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Frequently Asked Questions

What does an acoustical consultant do and what can he do for me?

In general, an acoustical consultant utilizes his expertise in the fields of acoustics, noise, and vibrations to make recommendations to clients. These recommendations could be made to solve existing problems or to ensure the success of new designs. Typical clients for most acoustical consultants include architects, contractors, business owners, and homeowners.

The benefit of hiring an acoustical consultant lies in his knowledge, experience, and motivation. As opposed to relying on limited personal knowledge, advice from friends or other “experts,” or vendors trying to sell their products, a good acoustical consultant will provide consultation services based on knowledge and experience and without bias. A good acoustical consultant is not trying to sell you his product, he is selling his expertise and knowledge and will use this expertise and knowledge to choose from a wide variety of options and products to recommend the best solution to fit each client’s specific needs.

What is sound?

Sound is produced by pressure variations about a mean value within a medium. In air, sound propagates via very small rapid changes in the air pressure above and below atmospheric pressure. These pressure variations travel in the form of a pressure wave with an inherent amount of energy. When the rapid variations in pressure occur between about 20 and 20,000 times per second (i.e., at a frequency between 20 hertz and 20 kilohertz), sound is potentially audible by humans even though the pressure variation can be, and typically is, very small with respect to the mean pressure (e.g., atmospheric pressure). Louder sounds are caused by greater variation in pressure – 1 Pascal, for example, will sound quite loud, provided that most of the acoustic energy is in the mid-frequencies (1 to 4 kilohertz) where the ear is most sensitive.

These pressure variations in air are typically induced by a vibrating object. A speaker cone serves as a good illustration. As the cone moves forward, the air immediately in front is compressed causing a slight increase in air pressure. As the cone moves back past its rest position, it causes a reduction in the air pressure known as 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.

How is sound measured?

A sound level meter is the principal instrument used for measuring sound. Sound level meters measure sound pressure level and can usually report maximum, minimum, peak, equivalent, and instantaneous sound levels for specific periods of time. In addition, sound level meters can typically measure overall sound levels (weighted or flat) over the whole audible spectrum as well as sound levels in smaller, discreet frequency bands.

What is a decibel (dB)?

The decibel is a logarithmic unit which is used in a number of scientific disciplines. Its use in acoustics arises from the fact that typical acoustic pressures can vary by a factor of one million or more. Expressing this great range in terms of a logarithmic scale (i.e., decibels) instead of a linear scale (i.e., Pascals) allows us to describe sound levels with more manageable numbers.

In acoustics, the decibel is most often used to compare sound pressure in air with a reference pressure. Decibels are also used to describe sound power and sound intensity, however the references for these quantities are different. Therefore, when discussing decibels, one must know what quantity is being measured and what the reference is to ensure that valid comparisons are being made.

How does sound power level differ from sound pressure level?

Sound power is related to the amount of acoustical energy produced by a sound source. If an object is rated at some sound power level, it means that that is the amount of power it is capable of radiating. Sound power level is based on the specific object and does not take into account its surroundings.

On the other hand, sound pressure level does take into account the surroundings. A piece of machinery producing a sound power level of X dB will be much quieter 50 feet away than at 5 feet away. Hence, the measured sound pressure level will be lower at a distance of 50 feet than at a distance of 5 feet although the sound power level of that piece of machinery has not changed.

What are “A-weighted” decibels (dBA)?

Sound levels can be averaged over the whole audible spectrum and reported as an overall sound pressure level in decibels (dB). More commonly, sound levels are reported as decibels, A-weighted. A-weighted sound level measurements are numerically adjusted to reflect the frequency-dependent nature of human hearing at low sound levels. Since humans are less sensitive to sounds at low frequency, A-weighted sound levels will almost always be lower than flat, overall sound pressure levels reported in terms of decibels.

Is 80 dB twice as loud as 40 dB?

No it is not. Due to the logarithmic nature of the decibel scale, sound levels do not add linearly. For example, if there are two sound sources in a room such as an air handler producing an average sound level of 90.0 dB and a pump producing a sound level of 86.0 dB, then the total sound level is 91.5 dB according to the mathematical rules governing logarithms. If two separate sound sources are producing the same sound level then the overall sound level resulting from the addition of these two sources will be 3 dB higher assuming they are not phase related.

