Art Noxon, PE Acoustical is ASC’s founder & inventor of the TubeTrap. This week Art discusses room modes and their effect on your Hifi room. Enjoy!
Most people who read a lot, maybe even, read a little too much, begin to design their home theater by trying to figure out what their room ratios should be. Room ratios are magic numbers that are supposed to proportion the room to make it sound great. But there’s a problem with room ratios. They were originally developed for engineers who had to design reverb test chambers. By itself, that doesn’t seem like much of a problem. After all, reverb chambers have always been used for testing of acoustical materials. To get the job done, they were designed to create the most uniform sound field possible. That way, it wouldn’t matter which part of which room a test was done, the results would always be the same.
The acoustic engineering that went into reverb chamber design is about 80 years old, just about as old as audio. This reverb test chamber theory may or may not be a good way to design your home theater, but it certainly is the reason behind the quest for the original mode-perfect room ratio. By the way, the room ratio that has the top “acoustic quality” is 1.0 to 1.9 to 1.4.
But if you ever get to step inside one of those reverb chambers, or even look at some pictures of it, you will notice three things. The marble-walled room has a reverb time of about 15 seconds. The speaker position is always in one of the tri corners of the room. And the “listening position” is where the microphone is located, always in another tri-corner of the room. Only when the speaker is in a tri-corner can all the room modes be generated and only when the listening position, the microphone, is in a tri-corner will all the modes be heard.
The perfect reverb chamber may sport the most uniform distribution of modes possible but it doesn’t sound musical inside, it doesn’t even sound good. It’s horrible to be inside of one. It’s an acoustic torture chamber, not a listening room. But, why then is so much attention being paid to room modes these days? Most every home theater magazine runs an article in most every issue on room modes. Can everybody be wrong? Well, yes and no.
Modes do play a part in room acoustics. Problem is that mode acoustics is not as simple as the magazines make it out to be. For example: If you play a very short tone in a room, just a toot, you hear just a toot. You won’t hear “room modes”. The toot started and stopped before the room mode ever got built up. It takes time to build modes in room. It is not built instantly. Before the mode is built up, the sound from the speaker sounds just fine. Many sounds in movies and music are so short lived that they never create a standing wave, otherwise known as a mode.
It takes time to build up and create a mode, typically about 1 second. This means that the tone has to be held steady for at least one second in order to create a mode. Actually the amount of time it takes to create a mode is exactly equal to the reverb time, the RT-60 for the decay of the mode. If you like listening to a lot of slow paced organ music, you probably will care a lot about room modes. But if you listen to most music, tones aren’t held long enough to even create modes.