Modified from an article by Arvle Marshall, DVM PhD In the Summer 1997 issue of Sports Medicine Program Newsletter.
The hearing range and sensitivity of an animal can best be depicted with an audiogram, which is a graph indicating both sensitivity (vertical axis) and
frequency (horizontal axis). Figure 1 shows a normal audiogram of a human and a
dog. Note that there is a middle zone of best hearing sensitivity. For humans
this is around 2,000 Hz and for dogs is 8,000 Hz. Also, the upper frequency
limit of sensitivity for the dog is much higher than for humans, dogs responding
to about 48,000 Hz. The human limit is about 20,000 Hz. At both extremes the
sound has to be much louder than the sound needs to be for detection at the
range of best hearing sensitivity. This is the reason the “silent whistles” work
with dogs. They are more tuned to the dog’s best hearing range than to the
humans’.
Hearing loss or deafness can be classified as to cause: induced or
congenital. Induced hearing loss can result from exposure to continuous loud
noise, and to exposure to ototoxic drugs or chemicals. Obviously, infections of
the middle and inner ear can result in hearing loss. Impulse noise is the single
most damaging kind of noise.
Some protection from loud continuous noise is naturally afforded by the
acoustic reflex. This is the reaction of the tiny uscles in the middle ear which
act to reduce the efficiency of sound transfer. This reflex occurs within 50
milliseconds following the onset of a loud sound. Impulse noise is sound which
rises to it’s peak intensity much faster than the acoustic reflex can operate.
Thus impulse noise can damage the receptor cells for hearing.
dB is a measure of sound intensity. Noise levels above 100 dB can damage
hearing over time. Noise produced by a gun is impulse noise, and it has a rise
time of about 50 microseconds and a duration of less than 1 millisecond. Note
that this is1000X faster than the acoustic refles can react. Also the peak
intensity of a 12 gauge shotgun is in the range of 140 dB. Even a .22 rifle
produces about 125-130 dB peak intensity, but it is perceved as “quieter”
because the duration of the sound is shorter. Impulse noise above 130 dB can
produce instantaneous hearing damage. The dog experiences the same impulse noise
that the shooter does. If the shooter chooses to wear hearing protection, what
bout the dog in the same environment? We do know from experience with hearing
aids in dogs, that they are easily trained to wear the foam type ear plugs,
which muffle the noise by 20 to 30 dB, enough to protect the ear from damage.
Hearing loss in older dogs (aged 12 to 15 years) is common. One study
indicates that this loss is due to cochlear degenerative changes, not to
otoscleroses (stiffening of the joints between the ossicles of the middle ear).
this is most likely a nautral consequence of aging.
Other common causes of hearing loss are drugs and chemicals that can be
ototoxic. Ear cleaning solutions can enter the middle ear if the tympanic
membrane is not intact, or is weakened due to infections in the external ear
canal. From the middle ear these chemicals can soak through the oval or round
windows into the inner ear and damage the hair cells. Several drugs,
particularly the aminoglycoside class of antibiotics, tend to concentrate in the
fluids of the inner ear and can damage the hair cells.
Congenital or genetic deafness is related to the lack of functional
melanocytes (pigment-producing cells) in the stria vascularis. The stria
vascularis is the part of the inner ear that produces the fluid, endolymph, that
is necessary for the hair cells to function. Congenital (genetic) deafness is
seen in breeds of dogs that have the piebald or Merle hair coat coloring
patterns. This defect is present at birth and is characterized by an absence of
hair cells in one or both ears.
Testing for hearing can be easily done by the owner, if attention is paid to
some subtle behavioral signs. A normally hearing dog will respond to any unusual
sound introduced into its surroundings with recognizable behavioral signs. The
dog will reflexively lift the pinna of its ears and may turn them toward the
source of the sound. Most dogs will show signs of alerting to the sound. It is
difficult to test the hearing of a dog in a veterinary clinic, because
everything in the environment is unusual to the dog. The trick is to introduce
an unusual sound into the dog’s normal surroundings when he is relaxed and other
wise doesn’t have his attention on something. Generally, if the dog hears the
sound the alerting response will occur. If the dog has lost some hearing
ability, then the test sound will have to be lounder than normal to elicit a
response. Also, the test works best if the sound is a very shourt one and at a
pitch of about 8,000 Hz, that is: a short “tweet” with a “silent whistle.”
The hearing of a dog can be assessed with a very specialized
electrodiagnostic test at Auburn’s Veterinary College. This is the brainstem
auditory evoked response (BAER) test, which records the brain’s response to a
click sound delivered to each ear with headphones. The BAER is very reliable and
can accurately determine if the dog has lost some or all of its hearing in one
or both ears. And, it does not require a behavioral response of the dog to
determine the result; thus if is an objective test rather than a subjective
test.
A research project that is needed at present is a survey study of the hearing
of working gun dogs; especially a comparison of these dogs at a young age, in
their prime and at an older age with dogs who have not been exposed to
ear-damaging noise. With this information improved recommendations can be made,
based on defendable data, for the protection of the hearing of these animals.
This should afford practical recommendations for extending the hearing part of
their useful working life.