Pianotech

  • 1.  Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Posted 07-14-2024 17:24

    We have had a lot of discussion on this listserver over the past year concerning use of magnetic (string vibration) sensors for ETD input as opposed to the far more common use of microphones. I thought that a scorecard might be in order and would be good preparation for discussions on the topic that might take place in Reno. I'll try to summarize in the following table the significant tuning practices, procedures, and other considerations that a tuner will address in considerations for use of a magnetic pickup versus a microphone. Regards, Norman.

    TOPIC

    TRADITIONAL CONSIDERATIONS

    MAGNETIC SENSOR CONSIDERATIONS

    (1) Attack Measurement

    Both aural tuners and visual tuners have mostly all favored using the sustain portion of a struck note to make tuning decisions.

    Claims from a new magnetic sensor indicate that the transient/attack portion must be used at the risk of major tuning errors on the order of close to one Hertz in octave 4.

    (2) Microphone Position

    Good tunings based on microphone input to an ETD are not considered by most of our colleagues to have a significant dependence on the spatial position of the microphone.

    Claims associated with a new magnetic sensor are that significant tuning errors can result from incorrect microphone(s) pickup positioning, again on the order of close to 1 Hz in octave 4.

    (3) Piano Audio

    Traditionally, piano tunings have been based on using the actual audio created by the piano's vibrating strings and coupled soundboard. This includes room acoustics, reflections, human binaural listening (for aural tuners), phasing, soundboard idiosyncrasies, false beats, etc.

    Claims associated with use of a new magnetic sensor claim significant improvements in a tuning can be achieved by not listening to the piano's audio during the tuning process.

    (4) Unison Tuning

    We all tune unisons with every tuning, and for my own tunings and those of others whom I have had occasion to witness, I find/expect unison accuracy to be on the order of 0.1 bps (beats per second) in octave 4, and up to 0.0125 bps in the highest octave.

    Recent reports are that only with a magnetic pickup can a unison be properly tuned. It is also asserted that false beats can be best tuned with use of a magnetic pickup

    (5) Partials Analysis

    Aural tuners focus on one coincident partial occurrence at a time, and can then integrate among different choices. Visual tuners often have algorithmic selections available (single-partial and multi-partial) in their ETD.

    A recent opinion from magnetic pickup advocates is that all visual tuners must use ETD's that incorporate multi-partial analysis.

    (6) Time to Do A Tuning

    It varies. A guesstimate for the "average" piano might be one hour.

    I don't believe that there are any claims that using a magnetic pickup saves tuning time. There have been reports of the opposite. I believe that we are still waiting for a video showing use of a magnetic pickup to tune a smaller upright, particularly in the upper half of the scale.

    (7) Quality of Tunings

    Attestations are available favoring use of traditional tuning means.

    Attestations are available favoring use of a magnetic sensor.

    (8) Cost

    ETD cost + perhaps $20 (?) for a microphone (if not using one already integrated into a laptop).

    ETD cost. A recently-available magnetic sensor I believe costs more than $20.

    (9) Obtrusiveness

    Not applicable

    Use of a magnetic sensor to measure a vibrating string, known in the industry as VWS (Vibrating Wire Sensor), if I have it right, can in turn (of course) affect the vibration of the wire being sensed. This is known in physics as the "observer effect". I personally doubt that such is significant in our case of piano string vibration, but I don't recall seeing any details about it.



    ------------------------------
    Norman Brickman
    Potomac Piano Service
    Potomac, Maryland
    potomacpiano@verizon.net
    https://potomacpiano.com
    (301) 983.9321
    ------------------------------



  • 2.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Posted 07-14-2024 19:38
    Norman - very helpful table.

    I've been researching the claims for the magnetic sensor and beyond my personal knowledge of the physics and which I'm familiar the claim with regard to No. 2, the positioning of microphones is based upon a misunderstanding of the paper https://www.researchgate.net/publication/249996381_Modeling_and_simulation_of_a_grand_piano by Chabassier, Chaigne and Joly which refers to spatial variations of amplitude across a soundboard, not frequency. A soundboard simply cannot be driven by the frequency of a string vibrating it and change it to another frequency in a different part of it. The soundboard either vibrates at the driven frequency or it doesn't vibrate. 

    The other paper of significance is https://www.mdpi.com/2624-599X/4/4/62 which examines the effect of damping of soundboard vibrations. Of course all the non sounding damped strings which are damped in the course of our tuning damp the soundboard, as the paper describes, to leave soundboard resonances, (Chladni modes or eigenmodes) irrelevant and for the soundboard to vibrate only with the driven vibration, the string under tuning consideration. As there may be time required for spurious vibrations to be damped, the academic papers would imply that the idea No. 1 that measurement should be made during the transient period is curious.

    With regard to the tuning of unisons, I'm sure that all will agree that any methodology including any device, microphone or app which can help us in the case where we experience false beats is helpful.

