Noise Control Basics

Noise Control & Soundproofing Basics – Memtech Acoustical

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Noise is simply defined as unwanted sound. A neighbor playing the TV too loud, a construction site near your home, or a factory machine that makes a “racket” in a plant are all forms of noise.

Sound (and noise) is measured in units of decibels, abbreviated as dB. Decibels are not linear increments (like a scale on a ruler or speedometer), but consist of a logarithmic scale, which means that the measurement increments are not “evenly spaced”. In units of decibels, 2+2 does not equal 4. If you add equal volume sound sources (e.g. 60 dB + 60 dB) the result is always 3 decibels greater than either of the single readings. For example, 60 dB + 60 dB = 63 dB and 80 dB + 80 dB = 83 dB. That might not make sense from a practical viewpoint, but it is the way that “sound addition” works. A more subjective view of decibels is expressed where each increase of 10 decibels equates to a doubling of volume to the human ear. Therefore, 60 decibels is twice a loud as 50 decibels 70 decibels is 4 times as loud as 50 decibels, etc. Most conversations and households (without the stereo blasting) measure in the 60 to 70 decibel range. Many factories are well above 90 decibels. The human ear can’t typically detect changes in sound of less than 1 decibel, and in many cases, individuals in noisy environments can’t even detect a 3 or 4 decibel change in background noise. But a change of 10 or 20 decibels is very noticeable, even by people who are not paying attention to the noise.

High levels of noise (above 90 decibels) can cause hearing damage, depending on how long a person is exposed. Federal, state and local governments, and many industries, establish noise standards to assure that hearing damage or high annoyance is avoided.noise-control-1

There is a great deal of jargon used in the field of acoustics. These include terms such as “weighting”, “peak”, “RMS”(Root Mean Square), “insertion loss” and a host of other terms that are usually confusing to the non-technical customer. To an engineer, these terms are essential to understanding and solving noise problems, but most customers simply want an answer to their problem. We are not going to attempt to explain every technical aspect of noise, but we can cover a few basics that relate to the noise control process.

In order to have noise, vibration must be present. It can be a shaking motor, a vibrating reed on a saxophone, or the vibration of a column of air in a pipe organ. As the vibration increases, the sound level will rise proportionally. Therefore, the best way to control noise is to limit (or dampen) the vibration. In many cases, this may be a simple process of placing a noisy machine on a rubberized base or stiffening up a loose structural element to keep it from shaking. However, in many cases, the cost and complexity of controlling vibration can become overly expensive and complicated. Therefore, we must attempt to reduce the flow of sound energy from the source to the user.

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Sound travels in waves from the source, similar to dropping a stone in a quiet pond. The waves move outward, losing energy as they travel, but instead of water, the sound waves consist of air molecules vibrating against each other. If we want to disrupt or reduce the strength of the wave, we have two choices: we must either block or absorb the wave energy to control the problem.

Blocking refers to the insertion of a barrier to prevent the wave from moving toward us. Absorbing refers to “capturing” the wave energy, and not allowing it to escape. The choice we make will depend on several factors

• The frequency (pitch) and content of the noise
• The environment
• The degree of noise reduction desired

Frequency

The first step in solving a noise problem is to understand the frequency content of the noise. Frequency is measured in Hertz, or number of vibrations per second. 100 Hertz can be described as a “rumble”, while 3,000 Hertz sounds more like a sharp whistle. Low frequency (bass) tends to cause nearby structures to vibrate, which means that we have to do a lot more than build a simple barrier to “block” the sound.

noise-control-4If you have ever stopped next to a car playing loud music, even though both cars have their windows closed, you will still “feel” the bass inside your vehicle. The low frequency sound vibrates the other car’s structure. The vibration of the car structure will cause the nearby air molecules to vibrate outside the car, and that wave will travel through the air to your windows, which will vibrate in response, and create a strong sound wave inside your car. You won’t hear any cymbals or singing, only the thumping bass. That’s because high frequency sound is effectively blocked by the windows. Only the low frequency energy will shake the structures and cause transmission of noise. You could stuff both cars full of absorbing foam and blankets, and the bass would still come through loud and clear inside your car. Resolving issues with low frequency noise requires significant modifications to the structures and/or adding mass to the structural system to reduce vibration.

 

Environment

Echo or reverberation can be a major irritant. In a gymnasium, the hard walls will reflect sound, which effectively increases the level of ambient noise.

noise-control-5In addition, it can “muddle” music and make it nearly impossible to understand someone speaking. This is where absorption techniques are used. By absorbing the sound through the strategic placement of panels and baffles, we can eliminate the “reflections” and greatly reduce noise and make it much easier to hear conversations. The design and construction of the baffles are critical to assure that they are absorbing the correct frequencies and will provide the desired result.

Required noise reduction

We often get requests to “reduce” noise, but how much reduction is needed? In some instances, where a regulatory auditor requires a specified noise level (for example 80 decibels), the amount of noise reduction needed is simple to determine. All we need to do is subtract the required noise level from the current noise level to set our reduction target.
noise-control-6 However, when a customer wants “less noise” in his home or place of business, the process becomes more challenging. A 10 decibel reduction will cut the subjective noise level in half, but in many cases this is not sufficient. The noise can often still be heard, and may only seem slightly less annoying. A goal must be established to assure that the solution will be acceptable to the customer. In these cases, we will often make an audio recording of the noise, and then play it back to the customer through a calibrated speaker/amplifier system to determine the level of noise that will be acceptable. Once a reduction goal has been established, then the engineering process begins. We must determine the appropriate materials and installation design to achieve this goal. Baffles, barriers, panels, and other engineered products can then be selected per the study. As a rule, the greater the noise reduction required, the greater the cost in materials to achieve the goal.

 

MEMTECH Acoustical is focused on providing each customer with the lowest-cost and most effective noise solutions possible. We are more than happy to delve into technical discussions with those who want to understand the measurement process and engineered solutions. However, for the non-technical customer who simply wants to reduce noise or reverberation in their home or office, we stand ready to understand your specific needs, and to deliver a solution that exceeds your requirements. Give us a call for a free quote!