Sound dishes

Many years ago, I was lucky enough to work in “Launch Pad” in the science museum. There were lots of wonderful activities and the following blog posts are about the activities. Launch Pad has been replaced by the equally wonderful “Wonderlab”

https://science-projects.org/wp-content/uploads/2016/10/ch2.pdf

https://www.sciencemuseum.org.uk/home

https://www.sciencemuseum.org.uk/see-and-do/wonderlab-equinor-gallery

Sound dishes

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This exhibit consisted of two large dishes (you can see them in the above photos). Their purpose was to show how parabolic reflectors work.

So how do they work?

When a person speaks their vocal cords vibrate which causes the air in front of them to vibrate, between a high and low pressure. Any materials between the vocal cords, the origin of the sound, vibrate and these vibrations radiate around the room.

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Sound is an example of a longitudinal wave. A longitudinal wave has a vibration direction parallel to the propagation direction and cannot travel in a vacuum (a completely empty space).

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Sound radiating around the room means that someone standing far away will only receive a small quantity of the vibrations that were started by the vocal cords of the speaker so they will perceive a quieter sound than that produced by the speaker.

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One of the dishes in the exhibit had a mouthpiece. If you spoke into it most of the vibrations from your vocal cords hit it and were reflected off in one direction. In this way the sound had been contained within parallel beams of vibrations which travelled across the gallery.

At the other end of the gallery there was another same shaped dish. The beams that contained the information of your voice were reflected off this dish and focussed into the ear of the person listening, if their head was in the right place.

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One little point to note here is that when you were in school (or you might still be there) you were probably taught that sound was a form of energy. The truth of the matter is that it isn’t a form of energy. The sound we hear is simply the brain interpreting the electrical signals which the ear helps set up when the ear drum vibrates because of the vibrating air molecules hitting it.

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https://www.slideserve.com/lacy/sound

Parabolic reflectors

Parabolic reflectors have many applications. For example, they are used as satellite receiving dishes, both for domestic (TV and internet) and scientific purposes. They are also used in radio telescopes and car headlamps.

Satellite dishes

Some home use satellite dishes, which receive television signals from satellites orbiting the Earth.

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

https://www.bbc.co.uk/bitesize/guides/z2dsgk7/revision/2

A satellite dish is a dish-shaped type of parabolic antenna designed to receive or transmit information by radio waves to or from a communication satellite. The term most commonly means a dish used by consumers to receive direct-broadcast satellite television from a direct broadcast satellite in geostationary orbit.

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They work in a similar way to the sound dishes. Television signals are sent down to Earth as beams from the satellites, and the dishes focus them to a point where the electromagnetic waves which carry the signals are converted to electrical signals, which eventually reach the television set.

The dishes can also be used to send signals to the satellites.

https://cybercollege.com/tvp065.htm

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Receiving dishes are also used to receive other signals from satellites such as telephone calls, the internet, other information, photographs, video, information about the weather and receive data from space probes that are orbiting the Earth, travelling within the Solar System and out of it.

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

A radio telescope is a specialized antenna and radio receiver used to receive radio waves from astronomical radio sources in the sky. Radio telescopes are the main observing instrument used in radio astronomy, which studies the radio frequency portion of the electromagnetic spectrum emitted by astronomical objects, just as optical telescopes are the main observing instrument used in traditional optical astronomy which studies the light wave portion of the spectrum coming from astronomical objects. Unlike optical telescopes, radio telescopes can be used in the daytime as well as at night.

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

In physics, electromagnetic radiation (EM radiation or EMR) refers to the waves of the electromagnetic field, propagating (radiating) through space, carrying energy. It includes radio waves, microwaves, infrared, (visible) light, ultraviolet, X-rays, and gamma rays.

Classically, electromagnetic radiation consists of electromagnetic waves, which are synchronized oscillations of electric and magnetic fields.

https://upload.wikimedia.org/wikipedia/commons/thumb/4/4c/Electromagneticwave3D.gif/220px-Electromagneticwave3D.gif

Electromagnetic waves can be imagined as a self-propagating transverse oscillating wave of electric and magnetic fields.

https://astronomy.swin.edu.au/cosmos/e/Electromagnetic+Radiation

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A pair of electric (red) and magnetic (blue) fields, propagating together as an electromagnetic wave in the direction indicated by the arrow at the speed of light. The two fields are always at 90o to the propagation direction.