    The academic papers are well worth the read, in particular the first one which notes sub-harmonic noise arising from difference frequencies deriving from the difference between high harmonic frequencies, and exemplifies noise from the region of the 17th harmonic. Of course this is irrelevant to us but I believe an interesting finding nevertheless.

    In my opinion it's important to start with the physics as in terms of what we experience, or what we think we experience or what we think we are measuring, however clever we think the process of measurement might be, I'm sure that none of us are free from self-confirmation bias. My academic training was in Physics at Imperial College, London, specialising in electronic engineering and ultimately developing the sonar based reversing alarms that most of us probably have on the back of our cars nowadays.

    With particularly best wishes for a great and illuminating time in Reno,

    David P





  • 3.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Posted 07-15-2024 15:26

    Norman,

    You should ask for public feedback from the users. Many have posted on PTG. You could consolidate all those posts. Then you could interview them personally and ask for more feedback. You could also have an independent lab look at this.

    The history of using electromagnetic (EM) sensing for guitar goes back to the early 1940's. Then for piano, there was a sensing device for the Accutuner. Then there was the Helpinstill device for the whole piano. Since then, we know there are many problems that needed to be solved. EM sensing can be very sensitive to interference such as 60 Hz power. Guitar pickups came up with the 'humbucker' device configuration, which helped. However, the guitar pickups in the market have limited frequency response and dynamic range, and have a pulling effect on the wire to some degree. These problems have been solved with high-tech engineering. I suggest you use an independent lab to verify.

    As for your table you list:

    1)   I am not aware of any claims that the transient 'must' be used.

    2)   I am not aware of any claims about 1 Hz, etc.

    3)   I am not aware of any claims that we 'must not listen' while tuning with a sensor.

    4)   I am not aware of any claims that 'only a sensor' gives proper unison tuning, but only that it facilitates greater accuracy.

    5)   I am not aware of any claim that one 'must use multi-partial' tuning when using a sensor.

    6)   There are many users using a sensor on upright pianos. Peter Grey and Paul McCloud, you should interview them on that subject. Paul has tuned over 300 upright pianos using a sensor.

    7)   The 'pulling effect' was a prior-art problem before, but now has been entirely eliminated and can be verified as such.

    Steve N.



    ------------------------------
    Steven Norsworthy
    CEO/President
    RF2BITS, Inc.
    Cardiff CA
    619-964-0101
    steven@rf2bits.com
    ------------------------------



  • 4.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Registered Piano Technician
    Posted 07-15-2024 18:13

    Hi Norman:

    As you know I've been using the magnetic sensor for about 8 months.  I'm always ready to answer any questions about my experience.  Not all of your assertions are correct regarding the sensor.  If I elaborate, my answer would probably be taken down.  I invite anyone interested to check out the booth in Reno where we can discuss any questions about it.



    ------------------------------
    Paul McCloud, RPT
    Accutone Piano Service
    www.AccutonePianoService.com
    pavadasa@gmail.com
    ------------------------------

    Original Message


  • 5.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Posted 07-15-2024 21:41

    Steven, thank you for validating my #9 on "Obtrusiveness". Not the best choice of words on my part, maybe "readout error" or the physics term "observer effect" or your use of "pulling effect" would have been better. Since it is a noticeable effect with use of magnetic sensors on guitars, and is always present in use of a magnetic sensor on any vibrating string, perhaps you would like to start a new,separate thread on the listserver on it and fill in any details that you have. "The Pulling Effect of Magnetic Sensors on Vibrating Strings"? 

     

    Paul, I'm afraid that I will not be at Reno this week. But I would still be interested in seeing a video of using a magnetic sensor on a smaller size upright – please. Very good on using a magnetic sensor for 8 months. Maybe you or Steven would want to gather and post, on a separate forum, a set of independent testimonials. There was Timothy Michael's recent post here on him stopping use of a magnetic sensor after 100 tunings, but as I say – getting a complete set of independent reviews would be best. Regards, Norman



    ------------------------------
    Norman Brickman
    Potomac Piano Service
    Potomac, Maryland
    potomacpiano@verizon.net
    https://potomacpiano.com
    (301) 983.9321
    ------------------------------

    Original Message


  • 6.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Posted 07-15-2024 21:56

    Norman, I engineered a solution to eliminate any measurable DC pull on the string that would change its natural vibrations in any way, as this design also greatly increases the AC field strength. The device has over 140 dB dynamic range, a flat frequency response from 0-20 kHz, a low impedance, EM shielding greater then -120 dB, and a thermal noise floor below that of the 24 bit ADC in the interface. How did we accomplish that? Hard engineering. It is a collaboration of engineering with an aerospace company who makes EM sensors for the DoD and supplies space satellites with sensors. If anyone tests the device in piano tuning, they can reposition as they please and watch a frequency detection and see there is no change!

