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ACOUSTIC DESIGN

Several steps have to be followed in planning and developing the layout and design of the areas within TV studios where the quality of sound recording, editing, monitoring and final critical listening are important: 1. Select site for least noisy area. 2. Carry out a noise survey to evaluate the acoustical isolation that is required. 3. Select position of the room within the building. 4. Select sound- isolation techniques and materials. 5. Control of structural and airborne noise. 6. Design appropriate room proportions and shape for a good diffusion of modes. 7. Select and place absorption materials in order to achieve the optimum reverberation time according to the room functionality. Numbers one to five are related to noise control, that actually is the most important aspect because broadcast applications require very low background noise levels. These are also the most expensive and difficult to achieve.

BUILDING LOCATION AND ENVIROMENTAL NOISE SOURCES

The environmental noise level that surrounds the studio complex is a key factor in determining the amount of insulation that is going to be required. The perfect environment would be the countryside but studio complexes are situated in the middle of the city and generally surrounded by high levels of environmental noise from motorways, railways and even aeroplanes. In order to assess the environmental noise, a noise survey has to be carried out. The results will provide us the amount of insulation and the materials that are going to be required in the construction of the external shell of the building.

NOISE SURVEY

In a very noisy urban environment a detailed noise survey should be accomplished. Measurements by a third- octave band, and in dBAs (those that give readings accordingly to Fletcher-Munson curves of equal loudness, based in the different perception of sound levels at different frequencies by the human ear) with a real- time spectrum analyser should be taken. The equipment required is a microprocessor-based recording noise metre. The noise survey will provide us with a graphical representation of the peak noise spectrum that dominates the site and this will be essential in order to determine the transmission loss requirement as we will see later. Another approach would be to contract an acoustical consultant to make the job and submit the results. By one way or another, this essential step should be carried out in order to establish the level of insulation that is required to achieve low levels of background noise within the studio.

NOISE CRITERIA (NC)

Another really important value in order to estimate the transmission loss requirement of the materials employed in the construction of the building, as we will see later, is the Noise Criteria curve. This provides values of maximum background noise allowable depending on room functionality. The Noise Criteria curves are showed in the graph below. Also a chart with appropriate NC values for different types of room within the TV studios is also provided.




 * // Studio Type // ||  ||   ||
 * Announce Both || 20-25 ||
 * Newsroom Set || 25-30 ||
 * Small Insert Studio || 20-25 ||
 * Midsize TV Studio || 20-25 ||
 * Audience Studio with Live Music || 25-30 ||
 * Large TV Studio || 20-25 ||
 * Sound Control Room || 20-25 ||
 * Gallery || 25-30 ||
 * Master Control Room || 25-30 ||
 * Edit Bay || 25-30 ||

POSITION OF THE ROOM WITHIN THE BUILDING

Noise sensitive areas should be placed in the central core of the building surrounded by offices, storages areas and equipment rooms in order to make these behave like barriers for the external noise. Also rooms with high levels of sound monitoring should be placed not next to other rooms where the capturing of sound is involved. The exception for that is when live rooms and control room should be connected due to the recording of music performance. In this particular case extra care should be taken in the acoustic isolation between both areas.

SOUND ISOLATION TECHNIQUES AND MATERIALS

INSULATION AND ABSORPTION Firstly a distinction between airborne sound insulation, structural-borne sound isolation and sound absorption, should be made, because they are concepts of a different nature and require a different treatment. - Airborne sound insulation refers to the noise that is produced by air from an instrument, the voice, the air conditioned and so on. This requires mass, discontinuity and resilience to reduce sound transferring from one area to another. - Structural-borne sound insulation on the other hand is produced by footsteps, hits from objects that are drooped in the floors, machinery vibration, etc. These are able to be transmitted through the structure of the building with less attenuation and travelling ten times faster than airborne sound, and are able to be radiated into the rooms. These require a similar treatment but has to be much more efficient due to these reasons. -Lastly absorption is achieved with certain kinds of material, places in the walls that will absorb the sound energy avoiding that reflections bouncing back into the room. This will be cover at the end of this report alongside reverberation time that is a parameter totally related with the amount of absorption in the room.

TRANSMISSION LOSS REQUIREMENT

In order to establish the transmission loss requirement in the construction of the walls a noise survey should be carried out and a NC has to be selected accordingly with the use for the room, as it was mentioned earlier. The peak noise levels from the survey should be plotted as the upper level of the graph with the NC selected, as the lower curve. The difference between the curves at each third-octave should be plotted and this will be the graph of the required attenuation that will help in the selection of the appropriate construction material, in order to achieve the insulation goals.

