Although this looks very technical, and it is, there is a profound meaning and truth here. If you examine the two graphs, the upper one with the sensor and the lower one with the mic, you can see clearly that the upper graphic shows much cleaner spikes, and that they are equally spaced, and they are taller. What we are seeing is that the mic picks up the information from the sound pressure through the air, and the sensor only detects only the movement of the string. The difference between these two sources is not just loudness, but frequency also. This is because the mic can't pick up and distinguish the frequencies like the sensor can. They are "smeared" by the mic, and so the ETD can't measure and indicate the exact frequency. Notice in the lower graph that there is a spike between the first and second partial that isn't there in the image above. The first partial is the first spike from the left-to-right, and there's no box for it. The ETD is going to try to "use" that spike in the calculation of the target frequency. Oops!
The information that is most important to know here is in the boxes showing X and Y axes. Click on the images above to clearly view the information. The "X" number is frequency, the "Y" is amplitude. Both amplitude and frequency are different of the mic vs the sensor. But the fact that the frequency is different is the most important take-away here. If you look at the table, you can see differences as much as 1.49 cents on the fundamental, and other differences of the higher partials. This is why using a sensor is much superior to using the built-in mics or even the best external mics.
Unfortunately, this is not intuitive to our minds. A mic is very good, right? We make nice recordings with them, and they sound amazing. But for creating an accurate piano tuning with an app, the mic is not ideal. The soundboard and bridge arrangement is like a filter, where the board is more efficient on certain notes and makes them sound louder, and others softer. The acoustic signature of the piano structure is not smooth, which is one reason we do voicing. We need to try to eliminate this inequality of sound pressure by softening or hardening the hammers, and other procedures. But even the best pianos are going to have volume and tone variations note to note, section to section. All of this is affecting the sound coming from the piano, but is undetected by the sensor. Yes, it will detect a change in amplitude of a certain partial if you voice a hammer, but otherwise the sensor is "deaf" to the sound of the piano.
For example, when you make a recording, you want the raw track to contain all the information. Then, using plug-ins, or outboard gear, the original waveforms are adjusted and edited. But if you use the plug-ins and other gear in the original track, you can't remove it. Similarly, we don't want the filter of the piano soundboard to affect the real information coming into the ETD. The only way to get the best input to your ETD is through a sensor picking up string vibration.
One question may come up, which is, "Can't I just get a guitar pickup and make it work? Why do I need the PIanosens?" Over months of testing, we met problem after problem doing just that. These guitar pickups are prone to noise, hum, and the magnetic pull on the strings. It is well known among guitarists that the pickup you use will affect the tone. Pickup makers have dozens and even hundreds of designs, which cater to the taste of the player, as their output is not equal over the frequency spectrum. In the end, Steve collaborated with an engineering firm who specializes in magnetic sensors in the aerospace industry to produce a product that has an even, uncolored spectrum, and is not affecting the string with magnetic pulling. They use million-dollar machines to wind coils with wire that is hair-thin, and make thousands of turns of that wire on the core. There is no hum or noise being picked up by the sensor, or at least the noise floor is almost not detectable. It is crucial that the noise floor is very low, so that the ETD can better detect the string vibration and not be affected.
Please also note that the graphs are not produced by any of our ETD's, but rather in a program called MatLab which is widely used by engineers, professors, and other companies around the world which have capabilities beyond what our cell phones and ipads can do. It is a laboratory standard in these fields. Steve has spent countless hours making the videos and spectral analyses on the subject of piano technology, making sure that any of the concepts he is presenting is impeccable and proven beyond any doubt. No one has done this kind of analysis before, partly because it would take too much money to pay someone to do it, and the tools weren't available that could do the job. His vast experience in the communications industry and signal processing has allowed him to make discoveries in our field that no one else has been able to make. Sometimes his presentations are a bit too technical for those of us who aren't engineers, but I have learned a lot from him over the past year. I hope these concepts are made a little more clear by my explanation to my fellow laymen.
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Paul McCloud, RPT
Accutone Piano Service
www.AccutonePianoService.compavadasa@gmail.com------------------------------
Original Message:
Sent: 01-12-2024 21:11
From: Steven Norsworthy
Subject: Documented Mics and Placements Affecting the Frequency Measurements
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Steven Norsworthy
Cardiff By The Sea CA
(619) 964-0101
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