Thanks for your introduction. I am looking forward to your participation in the ProGuitar Community.!
@Woy, you will find some of the tabs for the video lessons here:
Going to grab the guitar straight away!
@ProGuitar looks great! Thanks!
Hello Al and welcome to ProGuitar!
Im afraid I am not the person to help you. However, I got a recommendation through facebook on the book Six Decades of the Fender Telecaster: The Story of the World's First Solidbody Electric Guitar by Tony Bacon.
I hope someone else on this forum could give you a better and more fun answer!
This topic will act as a draft and basis to an article covering the work that Greg Byers ( @ByersGuitars) and I did in 2014. The aim of the "research" was to collect, both objective and subjective, data to deepen the understanding in how different construction parameters such as bracing systems, top, back and side thickness impacts the modes of vibration and sound characteristics of the guitar. Also, we further tried to cover the interaction between top and back plate. The full article will later be published on ProGuitar.
Thanks to Greg's experimental guitars we were able to make measurements on over 100 different combinations of bracing systems, back, side and top thicknesses. Most of the measured results verified the well known science in the field but we also found new interesting acoustic behaviours, to my knowledge, not earlier explained.
The experiment consisted of 6 (3 pairs of) top plates with the thickness of 1.5mm, 2.2mm and 3.0mm.
Type of wood:
Top plate: Spruce (Picea Engelmanni)
Bridge: Rosewood (Dalbergia maritima)
The experiment consisted of 4 (2 pairs of) back plates with the thickness of 2.2mm and 2.9mm.
Type of wood:
Back plate: Rosewood (Dalbergia latifolia)
Back struts: Mahogany (Swietenia macrophylla) and Spruce (Picea abies)
The experiment consisted of 4 pairs of sides. Two with a thickness of 1.4mm and two laminated sides with a total thickness of 2.7mm. The thick sides were also reinforced with ebony and equipped with closable sound ports.
Type of wood:
Sides: Rosewood (Dalbergia latifolia)
Neck- and tailpiece: Mahogany (Swietenia macrophylla)
Neck & Strings
The experiment consisted of 4 "identical" necks and the strings used were D'Addario Hard Tension Nylon strings.
Type of wood:
Neck: Mahogany (Swietenia macrophylla)
Fretboard: Ebony (Diospyros crassiflora)
The measurement setup
We needed to find a method to standardise the measurements to get comparable results which also were repeatable. This was done both when generating the input to the guitar but also when recording the output.
Getting a frequency response from an acoustic system is basically done by using all frequencies as an input, measuring the response and performing a Fourier transform.
This can be done in three ways;
We found the dirac pulse to be the most correct way due to the impact of transfer functions in other input systems.
I am not going to explain the physics but what you need to know is when using a very short pulse as an input to a system you are actually putting energy in all frequencies and the system will respond with it's acoustic behaviour, i.e. the frequency response. The frequency response tells you how different frequencies pass the system, or in this case, the guitar's amplification for every frequency. In a complex non static system ,as the guitar, a frequency response is not able to describe the complete acoustic behaviour. However, it can tell us many important main characteristics.
Practically, how do we enter a dirac pulse into the guitar? It is easier than it sounds. When using your knuckles and knock the guitar you are are actually creating a quite perfect dirac pulse, i.e the guitar will respond with it's transfer function. Equally, when you clap your hands in a room the room will answer with it's frequency response. The physics of acoustics is pretty beautiful. You give the system a knock, asking how it works, and it immediately respond with a great description of itself
We now needed to make a tool that repeatedly could knock the guitar with the same force every time. This was done with help of a pendulum and the gravity. The pendulum was made of a plastic ball, some wood, a rod, and then mounted on microphone stand. See image below.
To the inquisitive; the energy entering the guitar in each pulse was approximately 58mJ.
Greg had made a wooden construction to hang the guitar and position the recording microphone on the same distance for every measurement. The impulse response was measured at a distance of 1m from the guitar and 0.8m above the floor. The guitar bottom was placed about 0.59m above the floor. Measurements were made both inside and outside to compare the impact of room reflections.
For the top plate measurements the Pendulum was positioned to hit the bridge bone saddle from the bass to the treble side. For each construction combination the peak average of ten different saddle positions was measured.
For the back plate, the pendulum was positioned at three different spots (see image below). Each spot was hit three times starting with the middle of the lower bout, the down right of the lower bout and at last the right of the upper bout.
The equipment used was:
Modal Analysis -
When studying the vibrational pattern that arises on different resonance frequencies we entitle this as Modal analysis. Each vibrational behaviour/pattern is called a Mode. There are multiple ways to visualise these patterns and I will go through a couple of them and especially the method we used in this project.
Chladni figures is named after the german physicist Ernst Chladni. In the 18th century he described a method to visualise the vibrational patterns with sand on a metal plate using a violin bow. When drawing the bow on the metal plate the sand bounced from areas of great vibrations to areas of no vibrations, so kalled nodal lines.
Today many luthiers use a loudspeaker driven by a signal generator to get the guitar to resonate. Another way is to use a coil and a magnet which I will describe later on.
In this project we did not focus on "standard" Chladni patterns but as a reference, I will add an image on some chladni patterns I have done earlier with some nice earl grey tea.
This text will be updated continuously during July 2017 ....
We really appreciate comments, feedback and questions!
I have now made a practical test with a guitar and I have been able to verify some of the reasoning above. At first I made a subjective listening test both while playing the guitar but also with a generated white noise signal (through the pickup's magnetic field).
I was a little bit surprised when I generated the white noise and was lowering the volume. The noise sounded much more linear but I also perceived a treble increase. You can listen here:
Noise - Full volume
Noise - with volume roll-off
Does this mean an increase in treble? Let's look at the frequency response:
The frequency response when using full volume is represented by the red graph and the the yellow graph is when the volume knob is turned from 10 to 9.
When reducing volume the high treble range stays the same, i.e we get relatively more treble, as we could hear in the noise recording. However, the greatest difference is seen in the 1.8-4.2kHz where we loose amplitude when lowering the volume.
When playing the guitar I noticed two things when reducing the volume. I was able to verify the loss of perceived treble and clarity but I also noticed a loss in attack and dynamic range. The later is rather explained by the function of the amplifier. How is this possible? I had a hypothesis in the earlier post regarding the guitar's bandwidth and a guess that the first harmonics were more important to the perceived treble than the increase of cut off frequency. You can listen here:
E4 - Full Volume - normalised
E4 - with volume roll-off - normalised
Here comes the partials for the audio above.
Full volume is represented by red and when normalising the signal it is quite clear that we loose treble when reducing the volume.
I am pretty confident that there is not anything wrong with the simulation and this explanation. However, this results might change from guitar to guitar and setup due to variations in inductance, impedance, and capacitance. Also, my reasoning is not an exact explanation and answer to the question; why. This is just a verification to the phenomena. To fix this (if it is considered as an issue) we have to look into why the resistor in combination with the other filter parameters impacts the ~2-4kHz range.
I used a free online software to make the simulation and I will make the project shareable. I'll come back with a link soon...
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