What makes strings sound "sweet"?

I have always wondered about this. If you listen to many recordings of bowed string instruments, then you know that the upper registers can sometimes have a sweet tone. I define this by what it is not - edgy, brittle, dry and rough sounding. It is smooth and inviting. I used to assume this was due to rolled off highs or an emphasized midrange, but I am not so sure. It varies by recording, or course, but I have often wondered what, in the recording or reproduction process, causes strings to either sound brittle or sweet. Is it the acoustic of the original recording venue? A frequency balance issue? I would love to hear from those who might know. Thanks!
I believe that what it basically comes down to is harmonic balance, which in turn can be influenced by many things.

As you probably realize, a note consists of a "fundamental" frequency, together with harmonics (integral multiples) of that frequency, in particular proportions. Variations of those proportions (in other words, variations of the amplitudes of those harmonics relative to each other and to the fundamental) will produce the variations in sound quality you are describing.

Those proportions can be affected by mic placement and distance, the amount and character of reflected sound in both the recording space and the listening space, frequency response unevenness, distortion introduced by electronics and speakers, and many other factors.

Over-emphasis of odd harmonics (3rd, 5th, 7th, 9th etc.), and particularly higher-order odd harmonics such as the 7th and 9th, is generally considered to be the most objectionable form of harmonic imbalance.

-- Al
Agree with Al.

Fundamentally I think it has more to do with mic placement than anything else. Close microphones reproduce high frequencies and low level overtones that we never hear live. Can't think of a single occasion when I attended a live performance and heard screechy violins, even in a chamber. And, FWIW, I have found the balance between the strings and the rest of the orchestra on many recordings to over emphasize the strings, especially the violins.
My method is to very occasionally give my speakers a VERY light dusting with sugar. For a slightly lighter tone substitute Splenda. Seriously, a recording can be messed up at any stage and it is very hard to tell at what point it occurred. I have some CD transfers that sound very little like the original LPs; often different generations of transfers will be markedly different. The original CD issue of "WILL THE CIRCLE BE UNBROKEN' was unlistenable, the remaster sounds very close to the LPs, which were very good. On the other hand the original may be bad, in that case there is little to be done. The recording engineer has a tremendous influence over the sound and some are much better than others.
Als assessment seems accurate (as usual) and I would tend to agree.

It also helps explain why tube gear often produces better results in this department than SS gear, though this is not always necessarily the case.
Screw high order harmonic distortion. Harmonic distortion is related to the signal itself. IM distortion is, by definition, enharmonic, and much worse. Many factors contribute to sweet sound, frequency response, harmonic distortion spectra, amp behavior at clipping, room treatment, etc., but a lack of IM distortion is certainly one of them.
Agree also that distortion of any kind including IM will surely wreck havoc with the sound of strings especially. Getting strings to sound good along with everything else is one of the most elusive things to accomplish with a system I have found.
More even order harmonic distortions than odd order harmonic distortions.
Screw high order harmonic distortion. Harmonic distortion is related to the signal itself. IM distortion is, by definition, enharmonic, and much worse. Many factors contribute to sweet sound, frequency response, harmonic distortion spectra, amp behavior at clipping, room treatment, etc., but a lack of IM distortion is certainly one of them.
Viridian, I agree with your comments about the importance of having minimal IM distortion, and I would additionally mention TIM distortion.

But note that I referred to harmonic imbalance, not harmonic distortion. The distinction being that I was using the term "imbalance" to encompass a much broader set of ways in which the harmonic structure of a musical note can get messed up (just one of which is harmonic distortion), including frequency response unevenness and the other variables I mentioned in my earlier post.

-- Al
Strings shouldn't sound overly aggressive, but they also shouldn't sound too sweet. Depending upon the instrument and the player it can be very appropriate to have a little bite or even rasp to the sound. The choice of microphone and its placement are the biggest factors in string recorded sound quality.
What Onhwy61 said is exactly on the mark, as LIVE stringed instruments can sound very strident, when bowed aggressively. Good recordings will reproduce the edginess/bite, epecially if the mic was placed in front of the instrument.
I fall into the microphone camp of believers, at least, most of the time. So often, one sees a microphone just above and oh so close to the instrument during recordings.
I suspect this was done in the past to compensate for (older) home equipment that had difficulty reproducing high frequencies. Unfortunately, the practice still seems to be in effect, even though the compensation is no longer necessary, and now, is even objectionable.
The room acoustics do make a difference, and I think the folks are on the right track pointing to microphone errors & higher ordered harmonic distortion created by limitations of the recording/playback equipment. I also wonder if inaccuracy of dynamics can create some loss of 'sweetness' due to changing the natural rise and decay of the instrument to a more abrupt note.
On the other hand though, George Bush might somehow be responsible.
A couple martinis!
Bondmanp - To obtain sweet sounding strings pay attention to:

1. Pressure of the Bow: This certainly can affect tonal quality...experiment with adding slight pressure and increasing it as you draw the bow. The first finger of your bowing hand is where most of the pressure is applied. You'll want to work with a qualified teacher on this since bowing technique it's such a critical aspect of learning to play well.

