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Audio Theory And Acoustics
Harry E. Pistore 2021
The most delicate part of making an album, one might think to be hidden behind the genius of a songwriter or the skills of a musician and when the music is written and contextualised, it would all going to be a very smooth process of recording, producing, mixing and mastering until the finished product is completed but it’s not quite like that, the writing of a good song may be hard but the process involved in turning a song into a finished product for the consumer is as difficult, if not harder..
“The design goal for a mixing room is to be able to hear exactly what’s coming out of your speakers without the room affecting the outcome. ” (Gervais, 2011, p. 27)
One of the most important aspects of making a record is the room in which the music will be processed, the room in question would have to be a space designed to control sound frequencies, because “When sound strikes a surface, some of it is absorbed, some of it is reflected and some of it is transmitted through the surface.” (Smith, 2004, p. 6). Those unwanted reflections then must be controlled as much as possible to make sure the mixing person is able to judge the sound correctly and take the right decision in the mixing phase.
Important acoustic phenomena to be aware of for a mixing studio design are:
- – Standing Waves / Room Modes
- – Reverberation Effect / RT60
- – The Haas Effect
- – Phase Interference / Comb Filtering
ROOM OVERVIEW
This project’s goal is to turn my bedroom into a professional music production and mixing studio, it features a double glazed window, a door, carpet on the floor and furniture such as a single bed and a desk. The dimensions of the room are:
21.87 m3 – L 3.5 m, W 2.5 m, H 2,5 m 773 cf – L 11.5 ft, W 8.2 ft, H 8.2 ft
Small rooms do not sound great and are usually characterless, due to the sound travelling at 1130ft/s, which won’t allow time-delayed reflections to develop and won’t give the room a nice sonic acoustical space signature (Smith, 2004, p. 24), they also may incur into problems due to sound reflections that ultimately will fuse with the direct sound coming from the speakers, rendering any mixing job very challenging. The Haas effect is one of the issues caused by early reflections in an untreated room, where a sound that is followed by another, with a 40ms delay, will trick the listener into believing that the sound is one rather than two. This will affect the ability to judge a stereo image for example (McAllister, 2019). Another negative aspect of this room is the parallel walls, which can build up standing waves. A standing wave is a low frequency wave that keeps resonating due to the room’s characteristics, more specifically caused by parallel walls which help a certain length of sound wave to bounce back and forth. In an interview Han Zimmer stated that If the room in question were big enough the listener
would simply find an ideal mixing position away from the standing waves pattern (Steinberg, 2019).
The ideal mixing position in a rectangular room such as this one, would be facing the smaller wall, due to the fact that frequencies will have a longer space to decay behind the mixing desk and will bounce back weaker, less audible and disruptive, minimizing therefore the interference between the direct sound and the reflected waves. The problem with this setup is that the listener will have to cope with the angled ceiling, which is in fact 45° oblique on one side, while the rest is flat (refer to the photos below) and this will create unwanted reflections near the mixing position. The only location available will then be facing the bigger wall underneath the oblique ceiling, because sound reflections will easily bounce away from the listener’s location instead of accumulating around the desk (Smith, 2004, p. 24, fig. 3.3b).
(Smith, 2004, p. 24)
Due to the (SBIR) Speaker Boundary Interference Response, the monitors must not be placed too close to the wall otherwise some frequencies will bounce back and interfere with the direct sound, the proximity to a hard boundary will also change the speakers response mainly in the low-end (Gik Acoustics, 2009). It is important to keep them a bit far from the wall to avoid genaral sound reflections and high sound pressure level, SPL, which can transmit and excite the nearby surfaces producing vibrations and therefore unwanted frequencies. According to Senior, “The first thing the mounting hardware has to do, is present as much inertia as possible, so the speaker cabinet moves as little as possible in reaction to woofer excursions.” (M. Senior, 2011, p. 11).
A pair of professional grade monitor stands, with the option of locking to the floor is the best solution in this scenario. The position and distance between the two speakers and the listener should form an equilateral triangle, so in this case about 4ft away from each other to have a good perspective of a stereo image.
