Wikipedia:Reference desk/Archives/Science/2022 November 21

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November 21

Digital vs analog volume controls for fidelity

I've read from various non-authoritative sources online that given the choice between a digital volume control and a potentiometer to adjust the volume of sound, I should use the latter (as long as the former isn't set to cause clipping). This makes sense for minimizing quantization noise, since it keeps all the possible digital levels available; but how much difference does it make if other sources of noise are accounted for? For example, will a potentiometer always reduce the DAC's noise floor as much as it reduces the signal? NeonMerlin 02:01, 21 November 2022 (UTC)

Digital volume control does not diminish the number of digital levels. For a simple example, assume the levels of the source are 0, 1, 2, 3 and 4. Then perhaps on the max control setting you get 0 V, 0.50 V, 1.00 V, 1.50 V and 2.00 V. One notch lower may give you 0 V, 0.45 V, 0.90 V, 1.35 V and 1.80 V. So there are still five levels. There is no control setting that will turn input level 4 into output 1.90 V, but the jump from 2.00 V to 1.80 V corresponds to 0.9 dBV, which is barely noticeable.  --Lambiam 10:20, 21 November 2022 (UTC)

A potentiometer alone doesn't affect the Signal-to-noise ratio of an analog signal and has no useful effect on a digital signal. A potentiometer at the input to a DAC is useful to avoid overloading the DAC but any further reduction of the input signal leaves some conversion levels unused, resulting in poorer quantisation noise than the DAC with its given number of resolution bits is capable. A Digital potentiometer is a digitally controlled circuit that may be used to scale an analog signal, or it may be a multiplying function incorporated in a DAC. The Signal-to-(quantising plus random)noise ratio established at the analog to digital conversion cannot be improved but it may be worsened if the final digital output lacks extra resolution bits to encompass data MSB when gain >1 (amplification) is applied and to retain data LSB when gain <1 (attenuation) is applied. Philvoids (talk) 20:15, 21 November 2022 (UTC)

Is the question about controlling the volume at the input to a digital recorder, or at the output? My response above assumed the latter.  --Lambiam 19:16, 22 November 2022 (UTC)

There's a number of factors which can make potentiometers start getting a bit crackly especially if they've got a high resistance or are old. And if they are a wiper running over a coil you're digitising anyway :-) NadVolum (talk) 23:17, 21 November 2022 (UTC)

Potentiometers with logarithmic resistance tracks suited to audio volume control are susceptible to scratchy wear. A design advice is to add a capacitor to block direct current through the slider and its tiny contact point on the track. Philvoids (talk) 12:21, 22 November 2022 (UTC)

measuring oxygen level in air

Short video longer similar one The short (30 sec) video shows the experiment, longer one has explanation. The idea is that burning a candle inside a closed jar creates vacuum in the jar, as O2 is consumed. The vacuum is measured by water lifted into the jar. About 20% of the jar's volume is filled with water, so that means 20% of the air is gone. The part that is gone is the oxygen, and the part remaining is nitrogen and other insert gases.

Question: why isn't some of the gas CO2? Is there a way to calculate the amount? If I did that experiment in a pure O2 atmosphere, would the whole jar fill with water? Thanks. 2601:648:8200:990:0:0:0:4775 (talk) 05:29, 21 November 2022 (UTC)

There would be some carbon dioxide. The burning wax make water vapour and carbon dioxide, and later may yield carbon monoxide. The steam will mostly condense into the water anyway. Carbon dioxide will also dissolve a bit in the water. You might get a truer result if you burn something that has a solid combustion product, eg steel wool. Graeme Bartlett (talk) 06:22, 21 November 2022 (UTC)

Thanks. I did find some pages about using steel wool, but it apparently takes a long time, like days. You wait for it to slowly oxidize, at least on the pages I found. I don't need a very precise measurement so if the CO2 and CO don't affect it too much, I will try to find a way to do the candle method. The complication is that I'm trying to test the output of an O2 generator rather than atmospheric air, so I have to get the generator output into the jar and then light the candle, but I think I can find some hacky way to do that. 2601:648:8200:990:0:0:0:4775 (talk) 06:30, 21 November 2022 (UTC)

A small spark will ignite steel wool, so if you include a spark plug (as part of a powered circuit you can close from the outside), you can set the reaction off while the jar is closed off from surrounding air.  --Lambiam 10:29, 21 November 2022 (UTC)

