Scott Probert138951

28/12/85BTEC HND Diploma in Music (Production).Acoustics: Axial Mode and RT60 Analysis.

22/03/13

Introduction.

The aim of this assignment is to design a studio that has a homogenous sound field to create a well-balanced room for mixing audio. This will involve designing the size and shape of the room to create a diffuse sound field, use axial mode analysis to analyse the frequency response of the room and then discuss appropriate materials and acoustics treatments that could be used to help even out the frequency balance further.

When looking at how a room can affect the sound in a space, issues such as absorption, reflections, equipment placement, standing waves and reverberation time will need to be considered. Ways to control these issues will be to use absorbent materials, diffusers and place equipment in as an appropriate place as possible.

There are many things that can affect the frequency response of a room which in turn will have an effect on the sound heard by the listener and will eventually affect the mix created in that room. The main issues that can arise from a non-homogenous room is standing waves. Standing waves are “a particular pattern of constructive and destructive interference” (Hass. 2003). These constructive and destructive interference are issues that appear at certain frequencies and can boost and cut certain frequencies that leave the speakers (or monitors) and are reflected off the surfaces of the room. This, to the listener will result in them boosting or cutting certain frequencies in their mix to compensate for the inaccuracies in frequency response in the room. This may make the mix sound even and well balanced in that particular room, but when the track is played in another room or a homogenous room than the same mix will sound uneven. Not only will the particular frequency cause issues but its subsequent harmonics will also cause problems resulting in a very uneven room from simply on frequency. “The phenomenon is unavoidable; there are standing waves even in the best rooms” which leaves the question “how to minimize the problem?” (Leduc. 2009). The best way to do this will be to find the critical frequency and try to eliminate the reflections of that frequency which will also eliminate that frequencies harmonics and create a more homogenous acoustic space suitable for creating an even mix.

These boosts and cuts in the frequency response of a room are called modes. Using mathematical analysis to determine which frequencies will cause these modes is important to determine how the room can be treated to minimise the impact of these modes. However from doing previous modal analysis it is important to state that although the results may show many modes it is only certain modes that will need to be acoustically treated. An acoustic website dedicated in the acoustic field of room modes states that “if modes are widely separated, then the sound will be abnormally weak at the notes between the modal frequencies. At frequencies above 250 Hertz individual modes are seldom distinguishable” (Unknown. 2004). This will become

more apparent when modal analysis has taken place and the critical frequency has been found.

A diffuse sound field is a sound field that has a completely even spread of frequencies in the entire space of the room. However as a diffuse sound field requires mathematical analysis to determine the exact frequency distribution and as a room will such as a control room or studio space will not be completely empty, a diffuse sound field “does not exist in any room” (Jacobsen. 2007). This is due to the many factors that will affect the diffusion throughout a room: Speakers, desks, chairs, instruments, computers and even people will affect the diffusion in a room. With this in mind we can only hope to minimise the issues that occur in a non-diffuse sound field and create a room that is as diffuse as possible when taking into consideration the purpose of the room. The diffusion of a room can be analysed by carrying out axial mode analysis upon the dimensions of the room and can be treated through various acoustic treatments such as placing diffuser around the room and various adjustments to the rooms shape when applicable. The diffusion of frequencies in a room will also affect the balance of frequencies throughout the room. This will cause many problems in a control room as the listener may here the frequency boost or dip and compensate for this in the mix by attenuating or boosting the frequencies that are unbalanced in the room. This will affect how the mix will sound in different rooms where the frequency balance is different and will alter the sound of the entire piece. As a control room is used for tracking instruments and mixing audio projects this would be a particularly major issue as in a control it is important for the frequency balance in a control room to be even and dispersed throughout the entire room evenly. This allows the user to make critical judgments on the recording and mixing of instruments to create a well-balanced mix. In a live recording environment modal problems could cause issues when placing microphones. If a microphone is placed in an area that has unbalanced frequency dispersion, then while the player or people in the room might believe the frequency balance to be flat, the microphone may be placed in an area that has a frequency boost or dip and will capture this in the recording. Adversely an unbalanced frequency response in a live room may also cause the player to hear a coloured image of the sound coming from the instrument. This may then encourage the player to compensate for the colouration they are hearing by playing the instrument in a different way and when it comes to listening to the recording in a homogenous sound field the overcompensation would be noticed and the recording may have to be done again. However this can be a problem in itself, as most microphones do not have a flat frequency response that will need to be accounted for when recording. This however can be checked by looking at the frequency response chart that should be provided by the manufacturer.

One of the most important factors to consider when designing a control room is the frequency balance. To get a well balanced mix that will sound good in a variety of different spaces, the mix should be performed in a room that has “a relatively flat frequency response over the entire audio range without adding its own particular sound coloration” (Huber & Runstein. 2010). This will ensure that there are no boosts or dips in the frequency spectrum of the audio signal that reach the listener after leaving the monitors. A well-balanced room can be achieved through a studio

design that compensates for the fluctuations of frequencies by using well-placed materials that will affect the reflection and absorption qualities of the room in the way that the user requires. Depending on the materials used throughout the room the absorption and reflections of frequencies will change the tonal balance of a room. Using an absorption coefficients chart the absorption of a rooms surfaces can be analysed to determine which frequencies are being absorbed by the surfaces and by how much, this is given a measurement of a. This measurement is given as a number and 1.00 is equal to the total absorption of that frequency resulting in no reflections from that surface. This can also be given as a percentage with 1.00a being equal to 100%. While a rating of 0.15 for example will mean that not much of that frequency will be absorbed by the surface and instead most of the frequency will be reflected back into the room. By carrying out axial mode analysis on a room and analysing the materials of a room using a coefficient chart, acoustic treatment can then be chosen and placed throughout the room to help create a well balanced homogenous sound field resulting in an almost perfect space to record and mix an audio project.

