#037: Misconceptions in Acoustics

Having worked with many students and acoustical engineers over the years, I see some topics being perpetually misunderstood. Below, I have picked three that I see come up time and time again...

Avoid parallel walls, and you avoid standing waves

So, the idea is that modal behavior in a room or an enclosure hinges on the walls being exactly parallel, otherwise the modes cannot 'exist'. One quick way to disprove this, is to think about the extremes; what happens if walls are not perfectly parallel, which is probably the situation you would have in any room. Do you get rid of modal behavior? Obviously not. What if you don't have any parallel walls at all? We could look at a spherical room, and quickly realize that we will have infinitely many spherical modes (*). So what gives? Well, what you can change is the distribution of modes in frequency and their shape. But you cannot get rid of them by avoiding parallel walls. Remember, modes exist independent of sourcing/loading. They are an intrinsic consequence of the geometry and the boundary conditions. Whether they are excited or not depends on source placement seen from each mode on its own, and you may be positioned somewhere where this excitation is not an issue.

Modes are present even when walls are not parallel.

A little tie-in with the above is that you don't have 'zero sound' in a node, where the mode in question is zero, which is another thing that I have often heard even experienced acoustical engineers say. At that particular frequency, you do have sound in a nodal plane, it is just not 'supported' by that particular mode. So when doing a modal analysis, you will see that the modal field does have planes where said field is in fact zero, but these are the modes themselves, and not the resulting field when a source is present. Place the source nearer the nodal plane and the mode will be less excited; place yourself nearer the nodal plane and the mode (if excited) will affect what you hear less.

By the way, you can certainly have dips in a measured frequency response in a room, but they are sensitive to position. For more details look up e.g. boundary interference and non-minimum phase considerations in rooms.

(*) Note that if the sound field has a shape that matches the shape of the room, you can in fact you have true zero zones, for example for a spherical source placed in the center of a spherical room. But that is a special situation.

Above a certain frequency, you will have modes in a tube

So this is often said and written, oftentimes when transmission lines representations are discussed. I have already written a blog post on modes in tubes, but let us take the very basics here: Unlike room modes where each mode has a particular modal frequency, tube modes each exist at any frequency. While this may not feel comfortable or familiar, that is how it is. The tube modes each have a particular cut-on frequency, which determines if they will propagate or not, but each mode can be excited at any frequency. The plane wave can always propagate; you can say that it has its cut-on frequency at 0 Hz. As a consequence of the above, if you send a plane wave down a perfect tube (same shape down the axis, perfectly hard walls, no kinks, ...), all you will ever get out at the other end is a plane wave at any frequency. The higher order modes are not suddenly 'brought to life' as soon as you go above their respective cut-on frequencies, although that is how is often presented.

The plane wave propagates alone well above the cut-on frequency for the first mode.

When transmission line literature states a high frequency limit it is to avoid the potential of higher order modes being excited, which could happen by attaching a number of tubes that are not aligned down the same axis for example. But you can have situations where this limit is not as hard as it might be made out to be, and you should try and understand tube modes to getter a better feel for this.

As a vibrating piston moves outwards, there is a positive pressure in front of it

Again, I have heard this countless times. And yet, it is generally not true. As a piston in a baffle moves outwards and stops at its outermost position, you will see a negative sound pressure in front of it. You can show this in a number of ways; the Rayleigh integral; thinking of how you are playing into a mass as opposed to a compliance/enclosure; simulate and animate it. Whichever way you do it, you will find that the total pressure goes down as the piston goes out. This is extremely important to understand when you deal with e.g. hearing aids, where you have feedback issues. Get it wrong (as I have seen), and what you think is a fix to a problem, suddenly makes things a lot worse.

Piston displacement, and resulting particle displacement and sound pressure.

The displacement is in anti-phase with the sound pressure, while the acceleration is in-phase with the pressure. One interesting consequence of this, is that the pressure on one side of the piston can be in-phase with the pressure on the other side of the piston inside an enclosure, although the piston obviously moves in opposite directions seen from each side.

Final thoughts

Intuition can only take you so far. That is why you study for years before you get into engineering, physics, or whatever field you venture in to. If you can give the students a good grasp of all the topics before they start their jobs, everyone benefits. But as there will always be some topics that are misunderstood, it is important to keep studying consistently in the industry. And this is where many companies get it wrong, and think that 1) their engineers are at a high level, 2) the engineers can learn what they are missing via the on-going projects. Both are generally incorrect, and it takes a focused effort with knowledge sharing and getting more outside of the company (conferences, presentations, courses) to leave the echo chamber and see that there are so many things that are not in place. The very basics like complex numbers and transfer functions tend to be an issue, and that should be an indication that there is a long way to go. What you should do as a manager is to set aside time for this on-going training; the time spent on the training is well worth it in the end. The publicity alone one could create with such a setup would be invaluable as to show leadership in engineering and to attract new talent.

At Acculution, every Friday is Research Friday: A complete focus on new technology, method development, blog posts, journal papers, and mentoring students. The work is First Principles based, as opposed to being based on working by Analogy (i.e. "this situation 'looks like' something else that I have encountered, and hence can be solved the same way."). I expect potential hires to want to work this way too, as it makes your work life a lot less frustrating. At any level, you have got to keep sharpening the sword. There is always something new (or old) to be learned. Contact Acculution, if your engineers and specialists need training, and let us raise the level together.