2. At what point on the strings is the bow applied. There's a "sweet spot" where the strings are the most responsive to the drawing of the bow without any harshness in tone. I've found this to fall right at the end of the fingerboard. You'll want to experiment yourself to determine where the "sweet spot" falls on your violin. Many time this can be directly between the end of the fingerboard and the bridge. If you play above the fingerboard, the violin loses presence and volume. Even so, you might some songs call for this mellower tone. If you play too close to the bridge, you'll notice that the sound becomes more trebly...and has a certain whistling quality.

3. The bow needs to be in a straight line above the strings drawing a perfect plus sign. As you draw the bow over the strings, you'll want to focus on keeping the bow as straight as possible...the plus sign should be as accurate as possible throughout your playing. Try practicing this technique by drawing the bow from the lowest part to the tip of the bow. As you draw the bow its entire length, carefully observe the angle you're playing...and make sure that you're drawing a straight line across the strings. Keep the speed of the bow consistent...be careful to allow the bow to exert its own pressure, against the string, through natural gravity.

4. Are you using the full flatness of the hair of your bow against the string. Many good violinists will use approximately 3/4s+ of the hair applied to the string as the bow is being drawn. To do this, simply angle your bow just slightly so that part of the hair is raised closest to you. Again, you'll have to experiment with this to determine if it improves the tone. You may find that variations of using both the flat surface of the hair and 3/4s will contribute to songs in different ways. Experimentation is your best research.

5. How tight is your bow hair? When tightening the bow hair, be sure to leave it slightly loose and not over tighten. Over tightening the bow hair leaves too much tension and can affect the tone and the arch in your bow. The wood of your bow should still have a very slight arch to it after tightening the bow hair correctly (it should not be a straight line). You should be able to tap the bow slightly on one of your fingers and determine that the hair will barely touch the wood part of the bow.

6. Did you apply just the right amount of rosin? If you have too much rosin, there will be a scratchy sound as you draw the bow...with too little, the tone will seem to disappear as you draw the bow. If you find that you've applied too much rosin, do not attempt to clean the hair. Cleaners and Oils can ruin the capability of the hair to grab the string and create a clear pitch. It's best to play until the rosin slowly dissipates. If you feel that the rosin doesn't appear to be applying itself to your bow, this can be determined by gaining a whispering tone instead of a full clear pitch, try sanding the cake of rosin with some sandpaper so that it has a rough surface.

7. Do you have the proper grip on your bow? A few things to consider include: do not grip your bow too tightly, do not add unnecessary pressure to the bow, be sure to rest your little finger on the top of the bow, and keep your entire bowing arm and hand as relaxed as possible.
Thanks, all. Very interesting replies. Kijanki - thanks for all the info, but my OP specified "recordings" of bowed string instruments. Music is, for me, stricktly a spectator sport. :-)
Ahh, yes. This is absolutely the hardest element for a stereo to convey. I used to think bass was the hardest. Then I thought the midrange was the hardest. Now I appreciate that the sweet, angelic, gripping sound, detailed, grain-free sound of the treble is one of the most magical, mesmerizing, and elusive pieces of the puzzle.

The recording is important, but so is your stereo. I have built my stereo in the quest for this sound. Everything contributes, although I place high emphasis on the Jade Audio cables, the preamp, the Tripoint Troy, and the diamond tweeter. I believe it really becomes an issue of striking the correct balance of frequencies throughout, and removing all the noise (both heard and not heard).

It is so fragile. Just changing one cable can kill the magic. If you have it, be careful not to touch the stereo again or all is in jeopardy.
Bondmanp- It was intended as a joke but in a process I learned how difficult it is to play violin. In addition to technique and intention of the player there is a sound of instrument, acoustics, recording techniques etc. Radiation pattern of instruments can be also strange with some instruments like cello projecting only to the rear at certain frequencies.

Strings sound much better now with my my new speakers but I can hear big difference between recordings varying from sweet earthy tone with beautiful harmonics to high pitch screeching sound. My gear have deficiencies, I'm sure, but some recordings are just plain wrong and no gear in the world would make them sound great.
"This is absolutely the hardest element for a stereo to convey. ... The recording is important, but so is your stereo."

