So, both myself and Cognitive Daily have highlighted the recent news story that high school students have hit upon a ringtone that teachers can't hear. CognitiveDaily instituted a poll over in one of their posts with a supposed 17kHz sound clip (hey, who would have doubted the New York Times?), but it was pointed out that the sound was, in fact NOT 17kHz, but 14hHz. The difference is this: as we age, high-frequency hearing is the first to go, and 17kHz is quite a bit "higher" than 14kHz, which means that older people can still hear it. Loss of hearing is called presbycusis (better known as age-related hearing loss). This process happens at different rates for different people, due to environment and genes. But two things is certian: it is inevitable, and it is (so far) irreversible. Addressing this problem is essentially what my PhD thesis is about, but I digress.
Many people who listened to the 14kHz sound reported not consciously hearing the sounds, but did perceive a ringing in their ear, or a small pain. There is an interesting reason for this. (But its below the fold.....)
We hear because thousands of tiny hair cells in the cochlea vibrate when sound waves enter the ear canal (for more read this). What determines how "good" our hearing is, is merely how many of these precious cells we have retained through life. The amount you are born with is it, and they gradually die as you expose them to loud noises (and certain drugs). High frequency sounds correspond to the base of the cochlea, getting progressively "higher" in frequency as you travel up the spiral. An entering sound wave will vibrate a certain portion of the basilar membrane (frequency is rate of vibration), and since the hair cells sit atop this membrane, they too will receive stimulation. The human cochlea's basilar membrane is long enough to accomodate hearing ranges of 20-20,000Hz (20Hz-20kHz). That is the hearing we are all born with, and it progressively worsens with age as we lose hair cells that sit on a particular portion of the basilar membrane. That means that even the young people who could hear the 17kHz beep have still had some hearing loss, and quite naturally. We lose hair cells starting at the highest (20kHz) first, working our way down the frequency spectrum as we get older. Luckily, very few salient sounds in the environment are high frequency. (Not so for bats, though, they must hear sounds up to 100kHz!). 
So, to the point. Although we lose the hair cells in that area, the basilar membrane continues to vibrate. There's just no sensory cells to interpret the vibrations as sound per se. But the cochlea is still able to perceive pain in the ear (this occurs with high frequencies often), even when hair cells are gone. In addition, the flanking region of the cochlea may receive a small amount of "proxy" stimulation which might lead to ringing in the ears. This ringing occurs when hair cells receive non-specific or "mixed up" frequency stimulation, and can occur when we perceive sounds on the very edge of our range or after particularly loud noises.
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Hi.
Although its not directly related, I thought it would be worth mentioning here that TRPA1 cation channels present on the tip of stereocilia have been associated with mechanosensitive functions.
Kind Regards,
Tarun
hello,
i liked your intro. it caught my attention from all the other website on tinitus. i have ear ringing - for years now - but it has mostly not bothered me. i also hear what loud, rhythmic beats that some attribute to cell towers or other phenomena. i am sometimes clairaudient, but not consistently.
i have given up trying to understand these sounds. i have theories, but they are only that. i believe that in part the answer lies in a super-sensitive consciousness, but it could also be hormonal. there is often a link between the two.
recently, i have begun to experience a lot of what i can only describe as ear-waves? like a wave that goes through my ear, but there is no sound. i have had this before in the past, but now it is more frequent.
if you have any information on this phenomena, i'd be happy to hear from you.
muc thanks,
renee bales
Shelly,
I would like to use information from your blog, including answers to questions of questions for a children's book I am writing. Would you give permission? Also, what resources would you suggest to help children understand the sound waves all around us?
Thanks for writing this up, Shelley! Great information!
No problem. I could talk about this stuff all day. :)
Quick question:
Why do we lose the high frequencies first as opposed to the low frequencies? I heard actually that the outer ear magnifies frequencies within the range of most speech -- say 2-5 kHz -- so you would think that those would be the first frequencies to go.
