The Haunted Workshop

[Nigel Graham 2Participant @nigelgraham2]

Dave, Eric –

That’s an interesting point about patterns.

The ghostly voices started quite suddenly, probably as the sound of the cutting changed along the billet. Or at any rate I was only suddenly aware of them. I would not say they formed a pattern, rather an impression.

In this case I was not conscious of pattern-making, in the way that looking at a cloud and imagining the shape of an animal, say, does seem to have a semi-conscious aspect.

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Andy –

Good point. The lathe in its previous home was very noisy. It clattered. It was driven by a massive single-phase motor, and I found the gearbox empy of oil. It really appreciated the modern 3ph conversion and a bottle of oil I gave it, and now runs very quietly unless I put on a heavy cut.

The odd noise now is not that clattering, but a rapid, fairly quiet ticking. I think it happens with the clutch both engaged and disengaged, but I will have a closer look.

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Nick –

Aha. That does seem a large-scale version of the effect I noticed in the workshop: the noise “concentrated” in a realtively small volume.

Sound does not travel any further in the dark than in the daylight, but will travel very slightly faster in colder hence denser air. Also, at night there are fewer sound sources out in the open anyway, unless you live next to a motorway or something similarly noisy all the time. So individual sounds tend to be more noticeable.

There could be a physiological effect here, too. We are diurnal animals so I wonder if at night we turn up the sensitivity of our hearing to compensate for our eyes being less effective in the dark.

Sound and electromagnetic radiation cannot be compared easily, as they are affected by different influences.

However, for sound at least the lower the frequency the less rapidly it is absorbed by the medium carrying it.

So the components of the music you heard were likely the bass notes, below the singers’ voices – and quite possibly amplified more than them.

…

On ething this thread does show is that although obviously my heading, The Haunted Workshop, is tongue-in-cheek, we may be touching on why ghost stories and accounts of “paranormal” experiences have always had such a grip.  We notice instinctively the odd, the out-of-place, and if we cannot immediately and easily identify or explain it, it can make us anxious.

We may not be as skittish as a horse frightened by someone having moved a familar roadside object, but in times past it was far harder for people to think rationally of things going “bump” in the night, palimpsests of mist, voice-like noises or the like. Many “ghostly” phenomena tales, such as the feeling of being sat upon in bed, were likely only hypnagogic dreams.

Some people are still like that, but these tend not to want rational explanations, or they lack the curiosity (or courage?) to determine what they really experienced. To them, it was a ghost and nothing will make them change their minds.

 

Luckily I know the “ghostly voices” were peculiar effects of the noise my machining was making – even if my mind found them unsettling for a few minutes!

 

 

 

[duncan webster 1Participant @duncanwebster1]

Nigel points out that low frequency sound travels further than high. That’s why foghorns used to have a low note, but when I last heard one it was much higher pitch. Cost saving? You need  a much longer more expensive trumpet to make a low frequency sound. I can still remember the mournful sound of the foghorn lying in bed at my aunt’s house in north east Scotland. 60 odd years ago. Magnificent beach, but bitterly cold sea.

[Andy StopfordParticipant @andystopford50521]

The speed of sound decreases with decreasing temperature – at 15C it’s about 760 mph, this falls to about 660 at around -60C, the sort of temperature found at 35,000 to 40,000 feet,

This has an important consequence for aviation, since high-flying aircraft may approach the speed of sound (which may lead to loss of control and structural failure if not designed for transonic flight), while the reducing pressure will increase the stall speed at which they will fall out of the sky (I simplify somewhat, mindful of Dave’s point about inaccurate or misleading posts).

The U2 has, I think without looking it up, a safe envelope of only about 5 knots at it’s service altitude, between stall and the speed of sound. The early ones had no autopilot and were, apparently, very challenging to fly.

[Nigel Graham 2]

On Nigel Graham 2 Said:

Dave, Eric –

…In this case I was not conscious of pattern-making, in the way that looking at a cloud and imagining the shape of an animal, say, does seem to have a semi-conscious aspect.

