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Enjoy daily concerts...in your own home
By Roy Johnson, loudspeaker designer, Green Mountain Audio, Inc.
Sitting in front of your speakers, you can certainly imagine you are at the original event, inside its soundfield. Here, we describe what can be perceived from a recording and how rooms and speakers can conspire to leave us at home instead of taking us to the concert.
Two dimensional images, never 3-D
A stereo image allows you to 'see' one person there...another one there...and so forth, from left to right and even beyond those two speakers' locations. Thus, the left-right direction defines one dimension. There is no height dimension, because microphones do not know 'height,' only distance. Someone's tone, dynamics, and surrounding echoes change with distance, and to us that sounds like 'depth,' the second dimension.
Expect all echoes to also lie directly behind those voices and instruments, as illustrated below -- perhaps the studios will spread them to the sides but they would still lay along that straight line. A surround-sound system simply extends these two dimensions around us.
That second dimension, 'depth,' can also be heard as the sense of spaciousness in a recording. This is a complex set of echoes better known as reverberation (or reverb), such as those heard at a symphony hall.
Echoes and reverb are artificially produced in most pop/rock/jazz recordings and in most soundtracks. The result could emulate the sound of a 'fantasy' space, perhaps a cave of cathedral size, but with rough walls and a smooth floor.
It could be the sound of an intimate nightclub or an indoor arena. Many 'spaces' are available through the ingenuity of the recording engineer and have been since the early days of 'spooky-sounding' radio shows.
One of the most cherished echo-generators is a vibrating metal plate (below. Made by EMT in Germany, it is more than a meter tall. Capitol Records in Los Angeles has its legendary echo chambers buried 30 feet underground.
Sharpness
of the image
The stereo image has a left-to-right sharpness limited by the spacing of our two ears.
Our research shows that a trained listener can point to one person's voice next to another's when separated by only two degrees.
This about 4" (10cm) apart when they are 10' (3m) away. Of course, this means two people singing cheek by cheek, which makes no sense from a performance aspect. It does, however, illustrate the sharpness of aural perception, with your eyes closed.
Microphone technique
When recording an orchestra, it is possible for their echoes to come back to the two (or more) microphones with particular timing differences. We might hear the result as height simply by chance, as it depends on the concert hall, the microphones' placements, and their pickup patterns. This happens sometimes on the older recordings by Mercury Living Presence and RCA Living Stereo, among others.
Multiple microphones on an orchestra produce less-sharp images but more natural reverberation.
An example would be the Telarc, Reference Recording, and Deutsche Gramophone labels.
However, these techniques' shortcomings are sometimes revealed by their effect on the sound of the strings and/or brass.
The most natural-sounding orchestral microphone techniques are the ORTF style, the M-S method, the 'Decca tree,' and the single stereo microphone. These are seldom used because all require much experience by the engineer and extra time to be positioned in 'just the right spot' for each concert hall and orchestral layout.
A jazz combo can sound like they are performing in a small (empty) nightclub when the microphones were placed just far enough away to capture some of the studio's natural ambiance.
On a live recording, the main microphones must be placed close to each artist with extra microphones often placed out above the audience.
Any rock, pop, R/B, soul, hip-hop, dance, trance, electronica, reggae, and jazz fusion recording has the individual instruments and voices recorded as pinpoints, then a copy of each is time-delayed and double-tracked back in. The intention might be to produce a 'smeared-out' sound from left to right, which is 'enveloping.' Expect the artificial echo in these recordings to produce a pleasing effect rather than emulate a particular type of space.
When your speakers can create precise images of whatever they are given from the recording, then your music and soundtracks are more enjoyable, because everything going on is more easily perceived, and the more quickly you forget you are at home. The clarity of the images should hold up as you move away from the middle chair.
Blurred images
Imaging and the artistry are both degraded by reflections from your room's surfaces. Different reflections change the sound in different ways:
Reflections off of the side walls nearest the speakers keep the image squeezed between the speakers- performers sound crowded together. When you turn up the volume, voices sound 'shouty' because these reflections are most strong in the voice range.
A bare wall behind the speakers reduces the depth of the image, because you hear its near-term reflection before you hear the longer-term recorded echo. That bare surface also makes the voices 'shouty' when the volume is loud, because it is throwing back to you all the echoes it receives from the rest of the room.
If something large is placed right between the speakers, the depth is destroyed along with the tone balance. A large surface in the middle reflects voices and highs very loudly (and very quickly), thus covering up the recorded echoes, smearing voices and highs (which is the irritation factor), and makes the mids and highs sound much louder than the bass. A large object would include any entertainment center, rack full of gear, and video screen.
A large, bare coffee table in front of you (in a small or medium room) and/or a bare floor in front of the speakers are of no help either. Their reflections make music and soundtracks tiring, and at loud volumes, hurtful to hear.
Eyeglasses make reflections, audible on brass and strings. Take them off when listening critically, such as when choosing a new amplifier or cables (or speakers).
