Odd audio puzzle

[Nickthedentist]

Hi everyone,

I was sitting on our bed on Monday evening, reading one of my daughters her bedtime stories. I'd put on some soothing music on the hifi in the adjoining study. But throughout the story, I was distracted by flutter spoiling the music. Annoying, as I'd only recently resurrected the cassette deck from decades of disuse, and it had been working so well. Surely not a faulty motor? Or maybe too much TU torque? Something to look into later in the year when I'm less busy (hopefully).

Yesterday, I was on the bed reading stories, and my other daughter came into the study to play us her piano pieces for her grade 2 exam. She played them nicely, but something didn't sound right. Maybe the Yamaha Clavinova was protesting about the heat?

Any ideas what the problem was? It only came to me this morning.

[Boulevardier]

Originally Posted by Nickthedentist

Any ideas what the problem was? It only came to me this morning.

Expansion of rubber belts and piano strings in the heat?


But that feels too obvious for a puzzle...


Mike

[duncanlowe]

Beat frequency with a fan, maybe? Either in the room, or attached to some piece of equipment?

[paulsherwin]

Originally Posted by Boulevardier

Expansion of rubber belts and piano strings in the heat?

No piano strings in a Clavinova. They're basically sampling synthesizers, but with some clever mechanical gubbins that make the keyboards feel like standard pianos.

I can't see how the heat would affect a Clavinova, but some sort of belt failure or cassette jam might have made the cassette deck sound strange. Or maybe the heat's just making you feel under the weather, and you're imagining it.

[Nickthedentist]

Good tries! The heat making me imagine it was what I first thought too.

But only duncanlowe gets the "prize".

There's a slowly-rotating ceiling fan above our bed, used only on the very hottest days of the year, when the temperature upstairs at night is often 30 degC, even with all the windows fully open.

Then penny dropped when a police car with its siren wailing went along the ring road in the distance at the bottom of the garden, and that sounded awful too!

[paulsherwin]

Interesting, I don't think I've ever experienced that effect. Makes sense though.

[Uncle Bulgaria]

I was going to say the common element was your ears...

[G.Castle]

A kind of Doppler effect?

[paulsherwin]

It will be some sort of interference effect between the normal sound waves and those bouncing off the slowly moving fan blades.

[kalee20]

Originally Posted by G.Castle

A kind of Doppler effect?

That would give frequency modulation (low-frequency vibrato effect)...

Originally Posted by paulsherwin

It will be some sort of interference effect between the normal sound waves and those bouncing off the slowly moving fan blades.

... and that would give amplitude modulation (low-frequency tremolo effect).

It would be great to know which is taking place! (I like the FM theory).

[JohnBHanson]

A leslie speaker - see


https://en.wikipedia.org/wiki/Leslie_speaker

[Lucien Nunes]

But which type of rotary speaker is it most like? There are many...

There are two main applications for rotary speakers. One is as a tremulant, as often used in rock, jazz, gospel playing etc. This very characteristic sound is intended to impart a flavour all of its own and contains both deep FM and AM. In the classic compression horn Leslies, the horn rotates around a vertical axis beaming its audio radially. FM occurs because it moves towards and away from the listener, AM and a certain amount of swept bandpass occurs because it points towards and away from the listener and passes various constructional features of the cabinet. The other main Leslie variants (styrofoam and Rotosonic) do similar things although typically milder. The styrofoam Leslie for example has a horizontal axis parallel to the listener's axis, so the radial source never points at the listener.

The other main function of a rotary speaker is to produce a chorale effect. This is a slow, gentle modulation that is not so much intended to add an effect to the sound, but to disguise the thin and bland tone of an organ that has all its voices locked together in phase due to originating from the same generators. This never occurs in a pipe organ because there are always tiny differences in tuning between pipes that impart life to the sound through many slow, subtle beats. As with tape noise-reduction, you don't necessarily want to be aware that moving-speaker chorale is happening.

Most Leslies can be switched to Chorale (low) speed and although this does not sound like a tremolo, technically, it's still the same Leslie effect just slower and, in regard to the FM, much shallower. But there are other rotary speakers for classical organs that never have to produce the deep, rock organ tremolo, and can therefore be optimised for chorale.