.

… 

 

Another interesting example: I’m currently 3D-printing a circular thingy.   The 3D printer’s table is moved X-Y under the hot nozzle by a pair of pulse driven stepper motors, and these emit musical notes, sort of, depending on the Manhattan Course needed to make the shape.

The gcode emits the X and Y pulse sequences needed to jog the table right-left and front-back so that blobs of melted plastic land in the right place, the blobbing done by another stepper.  Although nothing whatever to do with music, the motors often seem to be playing bits of melody, and this morning, I heard a snatch of “Happy Birthday to You”.

Might be possible to design a shape causing a 3d-printer or CNC machine to deliberately play a recognisable tune!    But what I heard today was a coincidence – my brain, recognising noises similar to a well-known ditty.  At the time, the printer hadn’t started my object: it was printing the supports needed under overhangs, and they were added by Cura, not me!

Does long-wave sound travel further than short-wave?  I think that’s true in squishy air, but not in incompressible media like water or metal.  Anyone know for sure?

Really long distance radio waves are all short.   Long radio waves travel long-distances around the planet’s surface because of ground and ionospheric effects. But for the same reason low frequencies have trouble escaping into space.  VHF and higher frequencies travel much further, hence microwaves talk to Voyager 1.  And still in touch despite being 15 billion miles away !

Dave

 

 

[Nigel Graham 2Participant @nigelgraham2]

Duncan –

I’d have thought the cost of a foghorn’s length a fairly small part of the total, as it’s basically just a rolled metal tube; but I think some have been replaced electrically, by arrays of loudspeakers. That on St Aldhelm’s Head (E. Dorset) certainly looks like that. The foghorn at Portland Bill still seems that – a horn – but I have not heard its low, mournful note for a long time now so I don’t know if it’s even still used.

 

Dave –

The time to worry is when the 3D printer tries Manhatten Transfer rather than Manhatten Course…

– and Andy –

Sound speed in a medium is a function of density, not frequency.

So the colder the air the higher the speed – but although it is ever so cold at >30 000 feet, the air is far less dense than at sea-level anyway, perhaps more than countering the low-temperature effect.

Sound travels through water far faster than through air: around 1500m/s in water, 350m/s in air at sea-level. Faster: not necessarily further.

 

Sound is a mechanical, compression/ rarefaction oscillation, so can be made to pass through a solid like metal and glass, and liquids. Although those are not compressible in the usual sense they are sufficiently elastic for the tiny magnitudes of sound-waves. If this were not so, objects like metal bells, and electro-acoustic transducers made from piezo-electric ceramics – natural quartz in the original ones – could not work as they do. The transducer designers have to know such characteristics as the Young’s Modulus of Elasticity for their materials.

However, all acoustically-conductive materials do resist the vibration, and absorb the energy, in specific ways and to extents varying by frequency; and generally the higher the frequency the greater the absorption.

There is also surface impedance, analogously to refraction of light trying to cross between two media such as air and glass. This can create a significant signal loss, so for example echo-sounder transducers, and ultrasound testing, includes a “matching layer” of a material of suitable density to ease the sound’s transit across the barrier. This is generally some sort of oil or plastic in the echo-sounder, and an innocuous gel for medical ultrasound.

I think the oily spermacetti in the Sperm Whale’s head is the animal’s own hearing impedance-match. In our ears, the middle-ear’s three tiny bone levers form a mechanical impedance match from ear-drum to cochlea. They are are on little muscles to act as a damper so we don’t deafen ourselves internally by for example, chewing. Amplifier too: the 0dB Sound Pressure Level for air acoustics is a mere 20µPa = 5 X 10^(-9) Bar. Chosen as being the faintest detectable by the fully-healthy human ear in the middle of its frequency range – and that sound pressure oscillates the ear-drum by about the diameter of a hydrogen atom. That puts whispered sweet nothings in rather a new light!

(Underwater measurements use 1µPa as 0dB: a difference of 26dB.)

