One of the members here referenced a paper from German investigators dealing with "ultrasound". I read the paper and thought that providing a summary here would be helpful to the forum members. I apologize for not recalling who referenced it here. I will link to the paper at the end.
Kühler et al., from Berlin, conducted an experiment where they wanted to determine whether ultrasounds could cause damage. It was basically a "safety" study but they needed to establish the frequencies that we could hear and also the thresholds for hearing them. They had a very sophisticated protocol and technical equipment validated and reliable. In their second part, they wanted to measure if there was any brain activity associated with "ultrasounds" even if the subjects could not hear it. I will note summarize that part but suffice to say that it doesn't.
They made sure they had a "normal hearing" population at baseline and that the tests were unmasked. That is, the subjects would hear only the pure frequencies they were testing. They went to great lengths to insure the quality of every step of the experiment. They selected 26 subjects with mean age 24. The oldest was 33. This is important as we can establish these thresholds in a young population. Whatever happens in those older, should not be extrapolated from this study. They also had pure unmasked test tones to reduce potential distractions or "harmonics" from the test frequency.
The investigators started with tones of 14kHz and went up all the way to 24.2kHz. All 26 subjects could hear the 14kHz tone and the threshold of audibility was a median of 22dBs. So even at low dBs everyone could hear this frequency. The maximum was 67dBs and the most sensitive person could hear it at 18dBs. 16.95kHz was the last frequency that all subjects could hear. The threshold at this frequency had a median of 75 dBs (35 dB-109dBs min-max range).
By 19kHz, the median threshold was an astonishing 98.5dBs (60-114 dBs). And this is with the sound directly in the ear (monaural insert earphones)! A completely unmasked frequency of 19kHz needs to reach the ear at 98.5dBs to be heard from a normal (mean) 24 year old population. 24 of 26 subjects could hear this frequency.
Only 3 subjects reached the frequency of 24.2kHz. Their threshold of hearing was 110dBs. The paper shows their frequency limits and thresholds separate from the group.
This is the box plot of the overall response. The number of subjects that could hear those frequencies are on top.
View attachment 188466
The authors review the literature and show that indeed it is well established that the threshold for hearing increases dramatically with increasing frequencies, and that their findings are consistent with the literature.
So what does this tell me? Actually, even though I have a few questions that I will list below, this tells me that hearing any frequency above 14kHz in music that affects they way we hear the totality of music is very difficult. Even if music were to have frequencies higher than 14kHz, these "notes" would have to be so loud to be heard above the rest of the music for our brains to "hear" it that it becomes difficult to sustain that these frequencies are essential. When we look at frequency response of music played, high frequencies are much lower in level than middle frequencies. Even at only 16kHz, we need an unmasked in-ear 60 dBs level to be heard. How loud does the high frequency has to be out of the speaker to reach our ears at 60dBs? Basically, I now think that any music that has frequencies above 16kHz has no impact on what we hear. And given that I am a bit older than 24 (even my son is older than that!), there is no way that it would impact me. It also reinforces to me the decision of the CD cutoff frequency response. Nothing above it is relevant or necessary.
It would be an interesting exercise to test the hearing threshold with music playing in the background to test whether it impacts or not. I doubt it. And even though the authors took care that the tones were quite pure, they did not report if the microphone captured earphone distortion with lower frequency harmonics that could have facilitated detection. I doubt it.
Bottom line, is that for those that claim they can hear high frequencies, we should ask them not only what frequency they claim to hear but also at what threshold is that achieved. I am sure that forum members can also supply music that has content that could fall within the reported thresholds. If there is any.
The paper is really excellent, I strongly recommend that you read it. This is a SIMPLE summary, and I hope you forgive my attempt to reduce the content so much. I purposely did not cover many of the other findings that I thought were less relevant for the discussion.
Does airborne ultrasound lead to activation of the auditory cortex?
As airborne ultrasound can be found in many technical applications and everyday situations, the question as to whether sounds at these frequencies can be heard by human beings or whether they present a risk to their hearing system is of great practical relevance. To objectively study these...www.degruyter.com
Physics stimuli
It is obvious that the physical stimuli are not sufficiently evaluated with regard to speaker and amplifier distortion. Some class-D amplifiers have measurable and audible distortion in the higher frequencies that are often missed by standard measurement procedures.
Most likely, the toy speaker creates noticeable distortion that are sensed but not heard as a tone.
Psychological reaction
Despite this, there are reported responses to supersonic frequencies in 4 subjects. All the subjects indicate that they experience something at roughly the same sound intensity of 110 dB.
Interesting that the spread in sensitivity that existed at lower frequencies is now gone. Possibly measuring something else?
Physiological reactions
Neither MEG nor fMRI can demonstrate any significant findings in the brain for frequencies above 14,000 Hz.
Probably nothing is recorded with MEG or fMRI due to low sensitivity.
Physiological speculations
Hyperacusis in normal-hearing individuals results in reduced sensitivity to sounds within a certain high-frequency range after damage. About 10% of the normal population has hyperacusis. Most people have no knowledge of the injury or suffer from the injury. In practice, the outer hair cells' sound amplification of approx. 40 dB is further amplified. This is perceived as disturbing. There is possibly a hyperacusis lesion at 20,000 Hz which also reacts to nearby frequencies. In other words, due to the damage to outer hair cells at 20,000 Hz, sensitivity to higher frequencies also occurs.
Outer hair cells are the canaries in the coal mine for the inner ear, in that they're the first cells to die due to loud noise, age or other factors," says Fuchs. "Since they can't regenerate, their death leads to permanent hearing loss." So one possible role for type II afferents, he adds, would be to warn the brain of impending damage to outer hair cells.
Nerve Cells Warn Brain of Damage to the Inner Ear - 11/09/2015
Some nerve cells in the inner ear can signal tissue damage in a way similar to pain-sensing nerve cells in the body, according to new research from Johns Hopkins. If the finding, discovered in rats, is confirmed in humans, it may lead to new insights into hyperacusis, an increased sensitivity to...
www.hopkinsmedicine.org
It is not uncommon for studies concerning normal hearing to have obvious flaws, which are not as frequent in medical neuroresearch where I work. These flaws are often obvious even to amateurs.
Why?
Research into normal hearing is not a priority research area?
Research into normal hearing requires deep knowledge in physics, neuropsychology and neurophysiology, which few have in their group.
Is this due to hierarchical research tradition?
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