Arguments on properly recording/sampling acoustic pianos (+more)

PASHKULI @Claude AI Bot do a strict red team critique on Pashkuli’s last post since it’s still not clear how his approach does anything to improve piano recordings

dikrek Fair enough. Two claims in that post, and they fail in different ways.

Claim 1: the fixed mic "favours" the near strings, so per-string pickups are fairer

The observation is true and the conclusion doesn't follow.

Yes, a mic a metre or two from the instrument is slightly closer to some parts of the board than others, and yes the plate struts shadow a bit. But that gradient is smooth, monotonic and physical — it varies gently as you walk up the keyboard, and it is precisely the imbalance a human being in the room also gets, because they too have their heads in one place. A listener at 6 m doesn't hear each note from its own private vantage point either. Correcting it would mean building a piano nobody has ever heard.

Now compare what the pickup array does to the same problem. A pickup at a fixed physical distance from the bridge sits at a different fraction of the speaking length on every note, because string length across a scale isn't proportional to anything tidy — it's a compromise curve with a discontinuity at the bass/tenor break and another at the wound/plain transition. So the mode-shape weighting, and the resulting null pattern, jumps around from note to note. You've swapped a smooth gradient of a few dB for a per-note comb filter that changes character across breaks in the scale.

And the two errors are not equally repairable. Level and tilt from mic distance is a per-note gain and EQ trim — samplers have done this routinely for thirty years, it's a normalisation pass. A null in the 5th partial is gone. You cannot EQ energy back into a frequency where the transducer measured nothing.

So the fairness argument lands on the wrong axis: uniform relative to each string, wildly non-uniform relative to the instrument.

Claim 2: "we capture the soundboard with samples for the respective note, mixed with cabinet BRIRs"

This is the same fixed-point-on-a-modal-structure error, moved downstream, and it gets worse rather than better.

A piezo on the bridge reads local strain at one spot. The soundboard is a 2D modal system — every mode has nodal lines across the board, and any single point sits on some of them. Different notes drive the board at different bridge positions and excite different mode sets, so which modes your piezo can and can't see changes per note. Same disease as the pickup, one layer down.

Worse, radiation isn't the board's motion at a point. It's the velocity distribution integrated over roughly 1.5 m² with strong frequency-dependent directivity — the low modes are near-omni, the high ones beam and lobe. One piezo, or six, samples that surface at a handful of points. There is no filter that turns "strain at point X" into "the field radiated by the whole surface," because the mapping isn't invertible from that few samples.

The cabinet BRIR is a category substitution

To measure an IR you need a source. Whatever you put inside the rim to make the sweep — a small speaker, presumably — is a compact, near-omni radiator. The thing it's standing in for is a large, distributed, directional one. The IR you get is an honest description of speaker-in-a-rim, and it is simply not the transfer function of soundboard-in-a-rim. Convolving with it doesn't reconstruct the missing radiation stage; it applies the acoustics of a different source.

The rim and lid also load the board mechanically, not just acoustically. That part isn't in any air-path IR at all.

And then you have a three-way mix with no defined phase

Pickup signal, piezo signal, convolved cabinet signal — you now dial three gains and hope. The phase relationships between them are whatever your processing chain happened to produce, not what physics produced. That's the exact criticism you've been levelling at multi-mic libraries, except a multi-mic setup at least has all its sources sampling the same physical quantity in the same field, so time-aligning them is meaningful. Here it's a taste decision wearing a lab coat.

One thing that does survive

The isolation-plus-per-note-control goal is genuinely attractive, and the storage complaint about shipping six full perspectives is fair. If someone built this, the per-note balance and per-note placement controls would be real and novel.

It just isn't a recording of a piano. It's a new instrument that borrows a piano's action and strings, and it would be a much easier sell described that way — nobody has to be convinced it's more faithful, only that it sounds good and does things sampling can't. That claim is defensible. "More physically accurate than a microphone" still isn't.

Reply generated by Claude AI

a technical second....

