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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.

HUG has two approaches to contributor's Posts. If you have, like us, a scientific mind and are curious about how the ear works, how it can lead us to make the right - and wrong - decisions, and about the technical ins and outs of audio equipment, how it's designed and what choices the designer makes, then the factual area of HUG is for you. The objective methods of comparing audio equipment under controlled conditions has been thoroughly examined here on HUG and elsewhere and can be easily understood and tried with negligible technical knowledge.

Alternatively, if you just like chatting about audio and subjectivity rules for you, then the Subjective Soundings sub-forum is you. If upon examination we think that Posts are better suited to one sub-forum than than the other, they will be redirected during Moderation, which is applied throughout the site.

Questions and Posts about, for example, 'does amplifier A sounds better than amplifier B' or 'which speaker stands or cables are best' are suitable for the Subjective Soundings area.

The Moderators' decision is final in all matters regarding what appears here. That said, very few Posts are rejected. HUG Moderation individually spell and layout checks Posts for clarity but due to the workload, Posts in the Subjective Soundings area, from Oct. 2016 will not be. We regret that but we are unable to accept Posts that present what we consider to be free advertising for products that Harbeth does not make.

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{Updated Nov. 2016A}
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Sound waves and smells

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  • #31
    Waves

    Originally posted by HUG-1 View Post
    Alan covered this with his explanation of push-pull. Think only about the wave generation at the speaker source.
    In which case I'm afraid I must beg your indulgence for my being rather slow on the uptake.

    Originally posted by A.S. View Post
    ... the vacuum will create a pressure shock wave towards it as it tugs the air molecules across the room. The hand clap sends a 'positive' pressure wave into the room, the vacuum creates a 'negative' pressure wave towards it. If we listened carefully we'd hear a 'schloop' sound. But we wouldn't actually be able to tell by listening whether the direction of the wave was positive going (proceeding towards us) or negative going (air being pulled from us). The sound would be identical.

    However, consider what happens if we replace the real human and his hand clap with a mono recording of the same hand clap which we are going to reproduce over a single loudspeaker corrected normally with red to +. The loudspeaker again sends out a positive going shock wave. And if we swap the connections so that red goes to the black socket? Then the reversed hand clap will initially suck the air from the vicinity of your ears right across the room.
    As was demonstrated earlier in the thread, a sound wave consists of a succession of compressions and rarefactions of the air molecules; a 'negative' pressure wave can no more 'suck' a smell across a room than a positive one can 'blow' it.

    A speaker cone when at rest can either move outwards first - creating an initial compression, or inwards first - creating an initial rarefaction, what follows thereafter will always be the opposite will it not? - thereby creating the compression-rarefaction series that we hear as sound.

    If we have two cones fed an identical sine wave with one moving in when the other moves out, then one will be creating compression when the other is creating rarefaction and they are 'out of phase'.

    However, if one is fed a sine wave of half the wavelength of the other, then on every half cycle of the longer wavelength the cones will actually be moving in the same direction at the same time (the applet that Pluto links to in post #26 is what I'm looking at here) irrespective of whether the cones are in phase or 180 degrees out of phase.

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    • #32
      'Tuning out' reflections

      Weaver - can we please clarify what you are asking?

      All stereo signals must contain a certain amount of out of phase material - they would not be stereo were this not so. The idea is to provide just enough to achieve a nice musical sense of width, but if you go too far down that road you arrive in nausea territory!

      The use of static sine waves is purely as a learning aid and has little bearing on real world use of stereo. It certainly isn't worth complicating the subject by thinking about the relative phase of sound waves reflected off your walls. While such aspects might change the sound within your room by a small amount, the big picture will remain relatively unaffected. The human ear is rather good at "tuning out" unwanted reflections - perhaps because evolution has made us used to listening within an environment a few tens of cubic metres in size. Thinks - has our hearing evolved to perform optimally in the smallish spaces in which our ancestors lived?

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      • #33
        Waves bouncing off walls

        Originally posted by Pluto View Post
        It certainly isn't worth complicating the subject by thinking about the relative phase of sound waves reflected off your walls. While such aspects might change the sound within your room by a small amount, the big picture will remain relatively unaffected.
        The 'reflected off walls' issue was just a way of trying to visualise (for myself) a situation where sound from a speaker would meet another version of itself that was slightly out of step.

        If we have a single speaker playing, and assume a 'perfect' reflection coming back at it, is that situation comparable with a pair of speakers playing the same mono signal but out of phase?

        Clearly, sound as it makes it way through the air does not exist as a set of discrete frequencies and amplitudes; they add and subtract from one another to give highly complex waveforms and I think that perhaps this is the root of my problem.

        Would it be correct to say that, relative to a point source of sound that the angle of our heads (ie the distance of each ear to the source) gives rise to a difference in phase between the sound reaching each ear?

