Since discovering this science it is easily one of my favorite topics to discuss. People who haven’t done a lot of research into how sound actually works concerning room vibrations, reflections, and absorptions are really missing out on an interesting science. In my opinion, it really changes the thought and perception of what sound truly is or can be. When I began reading into this subject is when home theater and audio turned into a passion instead of a hobby.
At an elementary level all sound is, is vibrations. Sound waves are projected that create vibrations at different frequencies of pitch which produce the sound we hear and create what our brains interpret. A another topic of discussion is how the mind itself can be tricked, or convinced, that to the brain what something should sound like. All the way to the point that it actually begins to sounds different than what it really is. That is another topic for later discussions though. What I wanted to touch on in this post is the ability to monitor the sound in a room and actually seeing the cause and effect of what is taking place. This process cannot be achieved alone and does require some software, time, and physical hardware. The best dedicated source of information I have found on this topic is the REW Forums. There is a wealth of knowledge to be had just reading what has already been discussed. The forum is very open to newcomers. At least newcomers that follow and read the base rules before making blind posts. This is not only a forum though. There are some great freeware programs to access. The REW Analyzer is what I want to focus on for discussion. It simply blows my mind that the program is freeware. I realize at a base level the program is simply performing mathematical calculations, but it really shows me that there are some very knowledgeable people out there that really do invent with the intention of just sharing. It’s not about money or recognition. It’s about sharing a passion. It’s a heartwarming feeling knowing those people still exist. Back on topic though. What the REW analyzer does is map the frequencies heard in a room and charts them in a display we can see and easily understand. This does require some physical components and setup in order to make sure what data you collect is accurate enough to work with. The REW analyzer is a Mac or Windows based program. You would need a decent microphone used for recording what is known as a frequency sweep. So REW will play the frequency sweep through your home speakers and your mic records the sweep and REW analyzes the data and graphs it. I won’t elaborate on the configuration or setup for REW as that can be found on the forums themselves with some research. I want to explain what is analyzed by REW and why. So the sound any speaker makes can vary greatly depending on the room it is in. Put a speaker in a metal room and notice a vastly different sound from an entirely carpeted wall to floor room. What causes this change are the absorption and reflection properties a room, wall, or object in a room has. Again, remember that sound is simply vibrations. Not only do the vibrational characteristics change in rooms but also their reflective properties cause sound to vary greatly. A carpeted room for example is highly absorbent and this causes reflections to be very minimal and will cause the sound to sound dead. There is no life to what is heard. However, the sound in an all metal room causes an effect known as reverberation, which is the echoing of sounds, and this causes the original produced sound to get lost in the echoes. The source no longer sounds clear. A perfect room, as known in a scientific aspect, is a room that causes no reflections. With no reflections being heard the sound produced is exactly as what the speaker is capable of producing. This isn’t practical in homes. Therefore this is not the sound we are accustomed to and expect when listening. Therefore, most tests are performed in typical home settings that are common to most. What REW does is analyze what I just described. So a sweep in sound occurs across the entire frequency range. What happens is each individual frequency in a room has a different length of time the frequency remains due to different rooms having different reflective properties, not only on the sound as a whole, but at each individual frequency level. Some rooms for example sound very bass heavy or boomy while some may sound very harsh or bright. The room may hold or linger frequencies longer which drown out the surrounding frequencies. A room that does this is not considered ideal. You typically want a room that is equal across the spectrum and therefore creates a very even and clear soundstage. So REW during the analyzing period of a sweep is determining the peaks of frequencies across the sound spectrum, but is also recording the length of time each frequency is ringing. This will then, based on calculations, draw a 3D map of the sound of a room. So what you see on the X axis, left to right, is the frequency being produced and the Y axis is the volume reading in DB’s. On the Z axis, we will call it, you see a time in milliseconds. The Z axis is simply a forward to back axis. So combining the three axis’s paints a picture of how long frequencies ring, or remain in a room. This really allows you to visually see the sound your room is producing. So now that you have this information what do you do with it? This is were absorption and reflection comes back in to play. You can reduce frequencies by absorbing them and increase frequencies by reflecting them. By doing that you reduce or increase the time frequencies remain in a room. Different materials have different absorbent priorities at different frequency responses as well holds true for reflective materials. This again comes back to the overall theme of what a home theater should be. You could do what is recommended, or “by the books”, but I would never recommend that. To anyone. Ever. Determine what YOU like to hear and go for it. I use to be a stickler on trying to mimic what I read and was told assuming then that someone else was right and that I should just adjust and learn to enjoy what is “right”. Boy was I wrong. Determine what YOU enjoy. In the end, it’s really about you. To paraphrase, the REW program is extremely powerful for the data it is presenting. It becomes even more powerful when you understand it and can work with it. You can really achieve the sound you want at a scientific level. If there is one rule to follow on this topic, it is that the best sound is equal. You want frequencies across the spectrum to ring in equal lengths. If frequencies are not equal in the time space you are left with an unclear and uneven sound. This causes what is known as a muddy sound. If a frequency remains for a long time it will mask any frequencies produced after it. With all frequencies equal in volume and time you have a very clean, clear, and accurate sound that allows all details and nuances to be heard. If you really take sounds to this scientific level, what can be achieved is truly rewarding. It’s a whole art in itself and due to the field being all based on opinion is what creates an interesting fascination to me. Anything can be achieved with knowledge and understanding. This is probably one of my favorite topics concerning sound. I could discuss for hours on end. I know I will be posting future topics that relate to this discussion in some way. It’s truly fascinating. I hope the above opened your mind just a little bit in creating a visual perception of sound. It’s amazing what we take for granted every single day. If you stop, listen, and visualize what you hear. It is pretty fascinating.
–Austin
Common Mans Home Theater 