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More About Sound

What’s the difference between sound insulation and sound absorbing products?

Soundproofing, as it is commonly known, is all about creating an acoustic barrier. Sound insulation and soundproofing products are specially designed and manufactured to act as an acoustic barrier in order to reduce the amount of sound leaving or entering a room or workspace.
Sound absorbers are designed to absorb the sound within a room and help reduce sound reverberation or echo. Sound absorbers do not stop the sound leaving the space but will reduce the amount of noise by reducing the reverberation (echo) within the enclosure.
If you want to soundproof a room as fully as possible you almost certainly need to install both types of sound insulating materials.

What is the difference between sound absorption & sound insulation?

There is often confusion between sound insulation and sound absorption. Sound is absorbed when it encounters a material which will convert some or all of it into heat, or which allows it to pass through not to return. For this reason good sound absorbers do not of themselves make good sound insulators. Sound insulators rarely absorb sound. Sound absorbers contribute little to sound insulation. They are treated separately in sound control design.
Sound insulation prevents sound from travelling from one place to another, such as between apartments in a building, or to reduce unwanted external noise inside a concert hall. Heavy materials like concrete are the most effective materials for sound insulation – doubling the mass per unit area of a wall will improve its insulation by about 6dB. It is possible to achieve good insulation over most of the audio frequency range with less mass by instead using a double leaf partition (two independent walls separated by an air gap filed with a sound absorber).

What is soundproofing?

This is a term used when a reduction in the level of noise being heard is being reduced. It is often thought that if something has been soundproofed then the noise being generated has been silenced. This may be the case in some instances but is not always possible so a soundproofed situation may also refer to a noise nuisance that has been reduced in intensity as much as feasible or possible.

What is sound absorption?

Sound absorption is normally required in enclosed spaces such as studios, halls and recreation centres to reduce reverberation (echo) of noise. An un-insulated hall is often unusable for many events due to excess reverberation. This makes speech difficult to comprehend and becomes more of a problem when people are speaking further apart.

What is a sound barrier?

A sound barrier is another way of describing a sound blocker and normally comprises insulation with a high mass which then reduces the amount of noise that can pass through it. A simple door can be described as a noise barrier when it is closed to reduce the sound of noisy kids playing just outside. Sound waves flow like water and air so it is impossible to use a sound barrier such as a fence or screen to stop noise but they are effective when used to reduce noise immediately on the other side of them. From distances farther away the noise reduction will be less efficient.
If you imagine a large stone in the middle of a river, you will see that the water flows quickly around it but leaves a slack area immediately behind the stone. Sound waves act in exactly the same way when presented with a sound barrier that is not complete.

What is sound damping?

Sound damping is normally required to reduce noise from resonating panels. Noise from resonating panels is annoying and addressed by stiffening the panels usually with a vibration damping pad that is glued on.

How small and rapid are the changes of air pressure which cause sound?

When the rapid variations in pressure occur between about 20 and 20,000 times per second (i.e. at a frequency between 20Hz and 20kHz) sound is potentially audible even though the pressure variation can sometimes be as low as only a few tens of millionths of a Pascal. Movements of the ear drum as small as the diameter of a hydrogen atom can be audible! Louder sounds are caused by greater variation in pressure. A sound wave of one Pascal amplitude, for example, will sound quite loud, provided that most of the acoustic energy is in the mid-frequencies (1kHz – 4kHz) where the human ear is most sensitive. It is commonly accepted that the threshold of human hearing for a 1 kHz sound wave is about 20 micro-Pascals.

What makes sound?

Sound is produced when the air is disturbed in some way, for example by a vibrating object. A speaker cone from a high fidelity system serves as a good illustration. It may be possible to see the movement of a bass speaker cone, providing it is producing very low frequency sound. As the cone moves forward the air immediately in front is compressed causing a slight increase in air pressure, it then moves back past its rest position and causes a reduction in the air pressure (rarefaction). The process continues so that a wave of alternating high and low pressure is radiated away from the speaker cone at the speed of sound.

What is a decibel (dB)?

