Room Construction and Characteristics

Common problems with sound booths

"Construction Safety Factor"

Moving a sound booth

The most accurate sound field test measurements are those which are conducted in an anechoic room.  In an anechoic testing room, the SPL varies smoothly and will vary gradually as the distance from the speaker is increased (Walker et al 1984).  Unfortunately, the use of anechoic rooms are not cost effective in a clinical setting and most sound rooms will contain some reverberation.  Because of this less than perfect testing environment, it is important that Audiologists understand some of the limitations and do's and don'ts regarding their sound booths.                

A study by Siegenthaler (1981) and Frank and Williams (1993), found very few hearing test booths in clinical use, meeting the ANSI S3.1* ears covered standard.  Audiologists can help reduce the chances of measurement variability if they adhere to the following simple steps.  Audiologists need to be aware that practically all attenuation measurements reported by the manufacturers of hearing test booth, are taken in a controlled laboratory environment.  These controlled environments are rarely encountered in the clinical setting and this is a reason why manufacturer specs may differ from clinical results.  To assure compliance with current standards, it is important that a careful acoustical evaluation of the clinic environment (where the audiometric room will be installed) be performed.  This evaluation needs to be done by qualified professionals using proper instrumentation before the booth is actually installed. 

Some of the more common areas of problems in tests booths are the door seals, door fit and ventilation fans.  It is important that these parts of the booth are thoroughly maintained.  The greatest cause of ambient noise in a test booth seems to be caused by the ventilating fans (Lankford et al 1980).  A study by Lankford et al. 1980 found that every booth tested which was over 5 years old had faulty door seals.  Simple maintenance and modifications have proved to reduce background noise in the room and helped improve the noise attenuating characteristics of the booth.  In a study by Frank and Williams (1993), 136 test rooms were evaluated, revealing that almost all test rooms contained excessive ambient noise at the lower frequencies (125-500 Hz) caused by the ventilation system.  They also found that most Audiologists knew that the ventilation system caused much of the ambient noise, and therefore conducted their ambient noise measurements with the fans turned off.  This is contrary to ANSI standards, which requires that fans be turned on (so as to best capture the typical test setting). 

In the Frank and Williams (1993) study, they found that replacing the door seals and/or tightening the door latch significantly lower the test room ambient noise SPLs by as much as 10 to 12 dB in the lower frequencies (for rooms that did not pass ANSI standards).  Audiologists need to be aware of the likely problem areas in their test booths so that they can present questions or constructive comments to those professionals who calibrate and help maintain their booths. 

Even if all the necessary steps are taken to ensure that  the sound booth adheres to manufacturer specs, Audiologists need to be aware that ambient noise levels are rarely steady and tend to fluctuate and increase over time.  Where a single walled test booth sufficed in previous years, a double walled equivalent may be required to deal with the changing environment.  Trained professionals should check test booths twice a year or at a minimum, annually. 

An Audiologist thinking of buying a new test booth should purchase a booth with enough  noise reduction  to achieve a noise measurement that is 10 dB less than the ears-open specification of ANSI S3.1-1991.  The reason for this is to account for the fact that real people are installing the sound booth.  This is know as the "Construction Safety Factor".  The Construction safety factor is a 10 dB window which accounts for the effects of adding doors, ventilation, windows etc… to the sound booth.  Keep in mind that the manufacturer specification refers only to the attenuation characteristics of the wall and not the actual sound booth.  Therefore, the attenuation characteristics for a 2.5” wall is actually closer to 10 dB less than the manufacturer specification.     

Audiologists should also have the supplier verify the noise reduction characteristics of the booth once it is installed.  In addition, the ambient noise levels of the booth with the ventilation turned on and off should be measured and meet the ANSI S3.1 - 1999 ears-open criterion. 

Relocation of a sound booth should be avoided if possible.  When a sound booth is disassembled and reassembled, it is highly unlikely it will produce the same attenuation characteristics as it did before.  A major concern in taking a room apart is the bending of the walls.  When a room is initially assembled, acoustic grade caulking is used in the construction process.  This caulking must be completely removed, walls properly cleaned and then replaced with new caulking when reassembled in the new location.  If you use a contractor to assemble your room who has never installed a sound booth before, then it might be wise to use a “Construction Safety Factor” closer to 15 dB.   

* The ANSI 3.1 Maximum Permissible Ambient Noise Levels for Audiometric Test Rooms (MPANLs) standard has since been updated to ANSI S3.1 – 1999.   

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