ROOM ANALYSIS
Having assessed the room crucial areas, a certain amount of room treatment is needed to overcome the most important problems described above in the Room Overview. As Dave Pensado puts it, the room treatment is as important if not more than the actual monitors (Landr, 2016). To understand what treatment to use, an analysis of the room frequency response is needed, this is done by a software which will emit, record and analyse a specific sound in the room and show how its frequencies interact in relation to
this space. The standard method is to use a very flat omnidirectional microphone, like the TC30 Earthworks, placed in the listener’s position along with the software called Fuzzmeasure. This application generates a sine sweep, records it and analyses the results which will translate into an array of graphs, it also gives an RT60 (Reverberation Time) measurement that indicates the time in which a reverberant sound will decay by 60db (Bigwood, 2015). When a sound is produced in a room, it bounces off of any surfaces that it meets, some of it gets absorbed to some degree, depending on the material encountered but mostly it is reflected until its gradual loss of energy, the interaction of these reflections, blending together, is known as reverberation (NTi-Audio, 2018).
To measure the reverberation time of a small room is common practice to use a T20 reading, it is in fact often impossible to get a full RT60 measurement simply because it would take an extremely loud sound to achieve it, when instead the time in which a sound decays by 20 dB will be precise enough (Markus, 2018). The critical frequency range bands to consider when reading an RT60 graph is from 250 Hz to 4 kHz and to check if the decay value is consistent throughout this range (Mellor, 2015).
(Dewesoft)
The image above shows how the RT60 is calculated. Instead of the -60 dB in the picture, the RT20 would refer to a -20 dB decay.
“Time taken for sound to decay 60dB (T60) should be between 0.2s and 0.5s from 250Hz to 4kHz. T20 and T30 should be within +/- 25% from 250Hz to 4kHz when using one third octave smoothed bands” (Mellor, 2015).
Fig. 1 – RT60 (RT20) Graph
According to the RT20 graph in Fig.1, the reverberation time of the room, recorded by the Fuzzmeasure, in the critical frequency range band mentioned above, is around 0.2 seconds, which means that a sound will take 0.2 seconds to decay by 20 dB, this isn’t great because 200ms is a good decay time for a full RT60 read according to Mellor and an RT20 should be around 1/3rd or 1/4th quicker, so in the region of 65ms. The picks are in fact in the critical 250 Hz and 2 kHz area. This means that attention is needed when treating the room to take care of those frequency ranges. The huge peak in the low-end is normal for an untreated small room and it will be tackled with bass traps and diffusers.
Fig. 2 – Frequency Response Graph
The Fuzzmeasure Frequency Response Graph in Fig.2, shows in detail how frequencies behave in this location, the flatter the curve, the better the room acoustic is, sudden big bumps and dips, more than +/- 2 dB, clearly highlight a frequency problem that needs to be checked. (Feinstein, 2016). Small bumps and dips can be clearly noticed between 1 kHz and 15 kHz, which aren’t a particular issue to tackle as they are around +/- 2 dB and this can be easily dealt with some absorbing material such as foam or acoustic panels. Where real problems become evident is towards the low-end from 500 Hz down to 80 Hz, as these frequencies are more difficult to tame due to the longer length of the sound waves. Those huge bumps and dips around +/- 10 dB, underline the issue of waves resonating and ringing out, due to the small size of the room and the flat concrete walls and ceiling, they basically accumulate and mix together with their own reflections (Lachot). Most critical is the biggest bump in the low-end, in the 150 Hz / 200 Hz range, this frequency is in fact fluctuating and creating a big resonance, most likely a
standing wave, that will cause major difficulties in mixing low-end instruments as their direct sound coming from the monitors will eventually blend with that resonating sound wave.
Fig. 3 – Waterfall Chart
The Waterfall chart in fig. 3 gives a more detailed insight about how frequencies take longer to decay and therefore likely to produce a resonance, because when a direct sound is produced, it always takes a bit of time to dissipate and this chart tells exactly what frequency will still resonate after the direct sound has stopped (Gik Acoustics). “An ideal room would have very even decay, longer at low frequencies, gradually shortening towards the highs.” (Fitzgerald). With the proper acoustic treatment, it will be possible to level out the overall decay time and tame those resonances that seem to ring out more prominently in the 2 kHz and the low-end range as shown in fig. 3.
ROOM TREATMENT
According to the graphs, the most critical problem to tackle is the low-end in general and relative standing waves. A standing wave is the interference between a direct sound wave and its reflection, the wave will keep bouncing back and forth from the boundaries of the medium that it is confined in and if it doesn’t encounter any absorbing material it will keep travelling back in the same original direction causing to overlap to itself, when the distance that the wave travels is equal to an integral number of half wavelengths, it will reinforce itself and without absorbing material it will keep going and resonate, causing great interference with the direct sound (Newell, 2013, p. 700). According to Gervais, during mixdown, certain frequencies can sound softer or louder than what they really are, due to the presence of standing waves (Gervais, 2011 p. 26).