Reflectivity too: sound does not easily pass from, say, air into water or vice-versa. The density mismatch is so great most of the energy is reflected back.

 

Back in the 1990s when climate-change was beginning to be taken more seriously I understand someone – I known not whom – proposed monitoring the Atlantic’s mean temperature change by using the time of travel of sound pulses across it. The warmer the water the longer the time – but salinity changes would affect it too. This was scotched when the marine biologists warned the sound source level would need be so high it would harm a lot of the wildlife. (From a quick calculation I suppose over 100dB re 1µPa to be reasonably audible to the hydrophones on the opposite coast 4380km, and just under an hour’s travel time, away.)

 

So yes, sound can travel further in a given medium the lower its frequency, all other things being equal.*

Sound speed in any medium is constant for that medium as a function of the density; a surface change in density across a joint between different media, giving refraction and/or reflection.

The elasticity of the medium plays a part too, but I am not sure how – affecting absorption with frequency I think.

.

I touched on resonance in buildings a few posts back. I should explain this is the resonance of the air within the building if a door is open to the outside world. So a cathedral or theatre can theoretically be a resonator, somewhat akin to a bottle when you blow across the top – but the frequency would be extremely low, just not detectable unless of sufficient amplitude to be felt as a slowly-oscillating draught in the doorway. It’s the misuse of the word by art-critics that had annoyed me: they really mean reverberation and should use the right words, just as they are careful to use the proper terms in other aspects of music.

….

It is not really fair to compare sound and radio. Although both obey similar wave physics in some respects their fundamental nature is totally different, and they are affected by very different phenomena, particularly for radio the Earth’s own magnetic field, and solar radiation.

….

*The distance in a given situation varies with geometry as well as medium. A sound from a “point” source in a homogenous medium surrounding it to sufficient distance away, decays with the square of the distance – in deciBel terms, the loss is 20log(distance) dB for pressure-level, or intensity, 10 times for power. (Logs to base-10.) This is what I used to estimate those figures above: the simple distance-attenuation alone would be about 73dB.

[Andrew CrowParticipant @andrewcrow91475]

Duncan, Dave, Andy and Nigel, thank you for that most interesting discourse.

Not sure my aged brain will retain all the detail but will certainly have a better understanding of sound.

Andy.

[duncan webster 1Participant @duncanwebster1]

According tohttps://journeynorth.org/tm/hwhale/SingingHumpback.html#:~:text=A%20whale’s%20low%20frequency%20sounds,how%20far%20this%20distance%20is.

Whale sounds can be heard 10,000 miles away

[RobinParticipant @robin]

If you are getting spooked, get a dog. They are never wrong 😀

[Robin]

On Robin Said:

If you are getting spooked, get a dog. They are never wrong 😀

Very appropriate emoji, obviously somebody owned by a dog!

Now dogs can really get you spooked!
They can stare intently along empty passages, keep looking around behind when out walking and that’s not to mention barking at nothing visible (etc).

Yes I was owned by a dog some time ago!

[Nigel Graham 2Participant @nigelgraham2]

Duncan –

I read that link….

… With some concern.

Human hearing, in good health, extends well below the 100Hz claimed as minimum. That is only slightly below the bass A two octaves below Middle A (440, 220, 110 Hz). Helmholtz showed us, in his major On The Sensations Of Tone (1877)  that keyboard instruments usually extend down to the C lower, at 33Hz. The human ear’s minimum frequency is generally taken as 20Hz. (Maximum: 20kHz.)

So that JourneyNorth value is wrong, for a start, and with it using miles not SI units for a scientific article, makes me question its general level.

The key phrase is “reseachers believe”… How are they sure the call they measured is from a particular animal 10 000 miles away, not a similar one much closer?

 

Now, I made a mistake too; in my sums above for that distance loss by not converting the km to m.

My calculation would apply if the reference-level distance is 1 kilometre from the projector, but it is normal to take 1m, and deciBel scales are not linear!