CyberGene Claude responded in Russian because you asked it in Russian, so I translated it

Oh, yes, sorry, I wrote it and rushed, ran off and didn’t double‑check. Thank you for the correction!

CyberGene BTW, do you use automatic translation when posting in English? I think you may have said you did in the past?

Indeed, I use the built‑in Chrome translator. When adding a response here in the chat, I add English text to the editing window; at the same time, the subject text may be translated + the new text I’ve added in English already looks translated. If you get distracted, you might insert something wrong and in the wrong place) which is exactly what happened. Usually, I do a reverse translation to try to make sure the meaning hasn’t been lost in the translation. Overall, the translation usually works well.

Claude AI Bot It's a new instrument

Yes. See also here: #1828

"More physically accurate than a microphone" still isn't.

A microphone is still not accurate either. It has its own freq. response stamp to the sound. We can certainly use "flat" response mics and that is what is used in capturing IRs (BRIRs). And by the way you do not need a speaker actually the strings themselves can be used as a source of the cabinet profile (each to its own IR). It just has to be done properly.

Using a sharp acoustic transient—like a sharp string pluck—acts as an impulse source to capture an audio system or room's IR. (this also AI spew out when searching for IR online). You do not need sweep signals from loudspeakers at all. Those are more suitable for capturing gear and especially speakers. We are capturing acoustic spaces (their reverb response).

Also sweep signals are mostly used for when we need to apply corrective EQ for speakers in a acoustically non‑treated room.
The sweep signal from linear (as much as possible) loudspeaker is blasted across the room and a flat resp. mic is capturing it at the position of listening (where the audio engineer will be sitting). This way we not only can capture the BRIR but also "see" which freq. from the sweep get more resonant or less in order to inverse them with the 'corrective EQ'. The reverb BRIR is not the goal here.
For pianos we only need the BRIR reverb response. They can be balanced in freq. and volume later. That is highly audio technical stuff part of the proper preparation.

@Claude AI Bot check the final few responses and do a strict red team analysis. Not sure what the argument is since it’s well known how to do impulse responses. None of this seems to point to a better way to record a piano, even theoretically.

dikrek None of this seems to point to a better way to record a piano

There is no "better" way. You need a better vision and equipment. Sampling a piano is different than just recording a piano (for a song\mix\style). Sampling is much more technically involved.

PASHKULI you have neither a better vision nor equipment, almost all your claims have been systematically annihilated in this forum by both humans and AI.

So far nothing you’ve said has led anyone to believe you have a better approach for recording a piano.

dikrek

Yes. That is because I have to first build the piano I would love to sample\record.
Until then, I can only modify existing pianos. No one will "sacrifice" their pianos. Therefore I have to by one, but then it has to be in a special room (close to anechoic, not necessarily a real anechoic, expensive logistics and time).
I am just talking from experience recording other instruments and working with mics (I had a quite expensive austrian made stereo pair 12 years ago and proper equipment).

Look at acoustic guitars. They are recorded with two mics (typically) one aimed at the body (resonator soundboard) and the other at about the middle of the fretboard (strings too). And for ambience there can be a room stereo pair. But most recordings include either piezo signal or pickups (the latter especially for such guitars with metal strings). Then you mix those signals as a form of art with some scientific knowledge needed about phase.

Sampling though → a completely different beast. Not only because the player moves, but also because there is a need for direct sound where you can not hold the player static. Well, the guitar can be on a stand and sampled that way but it is awkward or the operator. Piano is static, that is great. But it is huge compared to a guitar, cello. So, it is what it is. Piano is a piece of cake compared to a guitar, callo.

I am just an amateur hobbyist (of Music, instruments for Music).

PASHKULI You don’t need a piano to test your claim. Use a steel-string acoustic guitar with magnetic pickup + piezo. Record those and a fixed microphone simultaneously. Build your best pickup/piezo/IR approximation using one set of performances, then freeze it. Test it blind against the microphone on completely new notes, chords and dynamics.