Hmm you mean a car spark plug with an ignition coil? Seems way overboard. I might order some nichrome heating wire which could maybe also work. I had figured to use it to ignite a match tip, and use the match to light a candle. The idea is to just do a crude estimate in my kitchen, not anything lab grade. The electric ignition is because I'll have to immerse the jar in water, then collect oxygen in it from a plastic tube, then light the candle. Another way might be to use a flex-type barbecue lighter but idk how well that would handle being partly immersed in water. I'll keep thinking about this. Thanks. 2601:648:8200:990:0:0:0:4775 (talk) 10:48, 21 November 2022 (UTC)

I've no idea about your background, so this is simply a friendly warning. Please be aware that normal, slow, combustion can accelerate to very fast, hot and intense burning in the presence of pure oxygen. You need to ensure that your containers can tolerate the sudden increase in temperature and, depending upon your setup, pressure. As an absolute minimum wear eye protection in case of glass shattering. There's a video here which is designed for medics showing how oxygen makes fires far hotter and fiercer. Martin of Sheffield (talk) 12:05, 21 November 2022 (UTC)

If you use steel wool as the fuel, you do not need a candle, just enough steel wool. Apparently you can use steel wool directly as the heating element as well.^[1]  --Lambiam 18:40, 21 November 2022 (UTC)

Lambiam, I think I would have trouble igniting the steel wool with a spark since I don't see how to avoid immersing it in water while collecting the oxygen. Jayron32's suggestion wet steel wool sitting for a week might be doable, though I'd have to do a jar with ordinary air to compare. Actually just waiting 1 day or so might be enough to see a difference in the oxidation speeds. Martin of Sheffield, good point about safety glasses. 2601:648:8200:990:0:0:0:4775 (talk) 05:54, 22 November 2022 (UTC)

It is possible to ignite steel wool by passing electricity through it. You don't need a spark plug and thousands of volts. Just have a pair of wires and have a few amps passing through the wool will heat it up till it burns. Graeme Bartlett (talk) 10:20, 22 November 2022 (UTC)

If you follow the link in my reply above, you'll see a method not involving a spark. It may not work well, though, with wet steel wool.  --Lambiam 10:24, 22 November 2022 (UTC)

The "about 20% rise of the water due to about 21% original oxygen content of air" is a great example of a logical fallacy (and one that can easily be explained alternately and even disproved), though I can't remember the correct term for this specific mistake. The easy forgotten detail is that the air around a burning candle is hot whereas the jar is cool. Cooling the air near the candle causes a decrease in pressure and therefore water is pulled up into the jar. Here's a good detailed explanation of that, a way to prove it, and yet another interesting idea related to it: [2]. DMacks (talk) 18:15, 21 November 2022 (UTC)

The air in the container is initially in thermal equilibrium with the outside air. The burning will heat the air, so the pressure rises and will push the water down until the pressure is equalized. On the other hand, the volume of the hot air decreases because of oxidation removing O₂, so the water will rise. It is not immediately clear which of the two counteracting effects will be the stronger, but that is not relevant. In the end, the air will cool down to thermal equilibrium again, and only the effect of the volume reduction remains.  --Lambiam 18:52, 21 November 2022 (UTC)

I have to agree with DMacks. The crude way that the candle experiment is usually performed would mean that temperature differences (and hence pressure differences) likely play a large role in the result. Even just lowering a jar over a lit candle is going to trap hot air, which will lead to volume changes when it cools. In addition, burning wax will (mostly) create carbon dioxide and water, but the portion that is carbon dioxide will still continue to occupy space in the jar, so even complete consumption of the oxygen would not be expected to reduce the volume by 20%. Dragons flight (talk) 09:22, 22 November 2022 (UTC)

IIRC When I worked in a school lab (decades ago) we added some (can't recall how much) bench sodium hydroxide solution to the water to ensure CO2 absorption.Martin of Sheffield (talk) 09:38, 22 November 2022 (UTC)

My interpretation of the envisaged experiment was that by some still-to-be-determined method (like nichrome wire igniting a match lighting the candle) the candle was going to be lit after the container is sealed off.  --Lambiam 10:33, 22 November 2022 (UTC)

If I do the thing with the nichrome wire, it will also be easy to check whether there is vacuum created by just heating the air by electrical resistance, with no candle. Yes the idea of igniting the match with a nichrome wire is to be able to light the candle after the jar is sealed. It seems like a pain and I will keep thinking about alternatives. 2601:648:8200:990:0:0:0:497F (talk) 06:44, 23 November 2022 (UTC)

• I've done the activity many times, but with wet steel wool over the course of about a week or so. As the steel wool rusts, it will react away the oxygen from the air. Burning is faster, but not strictly necessary. I just set the apparatus up on the desk at the front of my classroom, and let it react slowly. The water will rise inside the test tube about 1/5 of the way and then stop rising. It's usually good enough for a quick demonstration for school children. See here for an example of how to do it. --Jayron32 19:02, 21 November 2022 (UTC)