SOUND TRANSMISSION CLASS

Once the transmission loss requirements and the required attenuation graph have been obtained, the next step is to look for the construction materials that match with the values of our graph. Construction materials have a specification called Sound Transmission Class (STC) that measures the transmission loss over a range of 16 different frequencies between 125 and 4000 H. The measurement is only accurate for speech sound but much less for amplified music, mechanical equipment noise or any sound with substantial low-frequency energy below 125 Hz. Sometimes acoustical labs will measure transmission loss at frequencies below 125, possibly down to 50 Hz or lower, thus giving additional values in order to evaluate the transmission loss at very low frequencies. A STC calculator is available on this website: []

We need to proceed filling the values that we obtained over the third octave centre frequencies in our attenuation graph, in the chart that appears at the bottom of the page on this website. This will provide us with an approximate STC in order to achieve our insulation requirements. In general doubling the mass of a partition the STC rating increases by 5 db, also installing absorption materials inside of a partition’s air space will improve the STC by 4-6 dB and another technique to increment the STC values is placing an air space in the partition, a 1 and ½ inches air space will increase the STC by 3 db, a 3 inches air space will improve the STC by 6dB and so on.

WALLS

STC values for different walls assemblies are provided in this website: []. Also STC ratings for masonry walls are available in this one: []. A chart with suitable STC values depending on room functionality is also included below:


 * //Studio Type// || //Shell Wall STC// ||
 * Announce Both || 65+ ||
 * Newsroom Set || 60 ||
 * Small Insert Studio || 60 ||
 * Midsize TV Studio || 60 ||
 * Audience Studio With Live Music || 55 ||
 * Large TV Studio || 60 ||
 * Gallery || 55 ||
 * Sound Control Room || 60 ||
 * Master Control Room || 55 ||
 * Edit Bay || 55 ||

FLOOR AND CEILING CONSTRUCTION

Not only high transmission loss is required for the walls, also floors and ceilings demands a special treatment. Due to any impact noise, caused mainly by footsteps on the floor above the room that is going to be transmitted through the structure with low attenuation and at a speed ten times greater than in air, being the noise radiated into the room. A very simple and straightforward approach would be the use of carpet on the floors instead of hard surfaces to soften heel taps but this is not going to be enough due to its low mass and little effect is going to have in the reduction of noise radiation. So special considerations should be taken in the construction of floors and ceilings in order to not worsen the low noise levels achieved with the construction of the walls. The worst case scenario would be when the room is located in the ground floor of a frame building or if it is located in the top floor because in the first case the ceiling will be the only barrier to all the structural noise coming from upstairs. In the second one the floor will be the only surface from where all structure –borne sound is radiated from below.

FLOATING FLOORS

In a situation like the one that has been just described or in those ones where the transmission loss of the standard structural floor is not enough to maintain the required low levels of background noise, a floating floor is a must. We mentioned earlier than doubling the mass of the original structure only an increment of 5 db on the STC rating is going to be achieved. So a 150 mm concrete structural floor that has a STC of 54, it will only be incremented to a STC of 59 if the mass is double. Floating floors can achieve otherwise STC rating of 76. Generally floating floors are required when the STC of the structural floor has to be higher than STC-54. Floating floors are based on the separation of the masses with an air gap in between that provides much more transmission loss than if the masses would be combined together as in a continuous floor. The same principle is also employed in the manufacturing of acoustic windows, as we will see later. The main issue is that floating floors are supported by mounts, and these are also able to transmit noise. So in order to avoid this a resilient under-layment or resilient isolators are placed to support the floor.

The most effective over a wide frequency range is the combination of neoprene and a spring in the hanger due to the effectiveness of the spring at low frequencies (making the floor very flexible being able to move up and down) and the neoprene at high frequencies. As we can see in the image above, there are two layers of a very absorbent material (polyethylene) that is placed just below the air gap, with the spring coils to support the weight of the decoupled floor.

WINDOWS

In order to achieve high isolation windows a complex manufacturing procedure is required. Firstly a double pane with a 6- inch air gap or gas between is recommended. Secondly they should be mounted in a frame without any craps or air gaps, wrapping the edges of the glass with absorption materials will also improve the insulation characteristics.