A decibel is one unit on the decibel scale, which is a logarithmic scale. The name means one-tenth of a bel, a bel being an eponymous unit named after Alexander Graham Bell and used to compare power in electrical communication, voltage, or intensity of sound. The abbreviation of bel is B and decibel, dB. 10 dB = 1 B
Eighty-five decibels is the threshold for the possibility of noise-related hearing loss, and this guideline is intended to prevent such hearing loss. This figure suggests that many people who do not currently use ear protection should consider it. The following chart reveals that a great deal of the sound we’re exposed to is above that 85-decibel threshold. Because conditions may vary and distances are not specified, these figures are approximate.

 Decibels  Sound Source
 0  low threshold of hearing – softest sound you can hear
 10  leaves rustling in the breeze; quiet whisper
 20  average whisper
 20-50  quiet conversation
 40-45  conversation between acts at a theatre; hotel lobby conversation
 50  rainfall
 50-65  loudish conversation
 65-70  moderate traffic; hair dryer
 65-90  train
 75-80  factory (medium) – washing machine
 90  heavy traffic – power lawn mower – busy city walk
 90-100  thunder – walkman – tractor
 100  boom box with volume turned high – chain saw – leaf blower
 110  shouting; synphony concert
 115  rock concert
 120  ambulance siren
 130  threshold of pain – loud fireworks – gunshot
 140  airplane takeoff from short distance away
 140-190  space rocket takeoff
 170  shotgun

Acousticians use the dB scale for the following reasons:

  1. Quantities of interest often exhibit such huge ranges of variation that a dB scale is more convenient than a linear scale. For example, sound pressure radiated by a submarine may vary by eight orders of magnitude depending on direction; expression in linear units carries with it the confusion of the location of the decimal point. Decibels values are characteristically between only -999 to +999.
  2. The human ear interprets loudness more easily represented with a logarithmic scale than with a linear scale.

Amplitude measures how forceful the wave is. It is measured in decibels or dBA of sound pressure. 0 dBA is the softest level that a person can hear. Normal speaking voices are around 65 dBA. A rock concert can be about 120 dBA.
Sounds that are 85 dBA or above can permanently damage your ears. The more sound pressure a sound has, the less time it takes to cause damage. For example, a sound at 85 dBA may take as long at 8 hours to cause permanent damage, while a sound at 100 dBA can start damaging hair cells after only 30 minutes of listening.
Frequency is measured in the number of sound vibrations in one second. A healthy ear can hear sounds of very low frequency, 20 Hertz (or 20 cycles per second), to a very high frequency of 20,000 Hertz. The lowest A key on the piano is 27 Hertz. The middle C key on a piano creates a 262 Hertz tone. The highest key on the piano is 4186 Hertz.

Measuring sound reduction – Decibels vs. Percentage reductions

Sound intensities are measured in decibels (dB). Decibels however cannot be expressed in percentages.
There are two reasons why you can never equate decibels to percentages. First, the decibel scale is open-ended like that of the Richter scale used for measuring earthquake intensities. To calculate a percent you need to know the maximum value possible. In both of these scales there is no limiting maximum value. Therefore, you cannot calculate a percentage. Any attempt to do so is just meaningless!
Second, the decibel scale is logarithmic, while the percent scale is linear. Numbers that appear to be similar have vastly differing meanings. They are as different as trying to compare apples to elephants!
A decibel is not a given intensity (loudness) of sound, but rather, it is a ratio of how many times louder (or softer) a sound is than a given reference sound level.
This means that 0 dB is not the absence of sound, but is an arbitrary zero. We define it as the faintest sound that a young sensitive human ear can hear. Furthermore, because the decibel scale is logarithmic, every 10 dB increase in sound intensity is actually a ten-fold increase. Therefore, a sound intensity of 20 dB is not twice as loud as a sound intensity of 10 dB, but is 10 times as loud, and a sound intensity of 30 dB is 100 times as loud as a sound intensity of 10 dB. Similarly, a sound intensity of 50 dB would be 100,000 times as loud (10 x 10 x 10 x 10 x 10). This is how the decibel scale works. It is totally unlike the linear percent scale.
It is a fallacy trying to compare the decibel scale to the percent scale. To illustrate this, let’s wrongly assume that 0 dB is equal to 0 percent hearing loss or sound reduction and that 100 dB equals a 100 percent loss or sound reduction. This would then mean that 50 percent would equal a 50 dB hearing loss, right? Wrong! Not by a long way! A 50 percent hearing loss would equal, believe it or not, only a 3 dB loss! Looking at it the other way, a 50 decibel loss is not just half as loud, like it would be in a percentage scale, but would only be one thousandth of one percent as loud!
So decibels and percentages just do not equate. They are absolutely meaningless!