In the picture, a standing wave at different amplitudes, with an integral number of half wavelengths that fits in the medium is travelling in, bouncing back and forth in phase, if not reduced by acoustic treatment it will cause resonance (Isaac Physics).
To gain more control over the low-end response and its relative RT response, (Vicoustic, 2019, p. 12), a good solution would be to place bass frequency absorbing
material in the key areas of the room where the low-end usually accumulates, which are the four corners, up to the ceiling. As every room has a natural occurrance of resonating frequencies, which are recognised as modes, distortion will appear in frequencies below 300 Hz down to 20 Hz, the energy stored by this modal resonance tents to fade slowly and will eventually build up in the farthest locations of the room trying to escape (Acoustic Frontiers, 2011). The presence of walls and corners will eventually cause “boominess”, this phenomenon is explained by the laws of reflection which state that “the angle of incidence equals the angle of reflection”, in audio terms, a sound wave that encounters a hard barrier will double its pressure because both the reflective and incident wave will add up to each other, causing destructive and constructive interference, which will create resonance or ringing (Nave, 2016) and it mostly manifests in the corners of a room as a consequence.
Budget bass traps that could be used are foam bass traps, for example those made by Auralex with an NRC of 1.50. “NRC is a laboratory rating of a material sound absorption quality.” (Acoustics.com). According to Acoustics.com the NRC rating is an average absorption coefficient of how much sound a material can absorb in the range of 250 Hz and 2 kHz (Acoustics.com), this means that foam bass traps can be quite reliable at absorbing sound in that frequency range but in order to be able to control lower frequencies than 250 Hz, a more absorbing material is needed.
A type of effective bass trapping is provided by Tri-Trap corner bass traps, this type will absorb frequencies as low as 50 Hz (Gik Acoustics) which is a very good rating in order
to control low-end frequencies of for example, bass guitars, synths and kick drums. A few panels of these will be used to cover the corners, 2 in the corners by the console where the ceiling gets lower and 4 (2 per corner) in both corners behind the desk where the ceiling is higher.
Another area to treat will be the wall behind the mixing desk, because sound waves will simply bounce back to the listener’s ears and interfere with the direct sound. A solution would be to place either a pair of acoustic panels or some diffuser directly on the wall. It’s very important, when treating a room, to blend absorption and diffusion material together, because by using only absorption materials, the room may lose acoustic energy and will become too dead and unrealistic, by instead adding diffusion panels the energy of the room will be maintained and not reflected back entirely at the mixing console. (B. Harris, 2012)
Five diffusers will then be placed behind the mixing position, that will also scatter the energy of the high frequencies making the room sound bigger (Gik Acoustics).
Diffusion material can be very expensive due to the material itself and the calculations needed to make it effective, so the other way would be to build a homemade quadratic diffuser with simple DIY material by following the BBC white paper. It’s a simplification and a departure from the theoretical construction but it will meet the acoustic requirements needed. (R. Walker, 1995). This design will have to still respect certain parameters and calculations to obtain an even and effective sound diffusion.