Another important aspect of studio design to consider is isolation. Isolating the studio (control room and ‘live room’) from the outside world and each other, can prevent sound leakage from the control room monitors from entering the ‘live room’ and causing spill on the recording microphones. This also works for the opposite and can help reduce sound leakage from the ‘live room’ from entering the control room. This would be very useful as the engineer may be mixing or critically listening to a musical piece and may have performers in the ‘live room’ rehearsing or setting up equipment so isolation from these external sounds would be a very useful quality for any studio design. Isolation would also help reduce sound leakage from external noise. As many studios are based on industrial estates and busy streets, it is extremely useful to isolate the studio as efficiently as possible. This will also help sound leakage from the studio (control room and ‘live room’) from leaking out and disturbing passers-by and nearby businesses or houses. The most efficient way to isolate a studio would be to use floating walls or two wall construction design. This is a consideration that would be more achievable while in the process of designing the studio, as it would require a complete refurbishment that could be very expensive. This is due to the work that is needed to create floating walls. The floating wall design requires floating joists to be added to the existing joists of the walls, floors and ceiling. There is then an absorbent material placed between the outer wall and the new inner wall creating an extra layer of absorption helping attenuate sounds from entering and leaving the studio.

The cubed room.

When designing a studio, in particular a control room, it is important that the shape of the room and its construction help create a homogenous sound field and help reduce unwanted discrepancies such as standing waves. According to Bruce Swedien a sound engineer known for his work with Quincy Jones and Michael Jackson the worst shape of a room and one that will produce extensive standing waves is a cube. He states that “cube shaped rooms with parallel surfaces” will cause “sound waves, at critical frequencies to be reflected back and forth across the room with surprising intensity” (Swedien. 2003). As this is only one persons opinion, axial mode analysis will need to be carried out to see just how true this statement is, if the results appear to be true then this will help the construction design when designing a studio space later in this assignment.

Finding the critical frequency will determine where the modes will become more apparent. This can be done using the following equation:

F critical = 1.5 x C ÷ (4V ÷ S)

C = Speed of sound (343m/s)V = Room volumeS = Room surface area F critical = Critical frequency in Hz.

This calculation will now be done on the control room to determine the critical frequency where the room modal issues become more dominant.

V = L x W x H= 3x 3 x 3

= 27V = 27m3

S = 2 (3 x 3) + 2 (3 x 3) + 2 (3 x 3)= (2 x 9) + (2 x 9) + (2 x 9)

= 18 + 18 + 18= 54

S = 54m2

(4V ÷ S)= (4 x 27) ÷ 54

108 ÷ 54= 2

C ÷ (4V ÷ S)=343 ÷ 2= 171.50

1.5 x C ÷ (4V ÷ s)= 1.5 x 171.50

= 257.25

Critical frequency = 257.25Hz

To find the first fundamental standing wave frequency of the 3m cubed room will be calculated by dividing the length of the room doubled by the speed of sound and will be written out like this:

The fundamental standing wave frequency and its harmonic frequencies for the height of the cubed room.

The fundamental frequency calculated will be the standing wave frequency between the parallel walls. However as each fundamental frequency will have its own harmonics that will also cause modal issues, a table will be drawn up containing the first eight harmonics of the fundamental frequency for each dimension of the room.

V = Speed of sound (343m/s)H = Height of room (3m)W = Width of room (3m)L = Length of room (3m)f1 = First fundamental frequency in hertz (Hz)

Fundamental frequency for the height of the

room.

= V ÷ 2H= 343 ÷ (2 x 3)

= 343 ÷ 6= 57.16

f1 = 57.16Hz

Harmonic number. Fundamental frequencies/harmonic frequencies in HZ.

f1 57.16f2 114.32f3 171.48f4 228.64f5 285.80f6 342.96f7 400.12f8 457.28

The fundamental standing wave frequency and its harmonic frequencies for the

width of the cubed room.

Harmonic number. Fundamental frequencies/harmonic frequencies in HZ.

f1 57.16f2 114.32f3 171.48f4 228.64f5 285.80f6 342.96f7 400.12f8 457.28

The fundamental standing wave frequency and its harmonic frequencies for the length of the cubed room.

Fundamental frequency for the width of the

room.

= V ÷ 2H= 343 ÷ (2 x 3)

= 343 ÷ 6= 57.16

f1 = 57.16Hz

Fundamental frequency for the length of the

room.

= V ÷ 2H= 343 ÷ (2 x 3)

= 343 ÷ 6= 57.16

f1 = 57.16Hz

Harmonic number. Fundamental frequencies/harmonic frequencies in HZ.

f1 57.16f2 114.32f3 171.48f4 228.64f5 285.80f6 342.96f7 400.12f8 457.28

Table showing the fundamental frequencies for all dimensions of the cubed room in ascending order.

Harmonic number.

Fundamental frequencies/h

armonic frequencies in Hz for height of room (3m).