DOORS

There are a great variety of soundproofed doors available in the market achieving STC ratings of between 60 and 70 db. The problem with high isolation doors is that they are difficult to construct and maintain because the sealing and gasketing deteriorates with time and the regular use. Sometimes is better, in terms of economy and reliability, to create what is called a sound lock. This consists of an outer door that leads to a vestibule and an inner door. This will achieve an adequate transmission loss without an acoustical door.



MECHANICAL EQUIPMENT NOISE CONTROL HVAC (heating, ventilating and air conditioning) is the primary cause of the introduction of noise in the noise sensitive areas, because of that: the planning, layout and installation of this equipment is vital to maintain low levels of background-noise. Firstly an anticipation of the heating loads from the various equipment and the lighting system should be accurately estimated to allow the MEP (mechanical, electrical and plumbing) engineer to calculate the appropriate air conditioning system required with the appropriate air-flow velocity regarding with NC selected according to the functionality of the room. In fact the velocity of the air-flow is one of the key aspects in order to control low levels of noise from the HVAC system. Diffusers (designed to reduce the turbulence noise) should be placed at the entry point of the air into the room. A chart of the maximum air velocities regarding to the NC values is showed in a table below. Supply || 575 || 675 || 875 || 1200 ||
 * |||||||| Recommended Maximum Air Velocities in ft/sec ||
 * || Slot Speed at Terminal || 10 ft before terminal || 10-20 ft || 20-30 ft ||
 * NC-15 supply || 250 || 300 || 350 || 425 ||
 * NC-15 return || 300 || 350 || 350 || 500 ||
 * NC-20 supply || 300 || 350 || 425 || 500 ||
 * NC-20 return || 350 || 425 || 500 || 650 ||
 * NC-25 supply || 350 || 425 || 550 || 700 ||
 * NC-25 return || 425 || 500 || 650 || 800 ||
 * NC-30 supply || 425 || 500 || 700 || 850 ||
 * NC-30 return || 500 || 600 || 800 || 950 ||
 * NC-35 supply || 500 || 600 || 800 || 1000 ||
 * NC-35 return || 600 || 700 || 900 || 1150 ||
 * NC-40
 * NC-40 return || 675 || 775 || 975 || 1255 ||

The layout of the HVAC systems can vary from large to small studios because the first ones are going to require a much bigger amount of air-flow due to great heat loads. So in large studios the air-handling equipment is placed outside the building in an adjacent mechanical structure, with all the ductwork going to the top of the building and then being distributed. For smaller studios interior locations for air-handling units can be founded but then floating floors is required. Another distinction between smaller and larger studios refers to the placement of the outlet and the inlets. In large studios with high heat- loads due to the massive lighting equipment the warm air tends to flow upwards. However the cold air supply is located at higher elevations to keep the cold air flow away from scenery or any other obstacles and there tends to be more space for larger ducts with low discharge velocities of the air flows and is also placed away from the microphones. The inlets are also placed fairly high but a lower level than the outlets. This system becomes even more important when the amount of air-flown is needed increase of the level of background noise criteria is lower. On smaller studios with less demand of air-flown sidewall air grilles and diffusers are acceptable. The figure below shows different examples of ductwork in a small studio with different levels of sound isolation achieved in each one.



In both systems duct insulation and silencers are required. Firstly we should get rid of the fan noise propagation and entering the room, to avoid that the duct is externally covered with absorbing material such an 1-inch glass fiber to reduce noise at the end of the duct. The images below show different types of silencers available on the market, absorbin g material is also placed inside the silencers in order to reduce background noise levels. 



Eliminating structure-borne transmission from the fan noise into the duct can be achieved by rubber coupling the fan motor assembly with the distribution duct system. Wherever the ducts enter a noise sensitive-room the opening should be sealed and also isolate the duct from the structure of the wall to prevent structure-borne noise. This video provides an amazing technique to make ducts soundproofing introducing absorbing material inside the ducts: http://www.ehow.com/video_4971322_soundproofing-ventilation.html

ROOM PROPORTIONS AND SHAPE FOR GOOD MODES DIFFUSION.

