The “HOme Audio - Old School vs What ever the hell they think that STULL is…” thread got me thinking about a little factoid I heard many years ago when I was a subject in a hearing experiment. During one of our breaks the guy running the experiment said that the human ear is as sensitive as it can get at least within the normal frequency range. If it were any more sensitive, we would be bombarded by the sounds of blood being pushed through out ear drums and cellular activity.
Is this true?
If you get rid of ambient sounds - as by sitting in an anechoic chamber - you can indeed hear a lot of the noises your body makes. cite. I’ve been in one (only briefly), and they are bizarrely quiet. You can have a conversation with someone just a few feet away, and you find that they are difficult to hear; suddenly you realize how much of the sound you pick up from them under normal circumstances is actually reflected from surrounding surfaces.
Under rare circumstances I can hear blood flowing in my body. Not sure if it’s flowing through my eardrums, or somewhere else, but there’s a definite rushing sound that attacks with each heartbeat and then decays slowly until the next beat.
No. The reason why you do not normally hear things such as the blood moving through your blood vessels is that you are not paying attention to them, and other sound are drowning them out. As Machine Elf points out, under certain circumstances, such as an anechoic chamber where there are no other sounds that you could pay attention to, you do hear your blood and other bodily sounds.
I have some floaters in my eyeball. They are always present in my line of vision, but I do not notice them most of the time because my brain has learned that it is not worthwhile to pay attention to them. As with sight, so with hearing. Furthermore, quite apart from attentional processes within the brain, the ear contains a dynamically tunable amplification system (implemented in the outer hair cells of the cochlea) that allows it to selectively amplify frequencies of current interest and consequently, de-emphasize those with which it is not currently concerned.
I have no doubt that, in principle, the human ear, considered purely as if it were a microphone, could be more sensitive than it is, and many animals do indeed seem to have more sensitive auditory systems than we do (although I doubt that any of them reach theoretically possible sensitivity limits either). Bodily sounds are not an important limiting factor. We tune them out.
Both upthread answers are correct, I would like to add a couple of things for clarity
1 In an anechoic chamber after you hear your blood rushing around and you tune that out or between heartbeats , you will hear a sound similar to rain. That is the individual molecules of air striking your eardrum. This is the threshold of hearing and probably the limit for any animal. Eardrum movement from air about 1 100th of 1 millionth of a centimeter. Cite Master Handbook of Acoustics 4th ed p 41,42
2 When we are talking about more sensitive hearing, we are talking about frequency range. Adult Humans can hear appx from 20hz to 20,000hz(cycles per sec) YMMV. Dogs for instance can hear much higher frequencies IIRC 40 to 50 khz IOW much higher pitches, hence the dog whistle
Some perspectives on hz, these are expressed in fundamentals(root tone) ignoring harmonics
Low string E on a bass guitar 40hz
Low E on guitar 80hz
Bass to Soprano voice 80hz to 1050hz
High note on a piccolo 4khz
Piano range 27hz to 4200 hz
These are all approximately stated for simplicity, if you double the frequency you go up an octave
Last thing to add, human ears are sensitive through almost 13 orders of magnitude or doublings of spl or volume
This can get very complex and everyones ears are different
Capt
Wow! I want one of those anechoic chambers. So I guess our ears are even better than I had thought.
The ear does also have some dynamic gain control. The ear contains two muscles, the Stapedius muscle, which controls the stapes bone, and damps out very loud sounds, and the tensor tympani, which is bigger, and which tenses when we chew, and thus prevents the sound of chewing roaring in our ears. Also the way our ears work is part of a curious feedback system. Some hair cells in the cochlea receive efferent fibres from the brain that can control the sensitivity of the hair cell. This is in addition to afferent fibres that transmit the hair cell’s activity to the brain, and thus transmit the sound to our brain.
All up, our ears have some automatic gain control, and this is part of the reason we can hear over such a huge dynamic range. Sitting in an anechoic chamber, making no noise, lets our ears open up the gain, and with no extraneous noise to drown out the internal sounds, we start to hear all sorts of things.
Recording microphones also live at the edge of useful sensitivity. Microphones with small diameters have a higher intrinsic noise figure than larger ones, simply because the larger the membrane, the more the impact of individual gas molecules on the membrane evens out. This is a consideration for a recording engineer in some circumstances, since a small diameter capsule tends to have a more even directional response, which may be needed, but the increase in noise may be enough to give him second thoughts.
In yoga, they’ve been practicing listening to inner sounds of the body for many centuries. The inner sounds are called nada and are categorized as sounding like drums, flutes, buzzing bees, etc. I don’t know if any acoustic scientists have correlated the nada descriptions with known body sounds like blood flow or air molecules hitting the eardrum. I’m guessing the heartbeat must be the bass drum.
Oh, I definitely can hear it in my ears. I have to be in a very quiet room usually, but I could hear the blood rushing and pulsing. And if I have an ear infection, even in a loud room I could hear it. (I used to get a lot of earaches and infections as a child, so the sound of blood rushing through my ears is very familiar to me.)
I already mentioned that, and you have not got it quite right.
The inner hair cells of the cochlea have afferent innervation and convert the sound energy to a nervous signal that is sent to the brain. Different inner hair cells at different distances into the cochlea vary in size, and thus respond to different frequencies. However, the larger outer hair cells (they are not outside, just in the early part of the cochlea, which is along, tapering, fluid filled tube) have efferent innervation and, instead of picking up sound vibrations, produce their own vibrations within the cochlear fluid. These mix with the incoming sound vibrations, and thus, through positive and negative interference, amplify some of the incoming frequencies, and produce new, beat frequencies which inner hair cells will also detect. Almost certainly, the frequency with which the outer hair cells beat is under fine dynamical control descending from the brain, so that the ear is finely attuned and retuned, moment by moment, in order to detect sounds of current interest to our cognitive system.
In a diseased ear, in which the beating of the outer hair cells has got out of control, they can cause tinnitus, and even, in extreme cases, cause the ears to emit a humming sound that other people can hear.
True, I was misremembering a few issues. The paper linked to is interesting, in that I had always assumed that the mechanical amplification mechanism was a Q variation, and provided some frequency discrimination capability as well, however it is clear this isn’t the case. There do seem to be some gain capabilities with the inner hairs, but it isn’t all that important compared to the outer hairs.
Anecdotal evidence: I’m presently having problems with a wax plug in one ear and all I can hear is chewing and swallowing sounds plus some tinnitus.
How about the rushing sound I hear when I yawn?