RealTraps - Acoustic Basics

Hi-Fi and Home Theater Edition


More RealTraps Articles
RealTraps Educational Videos


Click the photos below to see them full size.

Click to see a larger version




"The culprit is bass waves bouncing off the walls, floor, and ceiling. Nulls as deep as 30 dB are not only common, but typical."




Click to see a larger version




"A few milliseconds later, reflections arrive after the sound bounces off the side walls and ceiling. This is known colloquially as time smearing."




Click to see a larger version




"Forget packing blankets, egg cartons and non-acoustic foam: These do not work. That's just another Internet myth."




Click to see a larger version




"Poor room acoustics is the biggest cause of dissatisfaction among audiophiles and home theater owners, whether they realize it or not."




Click to see a larger version

By Ethan Winer

Most audio devices sold these days have quality ranging from very good to excellent, and can pass music and movie sound tracks with minimal coloration. Yet many hi-fi and home theater enthusiasts, unhappy with what they hear, wrongly blame their gear. The important metrics for audio fidelity are frequency response, distortion, and noise. Most modern devices are very flat with distortion and noise too soft to hear at reasonable volume levels. Of course, loudspeaker quality varies a lot, but the rooms we listen in vary even more.


Click to see a larger version

Click the image to see it full size.

Bass frequencies are the most difficult to tame in a typical home-size room. One common problem is that music sounds different in different parts of the room - even for two adjacent seats on the couch. The culprit is reflected bass waves that collide in mid-air creating peaks and nulls in the response. Nulls as deep as 30 dB are not only common, but typical. Most rooms have several nulls in the bass range, as shown in the graph above.

The finest loudspeakers in the world are of little value if your room skews everything you hear. The solution is bass traps placed in the room corners, and optionally on the walls and ceiling. When bass traps are added to a room the low frequency response becomes flatter, and also changes less around the room. The more traps you add, the better it gets. It really is that simple. Note that rectangle rooms have 12 corners: four where each wall meets another wall, four where each wall meets the ceiling, and four more where each wall meets the floor. All are viable for bass traps. The article Bass Trap Myths explains much more about bass traps.


In a standard rectangular room, it's best if the speakers fire the longer way down the room. This improves the response at low frequencies by putting the reflecting wall behind you farther away. Symmetry is equally important, so center your seating and speakers left-right in the room. Listening in the front part of the room also helps the low end response, though a large movie screen in a small room might require sitting in the rear third of the room. The ideal speaker height is with the tweeters at ear level, and aimed at the center prime seat (toed in) rather than pointing straight ahead. Regardless of speaker brand or type, you'll enjoy the flattest response when the tweeters are pointed at your ears. The article How To Set Up a Listening Room explains room and loudspeaker setup in more detail.


Click to see a larger version

Click the image to see it full size.

Another common problem is poor clarity and imaging caused by "early" reflections. Sound from the loudspeakers travels directly to your ears, but a few milliseconds later reflections arrive after the sound bounces off the side walls, floor, and ceiling. This is known colloquially as "time smearing," though the more proper term is comb filtering. Comb filtering is a specific type of frequency response error that creates a series of peaks and dips as shown above.

The cure for early reflections is absorber panels placed at specific points on the side walls and ceiling. Once these points have been treated, clarity and soundstage magically come to life. All of a sudden you can easily hear small details that were previously masked by excess ambience and echoes. Stereo imaging also becomes wider and deeper. See the articles Creating a Reflection-Free Zone and Early Reflections for more information.


Diffusion avoids the damaging echoes and comb filter effect caused by reflections off nearby walls, but without reducing ambience as much as absorption. The best type of diffuser uses the Quadratic Residue method. You don't need to understand the math behind a diffuser to appreciate how much better it sounds than a bare reflecting wall! Unfortunately, good diffusers cost more than good absorbers because they're more complicated to build. If the room and budget are both small, placing absorbing panels on bare surfaces works very well. But when cost is no object and you want to retain as much liveness as possible, diffusion is the key. Good diffusers also make a small room sound much larger than it really is. But forget bookshelves - that's just an Internet myth. Our two videos Hearing is Believing and All About Diffusion let you hear what diffusion sounds like.


Trying to fix room acoustic problems with an equalizer doesn't work as well as proponents claim. Every location in a room has a different response, so no single EQ setting can help everywhere. Even if your goal is to improve the response for only one seat, it's impossible to counter nulls. As an example, if you have a 25 dB dip at 60 Hz, adding that much boost with EQ will increase low frequency distortion and likely blow up your loudspeakers. And at other locations where 60 Hz is too loud, that boost makes the response even worse. EQ can reduce peaks, but it does little to reduce the extended decay time that accompanies most peaks. Our article Assessing Room EQ explains why EQ is not a suitable substitute for bass traps and other treatment.


Forget packing blankets, egg cartons, non-acoustic foam, and carpet - they don't work well enough to consider. Egg cartons are way too thin to absorb low frequencies which is needed for music rooms. Likewise, non-acoustic packing foam is not the open cell type that absorbs sound. Styrofoam panels meant for thermal insulation have no useful acoustic value either. The best bass traps and absorber panels are made from rigid fiberglass. Inch for inch, the rigid fiberglass used in all RealTraps products absorbs more, and to a lower frequency, than any other material available.

In my experience, poor room acoustics is the biggest cause of dissatisfaction among audiophiles and home theater owners, whether they realize it or not. Sadly, too many people consider everything but acoustics when trying to improve the sound of their systems. It's a real eye-opener the first time you hear every note clearly articulated by an electric bass, and every musical detail in a complex orchestra performance. It may seem surprising, but acoustic treatment will improve the quality of everything you listen to far more than your choice of CD or Blu-ray player, wires, preamp, and power amp. In that one moment when you first listen in a well-treated room, it's immediately clear what you've been missing all along!

Ethan Winer plays the cello and electric guitar, and loves to write pop instrumentals. Ethan has, at various times, earned a living as a studio musician, computer programmer, audio engineer, composer/arranger, technical writer, and college instructor. He's also author of The Audio Expert published by Focal Press.

Entire contents of this site Copyright 2004- by RealTraps, LLC. All rights reserved.
No portion of this site may be reproduced in any form without the express permission of the copyright holder.