Geometry of Bluetooth Field

[Twodogs]

TL;DR

How exacting is the Bluetooth field of a smart phone?

I am curious about an experience using earbuds with a smart phone music app. With smartphone on the porch, I was roaming the garden listening to music on a pair of earbuds. At a some distance from the smart phone the signal began to break up into a staccato of brief fragments which were just barely recognizable, but the melody was gone. Here is the question. At one point I sat down to listen and I found that in this boundary region I could get clear sound with my head in one place, but if I moved my head three inches to the right the signal broke up and I could co reliably back and forth with same result. This gave the impression that the Bluetooth wave form was much more defined than I expected, more concrete than fluid. Any thoughts on this? Thanks.

 

[Dale]

The Bluetooth signal is a radio wave. So it won’t have any features on the scale of three inches.

More likely you were hitting a signal detection or digital threshold.

Edit: I am wrong here. See below

 

[Baluncore]

This gave the impression that the Bluetooth wave form was much more defined than I expected, more concrete than fluid. Any thoughts on this? Thanks.

Bluetooth operates at about 2.4 GHz, with a wavelength of 125mm, or 5".
You can expect multipath reflections which can give some peaks and deep nulls over distances of about 2.5". Bluetooth overcomes those problems by changing frequency, jumping channels in the 2.4Ghz ISM band.
https://en.wikipedia.org/wiki/Bluetooth#Implementation

Two systems, communicating, will continuously search for a healthy signal pattern, without interference, so you will not notice jumping channels. When your headphones cannot tell the Bluetooth transmitter that they have a reception problem, the channel will not change, so as you move, you will get what is called a "picket fence" pattern in your reception. Depending on the paths and the wavelength, the pickets can be about 2.5" wide and 2.5" apart.

 

[renormalize]

Depending on the paths and the wavelength, the pickets can be about 2.5" wide and 2.5" apart.

That's also the reason that hot and cold spots on the order of that size exist inside the cavity of your microwave oven, which operates at the same frequency as Bluetooth.

 

[Dale]

Bluetooth operates at about 2.4 GHz, with a wavelength of 125mm, or 5".

oops, I should have done the math!

 

[Baluncore]

Multipath interference patterns, or standing waves, can have fine detail at quite some distance. The deep nulls are very narrow, so they are not a problem until you get near the range limit, as the OP observed.

It would be difficult, if not impossible, to form a narrow 5" beam. To do that would require an aperture significantly larger than one wavelength, which then sets the width of the beam to be greater than several wavelengths.

 

[sophiecentaur]

TL;DR: How exacting is the Bluetooth field of a smart phone?

This gave the impression that the Bluetooth wave form was much more defined than I expected

It's pretty non-intuitive without going back to Young's Slits and beyond. The pattern of field strength around your receiver and transmitter will be very much three dimensional with, at any particular wavelength, nulls with around a half wavelength between them. It will only be fairly close to a null that the signal will actually drop out. Using a very basic form of modulation and coding, those nulls would / could seriously affect the reception. You must remember the flutter fading that you would get when driving a car through some multi path environments when listening to FM radio. FM sound can't deal with serious drop-outs because it has no 'memory'. Bluetooth is several decades more modern and the coding and (as above from @Baluncore ) frequency redundancy can deal with this problem much better but the very low signal levels that battery equipment can make things worse and the 'geometry' is still there, producing the interference pattern and the resulting nulls are there, whatever you do. To avoid the cracking and fluttering, receivers use subtle muting. and buffering.

 

[Paul Colby]

oops, I should have done the math!

And, such a tractable problem at that.

 

[Twodogs]

Very interesting, nice to find some confirmation and understanding of my experience. Thanks

 

[berkeman]

TL;DR: How exacting is the Bluetooth field of a smart phone?

I am curious about an experience using earbuds with a smart phone music app. With smartphone on the porch, I was roaming the garden listening to music on a pair of earbuds. At a some distance from the smart phone the signal began to break up into a staccato of brief fragments which were just barely recognizable, but the melody was gone. Here is the question. At one point I sat down to listen and I found that in this boundary region I could get clear sound with my head in one place, but if I moved my head three inches to the right the signal broke up and I could co reliably back and forth with same result. This gave the impression that the Bluetooth wave form was much more defined than I expected, more concrete than fluid. Any thoughts on this? Thanks.

If you are out in your garden with no metal structures nearby, multipath interference does not seem like a likely issue, IMO. Multipath interference is usually an issue when you are inside a building or out in the open between buildings or other metal structures that reflect RF energy.

More likely you were just at the edge of the sensitivity of the Bluetooth (LE?) transceiver, and small changes in distance were enough to cause dropouts (especially if there were interfering RF sources nearby).

If it were me, I would try to replicate the experiment, and also look for reflecting metal structures nearby.