Fundamental frequencies/har

monic frequencies in

Hz for width of room (3m).

Fundamental frequencies/har

monic frequencies in

Hz for length of room (3m).

Frequencies arranged in ascending

order in Hz.

f1 57.16 57.16 57.16 57.16f2 114.32 114.32 114.32 57.16f3 171.48 171.48 171.48 57.16f4 228.64 228.64 228.64 114.32f5 285.80 285.80 285.80 114.32f6 342.96 342.96 342.96 114.32f7 400.12 400.12 400.12 171.48f8 457.28 457.28 457.28 171.48

171.48228.64228.64228.64285.80285.80285.80342.96342.96342.96400.12

400.12400.12457.28457.28457.28

A table showing the difference between the standing wave frequencies in ascending order.

Frequencies arranged in ascending order in Hz.

Difference between frequencies in Hz.

57.16 0.0057.16 0.0057.16 57.16114.32 0.00114.32 0.00114.32 57.16171.48 0.00171.48 0.00171.48 57.16228.64 0.00228.64 0.00228.64 57.16285.80 0.00285.80 0.00285.80 57.16342.96 0.00342.96 0.00342.96 57.16400.12 0.00400.12 0.00400.12 57.16457.28 0.00457.28 0.00457.28

From analysing the table above it is clear to see that a cubed room will be full of modal issues and a cubed room would be a very bad environment for a control room, live room, mastering room or any room where audio intelligibility is needed. There will be significant peaks at all the frequencies with a difference of 0.00Hz as well as significant dead spots at the frequencies showing a difference 57.16Hz and would make listening to any sound waves in the room very uneven.

Knowing that the critical frequency is 257.25Hz it can now be determined that all the frequencies below this frequency will cause modal issues. And seen as the room mode frequencies of the cubed room all match, this, with being the same frequencies “will reinforce each other to form very noticeable peaks in the room response” (White. 1998). This is because having reflections of every surface forces the same frequencies (due to their length) to be reflected and meet each other in phase causing

an extreme amplitude peak for that frequency, and seen as so many frequencies in a cubed room cause standing waves, the room will become completely unbalanced and unsuitable for any critical listening or recording. Even with acoustic treatment it would be very difficult and take a lot of money to turn a cubed room into any kind of suitable environment for critical listening.

So now that a cubed room has proven to be a terrible shape acoustically, it is now time to test the dimensions of a room that may perform better in axial mode analysis. For this the dimensions will be made according one of the ‘golden ratios’ of acoustics determined by acousticians such as Ludwig W. Sephmeyer, Richard H. Bolt or Philip M. Morse.

Using Sephmeyer’s golden ratio theory to design a control room.

According to Jay Torborg Ludwig William Sephmeyer’s golden ratios for a room with minimal modal problems are “1.00 : 1.60 : 2.33” (Torborg. 2013). Based on these ratios the control room for this assignment will have the measurements of:

The critical frequency.

Now that the dimensions of the room are known it is now possible to calculate the critical frequency of the room. This is the frequency that will be used to determine which modal issues will need to be addressed with acoustic treatment after the modal analysis has been conducted. The equation to find the critical frequency is as follows:

Height (H) = 2.80mWidth (W) = 2.80 x 1.60= 4.48mLength (L) = 2.80 x 2.33= 6.52m

Giving the room a measurement of:H = 2.80m W = 4.48m L = 6.52m Based on Sephmeyer’s golden ratios theory.

F critical = 1.5 x C ÷ (4V ÷ S)

Where:

C = Speed of sound (343m/s)V = Room volumeS = Room surface area F critical = Critical frequency in Hz.

This calculation will now be done on the control room to determine the critical frequency where the room modal issues become more dominant.

Now that the critical frequency has been identified as being 189.15Hz. We can determine that modal issues above this frequency will be very low in amplitude and almost inaudible, this means that frequencies above the critical frequency shouldn’t cause any problems or need any acoustic treatment.

V = L x W x H= 6.52 x 4.48 x 2.80

= 81.78V = 81.78m3

S = 2 (6.52 x 4.48) + 2 (4.48 x 2.80) + 2 (6.52 x 2.80)= (2 x 29.20) + (2 x 12.54) + (2 x 18.25)

= 58.40 + 25.08 + 36.50= 119.98

S = 119.98m2

(4V ÷ S)= (4 x 81.78) ÷ 119.98

327.12 ÷ 119.98= 2.72

C ÷ (4V ÷ S)=343 ÷ 2.72

= 126.10

1.5 x C ÷ (4V ÷ s)= 1.5 x 126.10

= 189.15

Critical frequency = 189.15Hz

Now that the measurements of the control room are known, and the critical frequency is known, axial mode analysis can now be carried out to determine the standing wave frequencies and the harmonic problems that they can cause. Ideally as the Sephmeyer golden ratios were used to determine the measurements of the room there will be little or no modal problems. However this will be determined after the analysis has been carried out.

To find the fundamental standing wave frequency for height of the room (the space between the floor and the ceiling), the same equation that was used during the experiment with the cubed room will be used but its measurements will be replaced with the measurements of the assignment control room that utilised the golden ratios.

The fundamental standing wave frequency and its harmonic frequencies for the height of the control room.

V = Speed of sound (343m/s)H = Height of room (2.80m)W = Width of room (4.48m)L = Length of room (6.52m)f1 = First fundamental frequency in Hz.

