Distribution of hearing loss

Question

Low-frequency sounds are more penetrating, damaging. Hearing damage caused by blasts typically occurs at frequencies around 2 - 8 kHz, while age-related hearing loss starts at the high frequencies. The frequency range for noise-induced damage is biased upward, because the low-frequency conductive structures damage first.

https://www.entandaudiologynews.com/features/ent-features/post/effects-of-blast-and-acoustic-trauma-assessment-of-hearing-status-on-war-veterans

Why does hearing loss occur at high frequency first? It should occur at low frequency, because that is the more damaging sound. The distribution of hearing loss is totally incompatible with the noise exposure explanation. It could be due to growth factors, i.e., the high frequency parts die first because more distal.

Answer 1

Short answer
Noise-induced hearing loss affects primarily the mid-frequencies, because the inner ear is most sensitive to these frequencies.

Background
Noise-induced hearing loss occurs typically in the mid-frequencies (Rabinowitz, 2000), typically around 4 kHz (3 - 6 kHz), see Fig. 1. This can be explained by the fact that the human ear is most sensitive in this frequency range (Fig. 2). In turn this relates with the frequency range where speech understanding is most important, namely between roughly 2 and 6 kHz (Killion & Mueller, 2000). Hence, noise exposure neither targets the base, nor the apical parts of the cochlea, as claimed by asker and answerer, respectively. High-frequency hair cells in the rat cochlea have been shown to be very susceptible to damaging noise, but the low-frequency hair cells may go unscathed (source: eMedicine).

The other answerer claims that

parts of our hearing apparatus responsible for perceiving these [low-frequency] sounds are more fragile...

Hence, the claim that low-frequency regions in the cochlea are more sensitive than the more basal parts is not a claim supported by the bulk of the literature. In fact, as a rule of thumb, cochlear damage (inner ear damage) occurs most frequently first in the base, where the high frequencies are resolved. This holds for age-induced HL where high frequencies are lost first (Ciorba et al., 2011), as well as chemically-induced HL (Campo et al, 2013, barred some specific organic solvents that apparently do not follow this rule (Cappaert et al., 2002.

^{Fig. 1. Typical audiogram of noise-induced hearing loss showing loss of functional hearing at the mid-frequencies (Rabinowitz, 2000)}

^{Fig. 2. Relation between perceived loudness and intensity (equal-loudness curves) for a normal-hearing person. source: Lumen Phsyics}

<sub>References
- Campo et al., Dis Mon (2013); 59(4): 119–38
- Cappaert et al., Neurotoxicol Teratol (2002); 24: 503–10
- Ciorba et al., J Laryngol Otol (2011); 125(8):776-80
- Killion & Mueller, The Hearing Journal (2010); 63(1): 10-7
- Rabinowitz, Am Fam Physician (2000); 61*(9):2749-56</sub>

Answer 2

One has to distinguish between the frequency of a sound, its intensity (i.e., energy that it carries), and its loudness.

• Frequency, known more conventionally as the pitch, is what distinguishes high and low sounds. E.g., male voices are on average lower than female voices, i.e., male voices have lower frequency. Frequency is what changes when we hit different keys of a piano or different strings of a guitar. Frequency is measured in Hertz (Hz).
• Intensity is the energy carried by a sound wave. Intuitively it is clear that the sounds of the same frequency can have different intensity, e.g., the same key of a piano can be hit stronger or weaker, producing a louder or quieter sound. It is the intensity rather than the frequency of a sound that causes hearing damage. Intensity is however not the same as loudness, as I explain below. Intensity of a sound is measured in bells/decibells (dB).
• Loudness is our perception of a sound. Two sounds that we perceive as equally loud, do not necessarily have the same intensity. In fact, the higher frequency sounds have much higher energy/intensity when perceived as equally loud with lower frequency sounds.
• Spectrum While musical instruments can produce continuous sounds of a specific frequency, this is not the case for many sounds that we encounter in a real life. Combination of frequencies in a sound is called its spectrum. Blasts are an extreme case of sounds that are very short and therefore combine large number of frequencies, ranging from very low to very high ones, all having approximately the same intensity.

Since lower frequency sounds generally carry lower energy, the parts of our hearing apparatus responsible for perceiving these sounds are more fragile, i.e., more easily damaged by high energy sounds. While blast is a complex physical phenomenon, it can be roughly viewed as a sound carrying many frequencies of the same high intensity, which will thus cause more damage to lower frequency hearing. On the other hand, the loss of hearing with age reflects the "normal wear" by hearing the sounds that our ear is adapted for. Throughout our life the high frequency hearing deals with sounds of high energy and wears out faster.

Disclaimer: I admit that my answer reflects my stronger background in physics than in biology, and I will readily accept help in improving the last paragraph, (making it more precise).