Age related hearing loss is called Presbycusis and refers to natural deterioration of our hearing as we age. It is the most common cause of hearing loss and often starts in your 50’s. It progresses to effect one in three people over the age of 65 and one in two people over 75 years of age. The rate and level of change can be influenced by hereditary, environmental factors including medications, chemicals and noise exposure. Over time it increasingly impacts your ability to understand speech. We are lucky to have modern science and medicine on our side to diagnose and manage the adverse effects of hearing loss.
Hearing loss Impact on Speech Understanding
Age related hearing loss usually starts in the higher frequency range above 2000Hz (2KHz). Over time it progresses to affects the lower frequency ranges. Because it affects the high frequencies first you lose the ability to hear the higher frequency sounds of speech such as consonants “s”, “t” “h” or “th”. You may hear that someone is speaking but just not be able to understand what’s being said. Ironically most of the valuable speech information is contained toward the higher frequency range where our hearing deteriorates first. Speech sounds at the beginning and ends of words help us differentiate words from one another such as “mat” vs “map” or for plural words “ship” vs “ships”. Not hearing the high frequency speech sounds properly can lead to misinterpretations or misunderstanding of what’s been said.
On an almost daily basis I hear people describe how a family member or work mate “mumbles” or doesn’t speak clearly. Although this may be partially true it is a symptom of high frequency hearing loss.
As well as being higher in frequency, the speech sounds at the beginning and ends of words are largely unvoiced. Unvoiced means they are produced by passing air through the mouth, such as “Ss”, “Tt”and “Th” whereas voiced speech sounds such as “Uh”, “Aa” or “O” require sound to be made by your vocal chords. Speech sound made without involving the vocal chords have less energy (are softer) so they diminish quickly across distance and are easily masked over by background noise. The relative energy levels and pitch of speech can be visually shown on a graph known as a Spectrogram.
Visually representing speech (Spectrogram)
The following figure 2 shows a spectrogram of a female speaker saying “children like strawberries” You can see the harmonics of the vowel sounds such as “aw” in strawberries produced sounds appearing with a stripy appearance toward the lower frequency range. They are also darker showing these sounds carry more energy. Whereas consonants like“d” and “b” appear as short sharp non-harmonic bursts in the higher frequencies and are lighter showing they possess less energy.
Whole system and regional changes
Age related hearing loss occurs from changes to any part of your Auditory system. This includes your Outer, Middle and Inner Ear and also to the nervous system right through to the Auditory Cortex of your brain. One of the most consistent aspects of Presbycusis is a loss of function of the specialised cells in the inner ear called the Outer Hair Cells (OHC). These are also called Stereocilia. Stereocilia transform the mechanical energy of sound waves into electrical signals which ultimately leads to an excitation of auditory nerves. The stereocilia of the outer hair cells have the the unique ability to move and twitch in response to sound. They twitch to actively amplify the very faintest sounds and to act as shock absorbers to dampen relatively loud sound. This also acts as a protective mechanism for the inner ear. It gives normal hearing people a broad dynamic range. That is the ability to hear very faint sounds with little energy and cope with much higher energy or louder sounds.
Deterioration of the outer hair cell function with age reduces our dynamic range resulting in hearing loss. It can also reduce our tolerance to loud sound and can result in extra sensitivity to loud sounds. There is a condition called hyperacusis (hyper-acusis) which is a hypersensitivity to louder sounds.
Why high frequency hearing loss occurs first
Although all structures of the auditory system deteriorate, much of the high frequency hearing loss occurs due to changes in the mechanical structures and physiology of the inner ear within the Cochlear. The Cochlea is the small snail shelled shaped organ of our inner ears where sound vibration is converted into neural impulses as shown in the figures 3A & 3B below.