Harmonic number. Fundamental frequencies/harmonic frequencies in HZ.

f1 61.25f2 122.50f3 183.75f4 245.00f5 306.25f6 367.50f7 428.75f8 490.00

The fundamental standing wave frequency and its harmonic frequencies for the width of the control room.

Fundamental frequency for the height of the room.

= V ÷ 2H= 343 ÷ (2 x 2.80)

= 343 ÷ 5.60= 61.25

f1 = 61.25Hz

Fundamental frequency for the width of the room.

= V ÷ 2W= 343 ÷ (2 x 4.48)

= 343 ÷ 8.96= 38.28

f1 = 38.28Hz

Harmonic number. Fundamental frequencies/harmonic frequencies in HZ.

f1 38.28f2 76.56f3 114.84f4 153.12f5 191.40f6 229.68f7 267.96f8 306.24

The fundamental standing wave frequency and its harmonic frequencies for the length of the control room.

Harmonic number. Fundamental frequencies/harmonic frequencies in HZ.

f1 26.30f2 52.60f3 78.90f4 105.20f5 131.50f6 157.80f7 184.10f8 210.40

Fundamental frequency for the length of the room.

= V ÷ 2L= 343 ÷ (2 x 6.52)

= 343 ÷ 13.04= 26.30

f1 = 26.30Hz

Table showing the fundamental frequencies for all dimensions of the room in ascending order.

Harmonic number.

Fundamental frequencies/h

armonic frequencies in Hz for height

of room (2.80m).

Fundamental frequencies/har

monic frequencies in

Hz for width of room (4.48m).

Fundamental frequencies/har

monic frequencies in

Hz for length of room (6.52m).

Frequencies arranged in ascending

order in Hz.

f1 61.25 38.28 26.30 26.30f2 122.50 76.56 52.60 38.28f3 183.75 114.84 78.90 52.60f4 245.00 153.12 105.20 61.25f5 306.25 191.40 131.50 76.56f6 367.50 229.68 157.80 78.90f7 428.75 267.96 184.10 105.20f8 490.00 306.24 210.40 114.84

122.50131.50153.12157.80183.75184.10191.40210.40229.68245.00267.96306.24306.25367.50428.75490.00

A table showing the difference between the standing wave frequencies in ascending order.

Frequencies arranged in ascending order in Hz.

Difference between frequencies in Hz.

26.30 11.9838.28 14.3252.60 8.6561.25 15.3176.56 2.3478.90 26.30105.20 9.64114.84 7.66122.50 9.00

131.50 21.62153.12 4.68157.80 25.95183.75 0.35184.10 7.30191.40 19.00210.40 19.28229.68 15.32245.00 22.96267.96 38.28306.24 0.01306.25 61.25367.50 61.25428.75 61.25490.00

From analysing the chart above it is clear that the Sephmeyer’s golden ratios have significantly helped minimise modal issues in this space when compared to the previous cubed room. However it is not completely free from modal issues and there will be issues at the frequencies of:

183.75Hz306.25Hz

These only have a difference of 0.35 and 0.01Hz respectively and will cause these frequencies to appear more dominant in the room causing an un-balanced frequency balance. However as the critical frequency for the room is known to be 189.15Hz only the frequency of 183.75Hz will need to be acoustically treated.

Constructive interference within the control room.

Treating the issue of the standing wave issue at 183.75Hz can be done simply by attenuating the problem frequency using acoustic treatment such as acoustic foam that will reduce the energy of the wave being reflected back into the room. Using 100mm acoustic foam will attenuate the frequency but wont eliminate the problem as 100mm acoustic foam will not absorb all the energy of the sound wave and will reflect the sound wave back into the room, only at a reduced level. However by covering the walls in carpet, using thicker acoustic foam and suspending the acoustic foam from the wall rather than attaching it directly will help attenuate the frequency even further helping reduce the problem and creating a more homogenous sound field.

The placement of the acoustic treatment will also help attenuate the problem frequency more efficiently. It is important to treat the problem frequency as soon as possible to help eliminate the problem occurring at more areas throughout the room. This can be done by placing the acoustic treatment at the places that the sound will first reach after leaving the monitors. The monitors in a control room should be placed at an equilateral triangle between the two monitors and the listener. This is known as the ‘sweet spot’ where both monitors outputs are reaching the listener at the

same time and with the same intensity. This provides a better stereo image for the listener and creates a more accurate listening environment for critical listening. However as monitors usually have quite a wide dispersion for their output, a lot of the sound will pass by the listener and travel directly towards the next surface, and in a well-placed control room this will usually be the surface of a wall. This makes it the ideal place for the acoustic treatment as can be seen from the image below.

With the acoustic treatment in place at the positions shown in the image above, the reflections, while not being completely eliminated, will be reduced to such an extent

that the peaks in frequency will be attenuated to a point that will be adequate for critical listening. This will also lessen the energy of the reflections, as sound

Adding diffuses like the ones pictured above will help spread the frequency balance throughout the room helping eliminate any null spots within the room creating a more well balanced environment for listening at different positions in the room. Although this will require the listener to remove themselves from the ‘sweet spot’ it is a sensible decision to reference material in this way to gain a better understanding of how the recording will translate in other listening environments with different room characteristics. By changing listening positions the rooms characteristics will become more apparent changing the overall tonal balance of the material heard and allowing the listener to make better critical decisions to help the material translate more efficiently to other listening environments.

