Small Room Acoustics vs Large Rooms
Small rooms have certain acoustical issues that are different from large room acoustics. Large rooms have certain acoustical issues that are not found in small rooms. Assuming we use the same full range sound source in each room size, small rooms are about sound pressure raising havoc between the room boundary surfaces. Large rooms are all about reflections off of surfaces that contribute to higher reverberation times.
Small Room Acoustics
The size of our small room environment has a major impact on the acoustical issues of the room, especially at low frequencies. Certain ratios of room height, width, and length contribute to low frequency pressure build up areas in our small rooms. These higher pressure areas are termed room modes and can cause acoustical distortions at our listening or monitoring position.
Room modes can blur and smother certain frequency ranges that we must use to listen to vocals or music. They can over exaggerate some frequencies within the mode or can completely smother others. They are areas of pressure created by the frequency that is bouncing or running against a certain room dimension that does not correspond to a ratio of wavelength to wall to wall length. Large rooms have more space.
Large rooms do not suffer from room modal issues. They have larger distances between their room boundary surfaces and longer wavelengths get a chance to spread out or run their full wavelengths. They can run free without striking a room boundary surface. I was in a church today that has walls that were 185′ apart. If we take a 20 cycle wave which the organ can produce and measure it, we would find it to be about 56′ long. With 185′ long distances to travel, the wave will fully form before striking any surface.
Small Room Reflections
Small rooms must deal with reflections from room boundary surfaces. All of these reflections get interlaced with the direct energy from our speakers and confuse the wanted direct energy. The direct energy is the energy that travels in a straight line from the speaker to the listener. This is the actual sound. When reflections from room walls mix with the direct energy, they superimpose themselves upon the direct sound from the speakers. This delayed signal reduces source clarity and staging. There is a much smaller time window to deal with in small room acoustics than with large room.
Large Room Reflections
Reflections from large room walls and ceilings, all add up to increased reverberation times. The reflections in large rooms increases the reverberation times, which plays havoc with everything. It mainly impacts the frequency range from 500 – 2,000 Hz., which is the range where vocals lie. This is termed the speech intelligibility range. If reverberation times exceed 1 second, then speech begins to become unintelligible very quickly.
Exceed One Second
If we exceed 1 second reverberation times for speech, the reflections in our large room become superimposed over the direct sound we need to hear. The direct sound is the sound that leaves from a source such as our loudspeaker and travels directly to the receiver’s ear. It is the straight line sound that does not have any room reflections interlaced with it to confuse and muddle the direct sound.
No matter where one sits, the reverberation times blur and confuse the signal. This produces room acoustic distortions and the direct sound becomes unintelligible. Churches built 50 years ago are a good example of large room acoustics and usually high reverberation times. It is difficult to hear the pastor when he gives the sermon. When the choir starts to sing there may be spiritual salvation but acoustically, all is lost.
Stereo Signal In Small Rooms
Most sound systems that are placed in small rooms are stereo or multiple channel systems such as home theater or simply two channel stereo. A stereo signal is composed of a signal that is designed to produce depth and direction perception. A properly set up two channel system will produce a sound image that can extend wider than the physical position of the left and right channel speakers. This is what audiophiles refer to as the sound stage.
Sound Stage Impact
Reflections off of the side walls in small room acoustical set ups have a direct impact on the sound stage. If the side wall reflections are not delayed enough in time below the 20 ms. mark, then sound stage image will suffer. We will also receive an image shifting if there is unequal distances between the side walls. The sound stage image will be pulled towards the shorter side wall distance dimension.
No Sound Stage
Side wall reflections, in larger room acoustical situations, add to the room’s overall reverberation times. There is no sound stage created between the speakers in a large room environment. The signal is mono and is not designed to create depth and direction. The mono signal is intended to project the sound out in a straight line array that matches the use and need of the building. Many speakers may be needed to allow for adequate sound coverage in the given large room size.
Multiple Mono Sources
In some large room acoustical situations, the area to be covered with sound is large and the rooms reverberation times are too high. One method for dealing with this is to use multiple speakers with tighter dispersion patterns across the sound surface area, say of a congregation. The smaller radiation area pattern produced by multiple speakers positioned in specific areas for coverage, produce direct sound in shorter patterns but one still has the high reverberation times within the room to deal with.
Small rooms have similar acoustical issues as large rooms just on a different scale and level. Small room surface boundary reflections, especially from side walls, can play havoc with our sound stage presentation in a two channel system. We can have image shifting and image size reduction. Large room reflection issues contribute to the overall reverberation levels and confuse the direct sound from our mono source. Low frequency issues are predominant in small rooms, where larger rooms have the physical space for lower frequency wavelengths to extend themselves.
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