Transcript
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In this episode of Home Theater Geeks, I answer a question from Daniel, who wonders if too much sound absorption in a room is too much. Stay tuned.
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Hey there, Scott Wilkinson here, the home theater geek. In this episode, I answer a question from Daniel in New Jersey, who writes, I just listened to your episode on Ryan's Amazing Home Theater. At the end of the episode, you mentioned that Ryan sat in his theater and it was disturbingly quiet. I've been told that you don't want to do too much sound damping in a room because it will sound dead. Does that make any sense? Is there such a thing as a room that's too quiet? Well, this is a great question, Daniel, that that theater was featured in episode 463, and actually you're conflating two separate but interrelated things, sound isolation, or noise control and damping. By the way, in your original email to me, you used the word dampening, which is a very common mistake. But I, being an educator, I want to make sure that you understand that that was not the right word to use. Dampening means to make something slightly wet. Damping means to reduce the amplitude of an oscillation, like a sound wave. So just. Just wanted to clarify that point for you. So, sound isolation and damping. Sound isolation is the process of limiting sound from penetrating a barrier, like a wall in a home theater or a commercial cinema or recording studio. You want to limit that sound transmission through walls as much as possible to keep external sounds out so you're not distracted by noises like traffic or kids playing or something like that, and internal sounds inside the room. So people in other parts of the house or the facility, whatever it is, are not bothered by those big explosions and those fight scenes that can really raise a ruckus. Now, damping seeks to reduce the level of sound reflections within a room, which is mainly accomplished using absorption panels on the walls and the ceiling. Let's start with damping. Now, you don't want to completely eliminate reflections. You are right about that. What you heard is correct. It sounds very unnatural. Our brains are programmed to expect some reflections and reverberations within a room. If we look around and we see walls, we expect to hear that. And our brain from birth, has been programmed to recognize what are called early reflections first. First you get the direct sound. You get whatever's making a sound. That direct sound gets to you first. Then it's reflected on the walls and the ceiling and the floor, and off of other stuff and Those reflections reach you at different times, depending on how many times they've been reflected. So the early reflections, your brain detects those and learns to ignore them. And it also learns to ignore the late reflections, the ones that have arrived later. It gives us the ability to localize where sounds have come from. And this was an evolutionary advantage because if you're in a cave, a reverberant cave, and there's an animal snarling about, ready to pounce on you, you want to know which direction it's coming from so that you can run in the opposite direction. And you can demonstrate this by recording a conversation in a room. So put a little recorder down in a room, record yourself conversing with somebody. When you play that back, it sounds very echoey. You hear a lot of the room because the microphone and recording device don't have the brains processing of the echoes and reverberation within the room. The early reflections occur within the first 50 or 80 milliseconds of the original sound. After that, the reflections sort of turn into this reverb mush that the brain really can't cancel out. And we, we can see a classic example, a classic depiction of that in the first graphic. Here we have a little violin player and we have a person sitting in a chair. The red line is the direct sound that gets to you first, always. The yellow lines are the early reflections that bounce maybe once before they reach you. And the gray lines are the late reflections, which happen later because they bounce around more than the early reflections. And the early reflections are what we really pay pay attention to. Late reflections just are reverb. So that is how reverb and reflections in a room work. If you deaden all the reflections, the brain freaks out. The room is too dead. Your brain says, I see walls, I hear no reflections, something's wrong. So how dead is too dead? You don't want a cathedral like reverb either, because as I said, the brain can't ignore that. And it sounds very, very well. If you've ever been in a cathedral, you know what that sounds like. So what acoustic engineers aim for is an RT60, which is the reverb time, the time it takes for the reverb to drop by 60 decibels, which is a lot. You want that to be about on the order of a third of a second, 0.3 second for a typical room in the 300 to 400 square foot area. So here you can see a picture, steady state sound source making a sound. Sound turns off, the sound decays. Now how long does it take for it to decay by 60 decibels is a parameter known as RT60, and you want that RT60 to be about a third of a second. To get there, you want to cover no more than 15 or 20% of the surface area of the walls with absorption panels or it'll sound too dead. Now, this RT60 rating of about a third of a second is based on extensive human listening trials that were done by the eureka project some 40 years ago, which was an international consortium to determine the ideal listening room for speaker manufacturers. Interestingly enough.