Destructive interference within the control room.

Other issues may occur between the frequencies of:

131.50Hz157.80Hz

This is because the differences in frequencies between these are quite large and fall in the ‘dead spot’ area of around 20-25Hz that can cause dips in frequencies (particularly lower frequencies) around the room as “sound waves meeting out of phase results in silence or substantially reduced volume” (Unknown. 2013). Issues such as these are particularly hard to compensate for in terms of room acoustics as there is no way to regain the frequency dips using acoustic treatment. Even placing a parametric EQ in between before the output boosting these frequencies would not combat the problem, as the problem is that the frequencies have wavelengths that equal the dimensions of the room causing the sound waves to meet each other out of phase causing destructive phase cancellation that attenuates these frequencies.

The only really way to eliminate ‘dead spots’ in a room is to change the rooms dimensions to a size that will allow these frequencies to meet in phase causing constructive phase where the two waves meeting increase the amplitude of the

frequency causing a boost. This way room acoustic treatment can be used to help attenuate the frequencies helping create a more homogenous sound field. This can be done using bass traps such as the ones designed by ‘Gik’ a European acoustics company that make the ‘Scopus Tuned Bass Trap’ that has an airtight chamber allowing the trap to capture and attenuate targeted low frequency waves. These can be purchased from the ‘Gik’ website that can be found at http://gikacoustics.co.uk/product/gik-acoustics-scopus-tuned-bass-trap-t70/ and are available with a centre frequency response at 40Hz, 70Hz and 100Hz. Although these don’t target one particular frequency they are suitable and will help avoid any issues when changing the rooms layout, as moving, removing or adding equipment in the room will change the room’s acoustic response and will change the offending frequencies. For this reason it is a sensible choice for any ‘dead spots’ as frequencies being played within a control room will usually consist of a fundamental frequency, it’s harmonics and small fluctuations in between, known as in-harmonics. This means that not only will the fundamental frequency and its harmonics will cause problems, but the in-harmonics in between will also cause issues, and as these will often be between the fundamental and harmonic frequencies, they will be of a similar wavelength (see image below). This means that creating a bass trap that only targets one particular frequency will only attenuate that particular frequency and its harmonics, this means leaving the surrounding in-harmonics that have similar wavelengths to reflect around the room causing phase issues of the constructive and destructive nature.

Considering this, a tuned bass trap that attenuates the problem frequency and it’s harmonics will be useful but using broadband absorber like the ones pictured below, may tackle the problem frequency and it’s in-harmonics ultimately aiding the goal of a more homogenous sound field.

From conducting the axial mode analysis using Sephmeyer’s ‘golden ratios’ and analysing the results, it is clear that creating a completely homogenous sound field is almost impossible when considerations for the rooms use are taken into consideration. When taking into consideration the need for equipment and even people into the studio environment, the changes to size, shape and surface will all alter the dimensions and results from the axial mode analysis. This means, “an analysis of an empty room is pretty pointless” (White. 2002). The only way to achieve an accurate modal analysis would be to perform the analysis on a fully furbished control room and make measurements for dimension of the room. This would be a very complicated process and would involve measuring the distance from every piece of equipment to each surface in the room. If any major modal issues were then found, the room would then need to be changed and the analysis would have to be carried out again. This would be highly impractical and time consuming and although axial mode analysis performed on an empty room isn’t entirely accurate, it is a great start when trying to create a homogenous sound field and will at least make the user aware of where issues may arise in the room and how they can be avoided or reduced.

As a “diffuse sound field is an ideal sound field that does not exist in any room” (Jacobsen. 2007) and a completely ‘dead’ room would result in it being classed as an anechoic chamber. The achievement desired by anybody designing a control room should simply be to reduce any modal problems as much as they possibly can while maintaining a useable environment that is fit for purpose, and to be aware of any issues when critically listening as this can help achieve a mix that will translate well in another environment. This means not relying entirely on one single room to mix audio, and instead the room can be used to form a solid base for a track to be mixed and referenced in other listening environments.

The aim when designing and using a control room should be: To be aware of the environment and it’s acoustically properties. Ensure that the space is fit for its purpose. Treat the space to create an as accurate ‘sweet spot’ as possible.

This may involve changing the dimensions of the room where possible. The use of sloping walls and ceilings can help create better diffusion and help attenuate modal problems such as oblique room modes that have “one quarter of the energy of axial modes” (Wieczorek. 2002).

Although advice can be given by professional acoustic technicians about a space you have in mind for a studio, by taking some simple measurements and performing axial mode analysis yourself you will have a better understanding of the type of treatments your space requires and how to place equipment. This can save you time, money and give you a better understanding of which treatments are needed, which aren’t and where to place equipment without compromising the tonal balance of the room.

The next task for this assignment is to design a ‘live room’ that will be used for recording instruments that will be tracked and mixed in the control room. The ‘live room’ will need different characteristics than the control room as instruments get

there character and sound more natural to the listener when played an a ‘live’ environment rather than a ‘dead’ environment like a control room. So instead of focusing on the frequency balance and standing waves of the space instead the reverberation time will be the focus as this can add character to the sound and help the instrument sound more natural as listeners hear reverberation in everyday life to determine space, shape, depth and intelligibility.

RT60.