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        • #34
          Originally posted by weaver View Post
          ...If we have a single speaker playing, and assume a 'perfect' reflection coming back at it, is that situation comparable with a pair of speakers playing the same mono signal but out of phase?...
          Logically it can't be can it. A pair of speakers = two speakers at two different points in space in the listening room. However you look at it, such a pair cannot ever be considered the same sound generator as one soiltary speaker at one point in space can it?

          Clearly, sound ... add and subtract from one another to give highly complex waveforms and I think that perhaps this is the root of my problem.
          Are you sure about that? Superposition? Do the sound waves "know" that they are "out of phase"? Unlikely. Their phase status must be relative to the observer, be that (a pair of) human ears or a measuring microphone.

          Best surely to think of a pressure wave? As I showed in my trumbone example http://www.harbeth.co.uk/usergroup/s...665#post14665?

          Would it be correct to say that, relative to a point source of sound that the angle of our heads (ie the distance of each ear to the source) gives rise to a difference in phase between the sound reaching each ear?
          Evolution has given us two ears on the sides of our head (not on the top like a horse) for resons to do with localising sound direction. One technique used by the brain is relative loudness left/right - the other is relative phase, another word for arrival time.
          Alan A. Shaw
          Designer, owner
          Harbeth Audio UK

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          • #35
            Waves ....

            Originally posted by weaver View Post
            If we have a single speaker playing, and assume a 'perfect' reflection coming back at it, is that situation comparable with a pair of speakers playing the same mono signal but out of phase?
            No. At least, not in the real world. While there are many experiments in physics that involve light beams interfering with one another, AFAIK they deal with monochromatic light (i.e. a single fixed frequency). I suspect that the idea of sound waves doing the same across a broad range of frequencies isn't practical. It would require an extremely large reflective surface if nothing else. It might be noteworthy that digital noise cancelling techniques operate by synthesizing the effect I believe you are thinking about - adjusting relative phase at different frequencies to achieve a cancellation.

            Originally posted by weaver View Post
            Clearly, sound as it makes it way through the air does not exist as a set of discrete frequencies and amplitudes; they add and subtract from one another to give highly complex waveforms and I think that perhaps this is the root of my problem.
            Very true. Sine waves are all very well for learning concepts, but real life ain't like that!

            Originally posted by weaver View Post
            Would it be correct to say that, relative to a point source of sound that the angle of our heads (ie the distance of each ear to the source) gives rise to a difference in phase between the sound reaching each ear?
            Indeed. The principal mechanism by which we locate the position of sound sources at lower frequencies (which for these purposes means frequencies at which the half-wavelength is larger than the size of a typical head - roughly 1kHz) is essentially one of phase difference.

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            • #36
              On reflection....

              A pair of speakers equidistant from the ears, radiating out of phase could never be compared to a single speaker plus its reflected sound...even if 100% coherent reflection at all frequencies was possible...which it isn't in the real world. Because....

              ...the velocity of sound varies slightlywith frequency so by the time the wave front has made the trip from a single point source, to you, to the infinite wall on which it is reflected and back again to you, the phase relationships between frequencies will be completely changed.

              The out-of-phase nastiness originating from a pair of incorrectly wired speakers is 100% incoherent - a situation that simply does not exist in nature, hence it's disturbing quality.

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              • #37
                Time and phase

                Thanks to the posts above the situation is starting to become clearer.

                I would like to just explore the time/phase relationship a little though.

                Can we assume a 'perfect' pair of speakers being fed 'perfect' test tones and reproducing them perfectly such that what we hear is just pure single tones.

                If we play a mono 100 Hz signal through both speakers and listen at the prefect sweetspot in our perfect room we should hear a tone from midway between the speakers shouldn't we.

                If we then introduce a delay to the tone to one of the speakers of 1/200th of a second (ie half the period of the tone) would the speakers relative to the listener now be 180 degrees out of phase?

                Assuming the answer to that is yes, we move on

                Keeping the delay of 1/200th sec we move to a tone of 50Hz, the time delay is now one quarter the period of our tone so are the speakers now out of phase by 90 degrees?

                Then again we move to a tone of 200 Hz but still keep the 1/200th delay; the delay is now equal to the period so are we now 360 degrees out of phase and in this particular (artificial) instance does that mean that they are effectively back in phase?

                Lastly, we remove our time delay and instead we reverse the polarity (black for red) on the back of one of our speakers and the replay each of our tones - 50 Hz, 100 Hz, 200 Hz separately, what is the effect on the phase of each of these tones (relative to the perfect listener) now?

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                • #38
                  Perfect tones, rooms and speakers ...

                  Originally posted by weaver View Post
                  Thanks to the posts above the situation is starting to become clearer.

