Jens,
Interesting question, and not so trivial.
In order to get perfect cancellation by inverting one channel, both channel
speakers should be located exactly at the same position (a physical
impossibility), or else everything should be perfectly symmetrical, including
the position of the receiver respect to both speakers, there should be no
reflections at all (a perfect anechoic chamber), you should listen with a
single ear and it should be perfectly symmetrical, which isn't the case for the
human ear :). The frequency response (particularly important in this case the
phase response) of both speakers should be identical, which is very difficult
for normal speakers which have loose manufacturing tolerances.
I suggest the following experiment: Create a single stereo sound (the first two
are irrelevant for the experiment) with inverted signals, as your third sound,
but much longer, say one minute, to have time to make position adjustments) or
save it to a wav file with wavwrite() and play it in loop mode using any player
or sound editor (suggested: Audacity). Then close one of your ears with an ear
plug or strongly pressing the tragus with your finger, locate your other ear
parallel to the plane of the speaker diaphragms, as close as possible to the
third vertex of an equilateral triangle whose other vertices are on the center
of each speaker (you can't see the speakers from this orientation). Then rotate
slowly your head, at some angle you'll note that the sound reaches a minimum.
The more absorbent the room, the more chances to get minimal sound.
Explore also other frequencies, such as low frequencies.
This works better if the distance between the speakers is about 60 cm to
minimize the influence of reflections.
Let mi know if it worked for you.
By the way, you used 20050 Hz, nothing ´particularly wrong with that, but if
you look for a standard frequency, you might use 22050 Hz.
Regards,
Federico Miyara
PD: Active noise cancellation, a technology similar to what you try to test,
uses another principle: an adaptive filter whose response is adjusted in real
time monitoring the result at the desired position with a microphone, which
adapts its response to minimize the sound level.
On 19/12/2023 15:52, Jens Simon Strom wrote:
fs=20050;
t=0:1/fs:1;
f=440;
a=sin(2*%pi*f*t); // harmonic
a_=asin(a); // distorted
silence=zeros(1,length(a));
y=[a, silence, a;
silence, a_, -a];
playsnd(y,fs);
// The 1st section (channel 2 silent) and
// the 2nd one (channel 1 silent) sound as expected.
// The 3rd section sounds like the 1st, no silence!
// Why?
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