Do Cups and String Work as a Telephone? PDF Print E-mail
Saturday, 09 September 2006 05:03
We all know about it, get a couple of cups, get some string, put it all together and you've got yourself a makeshift telecommunication device that would make Blue Peter proud. Does it really work? How does it work? If it's really that good, how come we pay stupid amounts of money to mobile network operators? Can you use it for text messages? Get the answers to all those questions (except the last two) from the one and only Science Gimp.

Do Cups and String Work as a Telephone?

This somewhat crazy idea certainly does work - I urge you to try it! It's great fun and you can impress you friends with the knowledge of why it works!

The science behind this one is relatively simple and is all to do with sound waves and their properties. Lets look at this first.

Sound is a form of energy which travels in waves. When you hear a sound you are actually interpreting the vibrations of millions of air molecules that are being stimulated by sound waves. These stimulated molecules hit your eardrum and vibrate it in exactly the same way that they are being stimulated by the waves (wavelength, frequency, amplitude etc..). Your eardrum then sends these vibration to the cochlea which reinterpates the vibrations and sends signals to your brain, decoding it all along the way and making you think you hear a sound.

Ok - so we know now that sound travels in waves. What's more important to understand is that sound is only a form of energy which travels by being transferred from molecule to molecule. When a sound is made, what happens is molecules at the source of the sound have been stimulated in a certain frequency and wavelength. These stimulated particles bump into neighbouring particles, transferring the same stimulation/vibration. These then bump into other neighbours and transfer the vibration…. And these to the next…and so on and so on. This happens very very quickly - so quickly that it's usually not possible to notice under a distance smaller than 50m.

So in essence this is sound - a form of energy that travels in waves and is transferred from molecule to molecule at an incredible rate.

So how does this work in the cup and string situation? Lets assume that the 2 cups have had a hole made in their bases and string is tied in a some sort of knot so that it can't be pulled through the hole. At this stage it's important to note that the cup must be of a sufficiently "flexibly" material (paper or Styrofoam is fine - a china tea-cup would work less well) and the string must be pulled tight between the 2 cups.
When you speak into one cup, the sound waves that you produce will travel (via the vibrating molecules) and hit the cup. As the vibrating air molecules hit the cup, they will transfer some of their energy (the rest is reflected) into the cup molecules - causing them to vibrate. The cup molecules will vibrate (even if the amplitude is very small compared to the original) at almost the same wavelength and frequency as the sound waves. Because the cup is attached to the string and the string is pulled taught, these vibrations will travel down the length of the string and be transferred into the molecules which make up the cup at the other end. That cup will begin to vibrate at the same frequency and wavelength, transferring it's sound energy into the molecules in the air at the other end. And so, acting as a small speaker, the cup stimulates the molecules in the air within it and - hey presto - sound. Put simply, it's all to do with the energy transfer from air to cup to string to other cup to air!

Even though there are lots of inefficiencies in the system and energy and vibrations are lost in almost every part of the transfer from medium to medium, it is still a fun and effective way to communicate without disturbing others over a short distance.

You can even calculate the speed the sound is travelling in the string by using this simple equation
The speed of a wave travelling along a string (v) is directly proportional to the square root of the tension (T) over the linear density (µ) :

That equation is not strictly true at providing you with the speed of sound in the string - it's more appropriate to a wave pattern travelling along the string (as in a guitar). But that's a different kettle altogether.

Fascinating Sound facts

  • Sound travels at 345m/s at normal atmospheric pressure - which is equivalent to approx. 776mph! Relatively slow compared to the speed of light - which is 299,792,458m/s
  • Sound cannot travel in a vacuum because it requires particles to transfer the wave properties (energy) to adjacent particles. So all though loud explosions roaring through your speakers in Star Wars dogfights, just ain't accurate.
  • Sound travels faster in denser mediums (water for example) because the particles are closer together therefore transfer the sound energy to neighbouring particles quicker.
  • Sonic weapons tested by US DoD - firing very focused low frequency waves - have had varying success in incapacitating their targets. Some of the more violent ones work on the principle of quite literally shaking apart the target's internal organs. The test subjects in this instance were a heard of cows…..
  • The first time the sound barrier was broken on land was in1948 by a rocket train in California. It was powered by 50,000 pounds of thrust, reaching 1,019 mph
  • The saying "Sound as a pound" originates from the introduction of the pound coin in London, referring to the favourable exchange rate at the time.

Add New
Death via sound Posted by Obbop 2006-11-22 20:58:38
To assist in keeping Viet Cong and others with harmful intent from attaching
explosives to ships in South Vietnamese harbors warships with sonar would take
turns "pinging" their sonar at regular intervals.

Some of those
sonars were powerful enough to kill a swimmer up to 300 or more feet away from
the sonar dome where the ping originated.

Loud stuff, boys and

Makes me wonder, though, what the Union would have thought of a
foreign nation entering OUR civil war on the side of the southern (Confederate)
states. Gosh, the USA is sure two-faced. Butting into other's affairs yet
demanding everybody keep their hands off us.

Please remember, people. The USA
federal government IS owned and operated by an elite class and BIG corporations.
We, the common folks, are akin to the rest of the world. We have no say-so over
our government. Voting is a farce, the government does what is best for the very
wealthy sm...

Mom Posted by Randie 2011-01-23 19:08:01
We are working on a science project and would appreciate a bit of help. We are
trying to determine which medium carries sound most clearly. We made a string
telephone using Rope, Nylon and Metal wire. It seems like the Nylon performed
the best and we are trying to understand why this would be. Please feel free to
email me or call at or 631-537-8390. Thanks very much,

MEDIUM Posted by Herbert Mangolomet 2014-01-15 23:36:39
Hi I am having trouble with identifying the medium that sounds travel
through when using a string phones. You know what I mean? Please reply
ASAP!!! I need help for this lab I am performing at school...

