Flute Tone Investigations
Appendix 1 - Technical Resources

Technical resources

What equipment and software are being used in these investigations?  What are their limitations, and are these impacting on the quality of the measurements?  Do they impact on the findings?  How good enough is near enough?


The Hardware

We'll start by looking at the hardware that will be involved in the investigations.

The Recording Microphone(s)

The item most likely to affect the accuracy of our analyses is the recording microphone, but here, luck is definitely on our side.  As our initial skirmish with measured flute sound, the analysis of the recording of Paul Davis, showed, we are not expecting to see a particularly wide range of signals, at least in frequency terms.  The lowest note we could encounter on a C flute is 261.6Hz (C4).  By about 4kHz (around C8), the harmonics are some 40dB down and will be approaching inaudibility.  So if we can smoothly cover the range 250Hz to 5kHz, there won't be much that escapes our interest.  The whole audio band covers 10 octaves, we're concerned with less than half that.

The mic I'm hoping to use is the Behringer ECM 8000, sold as a "reference mic" for sound system installers.  It's omnidirectional, based on an electret microphone capsule, balanced and runs on P48 Phantom Power.  Being omnidirectional, it is a simple pressure sensor, and is not prone to proximity effect as are the more typical cardioid microphones encountered in recording and stage work.  It should measure equally well in the diffuse field and inline with the incoming sound.

I'd like to get away with that mic if we can as it is cheaply and readily available everywhere, making it possible for anyone else to replicate my results and carry on with their own studies.  Here's a calibration chart for a typical example of this type.  For convenience, I've marked in red on the chart the range of frequencies we'll be expecting.  As you can see, the microphone is substantially "flat" over that range, just starting to rise a little at the top end.  The degree of rise on this mic was 0.89dB at 5kHz compared to the midrange 1kHz, so less than the 1dB change that a musically trained ear is capable of perceiving under ideal listening conditions, listening to a constant tone.  Looks good so far!

Where we might run into problems with the Behringer mic is when conducting noise or clarity measurements.  The small diameter capsule that ensures a good frequency response particularly in the diffuse field, and the use of a pre-charged dielectric (an electret mic) reduces the mic's signal to noise ratio.  It's possible that, if we run into trouble here, we can switch to using other mics, eg the writer's own microphones based on AKG full condenser capsules. 

We'll also be looking into the use of miniature electret mics to investigate what's actually happening inside the flute, eg at the stopper, and at the foot.  We may find that such mics are not that accurate, but if necessary they can be calibrated off our reference mic.

I'm not committed to the use of the Behringer mic as our primary reference; if we find we need to get something better, we will.  The price of a Type 1 measuring microphone is not out of the question (about one 6-key flute, to express it in an internationally understandable currency).  But such expenditure is likely to put off others.

We will also want to accept and analyse recordings made by others, and of course here we have no knowledge of or control over the mic or the systems connected to it.  And they are unlikely to have access to calibration equipment.  Fortunately, there is a simple test we can rely on.  The range of frequencies we need to cover is approximately the same range of frequencies represented by the human voice.  If a recording of a voice plays back realistically, we can take that as a very good sign that a recording of a flute will contain no huge surprises.  This makes it look very probable that flute players around the world will be able to participate meaningfully in these investigations.

Microphone Preamplifier

I'm not particularly happy about my current microphone preamplifier - an "Ultragain Pro" "High Precision Tube Mic/Line Preamp Model MIC2200", also made by Behringer.  When it works it seems OK (but with no remarkable capabilities).  Like many companies these days, Behringer seem unaware that switches for audio need to be based on sliding contacts using precious metals.  (Oxides formed on regular metals soon render snap type switches unreliable for audio.)  Consequently, the switches on the device cannot be trusted to remain in contact, and often need to be "exercised" (pushed on and off repeatedly) to make them work again.  Hardly "precision", hardly confidence inspiring, and rather annoying!  I admit I bought it in a moment of weakness when I was too preoccupied with other matters to do the decent thing and build my own.  I will use it for the time being but the future control unit will incorporate a pair of precision microphone preamps to replace it. 