    Steve



    ------------------------------
    Steven Norsworthy
    CEO/President
    RF2BITS, Inc.
    Cardiff CA
    619-964-0101
    steven@rf2bits.com
    ------------------------------

    Original Message


  • 7.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Posted 07-16-2024 09:57

    In use in an acoustically noisy environment a magnetic sensor is going to be a boon for many and in that cause I'm wholly supportive.

    However, in relation to recent marketing, big complicated words are often used whilst the effect observed is due to something else.

    In the video https://youtu.be/85oup6ub8YE?t=126 demonstrates a variation of pitch between three strings which is ascribed to the Weinrich effect.

    The effect heard there is not of the Weinrech effect as claimed but typical of a more common experienced effect with which we've all battled. The response of the ETD reaches 0 only momentarily and the instability isn't a function of the strings but something else we've all experienced and however hard we've tried.
    A spectrogram shows the peaks 
    below 441.5 and the other between 442 and 442.5 

    The resulting beat blooming at around 0.3 seconds and minima at around 0.6 seconds is no doubt the experience that encourages the proponent to tune on the pitch measurement before around 0.7 seconds into the note rather than on the sustain.

    The marketing of the magnetic sensor reinforces upon us that we should always be using our ears, not rely blindly upon what we think we see on a machine, and understand with our ears what a machine is showing us.

    Whether we tune orally or not this is the reason why PTG at its conventions is invaluable in the promotion of aural tuning and understanding what we're doing by ear, as it's ultimately the ear which hears the music rather than the engineer looking at machine indicators.

    The reason why I've been possibly perceived to be fussing about technical matters is that before someone starts ascribing a phenomena to some effect, it's important to understand the physics and through that sort out whether some observation is due to something or something else. In some cases people want to see ghosts in the ectoplasm rather than causes right in front of us.

    To avoid disappointments any devices marketed or bought should be bought to sort out particular problems which are experienced for identified reasons which are real, rather than merely mistaken for other reasons. A device won't solve a problem if the cause is something else.

    Best wishes

    David P



    ------------------------------
    David Pinnegar BSc ARCS
    Hammerwood Park, East Grinstead, Sussex, UK
    +44 1342 850594
    "High Definition" Tuning
    ------------------------------

    Original Message


  • 8.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Registered Piano Technician
    Posted 07-16-2024 10:34
    Hi Norman:
    I agree that we need independent reviews.  We all have our experiences tuning with many styles, temperaments, ETD's, aurally, etc., and have our own conclusions.  There's no one way to do it.  But on this forum we aren't allowed to name a particular product, app, tool, etc.  I created an electronic tuning discussion group but no one seems to use it.  I will try to make a video tuning a console piano using the sensor, but I would have to post it elsewhere.  It's not rocket science how to place it, and all im doing is the best I can.  It does work, but somewhat fiddly on the small pianos in the treble section with dampers in the way.
    I agree with David that we need to try to understand scientifically what is actually going on vs what we assume is true traditionally.  

    Sent from my iPhone



    Original Message


  • 9.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Posted 07-16-2024 11:33

    Hi Norman, many are appreciating your starting this thread. It appears some want  to go off topic and debate 'spectrogram signal processing' on this thread and that is not the topic you started. Thereforethey could  start a new thread on such a topic and keep Norman's thread as his intent was originally. The use of magnetic sensors is uncorrelated with how we process spectrograms. Thanks again, Norman. ---Steve



    ------------------------------
    Steven Norsworthy
    CEO/President
    RF2BITS, Inc.
    Cardiff CA
    619-964-0101
    steven@rf2bits.com
    ------------------------------

    Original Message


  • 10.  RE: Use Of Magnetic Sensors For Piano Tuning – A Scorecard

    Posted 07-15-2024 20:27

    Thanks Steven but with respect to (1) 

    https://youtu.be/MraLJ4wx-Sk?t=583 it would appear that you're recommending tuning within the first 0.7 seconds.

    I downloaded the sample at https://youtu.be/5U3sq7ULIOg?t=194 apparently tuned perfectly but on which a beat is audible and put the signal through a spectrogram with resolution of 0.05Hz and then 0.01Hz

    As predicted by the academic paper a lot of sub-frequency noise was apparent within the first 1.2 seconds or so and the spectrogram showed

    (1) no difference whatsoever in pitch between strike and sustain over the first 3 seconds

    (2) two frequencies present - a stronger line at 525.9Hz and a weaker line at 525.5Hz so the three strings were indeed not tuned to perfect unison

    which suggests that the ear detects the beats which the best of normal ETD tuning here was unable to detect and that the first 1.2 seconds is noisier and presents no difference in frequency sampled later in the sustain.
    Not shown here but also splitting of the higher harmonic lines particularly above the 8th harmonic upwards was apparent indicating a difference between the three strings.
    Best wishes
    David P


    ------------------------------
    David Pinnegar BSc ARCS
    Hammerwood Park, East Grinstead, Sussex, UK
    +44 1342 850594
    "High Definition" Tuning
    ------------------------------

    Original Message