The Allen Gyrophonic starts to approach more closely what is going on with the ceiling fan. In the Gyrophonic cabinet, the speakers are mounted in a rotating baffle board, so that although their dispersion axes orbit the baffle board centre, they don't rotate relative to listener's axis. This causes a different and arguably more 'pipe-organ-like' modulation that is available at both chorale and tremulant speeds.

Then you have the Compton Rotofon, in which four (or in late units eight) paddles rotate on a horizontal axis parallel to the listener's axis, each carrying a drive unit aimed tangentially (rather than axially in the Gyrophonic or radially in the Leslie). Their effect is different again, and contains very subtle and complex FM and swept comb-filtering as there are eight or even sixteen moving sources interacting (including rear radiation) but virtually no AM, which is representative of pipe organ tone. The Rotofon has no fast setting - the organ itself contains an FM tremulant.

There are others; the Dereux organ speaker (which I don't think has a catchy name) has an upward-firing treble unit above which an angled duct slowly rotates, sweeping the dispersion pattern around the room without ever pointing it anywhere near the audience. That is possibly the nearest to the ceiling fan as it uses primarily the characteristics of the room, not its own construction, to create the modulation.

But have you ever looked inside the back of a harmonium? The tremulant, often called 'Vox Humana' is actually a fan. I can imagine the effect of the Vox Humana on a Clavinova, and it is not an ideal combination.

Crikey, that post got long.

[vidjoman]

Excellent descriptions. I now know what the differences are. I was always aware that they were different but not aware of how this was achieved. Thanks Lucien.

[knobtwiddler]

Reading Lucien's wonderful post makes me think that a dedicated thread on rotary speakers and their relatives is needed. I've never given much thought to the topic beyond the obvious Leslie (and its digital emulations), but Lucien has made me think that there must be many systems utlising similar principles.

I have a magazine article somewhere that talks about a pair of Italian brothers who made 'sound intonators' (if I recall name correctly) as a means of mechanically synthesising sound. I seem to remember each unit being tuned differently. They had rotors inside boxes, with horns on the outside. Not quite the same thing as a rotary speaker, but along similar technological lines.

Did you buy the house from an old organ player, Nick? Maybe his ghost came back to haunt you -)

[Bill]

Wurlitzer's answer to Leslie was their "Spectratone" system. A small HF speaker rotating in a vertical plane with slip rings on a disk belt driven by a two speed motor . IIRC there is a circular baffle with cut outs enclosing the rotating parts. I think there was an electronic tremolo included within that system.

[Techman]

Originally Posted by Lucien Nunes

But have you ever looked inside the back of a harmonium? The tremulant, often called 'Vox Humana' [I]is actually a fan.

Same as in the one seen behind the Ekco telly in my avatar at the side of this post. Mine needs a helping hand to get it started by opening the top and giving it a spin - I suspect a bit of air leakage somewhere...and even cobwebs will stop it from spinning up.

[Bobdger]

Hi Techman.
Dose the motor have a small start cap, it will be a low value 4 - 20µf at 400V.

Bob

[Lucien Nunes]

Wurlitzer's answer to Leslie was their "Spectratone" system.... I think there was an electronic tremolo included within that system.

Indeed there was, and it was implemented in two different ways. One has a disc eccentrically mounted on the speaker shaft, that lies between a lamp and an LDR, modulating the LDR resistance. The other has crank attached to the shaft that moves a core back and forth inside a solenoid that is part of the crossover, modulating its inductance. The rotating speakers are two small (4"?) paper-cone units, mounted at either end of a baked-bean can on the end of a radial arm, firing tangentially.

Does the motor have a small start cap

In a harmonium it's an air motor, powered by the bellows.

a dedicated thread on rotary speakers and their relatives is needed.

It might be worth a kind of taxonomy that classifies the various models according to which components emit which sounds and rotate on which axes and which patents were thereby circumvented. In that regard a picture is worth a thousand words but I failed to include any above.
I leave you with a link to some pics of the mighty Rotofon.

http://www.electrokinetica.org/d8/2/2.php

[G.Castle]

Absolutely fascinating!
Thanks Lucien (+others )

Greg.

[Hybrid tellies]

Very interesting and an affect that I have not experienced before.