According to this:
"some people can tell if the sound is in front or behind them."
I've never heard of that before. Can you?
Depends on the shape of the pinnae and the quality of your "auditory meatware".
Azimuth (left/right) is relatively easy for most folks. Elevation gets
a bit harder for many. Front/rear is the hardest (esp if you want to
express WHERE to the rear, etc.).
Your brain tends to rely on time differences, amplitude differences
and frequency attenuations to discriminate direction.
A sound coming from the left is louder at the left ear than the
right (because the right ear is in your "head's shadow"). It
is also slightly earlier than its arrival at the right ear
(a foot is about a millisecond delay). Additionally, higher
frequencies are attenuated more than low due to the "meat"
in the middle. And, finally, environmental characteristics
(room acoustics, etc.) come into play (your brain adds context
to better qualify the data that your ears are providing.
For a hoot, visit an elliptical room (my favorite exhibit at the
Museum of Science & Industry, Chicago) and stand on a focus
(ellipse can be considered a circle with two "centers") while
a friend stands on the other. Your BRAIN knows your friend to be
~20 feet away. And, you are SURROUNDED by noisey tourists
walking in every direction -- including directly behind you!
Yet, when your friend *whispers*, you will swear their lips are
inches from your ears!
You'd be wrong. ITD (Interaural Time Differences -- differences between
left and right ears in the time domain) are used for low frequencies
(e.g., below about 1KHz). Low frequencies are not attenuated much
(which is why you can hear the bass notes from the guy's sound system
in the car next to you through closed windows -- but can't hear any of
ILD (Interaural Level Differences) are used for higher frequencies -- where
the signal experiences *more* natural attenuation.
*You* can perceive time differences on the order of 10 MICROseconds.
You can probably resolve azimuth (left-right) to within *1* degree
if the sound is sort-of in front of you. Once it starts to move off to
the sides, your "resolvability" gets much worse (10-15 degrees).
The most interesting aspect of this is that you can recreate these
"situations" on a computer -- with headphones -- and the listener
will hear the sound as originating *inside* his/her head!
[I've written a "3D spatializer" that allows me to present sounds
to a (blind) user in much the same way that your eyes would perceive
"notifications" in the visual field. Ideally, it has to be tailored
to each individual user as each user's "audio meatware" is different.]
Notice how often you MOVE (tilt) your head when trying to identify
the location/source of a sound! Or, "focus" on it.
Ever notice how you can hear someone mentioning your name in a
crowded room -- despite being unable to hear anything else they
Hearing (like vision and the other senses) is delightfully complex!
If you aim one ear towards a bass note and the other away from it (i.e. the note is on your left), your left ear will hear it louder, that is a fact. Not because it's closer, but because the sound doesn't have to go round corners to get to that ear. You simply don't need any other method.
If a person with multiple personalities threatens suicide, is that person considered a hostage situation?
small to be of use. The volume is much more obvious.
OK, don't believe it. It's true just the same. The human brain is *very* sensitive at
distinguishing the timing of sounds. We're much better able to tell which of two nearly
simultaneous sounds occurred first than we are which of two nearly simultaneous sights.
Agree. The human brain is exquisitely capable of detecting slight phase
shifts. Even tiny bat brains can do it quite well.
<<David Corey compares the arrangement to the strings of a grand piano, with
the high notes at the base of the cochlea, where the basilar membrane is
narrow and stiff, and the bass notes at the apex, where the membrane is
wider and more flexible. Hair cells also convey basic nformation about the
intensity and duration of sounds. The louder a sound is at any particular
frequency, the more vigorously hair cells tuned to that frequency respond,
while their signaling pattern provides information about the timing and
rhythm of a sound. Konishi hypothesized that such timing and intensity
information was vital for sound localization. So he placed microphones in
the ears of owls to measure precisely what they were hearing as the portable
loudspeaker rotated around their head. He then recorded the differences in
time and intensity as sounds reached each of the owl's ears. The differences
are very slight. A sound that originates at the extreme left of the animal
will arrive at the left ear about 200 microseconds (millionths of a second)
before it reaches the right ear. (In humans, whose sound localization
abilities are keen but not on a par with those of owls, the difference
between a similar sound's time of arrival in each ear would be about three
It takes a rather simple neural comparator "circuit" in the brain to detect
which sound arrived first. That type of ability is found in pretty
primitive animals. But clearly intensity and timing are both integral parts
of 3D auditory location techniques. IIRC, bats, owls and certain prawns
easily can detect phase shifts and use the doppler effect to determine the
speed of their prey.