The early portion or basal end of the cochlear has the role of picking up the higher frequency sounds (as high as 20,000Hz). As sound vibration moves further down around the spiral you get lower and lower frequency sounds being picked up by the adjacent nerves. It is a piano keyboard like layout. We tend to get most of the mechanical wear and tear and physiological changes occurring in the early portion of the cochlear. This is partially because all sound vibration has to pass through this region of the cochlear. We refer to a hearing loss related to changes of the inner ear or beyond as a sensori-neural hearing loss. This is more of a permanent type of hearing loss and can impair or ability to understand speech or speech intelligibility. It seems ironic that the big proportion of speech information (with less energy) occurs toward the higher frequencies where our hearing sensitivity naturally diminishes first.
What your Audiogram Shows
The following graph is called an Audiogram which forms part of the battery of test results obtained when you see an Audiologist. This graph is a measure of your hearing sensitivity to tones presented to each ear across the frequency range. Usually ranging from a low 250Hz right up to at least 8000Hz or 8KHz. When the tones are presented you are requested to respond usually with a push of a button to every tone you hear including the very softest tones you can hear for each tone presented. Your responses to the softest tones you hear at each pitch are plotted on the Audiogram. This minimum volume for you to hear a tone at a particular frequency is called your threshold for that tone and is represented on the graph by an X for your left ear and a O for your right ear.
The Audiogram in the following example shows the frequency ranges for varying levels of hearing loss. Normal hearing is considered when the tones are heard at 25dB or less. If you had normal hearing across the full frequency range you would have a fairly straight line lying horizontally across the light blue shaded area below.
As you can see in the above Audiogram, this person’s hearing starts in the normal range for the lower tones tested and his hearing sensitivity slopes down to a severe hearing loss in the higher frequencies. As hearing sensitivity can vary so markedly across the frequency range we avoid talking in percentage of hearing loss. We prefer to describe the hearing configuration. In this example we’d say this person has a mild to severe hearing loss. This Audiogram only shows the person’s air conduction thresholds so we can’t tell whether the hearing loss is purely inner ear nerve related (sensori-neural) or whether there’s a conductive component to the hearing loss. i.e. sound not getting through the ear drum and middle ear system as efficiently as it could. Further testing to include bone conduction testing would need to be carried out to determine the proportion of each. However, this does look like the typical configuration of progressed age related hearing loss.
The Speech Banana
Displayed on this Audiogram in red letters are the speech symbols. The speech symbols represent where speech sounds occur in pitch and volume at average conversational levels, which is at about 50dB HL. Softly spoken people may be around 45dB but it averages 50dB. You can see that the individuals hearing levels are above the lower frequency speech sounds that carry more energy including the vowels “a”, “o” and “ei” and even the deeper consonants such as “v” and “z”. He will hear these quite readily at average conversational levels. You can also see that his Audiogram for both the left and right ear dive beneath the higher frequency consonants of “sh”, “ch” and “g” and even further below the very high frequency ‘s’ ‘f’ and ‘th’ speech sounds. These sounds will be particularly difficult for him to hear at average conversation levels especially if there’s background noise present.
Speech cues in higher frequency Harmonics
Studies have verified that there are speech cues in the high frequency harmonics in speech that provide additional separation and localisation of speech from other competing sounds around us. Having access to these higher frequency speech cues can reduce listening effort, improve comfort and speech understanding in background noise. Higher end hearing aids usually have a wider band-with up to 10,000Hz and even 12,000Hz. This helps them detect and amplify these higher frequency speech cues as required. Providing these higher frequency cues improves localisation. Localisation is the ability to pin point where sounds are coming from. There’s also improved separation of speech from competing noise and reduced listening effort.
For a good article on the “Hidden dangers of untreated hearing loss” please see https://www.healthyhearing.com/help/hearing-loss
If you or a loved one are experiencing the adverse effects of hearing loss I would urge you to broach the topic with them. The best first step is to book in for an In-Depth hearing test. Please see our page In-Depth Hearing Test to find out more. It is a good starting point or baseline in the journey to optimal ear, hearing and communication health. Contact us today!