How are decibel sound levels added?

If there are two uncorrelated sound sources in a room – for example a radio producing an average sound level of 62.0 dB, and a television producing a sound level of 73.0 dB – then the total decibel sound level is a logarithmic sum i.e.
Combined sound level = 10 x lg ( 10^(62/10) + 10^(73/10) )= 73.3 dB
Note: for two different sounds, the combined level cannot be more than 3 dB above the higher of the two sound levels. However, if the sounds are phase related (“correlated”) there can be up to a 6dB increase in SPL.

How does the ear work?

The eardrum is connected by three small jointed bones in the air-filled middle ear to the oval window of the inner ear or cochlea, a fluid- filled spiral shell about one and a half inches in length. Over 10,000 hair cells on the basilar membrane along the cochlea convert minuscule movements to nerve impulses, which are transmitted by the auditory nerve to the hearing centre of the brain.
The basilar membrane is wider at its apex than at its base near the oval window; the cochlea tapers towards its apex. Groups of the delicate hair sensors on the membrane, which vary in stiffness along its length, respond to different frequencies transmitted down the spiral. The hair sensors are one of the few cell types in the body which do not regenerate. They can therefore be irreparably damaged by large noise doses.

At what level does sound become unsafe?

It is strongly recommended that unprotected exposure to sound pressure levels above 100dB is avoided. Hearing protection should be used when exposed to levels above 85dB (about the sound level of a lawn mower when you are pushing it over a grassy surface), and especially when prolonged exposure (more than a fraction of an hour) is expected. Damage to hearing from loud noise is cumulative and is irreversible. Exposure to high noise levels is also one of the main causes of tinnitus.
Health hazards also result from extended exposure to vibration. An example is “white finger” disease, which is found amongst workers who frequently use hand-held machinery such as heavy drills or chain saws.

What is the sound absorption coefficient?

The absorption coefficient of a material is ideally the fraction of the randomly incident sound power which is absorbed, or otherwise not reflected. It is standard practice to measure the coefficient at the preferred octave frequencies over the range of at least 125Hz – 4kHz.
It can be determined on small material samples with an “impedance tube” or on large samples in a laboratory “reverberation room”.

What is sound insulation and how is it measured?

Sound insulation is a measure of the sound stopped by a barrier such as a partition wall. We can measure the sound reduction index in a laboratory transmission suite. This consists of two adjacent reverberant rooms, the difference between the level of the sound in the source room and the receiver room is measured, and the properties of the receiver room are taken into account by calculation.
The measurement method depends on the particular situation. There are standards for the measurement of the insulation of materials in the laboratory, and for a number of different field circumstances.
Usually test procedures generate a loud and consistent broadband spectrum of steady noise on one side of a partition or specimen of the material under test, and then measure the amount of this sound that passes through that material. The ratio of the incident sound to the transmitted sound is the “noise reduction”, usually expressed as 10 times the logarithm of this ratio. If the noise reduction is also corrected for the amount of sound absorption to be found in the receiving room, 10 times the logarithm of the corrected ratio is called the “transmission loss. This is performed for 1/3 octave bands of noise from 100 to 4000 Hz.

Modern myths about soundproofing

Attempts to quiet rooms over the years have created many fallacies. Even today, some companies and builders merchants sell a variety of materials to unsuspecting contractors and homeowners based on fallacies which have been pervasive for years. A few of these are:

 Fallacy  What they said  What it actually does
 Fill the wall with egg cartons  “Will improve loss by 10dB”  No measurable effect
 Put acoustic insulation in wall  “Will fix everything”  Typically 3 – 4dB improvement
 Put mass loaded vinyl under drywall  “Will improve loss by 27dB”  Actually 3 – 9dB
 Add another layer of drywall  “Will stop the bass sounds”  Actually 2 – 3dB per layer
 Use foam as a barrier  “Regarded as a great barrier”  Actually <2dB

As you can easily see, if we are trying to make a 30dB improvement, it will not be achieved with egg cartons and vinyl.

Do you have a question about Soundproofing, Sound Insulation or Sound Absorption?

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