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.

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.

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

(more…)

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. In general, a barrier with higher STC rating blocks more noise from transmitting through a partition. The STC ratings allow accurate ‘apple to apple’ comparison of materials for soundproofing.

First of all knowing STC rating of a barrier helps to measure your expectation of what can be achieved when using various types of soundproofing materials.

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When building your home recording studio one of the important factors you have to deal with is the sound 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.

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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 for acoustic 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.

 

(more…)

When building a home recording studio or a vocal booth, it is important to understand the different approaches one needs to take when tackling soundproofing and acoustic room treatment of the recording space.

The first thing people do when they get into the audio recording business is purchase recording equipment. Instead, they should consider spending their money on things that don’t make a sound. If you are converting a closet into a voice over booth, building a DIY vocal booth in a garage or turning a bedroom into a recording studio, investing money into the room itself may be a very good idea.

It boils down to this: you need to take care of acoustics and/or soundproofing because if you don’t, you will have a problem – noise problems. Noise problems related to home studios and neighbors in attached houses. Noise caused by a flying jet or lawn mower. Noise from family members or the dog.

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