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RealTraps News - The 38 Percent Rule
Many RealTraps customers know about the "38 percent rule" to find the best place in a room to listen, as explained in our Room Setup article. In the article's sidebar, studio designer and acoustics expert Wes Lachot shows that the inevitable low frequency peaks and nulls present in all rooms are the least damaging when listeners are seated 38 percent of the room's length away from the front wall.
The 38 percent figure isn't really a "rule" as much as a starting point, because other factors such as room construction and wall materials also have an affect. So it's always best to measure the response as a final confirmation. But 38 percent is usually very close to the ideal distance, and it also applies for the rear wall behind listeners. Often the seating in home theaters needs to be farther back in the room, especially when the video screen is very large. So if being 38 percent away from the front wall puts listeners too close to the screen, sitting 38 percent away from the rear wall is a good second choice.
To prove this point I used the Room Sim module included with the fabulous Room EQ Wizard software. This "donationware" program measures the frequency response, ringing, and other acoustic properties of a room, and is described fully in the Room Measuring Primer on the RealTraps web site. This primer also includes a link to download the software for free.
The Room Sim feature helps you find the best speaker and listener positions without having to physically move things around! This is especially useful for people who have large heavy speakers. Although the examples below show only the main stereo Left and Right speakers, the Room Sim module can accommodate up to four subwoofers.
For the examples that follow I simulated a room 16 by 11.5 by 8 feet high, which is typical for a bedroom recording studio or small apartment home theater. I placed the left and right main speakers 2 feet from each side wall and 3 feet from the front wall, which the simulated response showed as reasonable distances. Of course, you can place the speakers anywhere you'd like to see how the response changes. The loudspeakers and the listener's head were both set to 3'7" above the floor, which is about right for an average adult when seated.
The first two graphs let you compare the frequency response at the front and rear 38 percent listening positions. The absorption on every room surface except the floor was set to 10 percent, which is a reasonable average for untreated rooms. The floor was left at the default 0 (fully reflective). If you're not familiar with room measurements these responses probably seem pretty awful. And they are. This is compelling proof that all music rooms need bass traps!
The good news is as more and more bass traps are added, the response gets better, and both the speaker and listener positions become less critical. The last two graphs show the response 38 percent from the front and rear walls with all five surfaces set for 50 percent absorption. I'm sure you agree that this is quite an improvement! It also makes the rear 38 percent location response much closer to that of the front.
That's all for now. Until next time I remain your faithful RealTraps friend and advisor.
RealTraps was founded in 2003 by Ethan Winer and Doug Ferrara, and is now owned and managed by longtime employee Sean Kollar. Sean handles the day-to-day operations at the factory, and Ethan still pitches in to help customers with technical and pre-sales advice by phone and email. Ethan is known throughout the audio industry for his technical articles in audio and computer magazines, and his popular book The Audio Expert published by Focal Press is now in its second edition.
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