So over 4380km, a rough mean of the North Atlantic’s width, the loss by pure spreading alone would be:

20log(4380 000) dB re 1µPa at 1 metre source level

= 133dB. (Rounded)

Not 73dB.

.

Now let’s apply the same calculation to two whales 10 000 miles apart. (Does the particular species roam over that separation?) The biologists recording the call at source might be a km or so away from the whale so as not to alarm it (giving poor results as well being cruel), but that does not matter as long as they can know that distance.

10 000 miles =  16093 km.

Speading loss = 20Log (16 093 000) re 1µPa at 1m from the calling animal.

=  144dB.

I don’t know the Humpback Whale’s otological sensitivity, but deciBel arithmetic is by adding and subtraction (because they use logarithms). So if the call is at 100dB re the 1µPa standard for marine acoustics, the other will detect it at 44dB. Feasible, perhaps, but this ignores all sorts of complexities particularly including absorption losses.

After all, we live perfectly happily with Sound Pressure Levels of 40 – 50 dB re 20µPa, typical in a reasonably peaceful home or office; but the airborne acoustics’ scale is some 26dB higher than that for marine sound-levels; making a considerable difference to the actual pressures detected physiologically.

 

And how can we be reasonably sure if the biologists’ hydrophones at the receiving end really are picking up calls from their colleagues’ animal, or from another much nearer!

I do not know, but can offer an educated guess.

If the scientists near the calling whale transmit the call by radio as it happens, the listeners can record it then compare it with what their hydrophones detect some 3 hours later (1500m/s sound speed), with due allowance for distortion and further loss by absorption, scattering and reverberation.

The two teams know their own separation by normal navigation techniques. The people near the caller can also calculate the call intensity if they know their distance from the whale, reasonably accurately. Then by applying appropriate calculations the two teams can accept or reject the correlation as real or incorrect.

 

The JourneyNorth item over-simplifies the work, but it would be interesting to read the proper source reports. (If you can do so. Even if cited most academic research – and trade standards such as BSI – has been collared by a few huge publishers who charge very high fees.

In fact the radio-link methods echoes(!) how the speed in sound in water was first established, in a lake (Geneva I think). At the same time that an underwater gong was struck, a small gunpowder charge was fired above the water. The observers on the opposite shore timed the delay between the flash and hearing the gong by some sort of submerged listening device.

 

[Hz after Hertz, not Helmholtz, who uses c/s.)

 

I credit whoever is JourneyNorth for trying to convey scientific work to lay readers, but I do wish such publishers would be a more careful. At least the article does not make the mistake common in the ordinary newpapers of trying to compare marine animal call volumes reported by specialists, to those of discos and the like.

…..

V8Eng.

Cats can behave similarly. Our family home had a long open-space from front-door to the far end of the kitchen. The feline owning us would occasionally idle casually around one end then suddenly and for no apparent reason bolt to the other as if startled by Something Unseen!

Dogs… Well, I lived for a while in a rather grotty bedsit whose elderly landlady occupied the ground floor. One day I took her collie on a walk to a shop about half a mile away. All went well, dog trotting along, pausing frequently as they do (canines had P-mails aeons afore humans’ E-mails) until we had to cross a particular side-turning.

The dog would not cross that road.

Thinking to fool it I turned back – and was nearly pulled over as the hound urged us away from the Unseen – to enter the same housing estate from the next street back. That worked until we met the original side-road, even though this time a block further in.

Nothing for it but to take Laddie back home and go unaccompanied to the shop. I don’t suppose the dog realised I had faced and escaped our Deadly but Unseen Foe.

::::

 

[BazyleParticipant @bazyle]

Every human civilisation has invented (“discovered”) a god to explain what they cannot understand or explain with their current scientific knowledge. Frequently major religious activities have been driven by people who ‘heard voices’ which they surmised must have been from their god. So the scientific rationalisations expressed on this thread for converting white noise into speech would appear to be the explanation. The effect has a lot to answer for over the centuries.

[duncan webster 1Participant @duncanwebster1]

If you’re hearing voices, go and see your GP

😜

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