A very natural acoustic guitar sound can be achieved by using a single mic 45 cm (15 in) or more from the instrument. This extra distance results in:
• Less proximity effect, and a clearer sound.
• A natural balance of all of the instrument’s component characteristics.
• Less potentially undesirable off-axis coloration because more of the instrument is on-axis to the mic.
The tradeoff of this approach is that the close perspective is lost and replaced with a more distant sound.
Using an omnidirectional mic instead of a cardioid mic will eliminate proximity effect and make the off-axis pick up sound better, so omnidirectional mics can be posi-tioned a little closer than directional mics without sounding so isolated. But omnidirec-tional mics should only be used if the recording room characteristics are desirable – in most small home or project studio rooms this is not the case.
“Zoom Factor” @
Example 7.1: A mic set up close to an acoustic guitar’s sound hole primarily picks up the body of the sound colored by proximity effect.
Example 7.2: A mic set up close to an acoustic guitar’s neck primarily picks up the lighter stringy quality of the sound colored by proximity effect.
Example 7.3: A mic placed 45 cm (18 in) away picks up a clearer, more accurate, blended picture of the guitar’s natural sound, and is not colored by proximity effect.
microphone. It’s simpler to represent it that way, and yes, a lot of the sound does do that – but not all of it. Most acoustic instruments and sound sources radiate sound in all directions, but it’s more complex than that – different frequency content is radiated in different directions, and not all of that frequency content remains audible over the same distance as other frequency content! This sound travels outwards to the floor, walls, and ceiling, which reflect it back into the room towards the mic, and the other boundaries, which reflect it again. An infinite number of reflection paths are quickly created, which give the room its character. See Figure 7.4.
The closer a mic is to a sound source, the greater the ratio of direct to reflected sound – the sound will be more dry, dead, in-your-face, and up-front.
As a mic is moved further away from a sound source, the level of direct sound decreases and the relative amount of reflected sound increases. This means that the sound of the room or environment is heard more. A more distant mic’s sound can be anything from “a little less up close and personal” but “beneficially more live,” to quite set back, unfocused, reverberant, and distant.
Most of the time we listen to musical instruments and singers in a room – a space with a floor, walls, and a ceiling. The reflections from those boundaries are an essential part of the natural sound we are accustomed to hearing. An uncomfortable sensation of “deadness” is experienced when walking into a recording studio’s dry vocal booth for the first time because the reflections and room sound that we are used to hearing are missing.
Unnaturally dry sounds, lacking reflected content certainly have their place in contempo-rary music production styles alongside more natural or stylistically wet, reverberant sounds.
Figure 7.4 Direct and reflected sounds are picked up by a microphone, and complex “room signature”
reflections are quickly generated.
. Sound Source Direct
Sound Reflected
\ Sound
7.6 FLOOR REFLECTIONS – THE GOOD, THE BAD, AND BOUNDARY MICS
When a sound source is close miked in a large room (even a fairly reverberant one), the ampli-tude of the reflections from the walls and ceiling are of a relatively low level compared to the dry, direct sound reaching the microphone. This is due to the fact that the reflected sound’s amplitude drops as it travels a longer path to the walls and ceiling, and then back to the mic.
Still, these low level reflections can have a huge impact on some sound sources. The floor is much closer to both the sound source and microphone so the reflections from a hard floor are louder than the reflections from the walls and ceiling – and they have a fundamental impact on the record sound.
Floor reflections are not a long swishy reverb tail, but can be an extra sheen and dimen-sion – more “oomph” and power, indistinguishable from the dry sound. They can add life and excitement to a sound, and we are used to hearing floor reflections as part of a natural listening experience.
• A mic very close to a sound source picks up mainly dry sound which overpowers and masks reflected content – and the result is a dry, dead, in-your-face sound.
• Moving the mic a few feet away allows floor reflections to become audible.
• Moving the mic many feet away decreases the dry, direct sound and increases the amount of wall and ceiling reflections (room sound or reverb) that the mic picks up.