                  I would like to just explore the time/phase relationship a little though.

                  Can we assume a 'perfect' pair of speakers being fed 'perfect' test tones and reproducing them perfectly such that what we hear is just pure single tones.
                  You must add something rather abstract to your list of assumptions; an infinite room to listen in. Tones do not sit well in real rooms but OK, let's continue.

                  Originally posted by weaver View Post
                  If we play a mono 100 Hz signal through both speakers and listen at the prefect sweetspot in our perfect room we should hear a tone from midway between the speakers shouldn't we.
                  For the purposes of this discussion only, OK. In reality, pure tones are often quite hard to locate but I've never listened in a perfect room of infinite size.

                  Originally posted by weaver View Post
                  If we then introduce a delay to the tone to one of the speakers of 1/200th of a second (ie half the period of the tone) would the speakers relative to the listener now be 180 degrees out of phase?
                  Yes. At this frequency one speaker would be moving forwards, the other backwards in a symmetrical, complementary manner.

                  Originally posted by weaver View Post
                  Assuming the answer to that is yes, we move on

                  Keeping the delay of 1/200th sec we move to a tone of 50Hz, the time delay is now one quarter the period of our tone so are the speakers now out of phase by 90 degrees?
                  Yes.

                  Originally posted by weaver View Post
                  Then again we move to a tone of 200 Hz but still keep the 1/200th delay; the delay is now equal to the period so are we now 360 degrees out of phase and in this particular (artificial) instance does that mean that they are effectively back in phase?
                  Yes.

                  Originally posted by weaver View Post
                  Lastly, we remove our time delay and instead we reverse the polarity (black for red) on the back of one of our speakers and the replay each of our tones - 50 Hz, 100 Hz, 200 Hz separately, what is the effect on the phase of each of these tones (relative to the perfect listener) now?
                  They will all be out of phase - as will everything.

                  Finite time delays cause a complex patterning of boosting and cancellation across the spectrum. I suggest you explore the concept of comb filtering, particularly in the context of real room reflections and their effect on the response perceived by the listener.

                  A time delay may be thought of as a variable phase shift - a shift of 360 at a given frequency may be thought of as "no change" but this is untrue for a variety of reasons* - think of it as two people in a walking race in step with each other, but one of them exactly two, four, six or eight paces ahead. They may well be "in phase" but the one ahead still wins!

                  * Mainly that the phase relationship between two random signals based on a time delay will vary with frequency. This is why, for stereo to work properly, the timing relationship between the two channels must be maintained to within a very small magnitude.

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                  • #39
                    Reverse polarity vs phase shift

                    Originally posted by weaver View Post
                    There is what I assume must be a fundamental concept here that I am failing to get my head around and that is:

                    If we invert the phase of a complex signal (ie it is 180degrees out of phase) does that mean that all of the component wavelengths are 180 degrees out of phase with themselves?
                    ...

                    Is there something basic I have missed?
                    Firstly, thanks again Pluto.

                    Next - the basic concept I had missed?

                    When people talk about a pair of speakers being 'out of phase' there is in fact no 'phase shift' involved.

                    By reversing the polarity, the signal to one speaker has been inverted or mirrored.

                    Conversely, shifting the phase of a music signal by 180 degrees such that it is in anti phase wrt the original does a very different thing.

                    Trying to reconcile these two situations on the assumption that they were the same was my mistake.

                    It seems that the phrase 'out of phase' can mean two entirely different things depending on whether it is applied to a 'reverse polarity' situation or a 'phase shift' situation.

                    If we look at simple sine waves it can appear (it did to me) that we are looking at the same thing when in fact we are not.

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                    • #40
                      Phase shift and frequency

                      Originally posted by weaver View Post
                      ...By reversing the polarity, the signal to one speaker has been inverted or mirrored....Conversely, shifting the phase of a music signal by 180 degrees such that it is in anti phase wrt the original does a very different thing.
                      I'm still not sure you have it... The statement "shifting the phase of a music signal by 180" really has to be qualified by asking "at what frequency" because anything other than an electrical inversion is likely to have a phase shifting effect that varies with frequency.

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                      • #41
                        Phase, wavelength etc.

                        Originally posted by Pluto View Post
                        I'm still not sure you have it... The statement "shifting the phase of a music signal by 180" really has to be qualified by asking "at what frequency" because anything other than an electrical inversion is likely to have a phase shifting effect that varies with frequency.
                        Thanks Pluto, but what you have in red was what I'd assumed at the outset but couldn't reconcile with the reverse polarity out of phase concept; I had it as the relationship between phase and wavelength rather than frequency (post 29) but the concept is the same.

                        {Moderator's comment: Phase is one of the most difficult concepts to grasp even for those working in sound.}

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