D-A and A-D Converters

Once we have decent signals, we need to convert them from analog to digital so that the computer can deal with them.  The soundcards fitted to computers are "not bad", but far from top class, and it's the microphone input that is usually worst of all.  So, to do this properly, I wanted to put the Digital to Analog converters beyond reproach.  I've opted for an external unit, a PreSonus Firebox, connected to the computer by Firewire.  It offers an audio performance far greater than we could possibly need, especially considering the relatively undemanding nature of flute sound.

Speakers

I'll obviously be interested in listening closely to recordings I make, and others I may receive, so speakers better than those that normally come with a computer would be an asset.  I've chosen Fostex PM0.5 MkII Nearfield Monitors.  These incorporate bi-amped 2 x 35w amplifiers in each unit, and claim a free-field response of 50Hz to 20kHz +/- 2dB.  Of course our measurements do not rely on the qualities of the loudspeakers, so providing the response of the speakers is enough to alert us to issues, that's going to be plenty good enough.  Fostex offer a subwoofer to extend the response below 50Hz, but since we're only interested in 250Hz and above, that's not needed in this application.  Might add one later for personal gratification in other applications!

Control Unit

After a long career in professional broadcasting, recording and sound restoration, I'm used to having manual control over signal routing, levels, etc.  I find the "Mickey Mouse" mixers in computer sound ultimately frustrating.  So, I'm planning construction of a dedicated custom control panel to return full manual control to me.  I've mocked up a crude passive interim device to test out the concept while I confirm the final design.  Even so, it's a godsend already!

Essentially, the control unit allows me to select the inputs to the A to D converter, direct the outputs from the D to A converter, and let me choose to monitor signals coming in or out at will.  To be environmentally responsible, I can also shut down all the audio (including the powered speakers) when not in use, at a single convenient power switch. 

The final control unit will do much more.  It will also incorporate the precision microphone preamps and P48V phantom power, provide drive for outputs, speakers and headphones, and headset microphone for Skype, incorporate high impedance inputs for oscilloscope probes, and provide calibration outputs and measurement to enable absolute measurement.  An interim specification for the control unit is at Appendix 3.  I'll publish more details as the design emerges.

Sound Level Meter

We won't have a lot of need for absolute sound level readings, but I do have an old analog Realistic type II sound level meter that will probably fill the need.  I did have a more modern digital unit with all the bells and whistles, but it died spontaneously just after the warranty period, and the suppliers advised that such units cannot be fixed, at least not economically.  That makes it unattractive to replace it with a similar device from this modern era.  We seem to have entered a dark period in human development!

This was borne out recently by a power meter I bought.  When I plugged in the batteries, it didn't come on, and one of the batteries quickly became very hot.  I removed the batteries and ran an ohmmeter across the battery terminals - the battery holder was dead shorted.  This device could never have been operated.  It still had the Quality Check (QC) sticker on the front panel to assure me all was well.  Clearly, the QC sticker simply attests that the case isn't scratched. 

Recording and listening environment

The recording and listening environment is an important contributor to overall accuracy.  Ideally this work would be done in an anechoic chamber, but we're not looking for the top level of scientific repeatability, so hopefully we'll get away with an office well packed with books!  We will be conducting tests to determine to what extent standing waves within the room are influencing measurement accuracy.

Background noise is another issue that will impact on recording and measurement.  Fortunately, we are located on the suburban-bush fringe, and urban noise is negligible.  The dominant noise source (when the kids are out and the cat's been fed) is the noise from the office computer we'll be using to record on to.  A new computer has been ordered, and an ultra quiet power supply specified.  A lined cabinet will be made up to minimise interference from the remanent noise.

Test equipment

Now why should we need test equipment?  We just plug it all together and it goes, right?