It's remarkable that such complex structures arose within animals. The
different neural net sensors in our bodies is much like photocells,
microphones, thermocouples, piezo-electric pressure sensors and more
evolving on their own. Your hair follicles are sensitive enough to know
which way your hairs are aligned and to erect them on neural command.
On 3/11/2016 2:44 AM, Robert Green wrote:
Play two sine waves at equal amplitudes and fudge the phase of one
relative to the other.
Note that you "consciously" perceive sounds arriving within several
milliseconds of each other as "one" sound. (I think around 50ms
is where you start to become conscious of "echoes", etc.). Yet,
your brain is processing times 1000 times shorter than that as
it attempts to "localize" those sounds!
"Three times greater" cuz we have fatter heads! :> Three times longer
Some creatures rely on more "mechanical" means (e.g., directly coupling the
eardrums so the "difference" is available as a directly observable signal).
So much for the "intelligent design" theory! I.e., if you can create
EVERYTHING (including "radiation"), then why create a system that
requires so many different solutions to the same problem? Why don't
*our* ears move like those of dogs? Why aren't our eardrums directly
coupled like flies? Why don't all of our pinnae look the same? etc.
OTOH, if you are ADAPTING to an environment IMPOSED on you, you
EVOLVE solutions that address those particular needs in the context
of your own orgainsm, etc.
I think you're missing the bigger picture. Why would life forms even "want"
to solve problems in the first place? What propels different species of
animals to keep changing their design, so to speak? It happens at the
cellular level as well. Mitosis is a religously inspiring experience
because *something's" making all those strands of DNA and RNA dance in a way
that makes them replicate.
Convergent evolution may be proof of an over-arching intelligent force. Why
do life-forms evolve separate solutions to the same sorts of problems? Why
does life seem to *want* to evolve? What drives life-forms to solve these
problems at all? Why do insects with ultra-fast jaws appear to evolve that
trait independently with completely different gene sets and completely
(AFAWK) on their own on different continents? What force compels birds to
become more keen-sighted generation after generation?
What explains the creation of the incredible forms of mimickry that exist?
These could be just random mutations, as some suggest, but I believe they're
an evidence of the very essence of a "life force" that propels life forward.
Is it God? As we said in the Pentagon, that's above my pay grade, sir! But
it is food for thought.
The only thing it's evidence of is either your lack of intelligence, or your over active ability to be amazed by things. Don't forget, these evolutions happen over many many many generations. If there was a god involved, it would be far quicker. You don't need 6.5 billion people on earth through 100s of generations to decide you need to change something. And it would also change in everyone, not just randomly.
Watching his date from the corner of his eye while he poured her a drink, the young bachelor said, "Say when."
She replied, "Right after that drink."
In addition to time and intensity, there are also differences in how
particular frequencies are perceived. I.e., higher frequencies are
attenuated more (by the "acoustical shadow" cast by your head) than
The same sort of experiment (outlined above) is performed on people to
quantify their HRTF (Head Related Transfer Function -- how different
types of sounds are perceived IN EACH EAR CANAL based on where they originate
in the three-dimensional space surrounding them).
you can see microphones inserted in the subject's ear canals (as close to
the tympanic membrane as feasible).
The baffles in the anechoic chamber (floor/walls/ceiling) damp out reflections.
The speakers arranged in the arch over the subjects head present the
sound source in one of N "relative directions" (the subject's head
is located in the "center" of that arc).
The arc pivots so that the speaker array can be behind, above, infront, etc
of the subject. So, in effect, you can issue the sound source from any
position in a SPHERE surrounding the subject's head.
You can replace the human subject with a KEMAR head <http://kemar.us/
and obtain data for a generic head (vary the size, change the
shape/orientation of the pinnae, etc.).
Unfortunately, the data from one subject (or, a KEMAR) isn't directly
applicable to other subjects. Normal variation in our anatomies
means that the HRTF for *your* head is different from that of any
The takeaway from all this is that, in theory, you can manipulate
any sound source, mathematically, and present it to a user via
headphones and convince him that the source is at a specific
point in space relative to his head. *If* you have HIS particular
HRTF encoded in that mathematical algorithm!
