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PRINCIPLES OF SOUNDPROOFING

To create good sound insulation, one must utilize the following general principles:

Mass — use heavy materials

Air-tightness — cover the whole enclosure airtight

Flexibility — keep it limp, better to overlap, than stretch tight

Isolation — separate (decouple) from surrounding structure

 

Although each project has to be considered individually, the above principles are relevant in most cases.

MASS:

Massive, heavyweight barriers will block more sound energy than lightweight barriers. (Less noise will go through it.) This is because the high density of heavyweight materials suppresses sound vibrations inside the material, to a degree that the inside wall of a room, vibrates with less movement. Therefore, the amplitude of the sound waves re-radiated into the air inside the room, €œloudness€, is also minimized.

NOTE: Although a reduction in the amplitude of sound waves affects the €˜strength€™ or €˜loudness€™ of a sound, it does not affect the frequency (pitch) of that sound.

Mass Law

The Mass Law states that the sound insulation of a single-layer partition has a linear relationship with the surface density (mass per unit area) of the partition, and increases with the frequency of the sound.

Single-layer construction includes composite barriers such as plastered brickwork, as long as the layers are bonded together.

  • The heavier the barrier, the better the sound reduction.

In theory, for each doubling of mass sound insulation increases by 6 dB.

For example, the average sound reduction of a brick wall increases from 45 dB to 50 dB when the thickness is increased from 4 inch to 8.4 inch. This doubling of mass does not have to be achieved by a doubling of thickness, as the mass of a wall for sound insulation purposes is specified by its surface density measured in lbs per square foot (rather than per cubic foot). Similar sound reduction can be achieved by adding thinner, but heavier materials, like a layer of Mass Loaded Vinyl.

  • The higher the frequency the easier it is to block it.

Sound insulation increases by about 6 dB whenever the frequency is doubled.

Any doubling of frequency is a change of one octave. For example, a brick wall provides about 10 dB more insulation against 400 Hz sounds than against 100 Hz sounds. (100Hz = bass note, 400 Hz = Voice).  This change, from 100 to 200 Hz and then 200 to 400 Hz, is a rise of two octaves.  In extreme cases you might not even hear the sound but can feet the wall vibrating to the touch.
But increasing the mass alone is not enough. If you feel that Mass law does not work in your construction, that is because other factors such as air-tightness, rigidity and isolation have an effect.

AIR-TIGHTNESS:

Areas of reduced insulation or small gaps in the construction of a wall have a far greater effect on overall insulation than you might think. The effective soundproofing of a structure depends on air-tightness and uniformity

For example, if a brick wall contains a hole or crack which in size represents only 0.1 per cent of the total area of the wall, the average sound reduction of that wall is reduced from 50 dB to 30 dB by about 40% (!).

In general, ‘sound leaks’ should be considered as carefully as leaks of water.

Common air gaps: Wall€“floor gaps,  Gaps around doors, Poor window seals, Unsealed pipe runs, Unsealed cable runs, Porous barrier material (Cinder blocks)

Another aspect of soundproofing, that is often overlooked is consistency of the material€™s STC  (Sound Transmission Coefficient) used in construction.  Your construction is only as soundproof as its weakest point. For example, an unsealed door occupying 25 per cent of the area of a half-brick wall reduces the average sound reduction efficiency of that wall from around 45 dB to 23 dB.

MEMBRANE FLEXIBILITY:

Rigidity is a physical property of a partition and depends upon factors such as the elasticity of the materials and the way the partition is installed. High rigidity of the barrier can cause loss of insulation at certain frequencies due to resonances and coincidence effects. These effects diminish the expected results according to the Mass Law.

Resonance

Loss of insulation by resonance occurs if the incident sound waves have the same frequency as the natural frequency of the partition. The increased vibrations that occur in the structure are passed on to the air and so the insulation is lowered. Resonant frequencies are usually low and most likely to cause trouble in the air spaces of cavity construction.

Coincidence

Loss of insulation by coincidence is caused by the bending flexural vibrations, which can occur along the length of a partition. When sound waves reach a partition at angles other than 90°, their transmission can be amplified by the flexing inwards and outwards of the partitions. The sound-wave frequency and the bending-wave frequency coincide at the critical frequency. For several octaves above this critical frequency the sound insulation tends to remain constant and less than that predicted by the Mass Law. Coincidence loss is greatest in double-layer constructions, such as cavity walls or hollow blocks.

Flexible (limp) materials, combined with high mass, are best for high sound insulation. But even if you get the flexible high mass material such as Mass Load Vinyl, it needs to be installed in a way that keeps it limp: for example attached only at the top and allowed to hang freely, or installed in a loose wave-like manner, especially if sandwiched between two rigid surfaces, to keep its limp properties.