As we've seen above, what products offer doesn't always translates to what they deliver.  Sometimes, they simply give up the ghost, like my Digital Sound Level Meter.  Sometimes they linger, like my Behringer High-Precision Tube Mic/Line Preamp.  But sometimes they seem to work, but give performance far short of what they promise, and that is perhaps the worst sin of all.  As I mentioned above, we are living though a dark period, where we seem no longer capable of fixing things we've built.  It's become clear that the prices things are sold at these days are too little to allow the device to be tested in the factory.  The moral of the story is, if we don't test it thoroughly, we have no idea how well it is performing.

Fortunately, these days, most audio tests can be run using software designed for the purpose (and detailed below), providing you have appropriate interface to connect the device under test to the computer.  I've built such an interface device, and a version of it will be incorporated into the control unit being developed.  In addition, we have multimeters to handle the usual range of DC and AC current and voltage, and resistance, inductance and capacitance tests.

Artificial flute blower?

I hadn't intended this when I set out on this program, but I'm finding more and more reasons to consider building an artificial flute blower.  It clearly can't replace humans, but can take them and their weaknesses temporarily out of the equation while we study the underlying physics.  Such a system would include a low noise, adjustable, stable air source, a way to hold flutes, and an artificial mouth with artificial lips, adjustable for gap width and height, and for angle and distance.


The Software

This section deals with the software we'll be using to carry out the investigations, and to validate and calibrate the systems to be used.  Where possible, I'll try to confine myself to using freeware and shareware, so that others can try out what I'm doing without need for expensive software.

Some of this software I'm only coming to grips with myself, so you might find I shift horses midstream.

Audacity is a free Digital Audio Editor created by an international group of enthusiasts.  We should be able to use it for all our recording and replay purposes, and for editing recordings to remove false notes, etc.  We can also use it to view waveforms.  Audacity also incorporates filters and some analytical capacity, including FFT.  It can export files as .wav and, with the associated Lame plug-in, in the convenient .mp3 format. 

Scope or Soundcard Oscilloscope is a very nicely presented software test suite incorporating a two-channel oscilloscope, Fast Fourier Transform, a two channel signal generator and more.  It is not freeware, but available freely for non-commercial educational purposes.  It should help with fault finding, proof of performance and some analysis.

Visual Analyser is like Scope but seems to offer even more.  The downside is that it isn't quite so clear in its operation.  I suspect it will repay further investigation.

Rightmark audio analyser is a truly extraordinary test facility for stereo soundcards.  You plug the output of your soundcard back into the input ("loopback").  After setting the levels, Rightmark provides a burst of special signals, and analyses what it hears coming back in, showing the shortcomings of the soundcard.  If asked, it even writes a report in .html, with graphs, tables and evaluations.  Stunning! 

Of course it's not limited to testing soundcards.  Incorporate a mic preamplifier in the loopback and it will test that as well.  It even provides a facility for subtracting the shortcomings of the soundcard test from the combined soundcard and preamp test.  Neat-o!  We'll use it to prove our control unit and sound interface are up to scratch.

Room Mode Calculator.  The dimensions of a room predispose the room to favouring some frequencies over others, so it's worth checking out.  Calculating room modes is a simple enough job, but this calculator makes it a snap. 

Frequency-distance calculator conveniently lists those frequencies that might have quarterwave resonances at the distance entered.  Useful for identifying the reason behind any peaks notes in the room response.

Room EQ Wizard is a nicely presented Java application for measuring room responses and correcting modal resonances. It includes tools for generating test signals; measuring Sound Pressure Level; measuring frequency and impulse responses; generating spectral decay plots, waterfalls and energy-time curves; generating real time analyser (RTA) plots; calculating reverberation times; displaying equaliser responses and automatically adjusting the settings of parametric equalisers to counter the effects of room modes.  We can use it for tuning our recording and replay environment.  Available for free after joining the HomeTheatreShack forum.

Realtime analyser is another very comprehensive suite of audio test software, and another one that I haven't delved into deeply enough yet.  It is not freeware, but can be tried out for free.  It then degrades into limited functionality until purchased.  A feature that might interest us is the ability to synthesise complex waveforms by adding harmonics in adjustable amounts - the opposite of FFT.  This has the capacity to help answer questions about how we perceive these harmonics.

