Room acoustics and recording point for the Voice(Letto 65 volte)
Room acoustics and recording point for the Voice
A large room, with the same treatment, will always sound better than a small room.
In large rooms, in fact, some of the most serious acoustic problems, such as those caused by standing waves and from the misalignments from initial thoughts there are fewer incidents and they are much easier to resolve.
Let's take a brief look at these drawbacks.
First reflections
The sound wave that, before reaching the microphone (or the listener's ear), reflects off a single wall (a side wall or the ceiling of the room, or any other reflective surface such as a table, piece of furniture or other object) is called the first reflection.
The microphone (or ear) will therefore receive two copies of the same source:
- a live broadcast coming directly from the source
- the other reflected from the reflective surface, with lower volume and a sound colour that is more or less altered depending on the characteristics of the reflective body and, above all, slightly delayed (time lag) compared to the direct sound
The above time lag will produce an alteration, in the form of comb filter, of the resulting wave perceived by the microphone (created by the superimposition of the two overlapping waves), causing the cancellation of certain frequencies and, in general, significantly altering and impoverishing the sound.
The negative effect of this inconvenience will be more pronounced in cases where the reflected wave maintains a high volume compared to the direct wave.
This happens:
- when the source is very close to the reflecting surface
- when the surface itself has a high reflective power
It follows that the solutions may be as follows:
- cover reflective surfaces with sound-absorbing material capable of attenuating even the lowest frequencies of the source
- place the source as far away as possible from reflective surfaces (this means that in a room, a position close to the centre is best in this respect)
- reduce the distance between the singer's mouth and the microphone
- place a mobile slanted panel between the source and the reflective surface to deflect reflections away from the microphone
Obviously, the above solutions can also be used in combination (for example, 1 and 2 simultaneously).
On the left, a recording affected by the overlapping of waves generated by early reflections. On the right, a complete resolution of the problem, achieved by changing the position of the microphone and placing sound-absorbing acoustic panels in strategic locations.
Standing waves and bass traps
Resonances and frequency gaps (caused by standing waves, which depend on the size of the room itself) become lower and lower in frequency as we increase the three dimensions of the room.
Sufficiently large rooms (e.g. 7 m wide x 5 m long x 4 m high or larger) will not cause serious problems for male voices (which have a lower range) and will certainly not cause any noticeable problems for female voices.
If you have a smaller room, it will be essential to cover the corners and front and side walls, as well as the ceiling above the chosen microphone position, with so-called “bass traps”.
NOTE
For the voice, since it is not an instrument capable of reaching very low frequencies, bass traps are recommended to attenuate mid-low frequencies, such as those in the range between 80 and 400 Hz.
Therefore, do not consider bass traps tuned to lower frequencies (from 20 to 80 Hz) unless you need to record low-range instruments (bass drum and timpani, bass and double bass, etc.).
Some examples of bass traps: on the left, a cylindrical one for low frequencies; on the right, a rectangular one mounted on the ceiling for mid-low frequencies (highly recommended for vocals).
In any case, if possible, avoid using rooms with two or even three dimensions that are equal or nearly equal, or rooms in which one dimension is a multiple or approximately a multiple of another.
In such rooms, in fact, the resonance effect and holes at specific frequencies would be further exacerbated.
Flutter echo
Flutter echo occurs when opposing walls are close together and sufficiently parallel to create a rapid succession of high-intensity echoes.
Obviously, this problem can occur in both small and large rooms.
To prevent the onset of Flutter echo, we can:
- eliminate the parallelism of opposing walls by means of slanted counter-walls, or insert slanted panels on the sides of the source or leaning against the walls
- cover the walls, at least in crucial areas, with sound-absorbing panels with pyramid-shaped spikes capable of absorbing frequencies up to 120 Hz (lower frequencies will be very difficult to absorb with these panels); to achieve good results, panels with pyramid-shaped spikes 10 cm thick are recommended.
- Instead of sound-absorbing panels, it is possible to use diffuser panels capable of diffusing the same frequencies (this is a better but more expensive option).
On the left, the source (S) produces the maximum flutter echo phenomenon; to its right, two absorbent panels significantly dampen the effect; on the right, thanks to the non-parallel walls, the effect is dispersed.
Reverberation times
In every type of room, it will also be advisable to create a sufficiently diffuse but stable sound, with a reverberation time equal to or less than 0.5 seconds.
This result can be achieved by appropriately using sound-absorbing panels, diffuser panels and bass traps on the walls.
In the diagram, we can see at the top the long decay time of the reverberation caused by clapping (60 dB attenuation after almost 3 seconds). In the example below, with a large, thick carpet on the floor, we see a significant reduction in the decay time of the reverberation (60 dB attenuation after less than 2 seconds). Adding absorbent surfaces will reduce the duration of the reverberation.
For a better understanding of the above, I refer you once again to the book mentioned above, which deals with the subject of treating the recording room of a home studio in a more precise and comprehensive manner:
“Home studio for digital recording”
www.alessandrofois.com/home-studio-per-digital-audio-recording/
Microphone position
Last but not least is the choice of where to position the recording microphone in the room.
The purpose of this choice is to optimise the pickup point so that the main standing waves do not affect the sound, or affect it to a lesser extent, creating unwanted reinforcements or (worse) frequency gaps.