ISOLATION:

Sound transfers through any medium air, structural elements of buildings such as floors, walls.  As the sound converts to different wave motions at the junction of different materials, energy is lost and an incremental amount of insulation is gained. This is the principle behind the effectiveness of air cavities in windows, of floating floors, of carpets and of resilient mountings for vibrating machines. Decoupling of elements of construction can be effective in reducing the transmission of sound through a structure. Some broadcasting and concert buildings, and acoustic labs, achieve very high insulation by using completely discontinuous construction of a double structure separated by resilient mountings and rested on a springy support mounts.

Sound isolation can be easily ruined by strong flanking transmissions through rigid links, even by a single nail. Cavity constructions must be sufficiently wide for the air to be flexible, otherwise resonance and coincidence effects can cause the insulation to be reduced at certain frequencies. In small air gaps in conjunction with rigid walls air gaps couples with the walls and separation effect gets lost.

Soundproofing and sound isolation need to be looked at as an integral complex approach where all principals are observed.  Even an incremental increase in sound isolation can have a great effect on how it is being perceived.Because sound levels are measured using a logarithmic scale, a reduction of nine decibels is equivalent to elimination of about 80 percent of the unwanted sound.

SOUND AND SOUNDPROOFING

Many people who are starting to set up their home recording studio often get confused on what is soundproofing, and what is acoustic treatment.The materials and techniques used in soundproofing are very different from what needs to be used foracoustic room treatment. And, when you are preparing to set up your home recording studio, you need to understand the differences before shopping for soundproofing products. Understanding of basic principles of soundproofing will help you make right decisions, save you time and money, and a lot of headache down the road.

Many people who are starting to set up their home recording studio often get confused on what is soundproofing, and what is acoustic treatment.

The materials and techniques used in soundproofing are very different from what needs to be used foracoustic room treatment. And, when you are preparing to set up your home recording studio, you need to understand the differences before shopping for soundproofing products. Understanding of basic principles of soundproofing will help you make right decisions, save you time and money, and a lot of headache down the road.

 

Definition of Soundproofing

In a nut shell, soundproofing means that the sound has to be stopped from leaking in or out of an enclosure. Soundproofing, in essence, is reducing the sound pressure between the source of sound that is generating the actual sound pressure and the receiver of the sound – such as a microphone or human ear.

For example: If a lawn mower or airplane passes by your home, it generates a pressure wave of a certain frequency. That wave travels to your house and will be heard and possibly physically felt, depending on the frequency generated. To keep those sounds out, a recording studio needs to be isolated from the outside world. Sound isolation works the same — both ways — so there’s no difference in the approach of keeping sound in or out.

However, don’t be misled. It is very hard to achieve a 100% sound isolation on a small budget. But, knowing the physics of sound and understanding how sound transmits can help to achieve the best sound isolation possible.

The Science Behind “Sound”

The science of sound might have sounded boring when you had to learn it in school, but now, when you are building your own recording studio, it has a very practical application.

So, what exactly is “sound” ?

Sound is a type of energy made by vibrations. Vibrating objects create a mechanical disturbance in the medium in which it is directly adjacent to. Usually, the medium is air. So sound is actually a pressure wave.

When an object vibrates, it causes movement in the surrounding particles. These particles bump into the particles close to them, which also make them vibrate — causing them to bump into more air particles. The energy of their interaction creates ripples of more dense (higher pressure) to less dense (lower pressure) air molecules, with pressures above and below the normal atmospheric pressure. When the molecules are pushed closer together, it is called compression; when they are pulled apart, it is called rarefaction.

The back and forth oscillation of pressure produces sound waves. The frequency of the waves depends on the frequency of the vibrations. This movement keeps going until it runs out of energy.

The other thing to consider is that a sound wave is a form of a traveling wave, in that the air molecules disturbed by the sound source are unlikely to be the ones hitting your eardrum, but transfer their energy to other neighboring molecules. These mechanical vibrations are able to travel through all forms of matter: gases, liquids, solids. Sound cannot travel through vacuum because there are no particles to transfer the sound energy.

In summary:

  • For sound to be generated and transmitted something must be vibrating;
  • Sound waves travel by passing energy form a particle to a particle;
  • Sound dies out eventually when it loses its energy. (It happens due to friction in the air itself or in trying to move (vibrate) the barriers it encounters. Sound energy does not just disappear, but have to be spent on doing some work and while doing the work it is converted to heat. )
  • Sound can be transmitted through anything that has particles air, wood, concrete etc. (not vacuum).

