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Sound waves and smells

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  • #16
    Fill your room with water?

    Originally posted by STHLS5 View Post
    As you had mentioned, it shouldn't make any difference to the intensity of the smell in either room. Sounds do not travel by blowing or creating wind or air current. It is vibration of molecules (local oscillations) and they do not travel like waters in a garden hose. The air in front of the speakers remain where it was before and after the sound....
    Exactly so. Let's go over this to be sure we grasp this one ...

    1.
    Sounds do not travel by blowing or creating wind or air current
    Essential point. The loudspeaker is not like a desk fan. It is not producing a wind, does not producing a rush of air. If you turn on a 100W desk fan - see picture - and stand 2m away, you can feel the air rushing over your skin. If you turn on a 1 million watt PA concert hall speaker stack - see picture - can you feel the sound flowing over your skin even if you stand 2m away? No. You will feel no air flow because their is no air flow, no wind. The fan agitates the air in the room; the speakers do not.

    2.
    It is vibration of molecules (local oscillations) and they do not travel like water in a garden hose.
    Agreed. Sound does not flow like water. The speaker does not pour sound into the room from a tank. If you seal the room and put the hose pipe into it and connect it to a water supply, the room will fill up with water until it the walls burst. But you can play your speakers in the sealed room really loud all day and all night there will be no change in the quantity of air in the room, or its quality.

    3.
    The air in front of the speakers remain where it was before and after the sound.
    Agreed. That's because in 2. above, we said that the desk fan stirs or agitates the air in the room and the speakers do not.

    which means that 4. ....

    There should be little or no difference in the smell that we detect sitting at the sweet-spot in room A (playing the speakers really loud) or in room B (speakers disconnected) because the speakers are not stirring the air molecule particles. If we replaced the speakers in room A with desk fans and turned them on, then we would surely agitate and stir the air in room A and wherever we stood in the room the smell would be equally noxious. We cannot achieve that stirring effect with (sealed) loudspeakers no matter how big or how loud we play them. But if the speakers are vented, which means that they have a hole or port which acts as an air-pump, then some stirring is possible.

    So if we agree that the sound generated by the loudspeaker is not a gust of air, how does sound actually radiate from the speaker - or the birds high in the trees? Clue? What do barometers measure? (We hinted about this a few posts back...)

    >
    Attached Files
    Alan A. Shaw
    Designer, owner
    Harbeth Audio UK

    Comment


    • #17
      Concentrate and observe

      The answer dear readers is here.

      Pick any molecule in the medium and concentrate on it. Watch how it moves when the hands clap. As you move your mouse around the graphic you can see an explanation of various parameters. So how does the molecule move with the sound wave? And how does the initial at-rest position compare with the final at-rest position after the sound has passed through the medium? And how does that relate to the noxious smell in the room?

      I'm now off to London to show our overseas visitors around - pictures to follow in a few days.
      Alan A. Shaw
      Designer, owner
      Harbeth Audio UK

      Comment


      • #18
        Pressure Peaks Travel; Air Mass Doesn't

        Originally posted by A.S. View Post
        And how does the initial at-rest position compare with the final at-rest position after the sound has passed through the medium?
        It moves closer to neighbouring molecules. Then it moves back to its original position.

        Originally posted by A.S. View Post
        And how does that relate to the noxious smell in the room?
        I don't know!
        Ben from UK. Harbeth Super HL5 owner.

        Comment


        • #19
          Detecting out of phase

          Originally posted by STHLS5 View Post
          When we connect the speaker wires reversed in the case of a stereo system then we will have a general out of phase sound.
          Correct.

          Originally posted by STHLS5 View Post
          However, whether the sound will be out of phase or not depends on identical signal (not necessarily of the same strength) being fed to both speakers simultaneous.
          Equally true - for the output of the speakers to be be truly and mathematically out of phase with one another, they must be fed with identical material.

          This is precisely why it is easier to detect a "reverse polarity" [your words] condition with mono programme material. As I mentioned earlier, stereophony consists of mono material plus the additional information required to convey directional clues to the ear-brain of the listener. The former of these two (which we call the M component) is, by definition, fed identically to each loudspeaker. The latter (which we call the S component) is, by definition, fed to the right loudspeaker in reverse polarity compared to the left.

