Noise-induced hearing loss: The case of the 4 kHz notch
March 24, 2012 Leave a comment
A friend of mine recently asked: “Does a given frequency tend to cause hearing loss around that frequency? Or are high frequencies going to be lost first, no matter what the frequency of the offending sound?”
Thankfully, I could solve this mystery without getting the help of the admirable Hercule Poirot. The simple answer is – not always.
The complete answer is, of course, not so simple. Noise exposure may lead to a temporary or a permanent shift in hearing threshold or hearing sensitivity. A temporary shift in threshold is called a ‘temporary threshold shift’ (TTS). The amount of threshold shift, as I mentioned in a previous blog post, is usually a function of the level of noise and the duration of exposure. The spectral content (what frequencies the sound is composed of) and the temporal pattern of noise (how intermittent they are) also affect threshold shift.
If the level of exposure is moderate or the duration of exposure is short, then the maximum TTS occurs at the frequency of exposure (i.e. the frequency of the noise). If the level is higher or the duration of exposure is much longer, greater threshold shift occurs at high frequencies (4-6 kHz; see example below). If you were to expose the ear to a broadband noise (which has a flat spectrum), then you would see a dip in hearing at 4 kHz (Yost, 2000). In fact, that is exactly where a notch in the audiogram appears for someone who has noise-induced hearing loss. This amazing observation has a yet more amazing explanation. It is thought that this happens because of the unique resonance of the outer ear (Pierson et al., 1994).
What do I mean by the resonance of the outer ear? The outer ear refers to the pinna (the external ear that we can see), the ear canal, and the eardrum. Apart from funneling sounds to the eardrum, the outer ear gives a boost to sounds that enter the ear. This boost, as high as 20 dB at frequencies between 2-3 kHz, is termed as the outer ear resonance. When a broadband noise passes through the outer ear, it is amplified in this frequency range.
The amplified sound then passes through the middle ear (comprised of the middle ear bones, the Eustachian tube, and the oval window) and reaches the inner ear or the cochlea. Here it undergoes further transformation called the ‘half-octave shift’. This is a well-known phenomenon in auditory neuroscience that is attributed to cochlear mechanics. The half-octave shift refers to the fact that the point of maximum displacement in the cochlea for a given sound occurs half an octave above the frequency of the sound. What that means is that for a 3 kHz sound, the point of maximum displacement will be at 4 kHz, resulting in the 4 kHz notch! If you would like to know more about how the ear works, check out one of my favorite websites on the anatomy and physiology of the human ear here.
Bon jour, mon ami.
- Pierson, L.L., Gerhardt, K.J., and Rodriguez, G.P. (1994). Relationship between outer ear resonance and permanent noise-induced hearing loss. Am J Otolaryngol. 15, 1, 37-40.
- Yost, W.A. (2000). The abnormal auditory system. In The Fundamentals of Hearing. Academic Press, London, pp. 256-258.