Generically speaking, a person with average hearing would just barely perceive a difference of 3 dB. An increase of 10 decibels is generally accepted as the point at which a sound is subjectively perceived as “twice as loud.” Of course, subjective perception depends on factors such as the person and the nature of the sound.

At what level does sound become unsafe?

It is best, where possible, to avoid any unprotected exposure to sound pressure levels above 100 dBA and to use hearing protection when exposed to levels above 85 dBA if prolonged exposure is expected. Also, care should be taken to minimize direct exposure to impulsive noise, such as from gun fire. Damage to hearing from noise is cumulative and irreversible. The Occupational Safety and Health Administration (OSHA) has enacted regulations concerning permissible levels of noise exposure in work environments.

What effect does distance have on the propagation of sound?

The effect of distance on the propagation of sound depends on the nature of the sound source and the surrounding environment and climate. If the source is outdoors, and it is small compared with the distance to the measurement location, the sound energy will spread over an area which is proportional to the square of the distance. This “inverse square” relationship dictates that sound level will decrease by 6 dB for each doubling of distance. Noise sources such as a train or moving traffic will radiate noise in a cylindrical pattern so that the sound energy will spread over an area which is proportional to the distance. Thus, the sound level will decrease by 3 dB per doubling of distance. Of course, climatic conditions will have an effect on this.

What is vibration and how is it measured?

Vibration is the oscillation of an object about a static position. Vibrations typically, as in the case of a speaker diaphragm, produce sound. More often, however, vibrations are not necessarily desired as in the case of air conditioning equipment inducing vibrations which propagate through a building.

Vibration is measured with an accelerometer. An accelerometer is a device that is securely attached by some means to a surface and produces an electrical signal which is proportional to the surface acceleration. This signal is measured by a meter and can be expressed in terms of acceleration, velocity, or displacement of the surface. When making vibration measurements, the frequencies of interest generally range from below 1 hertz to about 1 kilohertz.

What is reverberation time?

Reverberation time, a fundamental quantity in architectural acoustics, is defined as the time it takes for sound energy in an enclosed space to decay by a factor of one million or 60 dB. Reverberation time is dependent on the volume and amount of acoustical absorption in the space and is typically measured and reported in octave bands.

What is the difference between insulation and absorption?

Sound insulation refers to the act of impeding the transmission of sound from one area to another or from a source to a receiver. Typical examples include the sound insulation between adjacent apartment units or between a busy highway and one’s bedroom. The best way to improve the insulation between two areas typically involves the use of heavy materials such as concrete or gypsum board.

Sound absorption refers to the phenomenon whereby some or all of the sound energy incident on a surface is either converted into heat or passes through the absorber. For this reason, good sound absorbers do not necessarily make good sound insulators. Absorption typically should not be the primary means of achieving good sound insulation.

How do you improve the sound insulation in a building?

The method employed to improve the sound insulation between two areas depends on the situation. For this reason, the advice of a competent person should be sought at an early stage. Following are general guidelines to consider in improving sound insulation.

In general, applying sound absorbing treatments will not greatly improve insulation conditions. Typically, blocking the transmission of sound by introducing barriers is the most beneficial route to take. Such a barrier could take the form of a barrier blocking the line of sight between a busy highway and house or a massive wall between apartment units. Common culprits in areas where sound insulation is an issue include windows, doors, ceiling plenums, and lightweight roofs.

What is meant by white noise and pink noise?

Technically speaking, white noise is defined as having a constant power spectral density independent of frequency. Broken down by octave bands, white noise will appear to exhibit a 3 dB increase in each successively higher octave band.

Pink noise is defined as having the same power within each octave. Thus, an analysis of pink noise in discreet bands of constant width will show a decrease in level with increasing frequency. Subjectively, both white noise and pink noise may sound somewhat like television static with white noise sounding more “hissy.”

What is an arpeggio?

An arpeggio is the sounding of the notes of a musical chord in rapid succession (instead of simultaneously).