Where this falls down is in the dynamics of perception: you will
tend to react to a sound stimulus by altering your position,
head orientation, etc. relative to your guesstimate as to where the
sound is located. I.e., you'll twist your neck, tilt your head,
etc. -- waiting for a repeat of that signal to give you a refined
estimation of where the source is located.
With the synthesized approach, moving your head MOVES THE SOURCE
(because the headphones don't know that your head is now pointed
in a different direction -- so, the signal that it presents is
STILL "off to the left" just as much as it was before you moved
your head!) You can work-around this by using a head-tracking
device and dynamically tweeking the math (signal processing)
to reflect the head's motions (to adjust the signals to each
ear so the sound source stays fixed in space as the head reorients
to try to refine its position).
[In my case, I rely on different *sounds* in different *places*
so the user doesn't have to know exactly where a sound originates:
"Ah, that signal off to my left means someone is at the front door;
had it appeared to my right, I would know it was the BACK door!"]
I doubt it, it's just not needed. There's the far simpler method of just analysing the volume.
Try this: sit in front of your stereo with the speakers in the usual place, equidistant from your head, one on the left and one on the right. Move the balance control around, and the sound appears to come from different places. This works equally well for all frequencies. All the stereo has done is change the volume of each speaker, it doesn't alter phase or timing at all.
Now you might be thinking of surround sound at this point.... "stereos" with many speakers around the room only work because you might move your head a bit. Try a fake surround sound generated by only two speakers using your computer, and it just doesn't work.
63% of men have had sex in the shower.
The other 37% have never been to prison.
Depends on the sound.
Most sounds: yes.
But the warning sounds that the local emergency vehicles make are so
loud or *something* that I cannot tell where they are coming from.
Whoever buys that stuff probably thinks louder is better - like the
morons who set the sound level in theaters... but it seems
counter-productive to me.... but maybe it's just me...
On 3/10/2016 3:46 PM, (PeteCresswell) wrote:
It is a combination of loudness, narrow frequency range and "anxiety".
Add to that the "canyon" effect in many roadways (buildings on each side
funneling the sound in a corridor).
There has been some research that suggests you need a combination
of an "attention getter" (like the current siren) coupled with a
wide-band noise source (imagine the sound of a crowd of people)
to help folks sort out the direction of the "alarm".
Each intersection, here, has a strobe light situated atop one
of the lamp posts/traffic signals *in* the intersection.
A strobe light on the emergency vehicle signals the intersection
that it is approaching. The traffic controls then automatically
adjust/abort the current cycle to favor the direction of the
approaching emergency vehicle (the thinking being to get any cars
sitting at the light MOVING, out into the intersection and "mobile"
so they can avoid the emergency vehicle).
We have accustomed ourselves to watch for the blinking strobe
light *in* the intersection (it blinks as an apparent acknowledgement
of the strobe on the emergency vehicle) to start looking (and
listening) for the vehicle -- expected shortly.
We have the auto-green for approaching emergency vehicles but, AFIK, not
the strobe lights.... OTOH, maybe they are there, but I have not seen
Last couple emergency vehicles I watched managed to charge into the
intersection before the light had even changed for them.... And when it
does change, it's just a *flick* and it's red.... no warning for the
Sounds made-to-order for revenue producing tickets.
The strobe light tells us (me), "watch to see whether you get a quick red
*or* an early green". This then tells me the vehicle is either coming
up from behind me/approaching head on (in the case of *me* getting a green)
OR coming from the right/left sides (in the case where I get a red).
In essence, it cuts my choices in half so somewhat improves my odds of
noticing the vehicle BEFORE it gets to the intersection.
Here, an emergency vehicle can often get tied up BEHIND traffic
queued at a light. As we have median strips at most of the
larger intersections, it's not easy for a vehicle to cross into
the "wrong" lane to navigate around cars "parked" at the light.
Prior to the strobe gizmo, there were numerous cases where I found
myself at the head of my lane and took the initiative to CAREFULLY
cross the intersection on RED just to make a hole for the vehicles
behind me to get out of the way.
[Of course, all the "seniors" sit their stupefied: "Should I go?
But the light is red!?" Instead, they wait until the emergency
vehicle is on their back bumper leaning on it's "foghorn" to
make up their mind that they MUST go...]
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