Agree 100%. Subtle weaknesses in source, pre-amp, ICs, and/or power amp together will often exhibit themselves here during a well recorded sustained string note. Speakers too but perhaps to a lesser extent in general in this regard. To get this detail to sound consistent and natural and not distorted in subtle ways from start to end is an ultimate test. I used to think that this was the blatant achilles' heel of CD digital. Now I think it may still be the weakest area of CD sound recordings, but not to the extent where it always must sound problematic due to the recording technology.
Agreed for the most part, Mapman and Kijanki. I have some recordings that have sweet string sound on almost any system I play them on - even my all-original factory-equiped Subarau car stereo! OTOH, I some recordings that have screechy, dry sounding strings on every system I play them on. So, IMHO, this seems like a more recording-dependent issue than some other sonic traits. And, Mapman, I have heard both of these traits on vinyl as well as CD.
I heard some very sweet yet articulate and impressive digital string reproduction off a VAC/VTL/DCS/Magico Mini system (mega mega bucks) at Sound By Singer a couple of years back. That became my reference in this regard and I determined from that that digital was not necessary the bane in this regard that I had suspected prior.
Sound by Singer is closing, perhaps the sound of the cash register ringing wasn't sweet enough.
I also heard an expensive system built around Peak Consort speakers at SBS also that was one of the strangest sounding I have heard and a total bust for me. Overemphasized highs and muddy indistinguishable lows. I suspect something in this system was defective. There were other QC issues with gear on display that I noticed that I thought totally out of line for a dealer like that. I think I also talked to the notorious owner of SBS on the way out. Not a pleasant conversation. Oh well.

They did have more nice stuff on display than any other shop I've been in in recent years. Reminded me of Tech Hifi years ago only factor in a X100 cost factor for inflation, Manhattan rent costs, whatever. The combo of all that nice and largely expensive gear to choose from with various deficiencies in customer service was apparently a lethal one.

I believe that would be "Peak Consult", though I wouldn't mind having a peak at a consort, no matter what she sounded like.
Steely violins are caused by bad mic placement. Violins radiate different frequency bands in different directions. For that reason, if you place a microphone right above a string section, it will sound shrill and steely. One solution is to try to equalize the shrillness out of the feed. A better one is to mike the orchestra properly, with a stereo pair flown above the front of the stage.

That's an interesting observation I have not heard before. but it makes sense and seems consistent with what I have heard live over the years!
I learned that from Toole's book, if you have access to it there's an interesting illustration of the weird directional characteristics of violins in Fig. 3.3.

Here's the paper Toole references, but they want $25 for it:


Also an AES paper called "Tonal Effects of Classical Music Microphone Placement":

I have a substantial collection of classical music. The strings sound fantastic on over 90% of the recordings. Folks, it's not the recording's fault.

The ability to portray both massed strings, and solo violin, is a major litmus test for system evaluation. Again, many components contribute, BUT, the cables are absolutely critical. I have come to appreciate this over the past several months. Specifically, having the right mix of materials. I must remark that Jade Audio cables really hit the nail on the head in terms of providing both that bite and sheen, along with the beauty, depth, and texture of strings. The real deal.

I agree with what you are saying. Getting a handle on massed strings has been one of the biggest challenges for my system I have found.

But I don't think that invalidates that different mikings produce different sounds in recordings.

I will likely agree that they should not sound overly steely with any decent recording, but at the same time I am not sure that they are always equally smooth yet detailed or "sweet".

Massed strings is definitely one aspect of sound that getting right definitely helps separate the contenders from the pretenders
I'm not invalidating the importance of the role of the recording engineer. I'm just saying that they almost always do a good job, at least in my experience across a wide variety of classical music. It still remains a stereo system issue. Besides, the point is moot. What can you do? Re-record all your favorite pieces. Try the cables. They aren't smooth or sweet, just real. Even Gidon Kremer sound good.
The problem I have is that it varies from recording to recording. So there's really no way to balance your system to make everything sound good, unless you roll the highs off so much that other instruments start to sound dead.

When I come across a recording with screechy strings, I typically grit my teeth and listen to something else instead.
There is one thing no one mentioned. That is the type of violin and strings used in that recording. That will have a significant effect on what you here (obviously). The recording is part of it, yes, but I can tell you you'll run screaming if I place Dominant's run-of-the-mill strings on my violin versus Pirastro "Olive" Gold strings which are silver/nickel wound over gut and you'll melt in your chair.

The point of my response, while the OP is geared toward the recording, it still failed to recognize the instruments themselves. Some people really just play on crappy strings. It may have been the recording of the century too but it still won't sound sweet.
I think it is due to tone woods and the type of varnish they use and the strings and the bow and the player too.
How to get to Carnege Hall - Practice!
Of course woods and varnish have a role as with the overall size of the instrument as well. Horsehair bow vs. synthetic. Type of rosin used, ivory frog vs. plastic/bone frog, type of wood in the bow, etc..etc. The player is probably one of the more important keys. A good player can help a not-so-great instrument sound just a little better.

Citing all of this I think it is nearly impossible to obtain a perfect recording, unless you are Maxim Vengerov or Hogan. Then you are just perfect whether it be an iPhone recording or professional. ;-)