          So it stands to reason that listening to pure mono material allows for the easiest detection of a "reverse polarity" error. The more stereophonic the material, the more S (out of phase) content has been intentionally added to provide directional clues that the listener perceives as "stereo".

          So if your pre-amp has a mono button, use it. This button, in effect, removes any S component by mixing two equal and opposite signals together resulting in total cancellation. Identifying a reverse polarity state on very wide stereo music can be tricky and ambiguous, hence you need to practice and be very aware of the sonic difference between correct and incorrect polarity. Like so many things in life, training combined with experience will get you there!

          Comment


          • #20
            Stereo sound

            So then, stereophony isn't simply differences in instrument loudness between left and right? That's what I always thought it was.
            Ben from UK. Harbeth Super HL5 owner.

            Comment


            • #21
              Listening to out-of-phase recordings on my P3ESR

              I've just got round to wiring up the P3 OOP. I've never heard OOP before, so didn't quite know what to expect.

              I selected Ella & Louis, a mono recording from the 1950s and pressed play. UGGHHH! It's really horrible to listen to. My brain got confused and thought Ella was inside my head at first, then I couldn't tell where she was at all. Really strange and confusing. I started to feel a bit sick too.

              Well, I really can't understand how any passing visitors couldn't identify that SOMETHING was wrong in the room at Munich. It sounds totally unlike a normal hi-fi system, at least with a mono recording.

              Let me now try a stereo recording...

              1) Arthur Rubinstein - Beethoven Piano Sonata No.14 (RCA Living Stereo SACD)

              Again impossible to locate the source of the sound apart from to say it is in front of me generally. I wasn't sure what to expect when using a stereo recording, the sound seems to be very unfocused and vague.

              2) Miles Davis - Kind of Blue (Columbia SACD)

              Instruments that are hard left/right are still obviously located there if lacking "solidity", but sounds that should be coming from the centre are very diffuse and impossible to locate. Miles trumpet is such a case, whereas Coltrane's tenor sax is on the left where it should be.

              3) Heifitz/BSO - Beethoven Violin Concerto (RCA Living Stereo SACD)

              Finally I thought I'd try a pure 2 track recording with no studio gimmickry to confuse the issue...

              Hmmm, I can still tell the violins are on the left, yet Heifitz own violin seems very vague spacially. I can tell the orchestra is in front of me but it's all fuzzy and vague. No better than the Miles Davis recording really.

              Conclusions

              If I were at Munich, I think I would notice this effect - particularly with vocals (which are normally centrally placed). However, unfamiliar music might lead me to believe I was simply listening to some poor speakers that don't image well, possibly. But these are my first ever experiences of OOP sound and I would expect seasoned reviewers/technical journalists and industry professionals to notice it straight away - just as Pluto did.

              Comment


              • #22
                Diffusing an odour

                And how does that relate to the noxious smell in the room?

                I don't know!
                It's really simple. A (heavy) molecule that we call an odour presumably either hovers in the air, at rest (but under gravity) or it somehow attaches itself to an oxygen molecule. If so, we've seen with the hand-clap illustration that after the sound wave passes through any given volume of space, the air molecules return to their original place in the room, just where they were before the sound wave jiggled them about.

                It must be true then that the reason a sound wave doesn't carry the smell is because there is no wind associated with the wave. For the smell to permeate the room there would have to be a real, progressive, irreversible motion of the molecules outwards away from the source. And we've seen that that's not the effect a sound wave has on the molecules suspended in your room.

                Agree?
                Alan A. Shaw
                Designer, owner
                Harbeth Audio UK

                Comment


                • #23
                  The smell remains the same

                  I agree Alan - the smell would be the same in each room.

                  Just a thought, the drive units move in AND out, does this make the air molecules move one way, then the other in synchrony with the dirve units as the wave passes them by (with no net movement overall, as shown in the hand-clap clip)?

                  Comment


                  • #24
                    Smalls sans sound waves

                    Originally posted by A.S. View Post
                    It must be true then that the reason a sound wave doesn't carry the smell is because there is no wind associated with the wave. For the smell to permeate the room there would have to be a real, progressive, irreversible motion of the molecules outwards away from the source.
                    Agree?
                    That can't be entirely correct, surely? A bad smell will permeate a room even in the absence of noticeable air currents.

                    {Moderator's comment: obviously, a) there is no such room as an entirely draught free one and b) the point we are trying to make is that no matter how loud the speakers are playing they will not waft the odour through the room at all.}

                    Comment


                    • #25
                      We listen to Pressure?

