Ambient noise attenuation: the key to reliable hearing assessment

Have you ever experienced loud interfering noise during testing? Has it ever been so loud that testing had to stop? Or have you ever wondered if the environmental noise (*also called background noise) was too loud but had no way to know for sure?

Well, you are not the only one. Ambient noise, also called background 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 noise has been the sound booth but rural and remote areas often don’t have access to sound booths. It isn’t practical to transport a sound booth to these areas nor is it affordable/ As a result access to hearing-related services is limited.

The focus should instead be on finding alternative and more effective way to attenuate noise. 

But, if we are to break with tradition, we first need to understand it.

What is ambient noise?

Ambient Noise is the sound level in a particular area. Ambient noise can transient or static. Transient noise changes over time while static noise is constant. A passing truck may affect the ambient noise levels for only a few seconds as it passes (transient) while static noise is consistent, such as that of an air conditioner, the hum of conversation or ambient music in a coffee shop, or sounds from the machinery in a factory.

What is noise attenuation?

Broadly, 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.

We are talking about sound or noise which is referred to as ambient noise, background noise or environmental noise. 

The blocking of noise is referred to as noise attenuation, acoustic attenuation or sound blocking. 

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 noise during testing may artificially elevate hearing thresholds due to a psychoacoustic phenomenon called masking.

Ambient noise blocking, or for the purpose of this article “attenuation”, is the key to accurate and reliable hearing tests.

When conducting diagnostic pure tone audiometry in clinical settings, noise levels should be low enough so as to allow testing of a patient down to -10 dB HL.

In occupational health or industrial health screening settings, these levels should be low enough a threshold measurement down to 0 dB HL.

But aren’t sound booths soundproof?

In short, no.

While there are some very quiet and very specialised rooms called anechoic chambers around the world that lay claim to being “soundproof”, the total exclusion of sound for audiometric testing is neither practical nor feasible when looking at the cost and infrastructure required.

Instead, we need to focus on removing unwanted 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 known as MPANL.

What are MPANLs?

Maximum permissible ambient noise levels (MPANL) standards stipulate the acceptable noise levels for an audiometric assessment room/environment. When the prescribed MPANLs contained in SANS, ANSI or any other standard is met, the clinician should be able to test without background sounds 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.

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 testing, they all present the same issue. Relying on the headset alone for attenuation is simply not good enough to produce reliable test results which often results in elevated thresholds and an increase in false referral rates. This ultimately increases the cost and time burden for patients on the back end.

The combination of a headset and a mini soundbooth provides far better attenuation than headsets alone and, if compliant with the standards, will produce reliable results.

It is also clear is the superior attenuation that a double-walled soundbooth provides. This allows patients to be tested down to -10 dB HL. If you have ever installed or investigated the cost of a double-walled sound booth you will also know the cost of this additional attenuation.

Added to this is the fact that the annual certification of sound booths are done once in twelve months, this means that clinicians are sometimes unaware of the transient noise that may be exceeding MPANL s inside the booths during testing.

So, while they do provide increased attenuation, double walled sound booths also massively increase cost which further decreases their suitability as a solution. 

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 during that day? Are they really a precise representation of the potential interfering effect of surrounding sounds on an individual audiometry assessment?

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

Getting back to practicality.

While this article is about attenuation, we cannot discuss it without discussing the impact the current solutions have on cost of infrastructure and limitations in providing greater access.

In both developing and developed countries, audiometric sound-booths are not readily available in schools, rural areas, clinics and nursing homes. (Lankford & Hopkins, 2000; Swanepoel et al, 2010a; Maclennan-Smith et al, 2013; Swanepoel et al, 2013).

The majority of patients in rural and remote areas have to travel a great distance (which is costly) to get access to audiological services, which is not always possible.

In thee instances where mobile booths are available, they seldom comply with MPANL standards, because they simply do not sufficiently attenuate low-frequency ambient sound. (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 where access to the same level of funding, expertise or build quality is limited. 

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

With the KUDUwave, we realized that we needed to focus on the real issue at hand. In this case, purely attenuating sound to the point where accurate hearing assessment 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 literally and figuratively managed 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.

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