I realized some measurements of the classical interference of (sound) waves from two "point" sources (headphones) from a smartphone using "Tone generator" of phyphox. In order to have a stronger sound I have added a small HiFi headphone amplifier as you can see on the given photo. The result of the interference measurements by a second smartphone with "Audio amplitude" of phyphox is not so bad. It could be better but somebody from a nearby laboratory was not too much happy hearing (reasonably) amplified 4000 Hz. For comparison I have calculated the expected interference pattern for exactly the same parameters:
Speed of sound Cair = 344 m/s
Frequency f = 4000 Hz
Wavelength (from previous two) = 8.6 cm
Distance between sources L = 54 cm
Distance from the sources to the measurement line d = 69.5 cm
Interval of measurements (second phone scan) x from -50 cm to + 50 cm
Knowing the time interval of measurements (with constant speed of the phone) one can recalculate time to x.
Additionally a small modification of the "Audio amplitude" was done to see the sound intensity in the direct scale (also attached here). Unfortunately, I had to keep only French and German translations because somewhere in many others an error existed and I could not find it.
Enjoy
Mikhail
Dear Mikhail,
I'm trying to reproduce your experiment. Turns out I don't understand if "the measurement line d" is parallel or perpendicular to L (the line marked added to the photo).
I attach my setup and first result. It's not even close to yours. Speakers are facing upwards and I'm moving the Audio Amplitude phone along a straight equidistant vertical line downwards.
Should I turn the faces of the speakers towards each other and move the Audio Amplitude phone along a straight line from the speaker to the left to the speaker to the right instead?
Was your sine wave sound really loud? Did you wear hearing protection (ear plugs or similar)? I'm asking not only for me reproducing this at home, but also whether you think this experiment can be reproduced in a classroom with students.
the measurements have to be done not between the speakers (it should work but it is not the classical geometry of the interference pattern from two sources), but along a line parallel to the line connecting the speakers. Distance from the sources to the measurement line is d. d can be much bigger than the distance between speakers L, which should be larger than the wavelength.
The sound level was not so high in my case and no protection was needed, but 4 kHz is not very pleasant to hear if you do not know that this is for a scientific reason ...
the measurements have to be done not between the speakers (it should work but it is not the classical geometry of the interference pattern from two sources), but along a line parallel to the line connecting the speakers. Distance from the sources to the measurement line is d. d can be much bigger than the distance between speakers L, which should be larger than the wavelength.
The sound level was not so high in my case and no protection was needed, but 4 kHz is not very pleasant to hear if you do not know that this is for a scientific reason ...
Regards
Mikhail
Thank you Mikhail. I'm starting to get results that look almost like yours and will try to find a frequency (and volume!) that is bearable for students with much better hearing than mine. Hope to post that soon.
Meanwhile, can I ask about the HiFi headphone amplifier you used? This is a bit beyond where my learning curve ended when I studied (basic) circuits, but I wonder 1) if that amplifier would be powerful enough to pull a 35 gram neodym magnet (see picture) hanging in a spring into a forced oscillation? and 2) Do you think you could force a spring to oscillate with frequencies as a low as 1-2 Hz if you used phyphox's MULTI TONE GENERATOR (e.g. 440 and 441 Hz)? Obviously you would have to connect an iron core electromagnetic coil of reasonable size to the headphone amplifier instead of headphones. And be careful with distance etc.
I'd really like my students to work with resonance and use the phyphox Spring experiment which has a very nice graph where relative amplitude is plotted against frequency. The video is great https://www.youtube.com/watch?v=VbL4IInVAO4 but as you can see from the snapshot at 3m44s, the whole setup with an electrical motor generating the vertical oscillation is quite elaborate and, I guess, could require time consuming maintenance. In particular if you'd have five setups for five groups of students and not only one for me.
I believe that the neodym magnets and iron core electromagnetic coils could be used in other experiments and should be "standard shelf equipment" anyway. Adding five small amplifiers to the shelf doesn't feel like a stretch.
(11-03-2021, 09:40 AM)Erik Josefsson Wrote: Meanwhile, can I ask about the HiFi headphone amplifier you used? This is a bit beyond where my learning curve ended when I studied (basic) circuits, but I wonder 1) if that amplifier would be powerful enough to pull a 35 gram neodym magnet (see picture) hanging in a spring into a forced oscillation? and 2) Do you think you could force a spring to oscillate with frequencies as a low as 1-2 Hz if you used phyphox's MULTI TONE GENERATOR (e.g. 440 and 441 Hz)? Obviously you would have to connect an iron core electromagnetic coil of reasonable size to the headphone amplifier instead of headphones. And be careful with distance etc.
Dear Erik,
I add a pdf file with all what I know about the amplifier. It is very useful when the generator has a high internal resistivity. But I do not think that it will work at 1-2 Hz, it is a sound amplifier.
You could use multi tone generator if you would add diodes and will keep only the envelope of the signal (4 diode bridge). Your resonance project is very useful and it would be great if it will work so using only smartphone with some minor addition. But I have not tried yet the forced oscillations of the spring with a mass. A headphone amplifier should be useful also to preamplify the smartphone signal before passing it to the diodes...
