Dominic Parker asked:
I've been an avid listener for only the past month or so when I've been listening to the podcasts on my walk to and from the library everyday to revise, and am absolutely loving it so far and have managed to get through a good 30 or 40 of the latest podcasts in this short period of time... Anyway just a few things I was wondering if you could help me with:
As an amateur musician and DJ I have forever struggled to work out how it is that, particular in cheaper more basic speakers, a single speaker cone/diaphragm is able to vibrate at so many different frequencies at any one time in order to produce the full tonal frequencies of a piece of music? So essentially: how is it that it is able to recreate a kick drum at say 70Hz whilst still producing a clear strings note of 1700Hz undisturbed.
Thanks very much for your time
Dave - If the speaker was to produce a single frequency, think what that actually means - it means that the speaker is moving backwards and forwards, and causing the air to move backwards and forwards in a sin wave pattern. You've probably seen a sin wave. Itís basically just a very specific zigzaggy wiggly line. Now, if the speaker moves in any other pattern than that, you could imagine itís moving slowly with a big wiggle and then on top of that superimposed, there's a little wiggle. So, moving in and out slowly but on top of that is kind of vibrating a little bit. So then it would be outputting sound with the low frequency, the big slow wiggle, and also a much higher frequency as well, at the same time. The way that sound works is you can superimpose the motion of the speaker, you can superimpose lots and lots of different vibrations and that will produce sounds of lots and lots of different frequencies, all at the same time by just making the right pattern for the speaker to move back and forwards. Itís not moving in a smooth wiggle, itís doing something incredibly complicated and that's a mixture of lots of different frequencies.
It's all due to sound being a wave. When you have waves coming from multiple instruments, all you have to do to get the total sound wave is to add them all up on top of each other. The resulting wave looks pretty ugly (there's a gallery of some here: http://www.floom.com/images/waveform_gallery.htm), but it's got all the information about what instruments play when, since they're all in there added up together. This wave would be what hits your ear and lets you hear the music if it's being played live.
Unfortunately, there are also subtle problems with trying to do this using a speaker. It works very well but even a theoretically "perfect" speaker also introduces some distortion. One of the main distortion products is intermodulation distortion. This results from having the combinations of frequencies present. A low frequency tone will be moving the cone back and forth and any high frequency tone is then "frequency modulated" by the lower tone because of the Doppler effect from the moving cone. This is minimised by having multiple speakers (also helpful in other ways) but this is a fundamental limitation to speaker quality. Earphones suffer very little from this because the movement of the tiny cone is very small as the power is so low, whereas high power speakers, with neoprene supported cones, have a long movements and will introduce distortion unless multiple speakers are employed.
As they work a bit like speakers in reverse, do our eardrums cancel out some of intermodulation? Geezer, Thu, 2nd Jun 2011
Unfortuantely they don't. This would only be the case if the ear drum was mechanically coupled to the speaker so that the speaker movement and eardrum movement was identical. This is much closer to being the case with an earphone. graham.d, Fri, 3rd Jun 2011
But doesn't the air, at least to some extent, mechanically couple the speaker cone to the eardrum? Geezer, Fri, 3rd Jun 2011
If in an enclosed environment (like the ear with an earphone shoved in it) and where the distance is less than the shortest wavelength, it is pretty good. Not with speakers though. When listening to live music from individual sound sources, the ear receives each sound as simple superpositioned waves. The movement of the receiving diaghragm is tiny. When this is all being sent from a single speaker cone there is non-linear mixing of the signals as can be understood from just two tones - a low one and a high one as described previously. The low tone will Frequency Modulate the high tone because of the Doppler effect. The Fane Ionophone (if it is still made) gets around this by having a continuous stream of air coming out which has its flow modulated by the signal. This does not produce IM distortion but regettably will (I expect) produce a slight hiss from the air stream. The simplest trick is to have multiple units and reduce cone speeds by having a large diameter for low frequency units. graham.d, Fri, 3rd Jun 2011
Right, no disagreement there Graham. What I'm wondering though, while the displacement of the eardrum is very small, isn't it still susceptible to a certain amount of intermodulation?
Yes, you are correct, but the movement is very small, and the velocity low compared with the speed of sound. The intermodulation effect is negligible compared with a typical speaker cone.
Thanks Graham. I think I get it now .
A speaker is not made like a piano or pipe organ.
Interesting that it should be said that distortion rises from the doppler effect of the speaker cone, on low frequencies, shifting the high frequencies in what must be an FM manner. Technically, that would have to occur. However, if that is significant, is it not also so that when live instruments play, a strong low-pitched instrument, by distorting the velocity of air in the vicinity of a higher pitched instrument, will have the same effect? As for causes of distortion, I had been under the impression that the biggest causes of distortion in speakers was first of all non-linearity in its deflection compared to the input current, and secondly, resonances in the speaker or its enclosure, and if you get rid of those two problems, you will have a very good sounding speaker. Atomic-S, Mon, 6th Jun 2011
I'm sure there must be some amount of intermodulation between instruments, but the great thing is (at least for those electronic engineers involved in designing the ultimate in hi-fi) you can't call it distortion Geezer, Mon, 6th Jun 2011
Atomic-S, I don't think that it is the case that the presence of a nearby instrument distorts the sound of another, at least via Doppler effect as described, but I have not done any maths to prove this; I think the waves superpose without intermodulation. What does happen is that instuments themselves do not produce sine waves (as Clifford suggests) but rather complex sets of harmonics; these often share a common resonance chamber whose walls vibrate in sympathy with the notes generated. I think it is in the art of instrument manufacture that the overall effect sounds "good" whatever the distortion components added. In this case the "distortion" is not regarded as such, as Geezer says. The sound reproduction system must simply not change this sound by adding to the distortion. Instruments could cause other instruments to vibrate, which could influence their sound I expect, but again this is something that we would tolerate and I think the effect would be relatively small in terms of IM distortion. One issue of electric pianos is that they do not model the sympathetic vibrations of "undamped" strings.
I think these gents sum the situation up quite well
When I was 16 I could hear the 20kHz whistle from a TV line output transformer running on the French 819 line system, now I am hard put to hear a 4kHz tone. syhprum, Tue, 7th Jun 2011
Actually my ears' frequency response is still quite good but I do hear some unpleasant distortion at high volumes even with live concerts. I was blaming it on poor stage reproduction equipment until I noticed it with an unamplified choir! I have also noticed I can't hear people as well (can't pick out their words) in noisy situations but I think that may be something in the brain as much as in the mechanics of the ear. My wife has a similar problem.
Dominic Parker asked the Naked Scientists: As an amateur musician and DJ I have forever struggled to work out how it is that, particular in cheaper more basic speakers, a single speaker cone/diaphragm is able to vibrate at so many different frequencies at any one time in order to produce the full tonal frequencies of a piece of music? So essentially: how is it that it is able to recreate a kick drum at say 70Hz whilst still producing a clear strings note of 1700Hz undisturbed. Thanks very much for your time Dom Parker What do you think? Dominic Parker , Tue, 7th Jun 2011