How to use that knowledge to soundproof a room, a door, sound booth or any type of soundproofing?

1 Since sound is transmitted by air you need to make an air tight enclosure, that does not let the sound waves in or out;

2 Because Sound energy can make particles in your enclosure vibrate and get through this way, you need to make it (a barrier) as heavy as possible. (Construct the barriers (walls) using materials that are hard to move, have a lot of mass and it take a lot of energy to get vibrating);

3 Because the sound waves can be transmitted through existing structural elements of the building ( like wall, floors, ceilings) you need to separate the vocal booth enclosure from other structural elements of the building it adjacent to, which may transmit the sound energy from the outside world.

Is soundproofing that simple?

This sounds pretty simple, isn’t it? In theory. But in practice you have to deal with materials that can achieve the level of isolation you require and the costs of those materials.

Theoretically one may suggest surrounding the room with a layer of vacuum, but that is probably for a Sci-Fi.

In real life you have to use what is available and it gets complicated.

This is where the secret is: What and How of soundproofing. What materials to use? How to install them?

Using mass for soundproofing

Although sound can’t escape directly from an airtight environment, its vibration energy causes the walls of the room to vibrate, and they in turn launch new sound waves. That is why it is important to make walls that would not move. And the heavier the walls, the more energy it requires to get them vibrating. So naturally, the simplest thing that can be done is to add mass to the walls.

But different materials have different sound transmission properties (see What is STC – Sound Transmission Class).

Brick is better than plywood, and then there are loss of materials like sand.

(For example, a given thickness of glass may transmit (let through) more sound energy than the same thickness and mass of sand, because the sand particles tend to lose more energy through friction between the individual particles.)

NOTE: As a rule of thumb, if the mass of a wall doubled (by doubling its thickness, for instance) amount of sound leakage will be reduced by 6 dB.

Using decoupling for soundproofing.

Separating the sound enclosure from structural elements of the building it’s adjacent to, (decoupling) helps to block structure-borne sound. Sound energy travels very efficiently, as mechanical vibrations, through wooden joists or steel girders. Special attention needs to be paid to floor supports as most unwanted energy gets injected into the floor.

NOTE: If sonic vibrations are injected into these components, they’ll bypass all soundproofing.

Sound frequencies and soundproofing.

Another issue that makes soundproofing complicated is that the sound waves have a range of frequencies and the isolation provided by a structure reduces with lower frequency. While high frequencies are easy to keep in or out, low frequencies are far more difficult to contain.

Since different frequencies have different wave length for every frequency above a certain, critical value, there’ll be an angle of incidence for which the wavelength within the material is equal to the wavelength of the sound incident upon the material, and when this occurs the attenuation drops significantly. This is why using layers of materials with different acoustic properties can help to improve soundproofing characteristics of a wall.

NOTE: The rule of thumb here is that for every octave drop in pitch the amount of sound isolation is halved.

Using air gap for cost effective soundproofing.

But, things are even more complex than this. Sheer increase in mass and thickness of the wall is not always feasible or cost effective. The best sound proofing method, which is used by most professional studios, is to build double walls with an air gap between them. The cushion of air between the walls separates energy from one wall to the other, and the wider the air gap, the better the isolation (most noticeable at low frequencies again).

NOTE: Unless the walls are separated by a considerable distance, the cushion of air between the walls couples energy from one wall to the other, reducing the isolation. But double wall structure will invariably perform significantly better than a single-layer barrier of similar mass, even if the air gap is only a few inches wide.

When soundproofing your voice over studio or recording room, combinations of factors must be considered. Doing your homework and understanding the science behind the sound will greatly help you to create a more successful home recording studio.

WHAT IS STC?

When building your home recording studio one of the important factors you have to deal with is thesound transmission. To determine their soundproofing quality materials are tested for sound transmission loss values, that can be measured, and rated in a single-number rating system – Sound Transmission Class (STC). The STC rating figure very roughly reflects the decibel reduction in noise that a partition can provide.

When building your home recording studio one of the important factors you have to deal with is thesound transmission. To determine their soundproofing quality materials are tested for sound transmission loss values, that can be measured, and rated in a single-number rating system – Sound Transmission Class (STC). The STC rating figure very roughly reflects the decibel reduction in noise that a partition can provide.

STC: Sound Transmission Class?

The Sound Transmission Class (STC) is a single-number rating of a material’s or an assembly’s ability to resist airborne sound transfer at the frequencies 125-4000 Hz. These transmission-loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour. Acoustical engineers fit these values to the appropriate TL Curve (or Transmission Loss) to determine an STC rating. The measurement is accurate for speech sounds, but less so for amplified music, mechanical equipment noise, transportation noise or any sound with substantial low-frequency energy below 125 Hz. Sometimes, acoustical labs will measure TL at frequencies below the normal STC boundary of 125 Hz, possibly down to 50 Hz or lower, thus giving additional valuable data to evaluate transmission loss at very low frequencies, such as a subwoofer-rich home theater system would produce.