DIVIDING THE AUDIBLE SPECTRUM



Firstly a division of the audible spectrum should be made, distinguishing four regions that are related to the dimensions of the room. In order to exemplify this we are going to assume that the dimensions of our room are 22 ft long, 18 ft wide, 14 ft high and a reverberation time 0.5 seconds. -Region X. No modal effects can occur in this region and is located below the lowest modal frequency that is established by dividing the velocity of sound (taking the commonly rounded value of 1130 ft per second at 34 degrees Celsius) by twice the length of the room: f1=1130/2L= 565/22= 26 Hz This being this the lowest frequency limit in the room’s response, smaller rooms will increase this value, this being a very important factor in order to value the appropriateness of the room for music reproduction. -Region A. This is a low-frequency region. The boundary (f2) between this region and the next region B can be obtained by this formula:

This region is also affected by room sizes. A smaller room will have this region increased being greater the audible spectrum dominated by modal resonances but with greater spacing of modal resonances and with an increase of colorations of sound. - Region B. This region is going to be dominated by diffraction and diffusion. Diffraction is the ability of sound waves to bend around obstacles when the wavelengths are longer than the dimensions of the objects. Diffusion refers to the even dispersion of sound avoiding hot spots or nulls in a room. This region also increases with the reduction of room size. -Region C. The boundary (f3) that separates region B and C is obtained by multiplying f1 by four. In this region sound behaves more like rays of light. As region A and B are increased by the reduction of the dimensions of the room region C is pushed being able to reproduce higher frequencies. STANDING WAVES This is the effect produced by the summation of the reflected sound added with the unreflected sound, producing an uneven response in the field. This is produced when f1= 565/L interacts with the length of the room producing a point of maximum pressure near the walls and a null in between, then a standing wave has occurred. As the frequency is increased when we reache f2 another standing wave is produced with three points of maximum sound pressure and two of minimum pressure as it is shown in the graph below.
 * f3= 4 x f1= 428 Hz **

Standing waves are also called modes and there are three different types of modes depending in the number of surfaces they bounce off: - Axial Modes: The sound will be reflected involving two parallel walls. The diagram below displays the three primary axial modes.

- Tangential modes: In this type four walls are involved, a diagram of the three primary tangential modes is showed below

-- Oblique modes: These are the more complicated ones in order to understand. They imply six surfaces and the diagram below will help us to understand what an oblique mode is.

As tangential modes they hit the front, back, left and right walls in the same places but hitting the ceiling and the floor in between.

CALCULATING MODAL FREQUENCIES

In 1896 it was demonstrated by Rayleigh that the sound enclosed in a rectangular room generates an infinite number of modes that can be obtained by the following formula:



Where, C is the speed of sound, about 1130 ft/s. L is the length of the room in feet. W is the width of the room in feet. H is the height of the room in feet. p, q, and r are the integers 0, 1, 2, 3, 4, and so on. If we consider only the length of the room q and r are 0 and substituting p=1, then the formula is exactly the same than the one we used to get f1, when we were obtaining the lowest modal frequency of the room, p=2 gives us f2 and so on.

Any mode can be described by three digits. For example 0,1,0 is the first order width axial mode, and 0,0,2 is the second order vertical axial mode. Axial modes implies two zeros, tangential modes implies one zero and the mode involves two pairs of surfaces. Oblique mode has no zero and three pairs of surfaces are involved. A room mode calculator is available on this website: [] At low frequencies the distribution of modes is very irregular with only axial modes, so this produces dips in the room response and contribute to coloration effects. To overcome all this, room proportions deserve special attention in order to make that tangential and oblique modes contribute to fill this gaps. As frequency increases the modes gets closer together obtaining a better diffusion of the sound In order to exemplify all this we are going to assume a room with the following dimensions: 34.95 x 24 x 15 ft. Using the room mode calculator on the website listed above we have obtained the results that are displayed in an organis ed manner in the chart below.