                      Originally posted by Pluto View Post
                      ....As I mentioned earlier, stereophony consists of mono material plus the additional information required to convey directional clues to the ear-brain of the listener. The former of these two (which we call the M component) is, by definition, fed identically to each loudspeaker. The latter (which we call the S component) is, by definition, fed to the right loudspeaker in reverse polarity compared to the left.
                      I am not sure if I understood you correctly. Not all stereophony effect is created by the net effect of differential phase reaching our left and right ears.

                      I have previously mentioned in
                      Originally posted by STHLS5 View Post
                      .
                      about the sound being slightly infront of the speakers. Previously, I have also mentioned that Elton John's voice in Candle in the Wind was way behind the speakers. I managed to correct them by moving the speakers closer to each other from 240cm to 164cm. So by moving the speakers I did not change the phase of the frequencies but the image moved closer towards me.

                      I also discoverd by chance while correcting the dip "a big W shaped response" from 65Hz to about 90Hz that by adding a thick Rockwool (4 x 4ft) on the floor in front of the speakers moved the sound from slightly "pinched" projection to the natural in between the speakers stage as described in the Tech Talk section.

                      In all the scenarios mentioned above, the image, sound stage and stereophonic effect changed by altering the distance between the left and right speakers and/or by reducing the reflected sound reaching our ears. So in my humble opinion phase difference alone does not determine the stereo effect, but it is a combination of intensity differential between left and right speakers and phase.


                      Originally posted by A.S. View Post
                      ....So if we agree that the sound generated by the loudspeaker is not a gust of air, how does sound actually radiate from the speaker - or the birds high in the trees? Clue? What do barometers measure? (We hinted about this a few posts back...)>
                      Ahhhh... I am under pressure to get the right answer. (wink)

                      ST

                      Comment


                      • #26
                        The nature of stereo

                        Originally posted by STHLS5 View Post
                        I am not sure if I understood you correctly. Not all stereophony effect is created by the net effect of differential phase reaching our left and right ears
                        Not necessarily differential phase - just a difference. Any and all stereo can be mathematically modelled in two ways. The more usual approach is termed AB (or sometimes XY), where A & B simply represent the signals presented to the left and right speakers respectively. This can be remodelled into the sum and difference domain where M = A + B and S = A - B. Each of these equations will be subject to a constant level reduction but we need not be concerned with this here.

                        This is fully reversible thus: A = M + S, B = M - S, an identical matrix to the one above. In theory you could convert from one domain to the other ad infinitum.

                        If you were given two signals, M & S, with the instruction to reproduce correct stereo on a pair of loudspeakers, you would have to send the M signal equally to each speaker and the S signal to the left speaker. You would also need to phase invert the S signal and send that to the right speaker.

                        The S component is the instant difference that results from A - B. Have a look at the applet on this page which will enable you to see the effect of adding two sine waves in different combinations of level and phase. Remember that subtraction is the same as adding, but with a 180 phase inversion!

                        Adding two identical (i.e. in phase) sine waves results in a 6dB increase. Phase shift one of the waves by 90 and you only get a 3dB increase. 120 is an interesting case as it results in a sine wave of the same level as the two being added, and 180 results in no output at all - total cancellation. So you can see that any phase shift at all between the two waves produces an output that is less than the best case (which we could call mono), that of the two waves being perfectly in phase.

                        You could use this applet to go on to explore the results of adding (or subtracting) waves of different frequencies and phase relationships but the important thing to realise for the purposes of this discussion is is that subtracting any two sine waves of non-identical frequency and/or phase will result in a signal, and it is this signal that we term "S". As long as an S signal exists, we have something that is not mono.

                        In real life these signals will all be vastly more complex than simple sine waves and the phase relationship between them will vary dynamically but the concept still holds good - the crucial point is that stereo signals can be illustrated in left & right (AB) terms or sum & difference (MS) terms. The former only considers levels, the latter tells us less about the real levels but more about the nature of the stereo. The greater the relative level of S to M, the wider the stereo will seem. No S at all, you get mono (A==B). All S, no M, results in mono which will be out of phase when reproduced on a stereo system. If you combined that signal electrically (to feed, say, a mono radio transmitter) you get total cancellation. Silence.