RT60 in acoustics refers to the reverberation time of an enclosed space. Reverberation can be defined as many complex reflections occurring in an enclosed space and should not be confused with early reflections or echo. The difference between reverb and early reflections is time. As can be seen in the diagram below reverberation starts at around 50ms after the original sound source. Before this are early reflections caused by reflections created by surfaces close to the listener and sound source.

RT60 is used to describe the time taken “for a sound to die away to a millionth of its original intensity (resulting in a decrease over time of 60dB)” (Huber & Runstein. 2010). Reverb plays an important role to our perception of a space and can help us determine a spaces size, depth, construction and contributes to our spatial awareness especially in a dark room. Creating a room with appropriate reverb in a professional sound environment is important as it can change the intelligibility of what is heard within the space. This can obviously affect the judgements made when making critical decisions concerning recording and mixing audio.

The speed that sound travels can be altered by the material it travels through. In a studio environment often the first material that sound travels through is air at around 343m/s (metres per second) depending on temperature and humidity. The listener first hears the direct sound form the sound source (usually monitors or the instrument), this is followed closely by the early reflections and takes around 5 – 50ms and caused by the surfaces between and close to the listener. Reverberation is then heard by the listener and is caused by the reflections of sound from the surfaces of the room. This can be changed by the surfaces in the room and the material they are constructed from. Every material has an absorption coefficient that describes the materials absorption resulting in the reflected sound being reduced in energy before striking the next surface. This process continues until the sound energy is reduced by 60dB resulting in the rooms RT60. However the absorption of sound in a room cannot be reduced simply as the absorption coefficient table shows that each material does not absorb sound at the same rate for all frequencies. Instead the table shows that each material has a value between 0 and 1 for each material shown at the frequencies of 125Hz, 250Hz, 500Hz, 1kHz, 2kHz and 4kHz. It is also only a rating giving the absorption of the material at 1m2. A rating of 1 shows complete absorption of sound at that frequency by that material, this results in no reflected sound at that frequency. A rating of 0 will result in no absorption at that frequency and the sound wave is reflected back into the room only loosing a small amount of energy as it travels through the air. This rating is given as a Sabine or . So with the information from the absorption coefficient table and knowing the room size calculations can be performed to determine the absorption

of frequencies in that particular space. From this the RT60 can be determined using Sabine’s equation of:

RT60 = 0.161 x V / n

n = (a1 x + a2 x + a3 x )

Where:

RT60 = Reverberation time.0.161 = Constant.n = The sum of multiplying the surface of each area by and summing together.V = Volume of the room. = The sum. = The absorption coefficient of the surface at a particular frequency given as a Sabine.a1 = Area of the ceiling.a2 = Area of the floor.a3 = Area of the walls.

So using the equation the RT60 for all frequencies in the coefficient chart will be performed to determine how well the fictitious ‘live room’ will be suited for its purpose and what can be done to help facilitate that purpose.

The room consists of four walls layered with wood panelling a plaster on brick ceiling and a heavy carpet floor with a heavy foam underlay. The diagram below shows the dimensions of the room and with this information tables will be made to determine the absorption of each surface at the frequencies provided by the absorption coefficient chart.

The room volume can be found by the following formula where:

V = Room volume.W = Width of the room.

H = Height of the room.L = Length of the room.

V = (W x H x L)V = (5 x 3 x 8)

V = 120m3

So now that one part of Sabine’s equation is known (V), it is now time to find out the value of n so that the equation can be completed to give the RT60 values for the ‘live room’.

W = 5mH = 3mL = 8m

= Absorption coefficient rating for the materials surface at the frequency shown in a measurement of Sabine’s at 1m2.S = Absorption coefficient for each surface area at the frequency shown and given as a measurement of Sabine’s.

Surface. Area. 125Hz 250Hz 500Hz

Ceiling(Plaster on

Brick).

a1 = (L x W)= (8 x 5)

= 40

a1 = 40m2

S = (a1 x )= (40 x 0.013)

= 0.52

S = 0.52

S = (a1 x )= (40 x 0.015)

= 0.60

S = 0.60

S = (a1 x )= (40 x 0.020)

= 0.80

S = 0.80

Floor(Heavy Carpet

with Heavy Foam

Underlay).

a2 = (L x W)= (8 x 5)

= 40

a2 = 40m2

S = (a2 x )= (40 x 0.150)

= 6.00

S = 6.00

S = (a2 x )= (40 x 0.250)

= 10.00

S = 10.00

S = (a2 x )= (40 x 0.500)

= 20.00

S = 20.00

Walls(Wood

Panelling).

a3 = 2 (L x H) + 2 (W x H)

= 2 (8 x 3) + 2 ( 5 x 3)

= 2 (24) + 2 (15)= (2 x 24) + (2 x

15)= 48 + 30

= 78

a3 = 78m2

S = (a3 x )= (78 x 0.240)

= 18.72

S = 18.72

S = (a3 x )= (78 x 0.190)

= 14.82

S = 14.82

S = (a3 x )= (78 x 0.140)

= 10.92

S = 10.92

Surface. Area. 1kHz 2kHz 4kHz

Ceiling(Plaster on

Brick).

a1 = (L x W)= (8 x 5)

= 40

a1 = 40m2

S = (a1 x )= (40 x 0.030)

= 1.20

S = 1.20

S = (a1 x )= (40 x 0.040)