Numb || FREQ Hz |||||| MODE DESCRIPTION |||||| TYPE OF MODE ||
 * Mode
 * ^  ||^   || P || Q || R || Oblique || Tangential || Axial ||
 * 1 || 16.17 || 1 || 0 || 0 ||  ||   || A ||
 * 2 || 23.54 || 0 || 1 || 0 ||  || T ||   ||
 * 3 || 28.55 || 1 || 1 || 0 ||  ||   || A ||
 * 4 || 32.33 || 2 || 0 || 0 ||  ||   || A ||
 * 5 || 37.67 || 0 || 0 || 1 ||  ||   || A ||
 * 6 || 40.00 || 2 || 1 || 0 ||  || T ||   ||
 * 7 || 41.00 || 1 || 0 || 1 ||  || T ||   ||
 * 8 || 44.41 || 0 || 1 || 1 ||  ||   || A ||
 * 9 || 47.08 || 0 || 2 || 0 ||  ||   || A ||
 * 10 || 47.26 || 1 || 1 || 1 || O ||  ||   ||
 * 11 || 48.50 || 3 || 0 || 0 ||  ||   || A ||
 * 12 || 49.64 || 2 || 0 || 1 ||  || T ||   ||
 * 13 || 49.78 || 1 || 2 || 0 ||  || T ||   ||
 * 14 || 53.90 || 3 || 1 || 0 ||  || T ||   ||
 * 15 || 54.93 || 2 || 1 || 0 || O ||  ||   ||
 * 16 || 57.11 || 2 || 2 || 0 ||  || T ||   ||
 * 17 || 60.29 || 0 || 2 || 1 ||  || T ||   ||
 * 18 || 61.40 || 3 || 0 || 1 ||  || T ||   ||
 * 19 || 62.42 || 1 || 2 || 1 || O ||  ||   ||
 * 20 || 64.66 || 4 || 0 || 0 ||  ||   || A ||
 * 21 || 67.59 || 3 || 2 || 0 ||  || T ||   ||
 * 22 || 68.41 || 2 || 2 || 1 || O ||  ||   ||
 * 23 || 68.81 || 4 || 1 || 0 ||  || T ||   ||
 * 24 || 70.62 || 0 || 3 || 0 ||  ||   || A ||
 * 25 || 72.45 || 1 || 3 || 0 ||  || T ||   ||
 * 26 || 74.83 || 4 || 0 || 1 ||  || T ||   ||
 * 27 || 75.33 || 0 || 0 || 2 ||  ||   || A ||
 * 28 || 77.04 || 1 || 0 || 2 ||  || T ||   ||
 * 29 || 77.67 || 2 || 3 || 0 ||  || T ||   ||
 * 30 || 78.92 || 0 || 1 || 2 ||  || T ||   ||
 * 31 || 79.98 || 4 || 2 || 0 ||  || T ||   ||
 * 32 || 80.04 || 0 || 3 || 1 ||  || T ||   ||
 * 33 || 80.56 || 1 || 1 || 2 || O ||  ||   ||
 * 34 || 80.82 || 5 || 0 || 0 ||  ||   || A ||
 * 35 || 81.97 || 2 || 0 || 2 ||  || T ||   ||
 * 36 || 85.29 || 2 || 1 || 2 || O ||  ||   ||
 * 37 || 85.67 || 3 || 3 || 0 ||  || T ||   ||
 * 38 || 88.83 || 0 || 2 || 0 ||  ||   || A ||
 * 39 || 89.59 || 3 || 0 || 2 ||  || T ||   ||
 * 40 || 90.29 || 1 || 2 || 2 || O ||  ||   ||
 * 41 || 94.16 || 0 || 4 || 0 ||  ||   || A ||
 * 42 || 95.54 || 1 || 4 || 0 ||  || T ||   ||
 * 43 || 95.54 || 1 || 4 || 0 ||  || T ||   ||
 * 44 || 95.75 || 4 || 3 || 0 ||  || T ||   ||
 * 45 || 96.99 || 6 || 0 || 0 ||  ||   || A ||
 * 46 || 99.28 || 4 || 0 || 2 ||  || T ||   ||
 * 47 || 99.56 || 2 || 4 || 0 ||  || T ||   ||
 * 48 || 101.42 || 0 || 4 || 1 ||  || T ||   ||
 * 49 || 103.26 || 0 || 3 || 2 ||  || T ||   ||
 * 50 || 105.92 || 3 || 4 || 0 ||  || T ||   ||
 * 51 || 113 || 0 || 0 || 3 ||  ||   || A ||
 * 52 || 113.16 || 7 || 0 || 0 ||  ||   || A ||
 * 53 || 114.15 || 1 || 0 || 3 ||  || T ||   ||
 * 54 || 114.23 || 4 || 4 || 0 ||  || T ||   ||
 * 55 || 115.42 || 0 || 1 || 3 ||  || T ||   ||
 * 56 || 117.53 || 2 || 0 || 3 ||  || T ||   ||
 * 57 || 117.70 || 0 || 5 || 0 ||  ||   || A ||
 * 58 || 120.59 || 0 || 4 || 2 ||  || T ||   ||
 * 59 || 122.41 || 0 || 2 || 3 ||  || T ||   ||
 * 60 || 122.96 || 3 || 0 || 3 ||  || T ||   ||
 * 61 || 129.32 || 8 || 0 || 0 ||  ||   || A ||
 * 62 || 130.19 || 4 || 0 || 3 ||  || T ||   ||
 * 63 || 133.25 || 0 || 3 || 3 ||  || T ||   ||
 * 64 || 141.25 || 0 || 6 || 0 ||  ||   || A ||
 * 65 || 145.49 || 9 || 0 || 0 ||  ||   || A ||
 * 66 || 147.09 || 0 || 4 || 3 ||  || T ||   ||
 * 67 || 150.66 || 0 || 0 || 4 ||  ||   || A ||
 * 68 || 151.53 || 1 || 0 || 4 ||  || T ||   ||
 * 69 || 152.49 || 0 || 1 || 4 ||  || T ||   ||
 * 70 || 154.09 || 2 || 1 || 4 ||  || T ||   ||
 * 71 || 157.85 || 0 || 2 || 4 ||  || T ||   ||
 * 72 || 158.27 || 3 || 0 || 3 ||  || T ||   ||
 * 73 || 163.95 || 4 || 0 || 4 ||  || T ||   ||
 * 74 || 164.79 || 0 || 7 || 0 ||  ||   || A ||
 * 75 || 166.39 || 0 || 3 || 9 ||  || T ||   ||
 * 76 || 177.67 || 0 || 4 || 4 ||  || T ||   ||
 * 77 || 188.33 || 0 || 0 || 5 ||  ||   || A ||
 * 78 || 188.33 || 0 || 8 || 0 ||  ||   || A ||
 * 79 || 211.87 || 0 || 9 || 0 ||  ||   || A ||
 * 80 || 226 || 0 || 0 || 6 ||  ||   || A ||
 * 81 || 301.33 || 0 || 0 || 8 ||  ||   || A ||
 * 82 || 339 || 0 || 0 || 9 ||  ||   || A ||