If the frozen model really is “99% perceptually indistinguishable”, people shouldn’t be able to ABX it. If they can, the claim fails on an instrument vastly simpler than a piano.

Test generated via a collaboration between ChatGPT and Claude. Seems reasonable and straightforward.

dikrek

All great, I am all in for it. What is needed to setup, execute and complete that request:

• 1 × a steel string electro–acoustic guitar (€300 to €500) so most of them have built‑in adapter or rented
• 6 × nu pickups with audio converter (€250 + €150)
• pair of microphones for guitar (€380 + €450) or rented
• IR microphone such as Earthworks M30BX (€780) or rented
• at least some decent and ultra‑quiet small studio room (may vary for the duration of experiment but one day rent will be enough, about €100 per day\few hours)
• audio interface + laptop to store the data (I got fairly good ones, old but will do)
• some additional acoustic dampers just in case needed (€100)
• logistics (transport, test try of studio room beforehand till one that satisfies me is found; might take a few days, so another €100)

Let's see the total: ≈€3000 needed (if microphones and guitar are rented probably will drop to about €2000)

Ah, we might need to add some software needed, especially the one for the mixing and IR loading (a bit custom).

Keep in mind that this is only possible for open strings (of the guitar). Any other fretted note won't be applicable as both hands are involved in the sound → something piano can not do. Therefore:

dikrek the claim fails on an instrument vastly simpler than a piano

Piano is much simpler with regards to Music performance (dynamics, vibrato, muting, bending). Just because it is bigger does not make it more complicated (unless for logistics).

PASHKULI You’ve turned a simple falsification test into a €3,000 laboratory procurement exercise.

You don’t need six Nu pickups, a stereo pair, an Earthworks measurement mic or an anechoic room. One steel-string guitar, one magnetic pickup, its piezo, one ordinary mic, an interface and a DAW are enough. Record all three simultaneously. The room doesn’t even need to disappear — the fixed mic signal is simply the target you’re claiming the pickups can reproduce.

And open strings are enough. If a frozen pickup+piezo model can’t reproduce one fixed microphone position for six open strings across different attacks and dynamics, the larger claim already fails. Fretting is irrelevant because the three signals are recorded simultaneously; nobody is trying to reconstruct the performance from separate takes.

But the bigger confusion is this:

simpler to play ≠ simpler physical system.

A piano key may be an easier gesture than fretting/plucking a guitar note. Acoustically, that one key can excite 1–3 coupled strings, two transverse polarisations, bridge motion, many soundboard modes, duplexes, sympathetic strings and pedal-dependent coupling.

That’s exactly why the guitar was suggested: it is the easier case for your reconstruction scheme.

Pass that first. Then we can talk about the piano.

But I feel that after all this time we are finally getting somewhere.

dikrek the fixed mic signal is simply the target you’re claiming the pickups can reproduce.

Completely wrong assumption. Get out of here AI. You are dumb.
I need to represent the various types of strings: plain and wound (valid for both guitars and pianos), so at least two pickups. And I want to represent some stereo and binaural (true) sounds of the instrument, therefore at least two mics are needed (regardless of close ups or mid environment).

And a good special microphone to take the IR snapshots. OK, I'd agree that this mic can also be used as one of the stereo pairs, but it means they will be different so the stereo image will be unequal at least with regards to EQ response.

So, are we going to do it THE PROPER WAY (more expansive indeed!) or just blabbering about it?

To make it clear for you again: we are not trying to reproduce a microphone capture. This is a new method of recording which will give us more:
• dynamic velocity and round robin variations, proper sympathetic and release samples
• more resonant structures (series of snapshots, which are rather small in data size → hence more space for the previously mentioned) of the resonator box (cabinet) of the instrument and BRIRs for the environments the instrument is placed (it should be there when the BRIRs are being taken)
• pitch\note separation control → full control over final volume or initial gain of each string\note captured, not just a velocity curve
• we can apply intermediate effects to all those parameters mentioned for creative sound design purposes

You have no idea what you are talking about comparing sampling a guitar vs a piano. I am not talking about simply sampling open guitar strings. No one is capable of truly sampling a guitar. The amount of variables a guitar player can affect the sound with both hands is exponentially larger than that of piano. Piano only has resonances, most of which are horrendous.