If you observe where the low frequency vs high frequency region of the cochlea is (in the diagram above), you'll notice that the high freq portion is near the oval window. Hence ALL sounds, low and high, vibrate that region to some extent while traveling to its point of destination. Also, high frequency equate more vibrations. This, in turn, equates more mechanical trauma for the region and likely contributes to a shorter life of the cell. There is also a line of thought that many cellular and maturational process occur in a basal to apical cascade: inital inner ear development goes this way, as does apoptosis following acute trauma. So, this may be another, albeit much slower, cellular cascade up the spiral.
As anecdotal evidence, I have personally observed more surviving hair cells in the apex vs the base when i intentially deafen animals. This suggests some protective effect that the apical (low freq) region has that the base (hi freq) doesn't. This makes a lot of sense considering that the freq which are most important are lower ones, like you pointed out, human speech. We would have evolved to conserve that.
I have two questions (maybe you already answer the first, but I am not a native english speaker, so maybe I simply just didn't understand):
1 - what about ringing tones that we hear constantly, even when there is no sound, often after having been exposed to very loud sounds (think about a rock n' roll concert for example) (this effect is called "acouphene" in french)? what exactly happens in the ear that leads us to hear something when there is in fact nothing?
2 - maybe not in your expertise but do you know if some people tried to build a mechanical ear? I mean the ear is doing some kind of Fourier transform of the incoming audio signal, did some people try to reproduce this with mechanical devices (so that we can calculate FFT of signals with a device instead of first sampling the signal and then applying some algorithms on it to get the frequency domain content of the sound)? (is it a nerd's question?)
Hi Sed, great questions.
1. Ringing in the ears is a prevalent disoder called tinnitis, and represents a disorder of the nerves that connect the hair cells to the brain. While everyone experiences this ringing or buzzing to a slight degree, some people experience it ALL the time. When hair cells in the cochlea die or become damaged, sometimes the nerves become "miswired" and send confusing auditory information to the brain. This is perceived as noise but is not discriminated as coherant sound. Prolonged exposure to loud noises (rock concerts, jackhammers, etc) is the most common cause, but there exist others such as certain tumors, some antibiotics, fluid in the ear, etc. There are no real treatments for it, except perhaps cognitive therapy or masking the tinnitus with white noise. The best bet is to protect your hearing so it doesn't occur.
2. People have certainly tried, and succeeded, in building something like a mechanical ear. It is called a cochlear implant, and instead of replacing the entire ear per se, it interprets sound waves as electrical impulses which it transfers directly to the auditory nerve. I personally don't work with these, by some of my friends in the program do. They are particularly effective in relaying sound where before there was none, but it takes a long time to train yourself to discriminate what you are hearing. Speech never sounds the same again (it is skewed towards hi freq so everyone sounds squeaky), and music cannot be perceived. The implant cannot recreate the ear's fine-tuned frequency and tone discrimination, but it can help a profoundly deaf person function better in their environment. I think that the state of these implants is still rather crude, but there is every reason to think their sensitivity will only get better.
A quick search leads to tinnitus, not tinnitis. :-)
(I like the french word more :-))
Damn lack of spellcheck. I have railed to the gods on high at SB, but to no avail. Anyway, hope you still get the point, "u" notwithstanding.
If you want a quick test of your higher-range hearing, go stand next to a television set. Their horizontal sync frequency is about 15,700 cycles.
I remember when I couldn't go into Birks jewellery store because of the screaming of the "ultrasonic" alarms (around 18,000 cycles), which the store staff insisted were off during the day. Obviously, just the alarm part was off.
I am no longer bothered by TVs unless I am really close to them, when I get that ringing or pressure feeling.
Another interesting place to find a stratospheric tone is at the end of the last track of Sgt. Pepper's Lonely Heart Club Band, "A Day in the Life." After the monumental final chord dies away, there follows several seconds of a 15-kHz tone and then a repeating snippet of "sampled" Beatle chatter (which, on the original vinyl album, would play forever). I could easily hear the high tone when I bought the album twelve years ago. I can still just barely perceive it if I crank the volume way up (while both the cats collapse to the floor, having seizures).