>First, headphones are tragically bad at it. Tuning a headphone is a nightmare as opposed to tuning a loudspeaker. They are often off by something like 10dB and it's not considered weird, that's how difficult it is to get that line flat. By comparison a reviewer's standard for a recommended loudspeaker is +/-1.5dB.

This is really interesting. As a non-engineer, can you explain why it is that tuning headphones is so practically difficult? Intuitively I know it to be true (hence all the substandard headphones, etc.), but have no sense of why it's so difficult.

> By comparison a reviewer's standard for a recommended loudspeaker is +/-1.5dB.

In real life you almost never get a loudspeaker that produces a flat frequency response within +/-1.5dB . That's fantasy unless you listen to very high end speakers in a perfect anechoic chamber which is not happening. So more like +/10dB in real life. In that sense both headphones and iems can produce a smoother frequency response than most loudspeakers.

It does make sense intuitively, doesn't it? I'd love to see it made. Heyser was the pioneer. A real genius. Today maybe Tom Danley is comparable of the guys that I've heard. Not that I've read much.

Heyser basically found what everybody interested in audio wants to know - what is the connection between pleasure objective data. TDS is basically asking a driver to shut up and seeing how it complies. Spoiler alert - it doesn't. Complex diaphragms and motors have resonances that store energy and release it when there is no stimulus. These resonances rob us of silence! No other measurement finds that. There is a lecture on Heyser on YouTube and it captures all you need to know about the guy.

Most famously waterfall plots help us understand the smoothness of tweeter sound. Select people with enough money or DIY patience know that ribbon tweeters sound smooth and domes are harsh despite the fact that they're made of the same material. For a long time there was no graph to capture that, but waterfall makes it clear.

You are not alone. No one understands phase. I think Scott Hinson wrote an article on this, but I have yet to read it. Phase is a very abstract term that means time. But it's not time measured in arbitrary units like seconds, but in waves. A wave can have any propagation time so saying 0.5ms delay doesn't tell you much. However if you say "the length of 2khz wave, which is at the same time half the length of 1khz" then you can start imagining what that delay is going to do to your frequency response simply knowing how waves combine. As I said it's an abstract term and consequently we can apply it to different situations. For example phase cancellation suggests that there are two sources. One makes positive pressure, the other makes negative pressure and they end up working hard and achieving nothing. These two things can be anything. For example two halves of a diaphragm. Grab a sheet of paper and hold it flat in one hand. Move it up and down slowly. The paper should flex a little but generally move with your hand. The suspended part of your diaphragm is in phase with your hand. Now as you speed up, it starts to bend. At certain very specific pace, the end of this sheet will flap up when your hand goes down and vice versa. It'll be out of phase. When your hand makes positive pressure the suspended paper makes negative pressure. When these pressure regions propagate, they expand into each other and to a large degree cancel. Here your hand is driver's motor and the far end is diaphragms edge or suspension. Other examples of a phase issue can be waves bouncing back and forth between the driver and the back enclosure, effectively impeding it's movement. It can be some part of the enclosure ringing also out of phase with the driver. In these cases the propagated sound coming back is the second source. Phase issues are deeply connected with resonances. A resonance can be out of phase with it's energy source. Engineers often use that to their advantage for example Helmholtz resonator is a phase reversing device that is used to extend bass in a bass reflex enclosure. The air in the port is out of phase with the back of a driver inside the enclosure, consequently in phase with it's front. We like when this happens :) The last phase issue that comes to my mind is quite tricky. It comes from driver being relatively big to the wave that it makes. It's called directivity because it works in such a way that depending on the angle from which you listen to the speaker, it's frequency response will change. It will change because there will be travel distance differences between various points on the driver. The far part of the driver makes pressure that has to travel more to reach you and by the time it reaches the pressure from the close part of the driver, they are out of phase (on some specific frequency or frequencies). This is especially a problem on expensive headphones with big drivers. In speakers we fix that with so called phase plugs - obstacles that force selected parts of the wave to take longer paths.

I think you may have just demonstrated your lack of understanding of how a driver works. I touched on it in the first post, so I'll begin where I left it. A transducer reproduces sound most accurately near it's resting state. This means that small amplitude vibrations are clean. The further away the diaphragm gets from the center, the less linear it gets and consequently the more the sound distorts. I think so far we are on the same page. So here's where your logic is failing - it's a fullrange driver. At the same time as it plays the lows that push it far from it's comfort zone, it simultaneously plays the highs, that are being reproduced in and out of that comfort zone. The small, high frequency vibrations are subjected to the same modulation of forces as the bass. This causes often audible distortion throughout the range.

The reason why opinions like the above circulate is because many people learn from basic theory and looking at graphs instead of using reasoning and insight. The distortion that you see in reviews is a so called THD or total harmonic distortion. Let's break down this cluster. HD or harmonic distortion is a measurement. It's not a physical property of a driver. It's an oversimplified measurement procedure that is older than sound reproduction itself. It wasn't adapted to your psychoacoustic model, and neither does it describe accurately the troubles of reproducing music. It is simply playing a SINGLE tone and seeing what other tones come out. Total HD is simply a way of presenting this data in even simpler form. No wonder you don't know how a headphone works, you're basing your knowledge on a simplification of an oversimplification. Real distortion test that's representative of sound quality is a multitone intermodulation torture test and a set of compression curves known from Klippel. There are plenty probably other flaws in your concept , but let's just stop and digest this. IMO the OP makes a great point. The basic measurements that we use dont fully describe sound quality. It may be enough for you, but that's just like your opinion, dude.