echo ah echo blah blah blah blah blah sound waves travel through different substances differently which means that when they transition from one substance to another substance that they often change their behavior sometimes the sound waves will reflect off of the surface of something or bounce back where it came from sometimes the sound will be absorbed by the substance and then sound can also transmit through the substance similar to the way it was already traveling what happens when sound comes into contact with a substance depends largely on a few factors smooth substances are smooth because the particles are spaced very uniformly leaving a smooth surface whereas rough substances have really random particle spacing so when you bounce an object off of a smooth surface it bounces off equal and opposite to the direction it came in like newton's third law says which gives us a reflection that's why mirrors reflect and this ball bounces off the floor when you bounce something off of a really bumpy surface then it can bounce any sort of direction which is a distortion or it can be absorbed and just stop moving all together and so rough surfaces tend not to reflect sound very well just like this ball just sort of bounces off to the side in these rocks hard substances are hard because the particles are very tightly packed into the space and soft substances are soft because the particles are very loosely packed into the space which means that you can squish them because the particles have room to move you can't really squish hard objects so when i try to bounce a sound through a hard object it will transfer or transmit and we can also see that things will bounce off or reflect off of hard things like this basketball on the driveway soft substances on the other hand do not let sound pass through them and they tend to absorb that energy like this chair or this dog bed or the grass because there's so much space between the particles when the sound energy comes into contact with them it just kind of gets bounced around and lost within the substance and so we see it sort of dissipate or be absorbed whereas when we put energy into a tightly packed surface then the energy is able to hit one after another after another chain reaction so that the energy can be reflected back off or through the substance so here we have a soft couch and a soft pillow no great sound transfer neither on the rug but if we go on the hardwood floor a little bit more sound and a little bit more sound out of the bricks as well so soft squishy things absorb sound and energy and hard surfaces reflect or transmit the key to sound transmission lies in the fact that we need something that's hard smooth and a little bit elastic it can sort of bend and flex to allow sound to transfer through like when we hit this marble the energy transferred through and then move the marble on the other side that's because all of the particles inside of this metal are so tightly packed together and in such a grid-like pattern that the energy can just chain reaction through one particle to the next particle to the next particle so if we're looking for something to transfer sound metals are a really great choice like guitar strings and trumpets hard smooth metal tuning forks actually vibrate with the sound because the material's slight elasticity allows the waves to travel through just like this glass to a point and the metal on this fireplace [Applause] [Music] how will sound interact with the bark of this tree did you say absorb good job how about this hard plastic slide did you say reflect way to go and how about metals did you say transmit nice job but you know better with this wooden fence which is porous so remember the attic soft squishy insulation no echo but in this hallway hard smooth walls and floor means good echo time so if we want an echo we're going to need a place very hard and smooth surfaces but if we want sound controlled and we don't want sound to echo then we're going to need a place that has lots of soft and rough surfaces