To have a better control over sound reflections in the most critical points in relation to the mixing position, other 5 diffusers will be placed on the wall in front of it, more
specifically behind the speakers and directly behind the desk. This will help to prevent the nasty effect of frequency cancellation known as comb filtering and an overall hollow and empty sound, as a result of the direct sound bouncing back to the listener’s ears arriving at different times (Rochman, 2009). Diffusion will eventually change the 3 domains of a sound wave, time, loudness and direction, in this way it would be harder for the human ears to understand where the sound reflections come from, drastically reducing the comb filtering effect and it will render the room bigger. (D. Shevyn, 2012) Early reflections must be also tackled on both sides of the listener’s mixing position which will cause comb filtering and Haas effect, since diffusion has already been applied on 2 walls already, absorption panels will instead be placed on the walls on both sides of the listener’s seat. There will be 2 vertical panels on each side wall. In regards to the floor, having carpet is already a very good form of sound treatment, because of its density, where high frequencies will likely to absorb but the other important area to tackle is in fact the ceiling, being flat and naked. It is a surface where waves will reflect directly to the listener’s ears even from the floor and create comb filtering, Haas effect and general unwanted reflections. The best solution is to apply acoustic panels above the mixing position leaving a gap in between the surface and the panel for scattering early reflections. A cheap way of doing this would be to hang an umbrella upside down above the mixing seat and fill it with acoustic foam (Elmosnino, 2018), although for a more professional looking absorbtion, 6 acoustic panels will be used, enough to cover both the desk and the listener’s position. Professional absorption panels can be expensive but they are essentially boxes with soft material inside with a calculated rate
of absorption, there are however ways to build something similar with a very effective absorption coefficient at home, by just using rockwool and cheap wooden frames covered with cotton sheets to avoid the rockwool particles to escape, as these particles are not safe to breathe, the working process in making them has to be very meticulous and attentive. To be very effective, the rockwool must be 5 cm thick or thicker but the sound coefficient must be taken into account, usually this is provided by the manufacturer but may not be 100% accurate from a physical perspective. Normally coefficient 1.00 equals to 100% absorption, so lower frequencies will have lower coefficients as harder to absorb, for example a type of rockwool may have coefficient 0.38 in absorbing 125Hz and 0.93 in absorbing 250 Hz, so whilst this will not be total absorption, it can be increased by adding extra layers of rockwool or by choosing different material with a better coefficient to frequency ratio (AcousticFreq, 2011).
The last area to treat is the ceiling perimeter where low-end can still accumulate and reflect on both the high sides of the console and also behind it. To treat these areas, bass traps will be installed, in particular 1 on each wall on both sides of the console and 2 behind it (refer to the studio plans below).
In conclusion, this treatment will guarantee an advanced level of absorption and diffusion, since all the material sourced are made to a professional high standard and applied to acoustically critical areas of the room, in line with tested audio acoustic guidelines. The amount of treatment material used will ensure an effective way to control unwanted sound reflections and resonance, whilst retaining a realistic tone for a real-world mixing environment.
LIST OF MATERIALS
- – 2 x Adam Hall Monitors Stands SKDB 039 – £40
- – 6 x GIK Acoustic Tri-Trap Corner Bass Traps – 50Hz (585 mm X 1200mm) £139.00 each (Corners)
- – 10 x GIK Acoustics Q7d Diffusor (498 mm x 1166 mm) £234.00 ech (Front / Back wall)
- – 4 x GIK 242 Acoustic Panel (60 cm x 120 cm) £53.50 each (Side walls)
- – 6 x GIK 242 Acoustic Panel (60 cm x 120 cm) £53.50 each (Ceiling cloud)
- – 4 Gik Acoustic Soffit Bass Trap – 40Hz (1215mm x 413mm) £208.00 each (Ceiling perimeter)
ROOM ACOUSTICS GRAPHS
3rd Measurement (used for the project)
Fig. 1
Fig. 2
Fig 3
1st Measurement
2nd Measurement
STUDIO PLAN FOR MUSIC PRODUCTION AND MIXING Software used: Room Acoustic Visualizer (Gik Acoustic).
1.
6 x GIK Acoustic Tri-Trap Corner Bass Traps – 50Hz (585 mm X 1200mm) in corners 4 x GIK 242 Acoustic Panel (60 cm x 120 cm) on side walls
10 x GIK Acoustics Q7d Diffusor (498 mm x 1166 mm) front and back of mixing console
2.
6 x GIK 242 Acoustic Panel (60 cm x 120 cm) above mixing position
3.
4 bass traps 40Hz absorption on ceiling perimeter
BIBLIOGRAPHY
Acoustics.com, Codes & Testing : Noise Reduction Coefficient (NRC). A vailable at: http://www.acoustics.com/nrc.asp (Accessed: 6 April 2020)
AcousticFreq (2011) How To Build Your Own Acoustic Panels (DIY). Available at: https://acousticsfreq.com/how-to-build-your-own-acoustic-panels/ (Accessed: 16 April 2020)
Acoustic Frontiers (2011) The Secret Of Bass Trap Placement. Available at: http://www.acousticfrontiers.com/201129the-secrets-of-bass-trap-placement-html/#com ments (Accessed: 19 April 2020)
Bigwood, R. (2015) SuperMegaUltraGroovy Fuzzmeasure 4. Available at: https://www.soundonsound.com/reviews/supermegaultragroovy-fuzzmeasure-4 (Accessed: 13 March 2020)
Dewesoft, Reverberation Time RT60 Measurement. Available at: https://dewesoft.com/applications/acoustics/reverberation-time (Accessed: 12 March 2020)
Elmosnino, (2018) Audio Production Principles: Practical Studio Applications. Oxford University Press.
