Hello Paul6001. All the sound waves move at the same speed. The wavelengths vary with frequency, but not the speed of their travel. We are trying to reproduce music that arrived sometime in the past on the diaphragm of a microphone. We do not want to add any more time "smear" when we listen to that recrded music. The microphone(s) "hear" what you would hear sitting in the same place as the microphone(s). We want to produce the same sonic patterns in our room that the microphones picked up. So we don't want parts of that recorded sound to reach our ears earlier or later than other parts. Don't mess up the puzzle! If it gets messed up, we won't be able to reassemble in our brains the sound pattern that hit the sensitive parts of the "mikes." If we distort that pattern, we will not be able to successfully reconstruct the performance captured on the recording. The feeling of "being there" depends on an accurate reconstruction of the sound patterns in the recording. Woofers are generally larger than tweeters. If we mount them on the same panel, the voice coils that vibrate to reproduce sound are at different distances from the panel (this is why flat panel speakers have an advantage in accurate sound reproduction - all the sound leaves from the same place). So the sound from the woofer arrives a bit later than the sound from the tweeter. That's the problem some speaker builders try to solve by either moving the tweeters backwards or moving the woofers forward. That leads to some odd loking boxes, but, if perfect reproduction is the goal, it's worth it. Ideally, all the sound producing parts of the speaker system are at equal distance from your ears. Happy listening.
A Question About Time Alignment
I was reading a review of the Wilson Alex V on Stereophile recently. (Published just in time. I’m thinking about picking up a pair. Maybe a couple for the bedroom, too.) And it raised a long-standing question of mine, one that I hope the wiser minds on this site can answer.
Wilson’s big selling point is aligning the different frequencies so they all reach your ear simultaneously. As I understand it, that’s why they have minute adjustments among the various drivers. The woofers put out bass notes that move slowly thanks to their long sound waves while the tweeters are playing faster moving, high frequency notes with short waves. Wilson lets you make adjustments so that they all arrive at the ear at once.
It seems to me, however, that live music isn’t time aligned. Suppose I’m playing the piano and you’re sitting across the room. When I stretch out my left hand to hit the low notes, those notes travel along the same long, slow wavelengths as the notes from Wilson’s woofers. Similarly, the treble notes I play with my right hand move quickly through the short wavelengths. The notes from the piano are naturally out of alignment. If Wilson’s goal is to achieve a lifelike sound, aligning the frequencies doesn’t seem like the way to do it.
Wilson has been selling lots of zillion dollar speakers for lots of years and people continue to gobble ‘em up. Something must be wrong with my line of reasoning. Would someone please point out where I’ve gone wrong? Nicely?
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@boomerbillone +10 Mike |
Two other things I can add to the discussion: 1- An interesting situation exists in the case of line-source loudspeakers, a good example being the Magnepan MG3.7i. This speaker has a long, vertically-orientated ribbon tweeter (a real good one), with magnetic-planar drivers for midrange and bass frequencies running along side the tweeter. In the instruction manual for the 3.7i, it is advised that the speaker be positioned so that the tweeter is slightly further away from the listener’s ears than is the midrange driver. The reason for that is that the speaker’s crossover creates a slight time lag in the midrange driver. With the tweeter and midrange driver equidistant from the listeners ears, the two drivers are not quite time/phase aligned. 3.7i owners need to experiment with varying degrees of toe-in, until the highs and mids sound coherent. With a dynamic loudspeaker (cone & dome drivers in a box)---with the drivers aligned vertically, as most are these days---tilting the enclosure forward or backward can sometimes be used the same way. Raising or lowering the enclosure instead achieves the same result, of course. 2- In a number of his YouTube videos, Danny Richie explains why tweeter and midrange drivers should be mounted as close together as possible. And why the higher the x/o frequency between them, the closer they should be to each other. That is because at the high frequencies tweeters are producing sound, the wavelengths are very short. Danny explains it all far better than can I, so if interested do a search on YouTube for GR Research. A free primer in loudspeaker design basics! |
Time Cohesion is making sure that the sound from the tweeter gets to your ears at the same time as the sound from the woofer. Lets say theres a piano sitting 10 feet in front of you and a cello sitting 6 feet in front. The sound of the cello hits your ears faster than the sound of the piano. Its the ratio of the speed that needs to be conserved by the speaker. This can only happen if the time cohesion is tuned perfectly. The master has spoken. |
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