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Listening Room Acoustics Vs. Free Space Listening

Dennis Foley May 7, 2014 No Comments
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Free space is a space free from objects that sound can run into generated from a source in that space. Free space listening is not in our listening room. When sound strikes an object, the sound is either reflected, absorbed, or diffused. Free space is just that, space that sound can travel freely without paying a toll by striking an object. It is sound that travels directly from the source to the ears of the listener.

Room Distortions

I had the idea for it one night at dinner with a friend. My friend was an accomplished artist and I began explaining the concept I had for free space listening. A space free from all surface boundary reflections like we have in our rooms. Free space listening is the opposite of listening room acoustics. A space where there is no room sound, only the sound of music. A space where we do not have the four acoustic room distortions : Room Modes, SBIE (speaker boundary interference effect), Poor Diffusion, and Comb Filtering.

Artist View

She listened intently, as I explained a two channel system set up using the speakers and seated position as the indexes of our triangle. This triangle would be in a space where it was quiet and had no background or ambient noise level. A space free from traffic, people, and all the associated human noise. A space free from room boundary surfaces with none of the four main room distortions. She took my thoughts and arranged them into the graphic you see here.

Free Space Listening

Free Space Listening

Room Modes

Since our graphic shows no “box” or room, we do not have any room modes. We do not have anything that can create pressure within our listening environment. The acoustics in this listening room example, are not bothered by low frequency, pressure build ups. Low frequency energy is free to move about and not be forced to fit into smaller room dimensions than the wavelength demands. A 30 Hz. wavelength of 37′, can move freely without striking a wall and creating a pressure pocket at certain room locations.

No Pressure

With no room walls, floor, or ceiling, we do not have the boundary surfaces for low frequency energy to pile up against and create room modes. You know from our past discussions, that room modes are areas of pressure build up caused by a longer low frequency wavelength trying to fit into a smaller physical space. If a microphone is placed in a room mode or pressure area, you may or may not hear any sound coming from that physical location of the room you are recording in or you may hear too much of a sound due to the room mode your microphone is placed in.

SBIE – Speaker Boundary Interference Effect

SBIE or speaker boundary interference effect, does not exist because we do not have any boundary surface to have our speakers positioned next to. SBIE occurs when a sound generating device is positioned right next to a room boundary surface and pressure and reflections from that close proximity to each other occurs. Without walls in our room, we have no surface to block or hinder the movement of energy. This close proximity to walls and room surfaces, creates pockets of pressure that can smother, blur, or completely eliminate parts of our audio signal. This is the major acoustical issues when it comes to the acoustics in a listening room.

Poor Diffusion

Poor diffusion in a room occurs because it is a room. It is defined by walls, floor, and ceiling and this “box” confines our sound energy. The usual solution to this excess energy which causes reflections and room modes is to use absorption technology to manage it. If we use absorption, then we cause a energy transformation from electromechanical sound energy to heat and thus our sound is changed and gone forever. A properly diffused room does not destroy sound energy in order to manage it, it simply takes the existing energy and distributes it out more evenly within the room.

Comb Filtering

Comb filtering can not occur because we have no room and no objects in the room for energy to get trapped between. Comb filtering is a series of similar reflections where sound energy strikes a surface and then another surface and continually repeats that procedure back and forth between the two surfaces. This is why it is called a comb filter. The reflections look like the teeth of a comb on a graph. A comb filter can act similar to a room mode and mask or exaggerate certain frequencies. Comb filters usually occur in the middle frequency ranges.

Free Space Listening

Free space listening is just that, it is listening in space that is free of obstructions. It is listening without a room and all the acoustical distortions that go with it. In free space listening, we have no walls, ceiling, or even a floor to produce reflections that cause SBIE and comb filtering. We have no room or box to try and force long, low frequency, wavelengths into, so they can express their discontent by creating room modes and comb filters. Without a room, we are acoustically free of these troublesome distortions.

In Summary
I hope this explanation helped. Please leave any comments below so I can get back to you. Don’t be afraid to hit those Facebook like, Google+ and Twitter buttons on the left hand side so other people can see this post. And if you want to learn more about this subject please sign up for our free room acoustic treatment videos and my Free Space Listening ebook which provide step by step instructions. Get instant access by signing up now.

Thanks
Dennis

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Dennis Foley

I am an acoustic engineer with over 30 years’ experience in the business. My technology has been used in Electric Lady Land Studios, Sony Music of New York, Cello Music and Films founded by Mark Levinson, and Saltmines Studios in Mesa, Arizona, along with hundreds of others.Connect with me on Google+

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