In general, a barrier with higher STC rating blocks more noise from transmitting through a partition. In the USA, STC is widely used to rate interior partitions, ceilings/floors, doors, windows and exterior wall configurations (ASTM Standards). Outside the USA, the Sound Reduction Index (SRI) ISO standard is used. The ASTM test methods have changed every few years and over many years have been changed significantly. Thus, STC results posted before 1999 may not produce the same results today, and this difference becomes wider as one goes back in time.

It must be noted that acoustical performance values such as STC are measured in specially constructed acoustical chambers, and that field conditions such as lack of adequate sealing, outlet boxes, back-to-back electrical boxes, medicine cabinets, flanking paths, and structure-borne sound can diminish acoustical performance. The as-built ‘field-STC’ (FSTC) is usually lower than the laboratory-measured STC. Nevertheless STC ratings are the ONLY way to accurately compare various noise reduction products. The STC ratings allow accurate ‘apple to apple’ comparison of materials for soundproofing.

For example loud speech can be understood fairly well through an STC 30 wall but should not be audible through an STC 60 wall regardless of the material the wall is constructed from.

HOW TO WASH PRODUCER’S CHOICE SOUND BLANKETS

Producer’s Choice sound blankets can be washed, unlike acoustic foam. If you spill coffee or other beverages on the blankets or they just get dusty, you can renew thSound Blanketsem by washing them.
Washing the blankets can also help to remove the “new blanket” smell.Directions on washing instructions.

Producer’s Choice sound blankets can be washed, unlike acoustic foam. If you spill coffee or other beverages on the blankets or they just get dusty, you can renew them by washing them.

Washing the blankets can also help to remove the “new blanket” smell.

Directions on washing instructions:

  • Wash the sound blankets at a commerical laundromat.
  • Blankets tend to get heavy when wet.
  • Note that this product has “Inside filler,” which is a recycled cotton fabric that is multicolored.
  • Note that colors from that filler may run and because of this, do not wash together with other clothes!
  • After washing you might see fuzz balls on the fabric.  This does not reduce the quality of the blanket or cause any problems.
  • You can fluff these blankets in the dryer too –making them nice and thick.
  • In general, for upkeep, you can vacuum it from the dust and wash them occasionally.

If you have questions, you can refer to the directions from this You Tube video:http://youtu.be/HTaGBLAAQTw

TIPS AND TRICKS FOR USING PRODUCER’S CHOICE ACOUSTIC SOUND BLANKETS

Sound blankets are an effective and economical alternative to acoustic foam. In other articles, we discuss the cost efficiency of acoustic sound blankets versus acoustic foam, superior acoustic quality as well as the versatility of applications. This article, however, was triggered by a question of the DVX user forum.

 

“I haven’t been able to get much feedback on this subject but does anyone have tips or tricks for sound blankets?

Indeed, acoustic blankets might seem like a simple product, and yet if used properly, you can get more for your money than with using acoustic foam.  It is also a misconception to think that the blankets are simply a “cheaper” alternative — and if you can afford, you should use acoustic foam instead.

On the contrary, Producer’s Choice Acoustic Blankets have been successfully used by professional film makers with multimillion-dollar budgets such as Century 21 Studios, HBO, and other professional film and music studios worldwide.

So, how do you use acoustic blankets effectively? Here are some tips and tricks to using Producer’s Choice blankets.

 

USING ACOUSTIC BLANKETS IN A HOME RECORDING STUDIO

 

This is the first and probably most popular use of the sound blankets.  If you are building a home recording studio, there are two things you need to take care of first:  make it quiet – Soundproof as much as possible, and make it dead – cut down on reflections, reverberations, and standing waves and so on. You want to do this so the sound does not bounce back from the walls and does not interfere with your recording.  This is where the acoustic blankets come in to play.

 

Tips on Turning an Entire Room into a Studio:

 

  • ORDER MORE ACOUSTIC BLANKETS THAN YOU THINK YOU WILL NEED.

When deciding on the quantity of the blankets – do not try to match blankets to the surface inch by inch.  First of all, blankets are not as square and precise cut to size as the foam.  The size of the blanket quoted in blanket description is a so called “Cut Size”, after the blanket is bound it takes away and inch or two. When the blankets get fluffed, they get thicker and it also takes away from the size a little. Finally, if you decide to dry clean the blankets or wash them, they can shrink a little more. So effectively, the blanket that is described as 72 x 80 can in fact end up being 70 x76 or so.