Feinstein, S. (2016) Understanding Frequency Response – Why it Matters. Available at: https://www.alesis.com/kb/article/2227 (Accessed: 14 March 2020)
Fitzgerald, D. A Beginner’s Guide to Room Analysis. Available at: https://realtraps.com/art_dan.htm (Accessed 20 march 2020)
Gervais, R. (2011) Home Recording Studio Build It Like The Pros. 2nd edn. Boston: Course Technology.
Gik Acoustics, Room Acoustic Visualizer. A vailable at: https://gikacoustics.co.uk/room-acoustics-visualizer/ (Accessed: 27 April 2020)
Gik Acoustics, Speaker Boundary Interference Response (SBIR). Available at: https://www.gikacoustics.com/speaker-boundary-interference-response-sbir (Accessed: 12 March 2020)
Gik Acoustics Tri-Trap Corner Bass Trap.Available at:https://gikacoustics.co.uk/product/gik-acoustics-tri-trap-corner-bass-trap-custom-size (Accessed: 5 May 2020)
Gik Acoustics, Understanding Decay Times and Waterfall Graphs. Available at: https://www.gikacoustics.com/understanding-decay-times/ (Accessed: 21 march 2020)
Harris, B. (2012) Home studio setup. Taylor & Francis.
Isaac Physics, Standing Waves P hysics Concept. Available at: https://isaacphysics.org/concepts/cp_standing_waves (Accessed: 27 April 2020)
Lachot, W. Bass Waves In A Control Room. Available at: https://realtraps.com/art_waves.htm (Accessed: 6 March 2020)
Landr (2016) 15 Dave Pensado Mix Tips Every Producer Should Know | LANDR. Available at: https://youtu.be/7AqNUNrO690 (Accessed: 12 March 2020)
Markus, (2018) What is the correct term for Reverberation Time?. Available at: https://blog.nti-audio.com/methods/correct-term-reverberation-time (Accessed: 25 March 2020)
McAllister, M. (2019) Haas Effect: What It Is and How It’s Used. A vailable at: https://producelikeapro.com/blog/haas-effect/ (Accessed: 3 March 2020)
Mellor, N. (2015) Understanding Small Room Reverberation Time Measurements. Available at: http://www.acousticfrontiers.com/understanding-small-room-reverberation-time-measure ments (Accessed: 26 March 2020)
Nave, C. R. Reflection Of Sound. Available at: http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/reflec.html (Accessed: 25 April 2020)
Newell, P. (2013) Recording Studio Design. 2nd edn. Taylor & Francis.
NTi-Audio, (2018) Reverberation Time RT60 Measurement. Available at: https://www.nti-audio.com/en/applications/room-building-acoustics/reverberation-time-rt 60-measurement (Accessed: 25 March 2020)
Rochman, D. (2009) How to Avoid Comb Filtering with Podium Mics. Available at: https://www.shure.com/en-US/performance-production/louder/how-to-avoid-comb-filterin g-with-podium-mics (Accessed: 2 April 2020)
Shevyn, D. (2012) How Diffusion Works. Available at: https://gikacoustics.co.uk/video-how-diffusion-works/ (Accessed: 21 April 2020)
Senior, M. (2011) Mixing Secrets For The Small Studio. Elsevier Inc.
Smith, E. T. (2004). Acoustic 101 – Practical Guidelines For Constructing Accurate Acoustical Spaces – Including Advice On Proper Material To Use. Version 3.0. Indianapolis: Auralex Acoustic Inc.
Steinberg (2019) Multi-Award-Winning Film Score Composer Hans Zimmer on Cubase | Steinberg Spotlights. Available at: https://youtu.be/14lF6hfbyhc (Accessed: 14 March 2020)
Vicoustic, 2019 Control Rooms White Paper. Available at: https://vicoustic.com/catalogs (Accessed: 4 April 2020)
Walker, R. (1995) Modular acoustic diffuser: The development and performance of a modular acoustic diffuser. Available at: https://www.bbc.co.uk/rd/publications/rdreport_1995_01 (Accessed: 12 April 2020)