Tuesday, August 25, 2009

Acoustic Diode?

My latest story at Physical Review Focus describes a layered structure that would transmit acoustic energy one way but not the other.

The authors call it an "acoustic diode," by analogy with an electric diode that passes electrical current one way but not the other. But I'd like to think that someone reading my story will have a more accurate understanding of what it is--and what it isn't--than they would get by skimming the article in Physical Review Letters.

The authors are upfront about a couple of issues. First, it's just a simulation. That's fine. Lots of publications don't like to cover simulations until they're backed up by experiment, but Focus has no strict rule, other than that it be clear to readers. Second, it works only in a narrow range of frequencies. Acoustics is hard in part because it often deals with frequencies that vary by orders of magnitude. A concert hall, for example, might need to gracefully handle sound frequencies from tens of hertz, with a wavelength of many meters, to above 10kHz, with a wavelength of a tens of millimeters. Getting both the geometry of the hall and its acoustic properties right over that large range is challenging, to say the least. A narrow-band solution like this one isn't likely to help, so it will only be relevant in applications using one frequency, like medical ultrasound.

The authors are less forthcoming about a third aspect. Their abstract refers to a "rectifying effect on the acoustic energy flux." Now, rectifying--which is what diodes do--usually means allowing only one direction of steady flow, for example of electrical charge. In other words, it affects constant (dc) signals. Its effect on an oscillating wave (such as turning an AM radio signal into sound) is a byproduct of this dc action. Earlier papers described "acoustic diodes" that would indeed work at zero frequency. In the present case, however, one-way transmission occurs only for a wave with a high frequency, which strikes me as rather different. The analogous device for electromagnetic waves such as light or microwaves is called an isolator.

More importantly, I think most casual readers would interpret "rectifying effect on the acoustic energy flux" to mean that if a wave has the right frequency and is going in the right direction, it will pass through unhindered, or maybe a bit weaker. That's not what will happen here. Instead, before acoustic energy can pass through, it is doubled in frequency using a nonlinear film. This doubled wave can pass through the filter part of the device to the other side. A wave with the original frequency couldn't get back through the filter, hence the "rectifying effect." But if the sound wave that is actually emitted--that is, the doubled wave--were reflected, it could come right back through the filter. Maybe it won't cause a problem, but the acoustic diode won't stop it coming back. All of this makes it harder to understand how the device could be used.

To be sure, this would all be clear to someone reading the paper carefully, but I suspect many casual readers would come away with a different picture. The structure still appears to be novel and interesting, and both of the independent researchers I interviewed liked it a lot. It may turn out to be interesting in itself, or it may provide inspiration for other researchers to do something even more interesting. But analogies have the power to both inform and mislead, and we need to be careful when we use a familiar word in a new way. In my story I tried to highlight the positive without getting carried away with the analogy.

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