measuring decibels and sustain?

[jim_n_virginia]

Has anyone ever heard of some sort of equpiment that will measure decibels and sustain in and instrument? Preferable like a tuner that will clip on or have leads that clip on the instrument but a bench type of equipment to.

I was thinking this sort of equipment would be a great tool to have when trying different mandolins (or guitars, fiddles or whgatever!) And if you could carry it in a shirt pocket it would even be better.

Probably analog so you could watch how high the needle climbs for decibels and how long needle is off the zero mark for sustain.

[dillo]

A portable SPL meter will do the trick (Sound Pressure Level). They can be had from $50 - $1500. Google or ask your favorite music store/pro audio dealer. The radio shack unit works fine, but it's no longer the cheapest.

[Chris Baird]

You are probably more interested in loudness rather than decibal levels. So, you will want to buy a meter with "weights" that give an aproximation of loudness using a decibal and frequency reading together to produce a unit measurement called phons. Phons are a best guess, based on research, on how loud a sound will be perceived.
#One thing to know, however, is that accurate and calibrated acoustic measurements require considerable control over a number of variables. #It would, imo, be far more effective to just bring along a friend and have them tell you how the various instruments sound out in front. You will be able to hear how they sound to you. There really isn't a need, for your purposes, to use any measurement equipment other than what you were born with.

[dave17120]

Sustain is easy, if its fairly quiet, use the second hand on your watch to see how long the note you play remains audible. Works for me. Dave

[Dave Cohen]

Sustain isn't as easy to characterize as most think. To begin with, sustain depends on frequency. The lowest open note on a mandolin is the 196 Hz G, but the lowest body motions start at around 260-320 Hz for f-hole mandolins. The notes on a mandolin below 250 Hz will have relatively more sustain, since there are no body motions stealing energy from the string. On the other hand, the notes above 250 Hz will decay much more quickly. How much more quickly? As much as several times more quickly!

Measuring "sustain" in the manner prescribed by Dave17120 will be very misleading. The total audible time for a plucked note depends on numerous factors, including how hard the note is plucked (i.e., initial amplitude) and will not be consistent. Physicists use the "characteristic time", which is the amount of time for a plucked note to decay to 37% (=1/e) of its' maximum value. That at least would be consistent. You would need to bring along a laptop computer with an audio analysis program to do that. But the biggest problem with measuring "sustain" or decay times is that you will ultimately find that there ar not huge differences in sustain between mandolins. Sustain in this case is strongly influenced by scale length, and the short scale length of mandolins insures that they do not have much "sustain" compared to, say, guitars. Further, a mandolin with relatively more "sustain" may very well _not_ be a mandolin which you judge to be "good". The only works done on quality factors of plucked stringed instruments thus far are contained in two papers by Meyer in 1983. One of the most interesting things he found was that while guitarists all professed to want more "sustain" in an instrument, all of the guitars which they judged to be "good" instruments had shorter decay times than the instruments which were judged to be "not as good". In other words, the guitars with _less_ sustain were judged to be better instruments!

Loudness is frought with similar pitfalls, though it need not be complicated with phons. The point is to get a quantitative or semi-quantitative comparison of sound pressure level ("spl"), and any available loudness meter (such as the Radio Shack one) would do that for you. You would just have to be mindful of the fact that the output of any instrument changes with frequency, and so does your perception of it. The much larger problem with comparing spl is that spl alone probably won't tell you what you want to know about a selection of mandolins.

[Chris Baird]

Using a meter that registers in phons is no more complicated than using a simple spl meter. All you do is turn on the a,b, or c weighting and read the measurement.

[Dave Cohen]

The a, b, and c weightings do -not_ give loudness measurements in phons. The a weighting curve is approximately the inversely of the 40 phon contour. You will notice that the meters are calibrated in dB. When you set the meter on a particular weighting, it simply gives you a value in dB relative to that weighting. But arguing about that is counterproductive. The fact is that a simple loudness measurement of a mandolin most likely won't tell one whether or not they will like the mandolin.

[jim_n_virginia]

Thanks all I think I found what I was looking for.

Found meter in Radio Shack to measure loudness and just using analog tuner to measure sustain.

Just wanted a way to A/B instruments and have some sort of documentation or way to graph out my findings.

Thanks to all for their suggestions all were good ones!

[Larry Simonson]

Dave's points are no doubt valid but beg the question: What, if any, objective measurement correlates with what we want to hear in an instrument? I give Jim-n-virginia credit for trying to find at least one well defined measure of tonal merit and I hope he will report his results for lots of instruments along with his conclusions.

[Dave Cohen]

Um, I don't think that I ever wrote anything suggesting that Jim should not attempt to make measurements. I am all in favor of measurements. It is just that the measurements which Jim suggested will most likely have a confusing outcome for him. That, and they could be very misleading to a lot of people if he were to report them via the internet.

Human hearing is logarithmic; the smallest difference in loudness that a typical human ear can discern corresponds to about a doubling of the sound pressure level - depending on frequency, of course. There are probably some mandolins that can put out double the spl compared to others. when you play a mandolin that sounds "thumpy", you are observing a noticeably greater spl. But those mandolins may not be the ones that project best. Also, loudness measurements are frought with difficulties. You can't put the meter too close to the mandolin; about a meter (i.e., 39 in.) away is good. You also have to deal with the room acoustics - acoustical tiles vs hard walls, 90 degree corners, clutter, etc., etc. Almost forgot the directionality of your meter, which is a mic, btw. My experience with professional players is that they don't always find the thumpiest mandolin to be the best for every job. You should consider numerous things, such as bass/treble balance, "timbre" or tone quality, etc. And you will probably find all of those things to be easier to base your decision on than loudness alone.

I already addressed the "sustain" issue in a previous post. If you plan on timing the duration, as suggested by Dave17120, you will need a mechanical plucker in order to get consistent initial amplitudes. For that matter, you would need a mechanical plucker to even begin to get consistent loudness measurements. If you use the characteristic time which I mentioned above, you will be able to get consistent values even when you pick the mandolin with a pick. I think that even Bernoulli knew that way back in 1750 or so.

I have done quite a few characteristic time measurements on mandolins. Because of the short scale, the characteristic times are short - on the order of a second or a few for the lowest notes, and much shorter for higher notes. One Calace Neapolitan had characteristic times on the order of ten times shorter for the notes above 500 Hz than for the notes below 450 Hz. That was because the body started doing its thing and stealing energy from the strings at about 500 Hz and above.

As to "What, if any objective measurement correlates with what we want to hear in an instrument?" There are several papers on quality factors in violins. For the sake of time, I won't mention them here. But there are also two papers by Meyer on classical guitars - a bit more pertinent to mandolins. If it can be done for classical guitars, it can be done for mandolins; it just takes some money and time. the references to the papers on quality factors in classical guitars follow. I have posted them previously on this board. If you are going to make assertions and assumptions about what science can and cannot do, you owe it to yourself to read the Meyer papers.

(1) Meyer, J. (1983) Quality aspects of the guitar tone. In "Function, Construction, and Quality of the Guitar (E.V. Jansson, ed.) Royal Swedish Academy of Music, Stockholm, pp 51-75.

(2) Meyer, J. (1983) The Function of the Guitar Body and its Dependence on Constructional Details, ibid, pp 77-108.