(11-03-2021, 09:40 AM)Erik Josefsson Wrote: Meanwhile, can I ask about the HiFi headphone amplifier you used? This is a bit beyond where my learning curve ended when I studied (basic) circuits, but I wonder 1) if that amplifier would be powerful enough to pull a 35 gram neodym magnet (see picture) hanging in a spring into a forced oscillation? and 2) Do you think you could force a spring to oscillate with frequencies as a low as 1-2 Hz if you used phyphox's MULTI TONE GENERATOR (e.g. 440 and 441 Hz)? Obviously you would have to connect an iron core electromagnetic coil of reasonable size to the headphone amplifier instead of headphones. And be careful with distance etc.
Dear Erik,
I add a pdf file with all what I know about the amplifier. It is very useful when the generator has a high internal resistivity. But I do not think that it will work at 1-2 Hz, it is a sound amplifier.
You could use multi tone generator if you would add diodes and will keep only the envelope of the signal (4 diode bridge). Your resonance project is very useful and it would be great if it will work so using only smartphone with some minor addition. But I have not tried yet the forced oscillations of the spring with a mass. A headphone amplifier should be useful also to preamplify the smartphone signal before passing it to the diodes...
Best
Mikhail
Thank you very much for the pdf Mikhail. That's exactly what I needed.
With regards to the diode bridge, I have to apologize for my ignorance, but is it really needed? My first simple thought was that since I clearly can hear the volume of the sound oscillating when I listen to 440 Hz and 441 Hz at the same time (generated by the MULTI TONE GENERATOR), then that oscillating volume simply represents a periodic variation of the energy in the magnetic coil of the phone's speaker. So, I would basically only have to make the energy in some other magnetic coil vary since it is only the variation (envelope?) I want. I don't want the sound of the variation.
Imagine you'd generate 25.000 and 25.002,23 Hz, you would still get a 2,23 Hz "energy beat" but you would not hear anything (at least I would not). And, in this case, you would not want to either.
So what I asked was if that 2,23 Hz "energy beat" (which is clearly there) could somehow drive change in a magnetic field of a standard education coil with a laminated iron core.
I made a ridiculously simple sketch with standard school equipment, see attachment. But again, I am sorry to say that I'd have to read up to figure out why I actually need a 4 diode bridge to make it work, and not only enough juice from the amp.
Erik, with 2 Hz beats you have variation of the amplitude of high frequency current. So, you will not see anything. You have to use at least one diode to recuperate 2 Hz current component. We are on vacations and I cannot realize your idea with beats (and diodes) now. You have to try yourself.
By the way there are a lot of similar headphone amplifiers nearly of the same price. I have chosen one just occasionally.
As for your blue coil, try to find or to make one with higher resistivity, about 10 Ohm. You may modify the existing one by adding much more turns.
(11-03-2021, 03:37 PM)solid Wrote: Erik, with 2 Hz beats you have variation of the amplitude of high frequency current. So, you will not see anything. You have to use at least one diode to recuperate 2 Hz current component. We are on vacations and I cannot realize your idea with beats (and diodes) now. You have to try yourself.
By the way there are a lot of similar headphone amplifiers nearly of the same price. I have chosen one just occasionally.
As for your blue coil, try to find or to make one with higher resistivity, about 10 Ohm. You may modify the existing one by adding much more turns.
The idea to use beats is really good!!
Thank you! I'll read up.
(btw, I just realised that the splitter jack shouldn't be there in the sketch, sorry)
1) If you will replace the permanent magnet by a ferrite which is remagnetized at high frequencies you will not need the diodes. The ferrite is attracted by a coil with both current polarity.
2) The spring is itself a coil which should shrink when current of any polarity pass through it. I tried it just now, but the current is too low. A powerful signal generator can be used. The frequency can be determined by the smartphone from acceleration signal... An advantage of this technique is that the driving force should not depend on the oscillation amplitude.
11-04-2021, 05:28 AM (This post was last modified: 11-04-2021, 05:53 AM by Erik Josefsson.)
(11-03-2021, 04:46 PM)solid Wrote: Two more things..
1) If you will replace the permanent magnet by a ferrite which is remagnetized at high frequencies you will not need the diodes. The ferrite is attracted by a coil with both current polarity.
2) The spring is itself a coil which should shrink when current of any polarity pass through it. I tried it just now, but the current is too low. A powerful signal generator can be used. The frequency can be determined by the smartphone from acceleration signal... An advantage of this technique is that the driving force should not depend on the oscillation amplitude.
....
Wow! You mean by 1) that I can replace the neodyme magnet with a ferrite rod antenna and use, for example, a 10.000 Hz + 10.002 Hz beat from an amplified phyphoxphone signal? But how would you know at what frequencies of the envelope's constituent sines would start to contribute to building of the "beat magnetic field" that is supposed to attract the hanging rod?
Regarding 2) on the disadvantage of the force being 1/r2-dependent(?), you'd still get a forced oscillation, no? Resonance-peaks in the amplitude-frequency-graph (phyphox "Spring experiment") would perhaps be narrower compared to a graph when the resonance comes from a rotating device, but still show?
I think I have one of those ferrite rods hiding in grandpas old radio that I have saved for "maybe I'll fix it some day"-reasons.
Oh no, I have no time for this...
//Erik
PS. A resonance experiment (or any experiment) that students make on their own or in small groups has to be fast to assemble and take down, require almost no maintenance, and be made from either "standard physics course" components" or cheap components. The ferrite rod idea sounds promising in all respects!
Just checked how slow I could make the beat, and it seems T=200 seconds is not a problem at all. It was the slowest beat I ever listened to. I guess that means that I can fine-tune resonance frequencies with ± 0.005 Hz precision or better if I could just get them out of the phone!
...