= 1.60

S = 1.60

S = (a1 x )= (40 x 0.050)

= 2.00

S = 2.00

Floor(Heavy Carpet

with Heavy Foam

Underlay).

a2 = (L x W)= (8 x 5)

= 40

a2 = 40m2

S = (a2 x )= (40 x 0.600)

= 24.00

S = 24.00

S = (a2 x )= (40 x 0.700)

= 28.00

S = 28.00

S = (a2 x )= (40 x 0.800)

= 32.00

S = 32.00

Walls(Wood

Panelling).

a3 = 2 (L x H) + 2 (W x H)

= 2 (8 x 3) + 2 ( 5 x 3)

= 2 (24) + 2 (15)= (2 x 24) + (2 x

15)= 48 + 30

= 78

a3 = 78m2

S = (a3 x )= (78 x 0.080)

= 6.24

S = 6.24

S = (a3 x )= (78 x 0.130)

= 10.14

S = 10.14

S = (a3 x )= (78 x 0.100)

= 7.80

S = 7.80

So now that the total absorption for each surface area is known at each frequency the value of n for each frequency can now be calculated by adding the numbers together. The measurement for the total will be in Sabine’s.

125Hz = (0.52 + 6.00 + 18.72)= 25.24

n at 125Hz = 25.24

250Hz = (0.60 + 10.00 + 14.82)= 25.42

n at 250Hz = 25.42

500Hz = (0.80 + 20.00 + 10.92)= 31.72

n at 500Hz = 31.72

1kHz = (1.20 + 24.00 + 6.24)= 31.44

n at 1kHz = 31.44

2kHz = (1.60 + 28.00 + 10.14)=39.74

n at 2kHz = 39.74

4kHz = (2.00 + 32.00 + 7.80)= 41.80

n at 4kHz = 41.80

Although the value for n at each frequency is now known, the equation can be shown in a simpler form by using this formula.

Formula to obtain the total absorption rating for a surface in Sabine’s:

n = (a1 x + a2 x + a3 x )

Total absorption rating in Sabine’s for each frequency where:

The surface area for each surface is:

a1 = 40m2

a2 = 40m2

a3 = 78m2

and:

= Absorption coefficient rating obtained from the absorption coefficient chart above for each surface at a measurement of

1m2.

125Hz

n = (40 x 0.013 + 40 x 0.150 + 78 x 0.240)n = (0.52 + 6.00 + 18.72)

n = 25.24

Absorption coefficient at 125Hz is 25.24 Sabine’s.

250Hz

n = (40 x 0.015 + 40 x 0.250 + 78 x 0.190)n = (0.60 + 10.00 + 14.82)

n = 25.42

Absorption coefficient at 250Hz is 25.42 Sabine’s.

4kHz

n = (40 x 0.050 + 40 x 0.800 + 78 x 0.100)n = (2.00 + 32.00 + 7.80)

n = 41.80

Absorption coefficient at 4kHz is 41.80 Sabine’s.

1kHz

n = (40 x 0.030 + 40 x 0.600 + 78 x 0.080)n = (1.20 + 24.00 + 6.24)

n = 31.44

Absorption coefficient at 1kHz is 31.44 Sabine’s.

2kHz

n = (40 x 0.040 + 40 x 0.700 + 78 x 0.130)n = (1.60 + 28.00 + 10.14)

n = 39.74

Absorption coefficient at 2kHz is 39.74 Sabine’s.

500Hz

n = (40 x 0.020 + 40 x 0.500 + 78 x 0.140)n = (0.80 + 20.00 + 10.92)

n = 31.72

Absorption coefficient at 500Hz is 31.72 Sabine’s.

So now that the values of V and n are known, Sabine’s formula can now be used to determine the RT60 value of the room for each frequency.

Sabine’s Formula:

RT60 = 0.161 x V ÷ n

125Hz

RT60 = 0.161 x V ÷ n= (0.161 x 120) ÷ 25.24

= 19.32 ÷ 25.24= 0.76

RT60 at 125Hz = 0.76s

250Hz

RT60 = 0.161 x V ÷ n= (0.161 x 120) ÷ 25.42

= 19.32 ÷ 25.42= 0.76

RT60 at 250Hz = 0.76s

500Hz

RT60 = 0.161 x V ÷ n= (0.161 x 120) ÷ 31.72

= 19.32 ÷ 31.72= 0.60

RT60 at 500Hz = 0.60s1kHz

RT60 = 0.161 x V ÷ n= (0.161 x 120) ÷ 31.44

= 19.32 ÷ 31.44= 0.61

RT60 at 1kHz = 0.61s

The room shows a very short RT60 at each frequency and would sound quite ‘dead’. However with the introduction of equipment into the room, this would change, creating more surfaces made form a variety of different materials causing the room to become a more ‘lively’ and natural sounding environment. This would help the character sound of a ‘live room’ become more natural as when people listen to instruments in the ‘real world’ they are usually in an environment with many objects and surface areas, so having a ‘live room’ with the same characteristics as the ‘normal’ listening environment will help people recognise the instruments characteristics.

The next step now that the RT60 values are known is to determine what type of recording environment is best suited to the ‘live room’ given its reverberation times.