                        Any worthwhile stereo will contain a good balance of the M & S components. Too little S, you get something near mono. Too much, and it will tend to sound out of phase-ish until you reach the extreme case of all S, no M, at which point you have what we call "out of phase".

                        Comment


                        • #27
                          The push-me, pull-you of phase

                          Originally posted by A.S. View Post
                          The answer dear readers is here....
                          Just to conclude this line of thought, we have seen in the animation how the instant after the hand clap the pressure shock wave radiates away from the hands that generated it. That pressure wave reaches the ears and presses on the ear drum - we hear the sound of the clap.

                          But suppose we imagine this: If we held in our hands two halves of a container sealed together with rubber O rings. Inside the container is a perfect vacuum, a complete absence of air. Gripping the two halves we sharply pull them apart. The vacuum is now exposed to the air of the room. What happens is the reverse of when we squeeze the air of the room between our hands. Now 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.

                          And if we have two loudspeakers, one connected normally and one in reverse phase? What then? What do we experience sitting at the sweet spot listening to the hand-clap recording over the out-of-phase speaker pair? We experience a really weird physical sensation. One ear is bombarded with a positive-going shock wave from the normally-wired speaker which presses the listener's ear drum inwards. The other ear, simultaneously, nearer the reverse-connected speaker has the ear drum sucked outwards.

                          Is there any example in nature of this push-pull phenomena? Not that I can think of. What does than mean? It means that evolution has not prepared us for that experience. When our subconscious searches our inbuilt mental Experience Look-up Table there is no entry to describe the sensation of one ear being pressed and the other simultaneously being pulled. So it feels odd, unnatural and even a little frightening.
                          Alan A. Shaw
                          Designer, owner
                          Harbeth Audio UK

                          Comment


                          • #28
                            Panning and sound stage depth ...

                            Originally posted by BAS-H View Post
                            So then, stereophony isn't simply differences in instrument loudness between left and right? That's what I always thought it was.
                            What a deep question this is!

                            It is of course true that a simple level difference between left and right will cause that source to appear anywhere between full left, then onto centre and continuing towards full right as the relative level is adjusted between left only, equal, right. This is precisely how an amplifier balance control works and a similar idea exists in each channel of a recording console, where the output of each individual microphone can be placed anywhere in the range between full left and full right: this control is called a "panpot" - panoramic potentiometer. However, simply "panning" mono mics into position provides, in and of itself, none of the more sophisticated directional clues which can give a recording real "life" and "sparkle" and elevate a run of the mill sound into one that excites and gives the listener a sense of "being there".

                            Two questions worthy of exploration:

                            1. Exactly how does a level difference between the two speakers of a stereo pair become interpreted by the ear-brain as a positional difference. Be warned - this is surprisingly subtle.

                            2. Having decided that a simple level difference between left & right will be interpreted by the ear-brain as positional information, how do we extend this idea to the perception of depth in a two channel stereo listening system?

                            Bear in mind that two speakers connected to two channels of signal can only convey differences of level and phase when presenting a sound, so how do we get from there to the perception of depth within the stereo "picture"?

                            Comment


                            • #29
                              the relationship between phase and wavelength?

                              Originally posted by Pluto View Post

                              In real life these signals will all be vastly more complex than simple sine waves and the phase relationship between them will vary dynamically but the concept still holds good - the crucial point is that stereo signals can be illustrated in left & right (AB) terms or sum & difference (MS) terms. The former only considers levels, the latter tells us less about the real levels but more about the nature of the stereo. The greater the relative level of S to M, the wider the stereo will seem. No S at all, you get mono (A==B). All S, no M, results in mono which will be out of phase when reproduced on a stereo system. If you combined that signal electrically (to feed, say, a mono radio transmitter) you get total cancellation. Silence.

                              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?

                              The visualisation I have is based on the notion of a sound wave proceeding from a speaker down a room and then being reflected from the end wall back along itself; depending on the wavelength of the sound and the length of the room that sound will be either re-enforced or canceled or somewhere in between.

                              Is it correct to say that the reflected sound will be 'out of phase' with the original?

                              If so, then for a given length of room, the degree to which the sound is out of phase will depend on the wavelength of the sound and there will only be one wavelength that is actually 'inverted'.

                              Is there something basic I have missed?

                              Comment


                              • #30
                                Alan covered this with his explanation of push-pull. Think only about the wave generation at the speaker source.

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