  • KEEP AWAY FROM THE WALLS

When hanging blankets along the walls, make sure there is an air gap between the wall and the blanket — at least 2 -3 inches.  This simple trick increases the sound absorbing efficiency of the blankets by about 40%.  For example NRC (noise reduction coefficient) of Producer’s choice blanket positioned 3 inch of the wall is 0.8 (80 % sound absorption) the exact same blanket if applied directly to the wall (laid flat against a hard reflective surface) has NRC of 0.5

  • ROUND THE CORNERS.

When you get to the corners, do not try to follow the straight angle of the room – round it up.  Let the blanket make a natural rounded curve. This will help greatly on cutting down of long waves and also cut down on standing waves by changing the shape of the room.

  • KEEP THE BLANKETS PLEATED

If you hang the blankets in a pleated fashion you add the capacity of the blanket to absorb lower frequency.  Allowing for folds in the blanket drastically increases its ability to absorb lower frequencies and adds to the overall NRC values. For example, Producer’s Choice Sound Blankets when tested flat measured at NRC 0.8. If hung in a pleated manner, it measures at NRC 0.95. This increase is contributed mostly by the added lower frequency sound absorption.

 

 

  • DO NOT FORGET THE CEILING

The ceiling and the floor are two reflective surfaces that often get forgotten when planning for the acoustic room treatment.  If you have a thick carpet on the floor that would usually be enough.  Ceiling can be covered by a blanket simply by nailing it in right through the blanket.  Thin nails with small washer will do just fine.  DO NOT use screws!  Screws will rip out the inside of the blanket and you will be screwed!   Let the blanket sag a little – creating air pockets of a sort.

  • COVER THE DOORWAY

You can use wall track mounts to cover the doorway and be able to remove the blankets out of the way.

 

Turning only a Section of a Room into a Studio

This is something that the acoustic foam cannot do well and the blankets really excel at!

ceiling-track-booth7Use a ceiling track kit and hang the blankets from it.   You can draw the blankets to enclose your recording studio when ready to record and pull them away when not in use.
By using ceiling tracks you can convert a corner of the room or split the room in half or add an extra coverage to your closet studio. Lots and lots of options!

 

CORNER BASS TRAPS

Sound absorption blankets can be very effective as corner bass traps.   But to effectively absorb low frequencies, the thickness of absorbing material has to be considerably larger than for mid or high range frequencies.

Corners are somewhat of a “hot spot” for lower frequencies, especially in rectangular rooms.  The reason is that the sound waves bounce from one corner to the other and at the right wave length they create standing waves on that corner.  Place a blanket in that corner with deep pleats.  It will change the room geometry from rectangular to odd shaped and will serve as diffuser and absorber at the same time.  We did not do any special tests for sound blankets used as a Bass Traps, but from what we have seen so far, Producer’s Choice Sound Blankets can be very effective bass traps when used in the corners folded.

In this graph below we compared low frequency sound absorption of Mega Bass traps from Real Traps.com   (http://www.realtraps.com/p_megatraps.htm) with Producers Choice blankets used Pleated.  To be fair, due to the nature of the product, the acoustical test method for the Bass Trap is different from acoustical test for sound curtains (G-75 mount), also the depth of absorption material in Mega traps is deeper than in the Producer’s Choice blankets, but as it was shown in previous tests folding acoustic blankets in larger folds dramatically increases the absorption at low frequencies.  Therefore if not apple to apples this comparison is an indication of possibility to effectively use Producer’s Choice blankets as corner Bass Traps.

 

lowfrequencychart_basstraps

We believe that if the blankets can be folded with at least 12 inch deep pleats they will outperform the Mega bass traps.  Further testing is required to have the actual proof.

 

STAGE SOUND ABSORPTION BLANKETS FOR LIVE BANDS AND “BAR-ROOM MUSICIANS”

Acoustic blankets proved to be very efficient for live bands and performances in a small venue. The blankets are just the right type of gear for musicians that play in different locations every weekend.  Some rooms can have extremely back acoustic, yet some venues may not have the room on stage to put tripod stands with premade sound absorbers and Bass Traps.

The tripod stands legs uses a good bit of space.  Sound blankets can be suspended in a way without creating a foot print on stage when space is limited.

Musicians told us that when they hung the blankets on the back wall of the stage in a poorly acoustically designed bar room, one that they’ve played multiple times in, it made a major difference in the overall sound quality.  In fact, most of the previous issues with “slap back” coming from the back wall were eliminated.