Why Hearing Aids Alone Can't Always Provide Speech Clarity! 

Some of the most common complaints I hear about hearing aids not working correctly is not the fault of the hearing aids but the laws of physics. 
The Complaints are:

• I still can't hear my wife/husband when they talk to me from the kitchen.
• The television is still not clear or I still have to turn the television up so loud it drives my spouse from the room.
• I still can't understand my friend across the table in the dining room at the retirement center.

These are some of the most common complaints I field on a daily basis and they all revolve around either range or noise but most often a combination of both. 

Review of what hearing aids can and can't do:

Today's modern digital hearing aids are wonderful devices and we have come a long way from what our parents use to wear.  With this in mind we must still remember that a hearing aid is exactly what we call it, an aid to hearing and not a device that regenerates our ears into being twenty-years old again.  The hearing aid can only apply pressure to our remaining auditory area.  It can not activate areas in the cochlea that have been so damaged or beat down there is nothing left to work with. In addition the hearing aid must work in every listening environment you move into, from quiet to very noisy, so is is kind of like a Jack-of-all-Trades and a master of none.  Fitting hearing aids has always been a balancing act of giving you the amplification you need to understand in your most important listening environments while while making loud or hostile listening environments bearable without having to turn the hearing aids off. 

With analog hearing aids we basically had a single program or setting that had to work everywhere you went and therefore worked well in none of the places you went.  Today's digital hearing aids equipped with multiple programs can be tuned to function, read amplify, differently in each of the programs.  This means we can have several smaller compromises with each program working better in the listening environment it is designed for.  But there is still two small problems we have yet to overcome and both of these problems relate to the laws of physics that can't be changed with simple electronics and computer programming.  The first of these laws we must deal with is called the Inverse Square Law of Acoustics.  This law states that every time we double the distance between the source sound and the receiver/listener the energy of the sound will drop by 6 decibels (dB).  The average male speaker is measured as talking at 65 dB SPL (sound pressure level) at three feet from his lips and the average female speaker has a pressure level of 62 dB SPL at 3 feet from her lips. 

The second big problem that needs to be addressed is that of noise.  The best definition of noise that I have ever heard is "Sound you don't want to hear right now".  With this definition noise can be tires on pavement, clatter of dishes and silverware, furnace/air conditioner fan, dishwasher, refrigerator compressor, television, small children or other conversations you are not interested in at this moment.  All of this adds to the background noise level in the soundscape you are trying to listen in and this background noise interferes with speech clarity.  The louder the background noise and the soft the speech sound you are interested in the less clarity there will be.  This is computed as a ratio that is given in a plus or minus value as measured in decibels (dB) and call the speech to noise ratio (SNR).  The higher the value the better the clarity.  Studies have shown that for normal hearing people they need a +10 dB SNR for best clarity while a person with a mild to moderate hearing loss does best with a +20 dB SNR but can generally get about 80% word recognition (all other factors being okay) at a +15 dB SNR.  At a +10 dB SNR this same person will achieve somewhere between 45% - 65% word recognition (all other factors being okay).  A person will have a very hard time following a running conversation if they are missing between 1/3 to 2/3 of what is being said. 

Noise Levels

Before we talk about noise levels we should mention what the range of hearing is for a normal hearing person.  Textbooks define the range of human hearing as being 30 feet in a quiet listening environment (we will define sound levels for listening environments shortly).  Effective communication range would be approximately 20 feet.  At 30 feet people yell at one another.

A quiet environment is never really quiet but we define a quiet environment as being less than 45 dB of ambient room noise.  We define a moderate noise environment as being between 45 dB and 60 dB with a noisy environment being between 60 dB and 75 dB.  Environments over 75 dB are considered hostile and  are difficult for everyone to listen in but are impossible for a person with a moderate or greater hearing loss to follow a conversation in.   The following chart can give you an idea of the problems encountered as noise levels go up with range.  We will use the female speaker for this example. 

As can be seen from the above chart the Speech to Noise Ratio falls off very quickly with range.  Remember that normal hearing people need a +10 dB SNR to understand easily and a person with a moderate hearing loss needs at least +15 SNR and a +20 SNR would be best.  As can be seen the range of effective communication shrinks rapidly with distance and as the range shrinks the clarity also is vastly reduced.  Most normal conversations are carried on at a range of eight to fifteen feet.  As can be seen from the chart above the speech to noise ratio is varying between +3 dB to -6 dB in a moderate noise environment.  We used 53 dB as the average noise for a moderate noise environment and guess what?  That, or slightly greater, is the noise level in the average living room in the evening after dinner.  Even a normal hearing person will have to work hard to understand speech in the living room, but they will since they can operate down to about a 0 dB SNR with a  lot of effort but a person with a mild hearing loss will be completely lost.  And this is true for people wearing hearing aids since the aid only works on the sound that is picked up by the microphone of the hearing aid and what we are talking about is the effects of range and noise on the speech we are interested in before it is picked up by the microphone. 

Yes, hearing aids are a great advantage, and in many cases the only help available, to improve speech clarity once the sound is introduced into the hearing aid but since the problem, in a lot of cases, is the effects of noise and range on speech before it reaches the hearing aids we  need to look for ways to overcome, go around or remove the effects that acoustical physics has on what we want to listen to and understand.  To achieve this we need to look at Hearing Assistive Technology and especially FM and BlueTooth systems focused on these areas.  That is what this section of our website addresses in depth.