Also a graph is displayed below with all the modes combined. As we can see axial modes have the greatest energy because only two parallel surfaces are involved and the path they have to make is shorter than in the tangential and oblique modes. Tangential modes have greater energy than oblique due to a shortest path and only for parallel surfaces and oblique modes are the weakest in energy with the longest path, greater reflection losses and six surfaces involved. If we consider axial modes at 9 dB, tangential modes will have a value of 6 dB and oblique will have 3 dB. A pretty even distribution of modes is achieved with this room proportions, the gaps between axial modes are filled by the tangential and oblique modes. MODAL FREQUENCY DENSITY

Modes get closer as frequency is increased. Between 20 Hz and 50 Hz the average spacing between modes is 2.7 Hz. In the region between 60 and 90 Hz there is an average spacing of 0.91 Hz. As frequency increased the modal frequencies starts to be so packed that individual resonances should not deserve any attention. The reason this is not showed in our graph is because we only obtained till the fourth order tangential modes and the second order oblique modes. If further calculations of tangential and oblique would have been carried out, we would clearly see that as frequency increases the spacing between modes is reduced in a great manner being filled by oblique and tangential modes. So we can see the importance of tangential and oblique modes in order to obtain a homogeneous dispersion of modal frequencies.

MODAL BANDWIDTH

In our graph where all modes were displayed the modal frequencies are represented by lines but in fact there is a bandwidth associated with this modal resonance. Modal bandwidth is strongly dependent on reverberation time. For reverberation times in the range of 0.3 to 0.5 seconds, bandwidth is in the range of 4.4 t0 7.3 Hz. It can be assumed that most audio rooms will have a modal bandwidth in the order of 5 Hz. This also reinforces the idea of a good dispersion of modes in our specific room. This being all this true when the reverberation time is uniform with frequency something that can be only stated from well- designed audio rooms .We will explain this in more depth in the last part of this report.

ROOM PROPORTIONS FOR RECTANGULAR ROOMS

As we mentioned earlier room proportions is one of the key aspects in order to obtain a good dispersion of modes at low frequency. //Sepmeye//r established several room dimensions ratios with the aim to achieve that, in fact the proportions of our example follows this ratio established by //Sepmeyer//. This explains why a very good dispersion of modes was achieved.


 * || Height || Width || Length ||
 * A || 1.00 || 1.14 || 1.39 ||
 * B || 1.00 || 1.28 || 1.39 ||
 * C || 1.00 || 1.60 || 2.33 ||

Following rule A: a room with a ceiling height of 15 ft, the room would have 17.1 ft width and 20.85 ft. legth.

MODES IN NON-RECTANGULAR ROOM

In non-rectangular rooms a better diffusion of sound is achieved even at a low frequency because modes do not follow a regular pattern, producing superposition of modes and a more homogeneous distribution.

