In William Butler Yeats' poem "The Lake Isle of Innisfree," the speaker is in the city, "stand[ing] on ... the pavements gray," but is imagining his favorite place, which he calls Innisfree. The "lake water lapping with low sounds by the shore" is both a memory and a call to the speaker. He has been there in his past, but now as a city dweller, he misses the quiet sound of the lake's waves gently brushing against the land. Unlike the sound of the crashing sea against the rocks, the sound of the peaceful lake is quiet, like a humming in the background. One could almost forget it is there, yet it creates that soothing kind of background noise that makes it easier for some people to think and sleep.
You may have heard the term "white noise." People today purchase sound machines that make a constant sound of static. However, you may not have heard of "pink noise." With pink noise, the higher sound frequencies have been eliminated, and only the "low sounds" are used. You can listen to both kinds of noise at the link below to see which one seems more soothing to you. Doubtless the speaker would have preferred pink sounds because they would more closely simulate the sounds of the lake he remembered.
The low sounds were not just a memory to the poem's narrator; they were also a call. Because his memory of the "lake water lapping" was so strong and persistent, something he heard "always night and day," he felt as if it was calling him to go back there. It created in him a deep longing and desire to return to the part of the world where "peace comes dropping slow." That longing came from deep inside of him, from "the deep heart's core," causing him to say, "I will arise and go now, and go to Innisfree."
The poem was autobiographical for Yeats. As you can listen to him relate in his own words at the link below, Yeats wrote this poem as a young man of 23 while living in London after he had passed a display fountain in a shop window where a small jet of water was balancing a ball. The sound of flowing water in the display reminded him of how peaceful he felt at Lough Gill, the lake in Sligo, Ireland, where he had spent time as a boy (pictured below [source: www.sligo-Ireland.com]). If you can imagine the quiet, soothing sound a small decorative fountain makes, you will have an idea of the sound that triggered Yeats to write this poem--the sound that Yeats describes as "lake water lapping with low sounds."