What is the lecture mainly about? Why was the experience of visiting anechoic chamber significant for Cage?

Listen to part of a lecture in a music history class.
    Professor:
    OK, we’re gonna continue our discussion of 20th century music. In the early 20th century, some composers in Europe and the United States, composers of so-called classical music had already started abandoning traditional forms… urn… in favor of newer and different ways. They were exploring new types of making music. As the century progressed through, the styles of avant-garde composers began to take further shape. People didn’t always appreciate new styles of music being experimented. You see, for many people, avant-garde music was too radical and very different. They even thought the government should put a ban on it. Now, as a case in point, let’s look at the composer John Cage.
    Cage is among the most famous composers of the 20th century avant-garde music. His earliest compositions were written in a traditional style, but then he quickly moved on to create unique kinds of works. So, what caused him to change? He had two particular experiences that entirely changed how he thought about music.
    One was when he met the avant-garde painter, Robert Rauschenberg in 1951. Now, what does painting have anything to do with making music? Well, avant-garde is a term that applies to a lot of different artistic genres. The famous painter, Rauschenberg had created a series of famous paintings that were composed mainly of white paint. There was basically nothing on his paintings… just different textures of white. I mean, literally… just white. However, the concept behind these paintings actually wasn’t so simple. He was trying to show that even if you don’t create any artwork, you can still have something… because even on a purely white canvas, there’s still plenty to see… shadows, reflections, dust. Rauschenberg’s white painting was highly influential for Cage and opened up a whole new way of understanding what art could be.
    The other important experience in Cage’s development came when he stepped into an anechoic chamber of Harvard University. An anechoic chamber is a room with walls that are designed to absorb all sounds made in the room. The word, anechoic means totally silent. So this is an ideal place where you can experience absolute silence. When Cage entered the room, he heard two unexpected sounds— one high, his nervous system in operation; and one low, his blood in circulation. He was deeply affected. It was at this point that he realized that music doesn’t need to be created intentionally. It is already all around us. This idea is what came to be called "found sound". Basically it’s about the sounds that are already there… traffic outside your windows, raindrops or whatever. Cage thought that they were just as musical as sounds made by musical instruments.
    It was these experiences that led Cage to create a composition that would express the idea of found sound. He wanted to provide an opportunity for the audience to identify random and natural sounds of the environment as music. So he composed his most famous piece called 4 33 " commonly known as the silent composition. This piece was completely silent. It consisted of the pianist going up to the piano and not hitting any keys for four minutes and thirty-three seconds. In other words, the entire piece consisted of silence. The only thing the pianist did was raise and lower the lid of the keyboard to let the audience know the beginning and ending of a movement. This composition had three movements but not a single note was played in any of them.
    Well, when it was first performed on stage, the audience was furious. People began whispering to one another, and some people began to walk out. It was called ridiculous and crazy by critics. For Cage, though, the music was just fine. He said that the audience was scandalized because they just missed the whole point of his music. He believed that there was no such thing as silence, no such thing as a complete absence of sound. During the performance, in fact, there were sounds of the wind in trees, raindrops pattering on the roof and people muttering. Cage had a different understanding of silence. He defined silence as simply the absence of intended sound or rather turning off our awareness. If we give up intentions, then we hear silence. So, to understand 4 ’33" as music, the audience had to pay close attention to the sound around it.
    Well, this was quite revolutionary so the reactions of people at the time were pretty much legitimate. I mean, it’s confusing and even confounding to people even today. Cage’s silence composition is still performed all over the world.
    Unfortunately, though, I guess it’s often misinterpreted. You see, it’s been choreographed, so sometimes it includes dance performances with the beat of the dancer’s feet against the stage floor. And they make some noises on purpose to call attention to the fact the piece is basically silent. Now do you think these performances reflect Cage’s ideas well?