SOUND ABSORPTION AND DIFFUSION TECHNIQUES AND MATERIALS SOUND DECAY IN A ROOM When a sound source is stopped in a room the sound do not decrease to zero instantaneously like in a free field case. The sound continues bouncing off the walls for a certain period of time till it is totally dissipated due to the absorptive qualities of the materials placed in the room surfaces and the room size. The amount of time that is required for the sound to decay 60 dB is called reverberation time: RT= K (V/ Sa) Where: RT is the reverberation time in seconds, K is 0.161 for metric units and 0.049 for imperial units, V is the room volume, S is the surface area of a room, a is the average absorption coefficient. The RT60 could be also calculated with a more practical formula: RT= K V/ (S1 x a1 + S2 x a2 +....) Where: RT is the reverberation time in seconds, K is the 0,0161 for metrics units and 0,049 for imperial units, S1 is the surface area of the material 1, a1 is the absorption coefficient of material 1, S2 is the surface area of material 2, a2 is the absorption coefficient of material 2 and so on. The chart below displays optimum reverberation times for different types of rooms depending on their functionality:

Also a RT60 calculator is available on this website: [|www.csgnetwork.com/acousticreverbdelaycalc.html].
 * // **ENVIRONMENT** // || **//RT 60 VALUE (s)//** || **// VOLUME (Cub met) //** ||
 * OPEN AIR || VERY SHORT ||  ||
 * POP MUSIC STUDIO || 0.15/ 0.12 ||  ||
 * RADIO TALKS STUDIO || 0.3/0.85 || 30-200 ||
 * RADIO GENERAL PURPOSE STUDIO || 0.6/0.85 || 250-800 ||
 * MUSIC STUDIO || 0.8/1.6 || 700-8,000 ||
 * THEATRE || 1.0 ||  ||
 * CONCERT HALL || 1.5-2.0 || 15,000-20,000 ||
 * LARGE CATHEDRAL || 10-12 ||  ||
 * TYPYCAL LIVING ROOM || 0.5 ||  ||
 * CONTROL ROOM || 0.25 ||  ||
 * **// TELEVISION //** ||  ||   ||
 * ANNOUNCE BOOTH || 0.5 || 27 ||
 * WORKING NEWSROOM SET || 1.0 ||  ||
 * SMALL INSERT STUDIO || 0.5 || 815 ||
 * MIDSIZE PRODUCTION STUDIO || 0.5 || 1631 ||
 * AUDIENCE STUDIO WITH LIVE MUSIC || 1.8 || 3262 ||
 * TV STUDIO || 1.0 || 3262 ||
 * GALLERY || 1.0 || 223 ||
 * EDIT BAY || 1.0 || 32 ||
 * M.C.R || 1.0 ||  ||

SOUND ABSORPTION The reverberation and flutter echoes (delayed sound reflections of enough intensity to be heard above the rest of the reverberant sound level) are controlled by placing sound-absorbing materials and other devices inside of the walls, ceilings and floors. The sound is also absorbed by the human body and the presence of a large number of people in a room is going to affect reverberation significantly. Materials have different absorption properties and their absorption coefficients vary with frequency. The sound on sound magazine provides a list of materials and its absorption coefficients in this website: []

SOUND-ABSORBING MATERIALS APPLICATIONS ECHO CONTROL Sound absorbing material can be placed in specific trouble areas to avoid echoes. As it was mentioned earlier this is caused from delayed reverberations with such a level that can be perceived above the rest of the reverberation sound level.

NOISE REDUCTION Sound absorbing materials are used also to reduce the noise level within a room by reducing the amount of reverberation and can be calculated using this formula:

In order to reduce the level of noise by 3 dB the total absorption must be doubled and to reduce it by 6 db should be doubled again.

REVERBERATION CONTROL As it was mentioned earlier the absorption coefficient of the materials change with frequency, so a very important aspect in the acoustic designed of a room is maintaining uniformity in the RT60 across the whole frequency range. In order to achieve this different types of materials and devices have to be used to obtain high frequency absorption compared to the ones use for low-frequency absorption, being the last one more difficult to achieve and one of the big challenges for a well designed audio room.