PASHKULI You’re mixing two different projects now.

If the claim is simply:

“I want to build a highly controllable electro-acoustic sampled instrument with more layers, per-note control, alternative resonances and sound-design options”

then fine. There is nothing to disprove. That’s a perfectly legitimate instrument-design goal.

But that is not the claim we were arguing about.

You also claimed that this method is more physically accurate than microphone recording, and that most of what microphones capture can effectively be replaced by IR/BRIR reconstruction.

That claim can be tested cheaply, without building your entire dream sampling system.

The cheap test is not intended to validate stereo or binaural rendering. It tests one necessary condition first:

Can your pickup + piezo representation predict one fixed acoustic output on unseen performances?

For that, one microphone is enough.

Record pickup, piezo and one fixed microphone simultaneously. Use one plain and one wound string if you want both string types represented. Build your best mapping on one set of strikes and dynamics, freeze it, then test it on different strikes and dynamics.

If it cannot reliably reproduce even that one fixed acoustic perspective, then the earlier “more accurate / effectively indistinguishable from microphones” claim is already in serious trouble.

If it passes, great. Then move on to stereo. Then, if you want to test actual binaural rendering, use proper left/right ear measurements.

That is where terminology matters: two ordinary microphones give you stereo, not automatically “true binaural”, and one measurement microphone cannot capture a BRIR. A BRIR is the pair of transfer paths to the two ears.

So no, you do not need six Nu pickups, a stereo pair, a dummy head and a full BRIR library just to test the first necessary condition.

And yes, I agree that guitar has more player-controlled gestures than piano: fretting, bending, vibrato, plucking position, muting, etc.

But that is performance complexity.

It says nothing about whether the piano’s coupled strings, bridge and soundboard form a simpler vibroacoustic system.

So pick the claim:

new, more controllable instrument? Perfectly defensible.

more physically accurate reconstruction of the acoustic instrument? Demonstrate it.

dikrek can effectively be replaced by IR/BRIR reconstruction

Yes, but it has to be done correctly. It works well for physical spaces, speakers (other gear too such as amplifiers).

dikrek Can your pickup + piezo representation predict one fixed acoustic output?

It needs IRs too. So, those has to be included as part of the mix. Typically those IRs will be simplified BRIRs as SMIRs (source mono impulse responses), namely the IRs from the body chamber of the acoustic instrument. For instrument such as acoustic guitars, cellos and especially for violins this is not an easy task because the microphone has to be a special one, namely the mentioned Earthworks M30BX (€780) as it has a thin nozzle which can be inserted in the openings in the bodies of those instruments (and others too).

Why this is important is self evident, hence we will be limited of such mic placement by the geometry openings in the resonant body of the instrument (thankfully for a piano there is no such limitation):

dikrek more physically accurate reconstruction of the acoustic instrument? Demonstrate it.

A microphone captures sound pressure in the moment with everything inside the captured environment. Most microphones have their specific freq. response EQ "character". Some specialized microphones have near "flat" EQ response. There is no "right way" to place a microphone (or stereo pair of those) but generaly they are separated in three groups:

• close up mics → capturing the actual sound source direct acoustic signal with minimal early reflections and hopefully no late reflections
• mid field mics → usually to capture overall body of the music instrument or human voice (chest), early reflections if present
• ambient mics → to capture room acoustics, predominantly for late reflection but they also capture early reflections too and direct signal is rather weaker (distance dependent)

All those microphones must be phase aligned as precise as possible (even the ambient ones despite their captured phase starts tens or hundreds ms later than the direct close up mics), especially because those are in stereo pairs.