There are several other suites similar to the above which I haven't yet had the time to delve into.  If they look promising, I'll note them here.

If you have come across interesting audio test and measurement software that appears to have something to offer our investigations, let us know!

And we shouldn't forget our old standbys that have served us so well in tuning research:

We may well be turning to them to help explain some of the correlations between tonal and  tuning issues.


Other instrumentation

We'll need facilities other than sound related.  These are available and should help:

Stereo zoom microscope:
Objective zoom range: 0.7x - 4.5x (zoom ratio 6.5:1)
Wide field eyepiece, WF10x/20mm
Working distance: 100mm - without converter lens
Total magnification: 7x - 45x
Round working stage, 95mmŲ
Coarse focus adjustment knob
Inter-pupillary distance: 54-75mm
Dioptre adjustment: ±5mm
45° inclined, 360° rotatable binocular head, wide fan-shape base, no light.

LED ring-light with 4 switcheable sectors and adjustable intensity

Microscope camera, 3.2 MPixels, High speed, USB2, with ScopePhoto data acquisition and annotation software.  ScopePhoto also facilitates measurements of distances and angles.

Digital Camera with Macro, Nikon 3.34 MPixels

Digital Scales, 500gm, 0.01gm resolution, with calibrated weight.

Temperature and Humidity meter, Testo 608, 0-50C, +/-0.5C, 10-95%RH, +/-2%RH.

Datalogger, for long term temperature and humidity logging

Magnahelic Flute Leakage Detector

Manometer for blowing pressure measurement

Measuring tools - micrometer, verniers, rules, bore gauges, magnifiers, etc


The Background

You might wonder if I have the required technical background to carry out these investigations.  After all, what would a common flute-maker know of precision measurement, high end professional audio, electronic design and construction, musical acoustics, software development, etc etc?  A fair question.

Like probably most flutemakers, I didn't start out that way.  My early career was in electronics, in the laboratories of the Research School of Physical Sciences, and later the Research School of Earth Sciences, at Australia's most advanced research university, the Australian National University in Canberra.  Our group designed and built the measurement and control equipment used by scientists investigating everything from diffusion at temperatures close to absolute zero, through nuclear research, to telescopes, to the age of the earth itself.  Because the research was cutting edge, the equipment we designed and built had to be cutting edge.  We were even favoured with moon-rock samples for analysis from the lunar landings, so we were definitely up there with the world's best.

After that, I decided I wanted to get into something with more human interaction and musical interest, so I moved into audio engineering in Australia's fast growing community radio sector.  Over the following ten years or so, I designed and built four broadcast suites (including a lot of the equipment), two recording studios and two transmitter sites for two radio stations. 

Then I got lured into sound preservation at two of Australia's top-end sound archiving institutions - starting at the National Library of Australia, and ending up as Head of Sound Preservation at the National Film & Sound Archive.

It was only after all that that I felt flute making was a viable full-time enterprise.  I'd been doing it since the mid 1970's but limited by the market for Irish flutes in this relatively sparsely populated land.  But when the Internet really got going, the world was suddenly my market place.

But science wasn't forgotten.  I was lucky to have one of the world's most respected musical acoustics experts, Professor Neville Fletcher in the next valley.  And I fell in with Professor Joe Wolfe at the University of New South Wales to help work on computer assisted flute design.  (Both flute players as it turns out.  As someone once said: "What is it with Australians and flutes?")  As you can imagine, I haven't been slow in learning what I can from them.

So, I think I can say with confidence, yes, I have the background to pull this off.  And the resources, and the determination.  I'm also just a few years off the official retirement age in Australia, 65, and while retirement is the last thought, I will be able to divert more of my time to research.  Clearly, there's still much to be done!


Conclusions

It's all rather good isn't it.  Our laboratory equipment and software available either for free or not much - were researchers ever so favoured?

 


Acknowledgements

Thanks to all those who are following this series and providing suggestions and comments!

 


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  Created 1 Dec 09