Different RT60 values in a space can have a major effect on the sounds that are generated, recorded and listened to in that particular environment. A churches design and shape (along with the usually hard materials their made from) make a very distinctive impression on the sounds that are generated and heard within that environment. A choir comprising of ten people can sound up to “3 times as large with larger reverb times” (Unknown. 2013) as the sounds produced take longer to die away in a ‘lively’ environment like that of a church. This helps the choir sound more powerful and although intelligibility may be compromised, the sustained

2kHz

RT60 = 0.161 x V ÷ n= (0.161 x 120) ÷ 39.74

= 19.32 ÷ 39.74= 0.48

RT60 at 2kHz = 0.48s

4kHz

RT60 = 0.161 x V ÷ n= (0.161 x 120) ÷ 41.80

= 19.32 ÷ 41.80= 0.46

RT60 at 4kHz = 0.46s

singing methods of a choir usually translate the message to the audience quite well and can be heard right at the back of the environment. However this may not be suitable for general speech as the sound would become muffled the further it travels as the listener at the back of the church would hear the original sound source (the voice) along with several reflections simultaneously. This would create quite an unbalanced and unnatural environment for recording or critical listening and would not allow a microphone to capture an accurate representation of the sound source.

As the RT60 values are quite small it could be assumed that the space would be well suited for recording speech for voice over work or dialogue for a film. Speech would come across as being intelligible due to the rather short RT60 values but also quite natural as these values aren’t too short which would cause the speech to sound too dry as the ‘normal’ listening environment for everyday speech isn’t devoid of reverberation. However although the RT60 values for 2kHz and 4kHz are quite small which creates good intelligibility as speech usually ranges between “ 300Hz to 3500Hz” where “consonants have most their energy above 1000Hz” (Ellis. 2001). Voices with a lower frequency range may become muddy as the lower frequency RT60 values are larger which would lower the intelligibility for speech in this frequency range. According to an article in ‘Sound On Sound’ “a typical living room as a T60 of around 0.5 seconds” (White. 1998), and with the results of the ‘live room’ being of similar value, we can assume that speech would sound similar to that of the natural listening environment for everyday speech. Recording speech in this environment could save the recording engineer having to use reverb units to try and recreate a natural sounding environment. This could also be enhanced by using an omnidirectional microphone to help capture the natural reverberations of the room. However some engineers may require a room that is almost completely ‘dead’ with even smaller RT60 values, as this would allow them to use convolution reverb software on the recording to emulate another environments acoustically properties. Starting with a very dry recording will help this, as when a signal that already contains reverb properties is sent through a convolution reverb unit, the unit will not only act upon the speech but also on the reverb that is already contained within the recording. This will cause the resulting audio to not sound as if it is entirely within the space set by the convolution reverb software but also in the room in which the recording was made.

From this it can be said that there is no optimum RT60 value for any one environment. There is however preferred reverberation times for what a particular environment will be used for. For example orchestral music usually is played in a space that has quite a large RT60 value across the frequencies. This may be due to the fact that a full orchestra requires a large space for performing and usually draws in a large audience and therefore a large space is required. This large space usually won’t be constructed of materials with high absorption ratings and as a consequence has quite large RT60 values across the frequencies. However according to some people even different types of orchestral music can be enhanced by different reverberation times and one particular genre should not be pigeonholed with one optimum value. David Bundler states that “Beethoven or Brahms sounds best in an acoustic of 2 seconds” while “Debussy and more

contemporary fare that involves complex harmonies is better at 1.6 seconds” (Bundler. 2001). This may be due to the type of piece played and the focus that is put on particular instruments within the orchestra. As low frequency instruments such as the double bass and large drums need quite a large space to propagate their sound waves it makes sense that the RT60 value at “125Hz is up to 1.5 times the value at 500Hz:” (Bond. 2008). This gives the instruments time to portray their sound waves fully and space to provide a reverberated version to the listener that adds character, depth and a sense of space that can play an integral part to how the piece is perceived creating the impact and style that the conductor and orchestra wish to portray.

Knowing that there isn’t an optimum RT60 value for overall recording a system that can be used is to design the room to accommodate different recording requirements. As the room can be modified using soundboards and curtains, the room can have a more permanent solution. By designing the room to be a different size and shape in different places, acoustic treatment can be added to the different sections of the room to help achieve a different RT60 value for the different sections of the room. This can help create a relatively dead zone at one end of the room while the other end of the room can be made quite lively and reflective. This would be useful when recording live performers, when the drums need a live environment and the singer needs a relatively dead environment. This can also be aided by the removable sound boards and walls to help reduce spill and isolate the character of the different sections of the room for recording.

From the research conducted it can be determined that when designing and constructing a recording space it is important consider:

What the space will be used to record. How much space is needed for the purpose of the room. What type of character you wish to add to the recordings through

reverberation.

With these points in mind it should also be said that the RT60 values of the room and the character that is added by them could be changed and altered by some simple addition of different materials around the room. This can really help when recording different material and can alter the character to suit the needs of the recording. The RT60 values of the room will also alter with the addition of instruments, people and equipment within the room, and by adding moveable soundboards covered in different absorbent materials, the RT60 values across the frequencies will be changed helping create a character more suited to the type of recording that will be carried out. This can also be done quite easily on a larger scale with the addition of detachable wall panels and curtains that will change the RT60 values of the room without having to alter the size of the room or reconstructing the design of the room. So with planning, knowledge, understanding and some simple calculations an appropriate recording space can be constructed to suit almost any recording situation required by the user.

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