anechoic chamber acoustic testWe have received several inquiries about constructing an acoustics testing lab.  There have been varying degrees of development of the plans we have received.  I thought this could be helpful, so I put together a list of factors to consider when designing an acoustics lab:

Inner and Outer Dimensions

The entire wall and acoustical absorption will be thick.  It is important to consider this thickness in your design.  The larger the room the lower the frequency you can test down to.  If you are working with tests involving low frequencies, you will need a relatively large room.  Generally, thicker wall construction is needed to reduce noise more and at lower frequencies.  In addition, acoustical absorption needs to be thick to absorb at lower frequencies.

Noise Reduction Needed

The noise reduction required will depend on the exterior noise levels and the noise levels needed inside the acoustic chamber.  You can measure the existing noise levels where the acoustic chamber will be located.  Be sure to measure during all possible noise conditions.  It is important to have the frequency data show how much noise is measured at each frequency band (1/3 octave or octave).  This frequency information will help determine what construction is needed.

Components – Doors, Windows

For reducing the noise from outside to inside, windows and doors require special attention.  The walls can usually be constructed of block, drywall, insulation and acoustic foam on the inside; however, special windows and doors are often times required.

It is important to know what access will be needed – size of items being tested, forklift access, how far open does the door need to go.  Perimeter seals, including magnetic seals can be used.  It may be necessary to get a sound rated door that has been tested to provide a certain noise reduction.

Visibility through windows will add cost and reduce the area of acoustically absorptive material; hence, windows should be kept to a minimum.  There are tests for the noise reduction provided by different types of glazing.  Be sure that the mullions are sound proofed as well.

HVAC / Ventilation

There are two issues with ventilation systems: noise from the system and the noise path.

Noise caused by the ventilation systems can increase the background noise level.  In addition, the path of noise from the HVAC system into the chamber may be a weak path.  Noise can break into the ductwork or travel from the ventilation system through the duct into the chamber.

Fiberglass lined duct or an acoustic silencer may be required.  The ventilation system manufacturer can provide the noise from the system.  The noise into the chamber can be calculated to determine if noise mitigation measures are needed.  The amount of noise reduction provided by the system can also be calculated to determine if noise mitigation is needed.

Vibration Isolation

The entire room can be floating on isolators, if needed.  Vibration isolators will prevent the transmission of vibration into the room.  Compressed fiberglass, a spring damper system, or rubber isolators can be used depending on the vibration source, level and frequency.  Target ambient vibration levels should be specified.

Lighting, Cables, Electric Power, Video, Floor Finish, Steel Rods

Planning is needed for any additional accessories for the acoustical chamber.  None of the accessories and finishes are difficult to include, but they must be identified.

Testing

After construction, the sound booth should provide free-field conditions.

Room modes create standing waves that influence free-field conditions.  Standing waves must not exist in the room.  Standing waves can be detected using the distance power law (the sound pressure level should decrease by 6 dBA per doubling of distance).  By plotting this decay vs. frequency, standing waves, if they exist, will be apparent.

Ambient noise and vibration levels should be tested to insure compliance with the specified levels.  The criteria is normally specified as levels for each 3rd octave band.  The duration of the measurement and indicator needs to be specified as well (Leq, Lmax, Lmin, L95 and L1).

 

Thank you for your interest in the Noise Engineers podcast.

Noise Engineers provides information and resources to help people address acoustical issues. In these episodes my goal is to provide resources, inexpensive tools, rules of thumb when dealing with acoustical issues. I would like to explain basic acoustic principles and answer any questions. I will describe actual projects to make this as practical as possible.

You can find our other podcasts at Noise Engineers podcast and iTunes

I welcome suggestions, comments, and questions. You can contact me on Facebook, Twitter , LinkedIn, email me (bill@ssacoustical.com) or call 520-979-2213.

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