Without explaining how to perform the calculations (which you will find in the book mentioned above), please note that:
The main modal nodes where frequencies are cancelled (those corresponding to standing waves, which depend on the dimensions of the room) are located at the following distances between the opposite walls of all three dimensions of the room (W, L, and H):
- 0 % and 100 % – incidence of first-level cancellation (maximum incidence)
- 50 % – incidence of 2nd level cancellation
- 33.33 % and 66.66 % – incidence of third-level cancellation
- 25 % and 75 % – incidence of 4th level cancellation
- 20 %, 40 %, 60 % and 80 % – 5th level cancellation incidence
- 16.66 % and 83.33 % – 6th level cancellation incidence
- 14.28 %, 28.57 %, 42.84 %, 57.12 %, 71.43 %, 85.72 % – incidence of 7th level cancellation
- 12.5 %, 37.5 %, 62.5 %, 87.5 % – incidence of 8th level cancellation (minimum incidence)
It is therefore generally inadvisable to position the microphone at the above distances for each of the three dimensions of the room.
Above, we have considered eight levels of modal nodes, whose incidence decreases from level 1 (maximum) to level 8 (minimum among those considered – in reality, there are infinite levels of decreasing incidence); of these, at least the first four should be considered carefully.
As an example, here is a schematic representation of the three dimensions of a room (expressed in inches). The points of maximum reinforcement and frequency holes that can occur at precise geometric distances depending on the distance between the walls are highlighted.
We can say that:
- Levels 1, 2, 3, and 4 should be strictly considered.
- Levels 5 and 6 should be taken into account especially in small rooms.
- Levels 7 and 8 could also be disregarded.
To put it simply:
Let us consider the percentage measures: 0 20 25 33 40 50 and complementary 100 80 75 66 60 50
Values to always consider are shown in bold.
In normal font, those to be considered secondary, but more important in small rooms.
The other measures can, all things considered, be disregarded for the sake of simplicity.
NOTE
Please note that these percentages refer to a standard room with a regular parallelepiped shape; in other cases, calculating these proportions would be very complex, but the above table can serve as a reference guide.
Please note that placing speakers too close to walls is not recommended in order to reduce the incidence of early reflections, which have minimal impact towards the centre of the room.
Consequently, the ideal distance for positioning the microphone is:
- IDEAL: the 46% and its complementary 54% for the distance between two opposite walls, including the distance between the floor and ceiling.
- GOOD: if the above is not feasible, consider 77/78% (and its complement 22/23%) or 69% (and its complement 31%).
- ACCEPTABLE: if no other options are available, use 64% (and its complementary 36%).
Let's take an example.
If you have a room with the following dimensions: 4mWx3Lx2.70H, these are the recommended distances for the microphone:
- W m. 4.00 = the ideal distance is m. 1.84 (46%) or m. 2.16 (54%)
- L m. 3.00 = the ideal distance is m. 1.38 (46%) or m. 1.62 (54%)
With regard to height:
- with a room height of 2.70 m = a singer with a height of 1.60 m would be at 59% of the height – looking at the table, we see that this is not ideal but is tolerable
- with a room height of 2.70 m = a singer with a height of 1.70 m would be at 63% of the height – looking at the table, we see that this is not ideal but is tolerable
- with a room height of 2.70 m = a singer with a height of 1.80 m would be at 67% of the height – looking at the table, we see that this measurement is too close to a 3rd level node, so it would be advisable to modify the height of the singer (and therefore the microphone) with a small platform
To conclude: for practical reasons, we can settle for an approximate height, avoiding (if possible) to match the height of the microphone with one of the incidence percentages from 1 to 4.
On the left, the microphone (M) has been placed near the centre of the room, corresponding to 46% of the room's width and length measurements, which is generally the ideal point for acoustic recording. As for the height (on the right), the position of the microphone will essentially depend on the height of the singer; the five examples represent the positioning of the microphone as a percentage of the height of the room, which in the example is 2.70 metres: 54% would be the ideal height but would correspond to a person only 146 cm tall; followed by 59%, which is the height for a person 160 cm tall, and 67% for a person 170 cm tall, both of which are not ideal but acceptable; 67% is the measurement for a person 180 cm tall. This measurement is not ideal and would be more tolerable with a few centimetres added by means of a platform, to reach 69%. In general, however, height can be considered a negligible factor for practical reasons, limiting oneself to correcting microphone heights that are in the centre, at one-third and one-quarter of the room's height.
Mobile panels and screens
Beyond the need to prevent problems caused by standing waves, reduce flutter echo and limit reverberation times, it is often useful to have some mobile sound-absorbing panels that can be positioned as required.
It would be even better if, on the opposite side, these sound-absorbing panels were covered with a diffuser panel, so that the most suitable solution could be chosen from the two options on a case-by-case basis.
With these panels, it will be possible to adjust and attenuate the environmental acoustic influence as desired.
Some mobile sound-absorbing panels.
For very small studios, there are some semi-circular sound-absorbing screens available on the market that can be installed on a pole and positioned behind and to the sides of the microphone.
This is not an ideal solution, but it can be adopted in small offices, especially where wall coverings are not sufficient to provide good overall acoustic correction for the environment.
Sound-absorbing acoustic screen for singers, a simple mid-level solution in cases of imperfect sound absorption in the room.
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