Ambient noise attenuation: the key to reliable hearing assessment

Have you ever experienced loud interfering background noise during testing? Has it ever been so loud that testing had to stop?  Well, you are not the only one.  Background noise, also known as ambient noise, is a huge concern as not only can it affect the quality of your results, but can also create a knock on effect for the patient.

Traditionally the go-to solution for blocking ambient noise has been the sound booth.

While tradition has its place, it is not always practical nor possible. Rural and remote areas have little to no access to sound booths, and it is impractical to transport a whole sound booth to these areas. As a result access to hearing-related services is scarce if anything.

Instead, the focus should be on finding alternative and more effective forms of attenuation rather than relying on the sound booth alone, which represents only a single viewpoint for sound blocking.

In order to break with tradition, we first need to understand it.


What is ambient noise attenuation?

The term “attenuation” refers to the reduction of the force, effect, or value of something. In some cases, this could be a virus or bacterium. In this case, it is ambient or background noise.

By removing or suppressing background noise we ensure that there is no interference with a patient's ability to hear the tones presented while testing. The presence of ambient noise during testing may artificially elevate hearing thresholds due to a psychoacoustic phenomenon called masking.

Ambient noise attenuation, or for the purpose of this article “attenuation”, is the key to accurate and reliable audiological assessments. When conducting diagnostic pure tone audiometry in clinical settings, ambient noise levels should be low enough so as to allow testing of a patient down to -10 dB HL. In occupational settings, these levels should be low enough for threshold measurement down to 0 dB HL.


But aren’t sound booths soundproof?

anechoic chamberIn short, no. While there are some very quiet and very specialised rooms, more correctly anechoic chambers, around the world that lay claim to being “soundproof”, the total exclusion of ambient noise for audiometric testing is neither practical nor feasible when looking at the cost and infrastructure required.

Instead, we need to focus on removing noise to the point that thresholds are not influenced by masking. Luckily, there are several standards available for just that purpose. The South African National Standards (SANS) and American National Standards Institute (ANSI) both indicate the acceptable noise levels that allow for accurate hearing testing.

These standards specify the maximum permissible ambient noise levels  for an audiometric assessment room/environment. When the prescribed MPANLs contained in SANS, ANSI or any other such standard is met, the clinician should be able to test without ambient noise affecting the results.


Attenuation of ambient noise during hearing assessment

Traditional testing relies on a standard set of tools such as supra-aural, circumaural headsets and a sound booth or sound treated room.

The following image shows the attenuation capability of each of these common tools as well as one not so common tool, the KUDUwave.

attenuation graphic.jpg

As you can see, supra-aural and circumaural headsets provide very little attenuation as they are designed for use in a treated environment or in environments that are already so quiet that further attenuation would not be necessary. While these headsets seem to perform better in the higher frequencies than the lower frequencies, for accurate and reliable diagnostic results, supra-aural and circumaural headsets should be used in noise environments similar to a very quiet room or in a sound booth.

It is clear that while there are multiple cell phone apps available on google play store and iTunes for hearing screening, they all present the same issue. Relying on the headset alone for attenuation is insufficient to produce reliable test results and often ends up in elevated thresholds and an increase in false referral rates which ultimately increase the cost and time burden for patients on the back end.

The combination of a headset and a mini sound-booth provides far better attenuation than headsets alone and, if compliant with the standards, will provide reliable and accurate results. Also obvious is the superior attenuation of a double-walled sound booth. This allows for a patient to be tested down to -10 dB HL in clinical settings and down to 0 dB HL in occupational health settings. If you have ever installed or investigated the cost of a double-walled sound booth you will also know the difference in cost of this incredible attenuation. Furthermore, annual certification of sound booths are done once in twelve months, this means that clinicians caring out hearing assessments are sometimes not aware of the transient noise that may be exceeding MPANLs inside the booths during the assessment of patients. Unmonitored ambient noise exceeding these levels may artificially elevate the patient's thresholds. Although the above-stated sound booths provide better attenuation to allow for specific and reliable results, there is increased cost and decreased accessibility to hearing testing when they are used as means of providing attenuation.


Let’s talk about the thick green lines.

When comparing the attenuation levels of ordinary equipment it is clear to see just how the KUDUwave outperforms the standard toolset.

By employing a combination of insert ear tips and specialised cups the attenuation is comparable to that of a headset and mini-booth combination. 

Studies have shown that double attenuation, provided through the simultaneous use of insert foam ear tips and circumaural headsets, provides superior attenuation values compared to those provided by mini sound-booth and supra-aural headsets alone (Berger, 1984; Berger et al, 2003). The KUDUwave uses this double attenuation strategy to provide increased attenuation during audiometric testing (As indicated on the image above). In fact, by combining a KUDUwave with a standard booth, you can achieve the attenuation ability of a double-walled booth.

But there is more to this problem than meets the eye when you consider just how busy and loud everyday environments are. With all this activity surrounding us at all times, noise levels can vary from one minute to the next. Passing foot traffic, people talking, telephones ringing, vehicles zooming past, and even the occasional overly vocal pigeon are just some of the unexpected yet commonplace events that generate noise and affect hearing test results.  

If noise levels constantly shift and change throughout the day, why then, are sound booths certified at only a single point in time? Are they really a precise representation of the potential interfering effect of ambient noise on an individual audiometry assessment?

As an additional step, the KUDUwave monitors environmental ambient noise in real-time, to ensure that the clinician is aware of the ambient noise exceeding MPANLs during testing, and offering a simple, real-time solution to this issue by monitoring ambient noise as and when it happens.


Getting back to practicality.

While this article is about attenuation, we cannot discuss it without discussing the impact of its inherent cost in infrastructure and access.

In both developing and developed countries, sound-booths are not always available in the spaces they are needed such as schools, rural areas, clinics and nursing homes (Lankford & Hopkins, 2000; Swanepoel et al, 2010a; Maclennan-Smith et al, 2013; Swanepoel et al, 2013). Majority of patients in rural and remote areas have to travel a great distance (which is costly) to get access to audiological services - this is not always possible.

When there are mobile booths available, they seldom comply with maximum permissible ambient noise level (MPANL) standards, due to their inadequate attenuation of low-frequency ambient noise (Lankford et al., 1999; Frank & Williams, 1994).

In fact, another study found that in the USA only 33%  of mini sound booths and 14% of sound booths for screening diagnostic testing complied with the ANSI standard for background noise attenuation

One can only imagine the ratio in developing countries that don't have access to the same level of funding, expertise or build quality.

"We cannot solve our problems with the same thinking we used when we created them."  Albert Einstein

With the KUDUwave, we relised that  we needed to focus on the real issue at hand. In this case, purely attenuating sound to the point where accurate hearing assesment is possible.  Focusing on historic solutions such as the sound booth, would have left us firmly and ironically "inside the box". 

Instead we focussed on  the practical implications of achieving the desired output while eliminating the downsides, in this case we have  both literaly and figuratively manged to get "out of the box" by going completely booth free.

For more detail on MPANL have a look at article we recently published on  maximum permissible ambient noise levels

Find Out More About KUDUwave Audiometers Thinking out the booth

 

If you have any thoughts or questions on this topic, why not discuss them in the comments below?

Hlolo Ramatsoma

Written by Hlolo Ramatsoma

Hlolo is a clinical, research & support Audiologist at eMoyo. He is involved in many parts of the business, from consulting to R&D to supporting and training customers. He earned his BSc in Audiology from the University of Cape Town and is an experienced clinical audiologist specialized in ototoxicity monitoring, product specialist and training audiologist.

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