Acoustically reflective floors can add beneficial floor reflections to a recording – but significant floor reflections can also cause phase problems and comb filtering due to the
Fi g u r e 7. 5 1: A close mic favors dry, direct sound. 2: A few feet away, floor reflections become part of the sound. 3: With a more distant mic position reflections from the entire room (reverb) are picked up by the mic.
Direct
Floor Reflections Distant : Wall Reflections
2
3 1If the sound of comb filtering is identified, the mic position should be adjusted to minimize the problem. Moving the mic by just a few inches can change the phase relationships of the direct and reflected sound at the mic position and make things better, or worse! Do not position a mic and assume it is picking up the best possible sound. Try a few different posi-tions – they may sound worse, but you’ll have verified that the initial placement was best.
Thin, lightweight carpet and rugs can cause poor sounding floor reflections because they only absorb high frequencies, so the reflections from a carpeted floor are muddy and dull.
Many instruments, such as acoustic guitars and drum sets, really benefit from the hype of bright floor reflections. Most studios have hardwood floors because the reflections sound great – and if necessary the room can be damped down with thick, heavy carpet or rugs.
Boundary mics positioned on the floor exploit floor reflections as part of their design, as discussed in Chapter 3. The boundary (the floor) is necessary for the mic to function well and have good low frequency response. Because the diaphragm is only a few millimeters above the floor boundary, the phase and comb filtering problems caused when the direct and reflected sound sum at the mic capsule are in such high frequencies that they are irrelevant.
This does not mean that a boundary mic will always pick up good sound though! Many instru-ments radiate their high frequency content quite directionally, and not necessarily towards a mic positioned below the plane of the instrument. So while a boundary mic on the floor does not suffer from phase and comb filtering problems between the dry and floor reflected sound, it is often better to use a conventional mic, up and away from the floor, to capture the best blend of direct sound and floor reflections.
Floor Reflections @
Example 7.4: An acoustic guitar recorded in a carpeted room.
Example 7.5: An acoustic guitar recorded in a room with a reflective wooden floor.
7.7 DISTANCE AND STEREO ARRAYS
The top diagram in Figure 7.6 shows a near-coincident array positioned relatively close to a sound source. The sound source extends through most of the effective pick-up area of the array. Panned hard left and hard right during mixing this results in an expansive image that takes up the entire width between the loudspeakers.
In the bottom diagram, the same array is moved back from the sound source. The sound source is now more concentrated in the overlapping center pick-up of both mics. Even panned as wide as possible during mixing the image will be narrower than when the array was closer to the sound source.
But – image width is not all that changes as a stereo array is positioned at different dis-tances from the sound source.
When the array is positioned closer to the sound source the amplitude of direct sound is increased and the relative amplitude of reflected room sound is decreased – the sound is drier and more up-front (just as it is with a single mic).
Moving the array further away from the sound source increases the relative amplitude of reflected sound and decreases the amplitude of the direct sound – the sound is more distant and reverberant (just as it would be with a single mic).
The positioning of stereo arrays is a balancing act between the desired image width and wet/dry balance:
• A stereo array up close = a wider, but drier image.
• A stereo array further away = a narrower, but more reverberant image.
You should not compromise desired image width for the sake of wet/dry balance or vice versa. As previously discussed, different stereo arrays produce different widths and imaging characteristics – you should experiment with different arrays and/or different microphones until the desired combination of image width and reverb is obtained.
Fi g u r e 7.6 Top: A near-coincident mic array close to a sound source creates a wide image. Bottom:
Moving the mic further away from the sound source results in a narrower stereo image.
Sound Source Sound Source
SOLUTION: Try a near-coincident pair in order to widen the image without changing the wet/dry balance too much.
PROBLEM: An acoustic guitar miked with a near-coincident pair is too reverberant or
“roomy,” but the image width is good.
SOLUTION: Try a spaced pair of cardioid mics a little closer.