These are all early-arriving reflections, which is the problem. When your speakers are placed at least 20" (50cm) out from the wall behind and/or the video screen (to their front surfaces), and a few feet (1m) from the side walls, then you will hear far more music, musicianship, and clarity even when you are off in the kitchen. This is because the brain uses that time delay to separate the sound of your room from the sound of the recording.
One could keep speakers close to the sidewalls and/ or to the wall behind (but not to the video screen in between) by treating those surfaces with diffusing pro- ducts from Acoustic Sciences, Auralex, Real Traps, RPG, and SRL Acoustics, among others. These devices scatter the principal reflection into many directions, so it is no longer loud in your direction. Diffusors make a small room sound large. One can place sound-absorbing panels in the large bare areas of a sparsely-decorated room, to reduce its overall echo. Some placements of diffusors and absorbers are described on our Room Acoustics page and in our Owner's Guides, but the best advice comes from their manufacturers.
Speaker placement
We recommend the Left and Right speakers be spread 50-53 degrees apart. We call this an 'Equal-legged T' layout.
An example would be if the speakers are placed nine feet (3m) apart, center-to center, then one would sit 9' or little more (3m+) back from the midpoint of that line connecting the two speakers.
This triangle is about 10% narrower than an equilateral triangle, and best suits how we hear via two ears. This is discussed in more detail in our section on Speaker Placement.
Speaker dispersion
A speaker spreads its sound from left to right, which is called its dispersion pattern (it should be called its radiation pattern). All speakers have different patterns of dispersion, and you can hear that as you walk around them.
When a speaker disperses all tones from bass to treble equally in all around itself, this is an omni-directional design. However, most recordings do not sound right on these speakers, because they create many reflections off your walls, ones the recording engineer never expected any speakers to make. One could heavily treat all the walls behind and beside these speakers to absorb those sounds, but then why use an omni-speaker?
A speaker theoretically could be totally directional, sending sound only to you in the one chair. While that is no fun for anyone else in the room, some speakers are highly directional, but only in their treble range. In the lower voice range, they put out a wide pattern, and low bass tones go everywhere. Thus, you must sit exactly in the middle to hear the best sound. Anywhere else in the room, the sound is quite 'dark' from the lack of highs. In fact, even sitting in 'the best chair,' most recordings typically sound 'dark' because of the lack of treble coming back to you from the room.
A speaker can vary in its dispersion, perhaps sending bass and treble out widely, but the upper-voice range only goes straight ahead. Singers then sound muffled to anyone not seated in the middle. For the person seated in the middle, these speakers have to be placed 'just so' for their voice-range reflections off the room's side walls to subjectively fill in that missing off-axis upper-voice range.
A speaker can vary in its dispersion and in its tone balance with your distance to it. The worst offenders are those designs using multiple woofers, mids, and/or tweeters. Multiple sources of sound add together and subtract from each other differently depending on your distance and angle to them.
As discussed above, the hard surfaces of a room, entertainment center, and video screen love to loudly reflect upper voice-range and low-treble sounds. These reflections also lie in our most acute tone-range, making them the most fatiguing to 'listen through.' Because of this, our speakers are designed produce less and less high treble, less middle voice, and somewhat less low-voice range as you move to their sides. This gradual downwards-tilt to their tone balance as you move to the sides means that they smoothly reduce what your walls would like to reflect, having no 'holes' in the spectrum such as a dip in the upper-voice range. This makes for the most natural sound in average room as long as the speakers are pulled out from the walls.
Time-coherent speaker design
Most speakers greatly delay the bass, the voice somewhat less, all compared to the highs. This is the same as saying their treble range comes out first. The effect on the image can be a sense of height on violins and cymbals. While this is a pleasing effect for some recordings, most are affected in negative ways, because highs that come out too soon make the sound excessively harsh, or make some music sound sluggish (from the bass arriving too late).
When a speaker has the least amount of time-delay distortion (minimum phase shift), the bass, voice, and treble tones always arrive at your ears in their original sequences. You will then hear all the sounds and images form along one horizontal line, which means they are then the sharpest in clarity and the concert hall behind them is most apparent. You will be hearing the most musicianship, the clearest soundtracks, the most instrumental textures, and the most realistic ambiance. You can play any recording, because distorted recordings are not being smeared out in time.
Time-coherent speaker designs are few, because they are difficult to design and expensive to make. Our speakers are time-coherent. More about this is found on our page, Time- and Phase-coherent Speaker Design..
The most difficult part
It might take some ingenuity in decorating, but when your speakers are not placed tightly against any walls or the TV or entertainment center, and there is not much echo in your room, many surprises await in all your media -- and certainly more enjoyment than you ever thought possible. When the speakers are designed to deliver a wide and uniform soundfield into the room, everyone enjoys the music and films as much the one sitting in the middle.
Recorded Sound - The Ideal Soundfield - Time and Phase Coherence - Math and Physics - Drivers - Adjustability - Cabinets - Ports - Circuits - Measurements
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