TYPES OF ABSORBERS

There are three basics types of absorber: porous, panel and cavity resonator:

- Porous absorber: These are the most common acoustical absorbers. The sound wave produced the air particles to vibrate inside the material producing frictional losses that convert the sound energy into heat. The most important factor is density and materials with less density have a better performance in frequencies below 250 Hz with an increment in performance from 500 Hz to 4kHz when the density is increased. Thickness is also an important factor and increment in thickness improves absorption mainly at low frequencies that is going to be also improved increasing the air space between the material and the wall, a graph of the effect airspace on absorption characteristics is shown below:

Glass fiber, mineral wood, heavy drapes and carpets are all porous absorbers although the one of most common use is glass fiber. Cellular foam absorbers should be also mentioned with geometric pattern of wedges and cavities. It should be distinguished between open-cell that is acoustically absorptive at high frequencies and close-cell that does not have absorptive properties and it is used only as a thermal insulator. An example of open-cell acoustic material is shown below:



Drapes and curtains have high absorption at high frequencies but low absorption at low frequencies. The same happens with carpets with a very poor absorption at low frequencies although a foam rubber under-layment can improve the performance at low frequencies. - Panels: Different types of materials are used in the construction of panel absorbers, also called diaphragmatic absorbers. This can be made of wood, pressed wood fiber, plastic or a mixture of various types of absorbers. This is placed in the walls with an air gap in between and absorbs sound waves due to vibration. This forms a resonant system where the air gap behaves as a spring and the panel as a mass with a certain amount of frictional loss resulting in the absorption of some of the sound energy. The peak of the absorption is determined by the resonance frequency and can be obtained following this formula:



Where

m is mass in kg/square meters,

d is air space in meters.

An increment in the absorption characteristic can be obtained by filling the air space with absorptive material, such as fiber glass. This video shows how simple is to install panels from the manufacturer elite: http://www.youtube.com/watch?v=xBCTPcE7yxo

- Cavity Resonators: There are two basic types of cavity resonators, also called Helmholtz Resonators: perforated panels and slot resonators.

1. Perforated Panels. The action of blowing across the mouth of a bottle produces a tone at the frequency of resonance as the mass of the air makes the air inside the bottle behave like a spring. So a perforated panel placed against the wall with an air gap in between could behave as an assemblage of many bottles providing an absorption characteristic of different frequencies that are directly related to the area of the hole and inversely related to the length of the port and the panel thickness, as the formula showed below states:



where:

c is the speed of sound (340 m/s at 30 degrees celsius),

p is the area of the hole in square metres ,

l is the length of the air space in metres,

v is the volume of the cavity in cubic metres.

2. Slot Resonators: Instead of perforated holes the slot resonators consist of a number of slats spaced away from a rigid backing with airspaces (slots) in between. The resonant frequency can be calculated as follows:



where: s is the width of slot (metres), d is the thickness of slat (metres), D is the depth of the airspace (metres), w is the width of slat (metres).

Such resonant cavities are particularly useful for low-frequency absorption. An image displayed below shows a slat resonator with parallel air spaces:



Broader absorption characteristics could be achieved by using slats and slots of different width. The image below shows a slat resonator of these characteristics:



This video explains the construction of a combination of a slot resonator, also called bass trap, with a front panel for high frequency absorption. This model also includes a membrane inside the bass trap to improve the low frequency absorption. http://www.youtube.com/watch?v=NFVB-9O2O1E&feature=related

SUMMARY



Several applications that have been described in the report can be seen in the lay out of the studio display above: Firstly it can be seen a double sound lock to access to and from the live room. Secondly panels absorbers and slat resonators are used in the recording and listening areas in order to absorb high frequencies and certain critical frequencies that are going to be reinforced by the geometry of the room, special treatment deserve the control room that include also a bass trap to absorb low frequencies that are produces by very small room, as we saw in the previous video and that can affect the mix. Thirdly parallels walls are avoided to get a homogeneous dispersion of modes. Lastly a double pane window to obtain a good level of isolation between the high levels of noise of a live room and the very quiet environment needed for a control room.

BIBLIOGRAPHY: Ballou, G. (1987) Handbook for Sound Engineers, 1st Ed, Focal Press. Claudy, L (2oo7) NAB Engineering Handbook, Focal Press. [] [] [|http://en.wikipedia.org/wiki/Fletcher–Munson_curves] [] [] [] [] http://www.diracdelta.co.uk/science/source/s/o/sound%20transmission%20class/source. [|http://www.audioholics.com/education/acoustics-principles/helmholtz-resonant-] [|http://www.ronnlynn.com/fr_equipment.] [|http://www.industrialacoustics.com/uk/building_services/duct_silencers.] [|http://www.prime-air.com/case_studies/vibro-acoustics-floating-floor-] [|http://www.sae.edu/reference_material/audio/pages/Low%20Mid%20Frequencies.] []