. 1) Speaker Boundary Interference. This is the direct and in-direct interaction of the early reflections from your room boundarys. Floor, ceiling, walls etc. 2) Comb Filtering These are the constructive and destructive interference between direct sound and early reflected sound. 3) Poor Diffusion This comes from sparse spatial and temporal reflection patterns due to mid and late arriving reflections 4) Modal Coupling The sonic coupling (or partnership) between the loudspeakers and listener with the rooms pressure variations (or room modes) As mentioned before, the lower down the frequency spectrum, the more obvious problems become. For this reason, the rooms acoustical signature is very much characterized by its low frequency modal response and speaker boundary interference such as hefty early reflections from hard surfaces. When sound from speakers or subwoofers hit the room boundaries, a very complex series of reflections occur. As its very difficult to isolate direct sound (and you wouldn't want to isolate all of it anyway) these reflections interact amongst themselves to produce a huge range of effects many of which are largely derogatory to the overall sound quality of the given system. So much so that even when spending tens of thousands of pounds on equipment, this sonic distortion will prevent the listener from hearing the full potential of those expensive amps, processors, and more importantly speakers and subwoofers. Room Modes, Sound waves produced in rooms coherently interfere as they reflect back and forth between hard walls. This interference results in resonances at frequencies determined by the room. As mentioned in an earlier thread, axial modes between two opposing boundarys is not difficult to calculate. To save my typing finger, I'll quote my earlier post, One of the reasons why the 2/3/5 ratio is popular is that the dimensional ratio's that uniformly space the modal pressures are easier to deal with. Speaker boundary interference response room modes happen because quite simply, reflected sound interferes with itself. This type of acoustic distortion occurs because of interference between the direct sound from the speaker/subwoofer and the reflections of the room. In particular the corner imediately surrounding it. *Note* As mentioned, although this "distortion" occurs right through the frequency range, it is more significant at lower frequencies. FOR THIS REASON ALONE, WHERE POSSIBLE, SUBWOOFERS SHOULD NOT BE PLACED IN CORNERS.*** Secondly, the rooms boundaries act as a mirror for the speakers causing virtual images of sound. When these reflections combine with the speakers direct sound, they will either enhance or cancel it to differing degrees depending on the phase relationships between the direct sound at the listening position. When the sound is "enhanced" this is known as a peak. When cacellation occurs, the sound "dips" thus a given frequency spectrum on a graph can be full of dips and peaks. This irregular pattern can be addressed to some extent by equalization. Many such systems exist, the Beringer and TACT systems are just two. However, purists will tell you this is adding more rubbish into the chain. In particular, electronically compensating for deep nulls or dips is not seen as good practice. Lets take a brief look at what is probably the most talked about acoustic problem. Poor diffusion! Many Home Cinema fans try to aspire to the very best mulitplex Cinemas by trying to create the diffuse, enveloping surround soundfields of these cinemas by using dipole speakers. In order to understand why, lets take a look at how the Leicester Square Odeons of this world acheive their all involving, enveloping surround. Very simply, they use what is in effect multiple arrays of monopole speakers and in best examples correct diffusion treatment. Thats it. Well, not quite, in the top cinemas there is a great degree of equalization that takes place which involves the use of very sophisticated and specialized equipement. Why the 'backwards' correction, why not design a cinema that is acousticly optimum in size and shape in the first place so they dont have to re-correct. Firstly, we must remember that in the days when some cinemas were built, sound was still in its infancy. In fact many of the very first cinemas were theaters or concert/music halls that had been converted. In later years when the Muliplex idea was born from the USA (where else) apart from building new complex's, these 'old' cinemas were once again redesigned and need I say wrecked, internally and acousticly. Usually, and as is the case of the Barnet Odeon which I visited for the first time last year, the circle upstairs would be 'cut off' and used as the biggest "screen 1". This would invariably leave the lower and larger stall areas which were split into a further two or even three smaller screens. If the Barnet Odeon cinema experience is anything to go by, I would rather be poked in the ears (and eyes) with a sharp stick! The old concert halls that were not tampered with, were believe it or not, acousticly the best sounding. Why. They already had acoustic treatments in place many decades before treatments were taken seriously at all. Many of these concert halls had design orientated space rather than the squeeze every bum onto as many seats as possible concept that is todays money driven world. These spaces were like mini areana's that were made up of fancy semi circular spaces, multi-tier shapes and even viewing box's at the sides for the upper classes. The result being the reduction of unwanted parellel surfaces to a minimum which basicly killed off any boundary interference. More importantly, the elaborate, ornate mouldings and other surface relief that was on virtually every wall and corner of the halls acted as superb 'natural' diffusers. Fantastic. Just imagine, perfect diffusion 100 years ago. A note here* By perfect diffusion I dont mean that each listener in the hall hears the same total loudness. Direct sound decreases as the distance between the sound source increases. BUT, by acheiving EVEN reflected energy around the hall or room, the problems caused by too little or to much reflected sound is to a great degree avoided. Anyone who has visited the Royal Albert Hall for example will no doubt testify to the incredible acoustical properties of this great place which are mainly due to the reasons I have outlined above. You can sit ANYWHERE in this areana of sound and still be rewarded with amazing sonics. Yes, there was surround sound before Luca$........ Getting back to our home cinemas, I regard the use of di-pole speakers a backward step most situations. The fact that these speakers are designed to actually produce a diffuse sound is suspect enough. Dipoles in effect try to achieve the diffused sound that results from proper reflective surface dispertion by trying to replicate these diffusive qualitys. Indeed you can say that they try to pre-empt these effects with their own diffusion. This diffusive sound that is produced is very far from ideal and by the time these 'diffused' soundwaves bounce around the room and into each other, you basicly have a mushy, incoherent mess, with or without treatments! Di-poles were a little more acceptable in the days when Pro-Logic and the limited mono rear channel with its even more limited frequency limits ruled. IMO this is a 'compromise' design. Diffusion should be achieved AFTER it has left the sound source be it singer, band, or home cinema system, not during. Having said that, di-poles do have their uses in smaller rooms and living room environments where, for various reasons, enough diffusion via the correct methods does not take place. However, in dedicated rooms where options are much wider and correct diffusion can be achieved, I always advise against the use of di-poles. Part 2 to follow.