In a vacuum, electromagnetic waves travel at the speed of light (3 x 108ms-1), commonly denoted c. The oscillations of the two fields are perpendicular to each other and perpendicular to the direction of energy and wave propagation, forming a transverse wave. The wavefront of electromagnetic waves emitted from a point source (such as a light bulb) is a sphere. The position of an electromagnetic wave within the electromagnetic spectrum can be characterized by either its frequency of oscillation or its wavelength. Electromagnetic waves of different frequency are called by different names since they have different sources and effects on matter. In order of increasing frequency and decreasing wavelength these are: radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays and gamma rays.

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The 64-meter radio telescope at Parkes Observatory as seen in 1969, when it was used to receive live televised footage from Apollo 11

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Plot of Earth’s atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation.

Car headlamps

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

A headlamp is a lamp attached to the front of a vehicle to illuminate the road ahead. Headlamps are also often called headlights, but in the most precise usage, headlamp is the term for the device itself and headlight is the term for the beam of light produced and distributed by the device.

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Light, like sound travels in waves. If the surface of a parabolic dish is made so that it will reflect light, then it will focus light, or make the light into a beam in just the same way as the sound dished did. In car headlamps, light from the bulb goes out in all directions (like the sound of a voice in the sound dishes), and it is reflected off a parabolic mirror behind the bulb in such a way that it is sent out forwards as a beam of light. Some telescope use parabolic mirrors to collect and focus light from space.

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

A reflecting telescope (also called a reflector) is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image.

https://theconversation.com/how-do-you-build-a-mirror-for-one-of-the-worlds-biggest-telescopes-49927

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Optical diagram of the Giant Magellan Telescope. Giant Magellan Telescope – GMTO Corporation, CC BY-ND

Below is a very helpful comment from Stuart as I was concerned about using the term energy, as what people understand by the term is changing:

“Regarding your sound dishes at the Science Museum I would describe what is going on in exactly the same way as I would with a radio transmitter and receiver, since shoving the word “energy” in is probably pointless unless it adds something.

E.g. A radio wave is produced by the transmitter dish and this travels through the air to the receiver dish, where the wave is gathered (and focused to a point) on the receiving antenna. In the same way, a sound made at the ‘transmitter’ dish is reflected out through the air towards the ‘receiver’ dish. This sound wave is then focused to a point (where you place your ear) and is then perceived by the hearer.

Is there energy associated with a radio wave? Of course, it is “in” the oscillating electric and magnetic fields. Is there energy associated with a sound wave? Yes, it is “in” the compressions and rarefactions of the air. Does it help to discuss either? Probably not.”

All the best,
Stuart in sunny Penrith.

2 thoughts on “Sound dishes

  1. I’m not sure if my original message was sent, so am re-posting it here. Apologies if you get it twice!
    Hi Helen,
    As there was already a load of discussion on the PTNC replying to your question I thought I’d reply privately, but couldn’t find an e-mail – hence why I am writing to your blog!

    Regarding your sound dishes at the Science Museum I would describe what is going on in exactly the same way as I would with a radio transmitter and receiver, since shoving the word “energy” in is probably pointless unless it adds something.

    E.g. A radio wave is produced by the transmitter dish and this travels through the air to the receiver dish, where the wave is gathered (and focused to a point) on the receiving antenna. In the same way, a sound made at the ‘transmitter’ dish is reflected out through the air towards the ‘receiver’ dish. This sound wave is then focused to a point (where you place your ear) and is then perceived by the hearer.

    Is there energy associated with a radio wave? Of course, it is “in” the oscillating electric and magnetic fields. Is there energy associated with a sound wave? Yes, it is “in” the compressions and rarefactions of the air. Does it help to discuss either? Probably not.

    I have a colleague in my department who absolutely loathes the new energy orthodoxy (especially the notion of a ‘store’ of kinetic energy!) but I am coming to see its benefits (albeit gradually!) My PGCE tutor was Ian Lawrence, who, together with Charles Tracy, was largely responsible for the “new orthodoxy” (he would probably rap my knuckles for calling it that!) During my PGCE (and for many years subsequent) I have interacted with him and can see why the IoP has taken this stance, although for teachers working in schools where very few students take things further than GCSE I can also sense the frustration felt by staff in trying to get something “right” that, say, 97% of students will never take any further. We always managed to get things right once they got to A Level, they would point out. Was it worth all the hassle just to have a purer set of descriptions at GCSE? For me, the jury is still out, but I’m happy to work with it.

    All the best,
    Stuart in sunny Penrith.

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