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Resonance, Reflectivity, and Reverberation

MikeSorensen June 7, 2012 No Comments
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Free Space Is Best

If we could work out in free space we would not have to deal with the room acoustic issues that our room “boxes” produce. Free space is just that, no walls or ceilings that act as containers for our music sound energy whether it is from speakers. Unfortunately, free space listening and living is not practical if one is in the music business. We all must do our work in different boxes. Our boxes or rooms cause many issues that must be overcome. Three such issues are resonances, reflectivity, and reverberation.

Room As A Box

When we introduce sound energy into a “box” which is what a room is we must address the sonic issues that this introduction of energy causes. Low frequency energy is the biggest culprit when it comes to room resonances. Low frequency waves are long and fat. They do not fit into our box, unless our box or room has the size to let them run free without causing issues which is more the exception than the rule. Since our low frequency energy waves are long they do not fit between our walls, they don’t fit within the space of our walls and then they create pressure areas. These pressure areas are termed resonances and they can occur between our side walls and opposite walls. We even have names for each of these particular resonance locations that occur between each of our room boundary surfaces. If our resonances occur between our room side walls that have horizontal, parallel alignments they are termed axial resonances. If the energy within our rooms does not like the space that runs from our room corners to corners, they are termed oblique resonances. Tangential resonances are resonances that occur between our floors and ceilings. Low frequency energy causes so many issues within our rooms that their locations get special names.

Resonances

These resonances cause many acoustical issues for those of us who make our living in a box. Low frequency energy resonances can do different things to our music. Resonances in our room are like little energy pockets that exist in different places in our rooms. If we place a microphone in one of these excess energy pockets or room modes two things can happen. If the resonances are strong in that area where the microphone is placed, certain frequencies will be picked up by the microphone and exaggerated in the recording. The opposite can also happen. Larger amplitude resonances can also smother certain frequencies, so that they will not be picked up by the microphone at all. Neither of these phenomenons are welcome.

Too Much / Too Little

In a playback environment such as a home theater room or listening room, we do not have a microphone to be concerned about, we have a listening position to address. We want to make sure our speakers are not positioned in a room mode, but we also want to make sure our listening position is not in a room mode. Our listening position must be placed in a position that is resonance free if we are going to hear everything in the recording. If our listening position is in a room mode, like our microphone, certain frequencies can be smothered within a room mode and not heard at all at the listening position. The flip side can also occur. Certain frequencies can be exaggerated to the point that they become their own sounds and can also overpower other closely related frequencies.

Reflectivity

Reflectivity is the ability of an object to give back as much as it receives. Hard surface materials such as concrete, windows, and metals produce reflections within our rooms. Absorption is the opposite of reflection. When sound energy is absorbed, it undergoes an transformation and is changed
to heat. This energy conversion means that the original energy is lost forever. Reflections bounce around our rooms and are confusing to our ears and brains. Reflections from our room walls and ceiling mix with the direct wanted sound from our speakers whether we are in a monitoring position or a playback listening chair.

Direct Sound

The direct sound from our loudspeakers is the sound that travels in a straight line from our loudspeakers to our ears. It is the sound that is reflection free, so it does not have the signature of the room in it. This is the very reason behind the professional process of near field monitoring. Sitting in a near field takes out the sound of the room and allows the listener to hear the direct sound from the speakers without the reflections from the room getting mixed into the mix. Our goal at the monitoring or listening position is to place both of them in a reflection free area which is just that free from wall or ceiling reflections.

Reverberation

Reverberation or reverb as it is called in the business has to do with frequency, time, and energy. Once any type of energy is introduced into our boxes or rooms, it has a life span. This life span starts with the introduction of the particular sound energy and follows this energy throughout its short lived life within the room. Once the energy enters the room, it is bounced around from one room surface to another. As it is moving through the room and bouncing off all the room surfaces, it interacts with other frequencies and sound energy doing the same thing. All of this juxtapositioned sound energy with its individual life spans adds up and this total accummulated energy mass is termed reverberation.

Incoherence

As sound energy is reflected from our room boundary surfaces, it dies away and it becomes what we call in acoustic circles incoherent which means all blended together. Our middle and high frequencies are the most affected by this phenomenon of incoherence. These are the frequencies that are most present in our vocals. If we draw a straight line from our speakers to our ears and use that as our reference, wanted energy then all other reflections we can view as suspect. The first order reflection is the first reflection that reaches our ears and is mixed with the direct straight line energy. The first order reflection usually comes from the surface closest to our speakers or sound producing device. This first order reflection then arrives at our ears and is mixed with the direct sound from our speakers and confuses our brains. All reflections must arrive at our ears after the wanted, direct energy from our speakers.

Resonances, reflectivity, and reverberation are the three “rs” when it comes to small room acoustic environments. Resonances are mostly the domain of low frequency energy and are very destructive. Room modal resonances can also occur at middle and high frequencies but are less prevalent because of their shorter wavelengths. Smaller room dimensions favor shorter frequencies. Reflectivity is the domain of middle and high frequencies because of their shorter wavelengths and occurs as our room sound energy is interjected into the room and then meets a surface. Reflections at our mixing or monitoring position confuses the wanted direct energy from our loudspeakers. All of these reflections, each with their own acoustic life spans living and expiring at different rates and levels, all contributes to room reverberation levels. All three of these “Rs” must be addressed in any quality professional monitoring or playback rooms.

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MikeSorensen

I am a structural engineer as well as a master furniture maker. I design cabinets for low frequency, activated carbon absorbers. Connect with me on Google+

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