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Since its inception ten years ago, the Harbeth User Group's ambition has been to create a lasting knowledge archive. Knowledge is based on facts and observations. Knowledge is timeless. Knowledge is human independent and replicatable. However, we live in new world where thanks to social media, 'facts' have become flexible and personal. HUG operates in that real world.

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Woofer driver positioning on baffle - in front or behind baffle?

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  • #16
    Lags in crossovers?

    Originally posted by A.S. View Post
    The velocity of sound waves coming from the bass/mid and tweeter are the same regardless of frequency.
    Yes, so if I were to assume that an ideal crossover had managed to exactly time align tweeter and bass/mid and then the bass/mid was moved back by 20mm would that mean that sound from the bass/mid now lagged behind the sound from the tweeter by the fraction I suggested?

    I am confused as to whether you're point is that the statement is incorrect or that it is simply not the right way to view this issue.

    Originally posted by A.S. View Post
    That is, approx. 340mS (very, very, very slow compared with the velocity of light).

    At 340Hz, the wavelength of each cycle of a sine wave is 1.0m. At 3400Hz (3.4kHz) the wavelength is 0.1m (10cm) .....
    340 Hz is around F above middle C isn't it; 2cm as a fraction of 100cm (the wavelength) now seems relatively large (1/50th). 3400 Hz is close to the top of the piano range; 2cm as a fraction of a 10cm wavelength is significant (but my assumption is that the bass/mid isn't doing anything at this frequency.)

    The relationship between wavelength and frequency (wavelength = velocity/frequency) is one that I had long since forgotten, thanks for the reminder.

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    • #17
      Time (mis) alignment

      Originally posted by weaver View Post
      Yes, so if I were to assume that an ideal crossover had managed to exactly time align tweeter and bass/mid and then the bass/mid was moved back by 20mm would that mean that sound from the bass/mid now lagged behind the sound from the tweeter by the fraction I suggested?
      Let's be honest, we're not really "time-aligning" the two drive units. We can't do that in a passive crossover. We could (theoretically) do that in a digital crossover because we'd hold the data that would otherwise feed the tweeter directly in a frame-buffer and release it at exactly the right time. But analogue passive crossovers don't have frame stores do they. So we have to say "time alignment" in quotes because that's impossible to achieve without digital technology, when, should you wish, it would be a trivial matter to delay the signal to the tweeter by a second, a day, a week .... just depending upon how big the frame store is. But "time aligmnent" of a compromised type is definitely what we are aiming at.

      340 Hz is around F above middle C isn't it; 2cm as a fraction of 100cm (the wavelength) now seems relatively large (1/50th). 3400 Hz is close to the top of the piano range; 2cm as a fraction of a 10cm wavelength is significant (but my assumption is that the bass/mid isn't doing anything at this frequency.)
      Right to say that 2cms as a fraction of 10cms is a huge percentage. Percentage of what? Percentage of the cycle. And we know that a cycle is another way of saying 360 degrees ......

      And most definitely, the bass/midrange is working nearly at full loudness to about 3kHz, is at about 1/3 loudness to 4kHz, tenth loudness to perhaps 6kHz. Again, crossover filters gradually fade out the woofer and fade up the tweeter. IT IS THIS LARGE OVERLAP BETWEEN DRIVE UNITS WHERE THE "TIME ALIGMENT PROBLEM" LIES. If the bass/mid's contribution stopped dead and handed over sharply to the tweeter the problem of mis-alignment would be inaudible.

      We've covered crossover slopes in this TechTalk issue.
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

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      • #18
        Consequences of signals out of phase?

        Many thanks Alan, that starts to become much clearer.

        Back up at the top of the thread you said:

        It's about path length adjustment.
        which appears to be the better way of thinking about it; as you say above, actual "time alignment" would be something else.

        Is the (possible) reduction in energy in the crossover region due to "time alignment" issues therefore a consequence of signals being slightly out of phase and cancelling each other?

        To come back to your query about the technical level at which you pitch the TT videos. In general I think you do very well at keeping things in 'everyday' language. In common with most industries though there are terms which can appear to mean one thing if taken literally but their accepted usage within that industry actually means something else -"time alignment" being the example here.

        That said, I believe your explanations are clearest when you describe things directly and don't try to over-simplify, perhaps it is just a case of making clear when a term being used isn't being used literally.

        Lastly, a point which crops up frequently is the difficulties audiophiles can get into when we believe we understand something, when we believe we know that such and such component is making such and such contribution - when in fact the science says something completely different. But if we don't understand the science, if the terminology doesn't actually mean what we think it means, it's easy to see how these issues are perpetuated.

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        • #19
          Time alignment of recording microphones

          Just crossing over with the comments from Paul G Smith in the mic thread regarding spot mics and so on got me wondering about delays in sound reaching us - or more specifically microphones - in an orchestral context.

          A thread on recording.org answers much of what I had been thinking about.

          It also talks about 'time alignment' in the way that I had originally assumed was meant in this thread.

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