Back to Basics/Chasin | November 2013 Hearing Review
On LRADs, Trumpets, and Loudspeakers
By Marshall Chasin, AuD
Several years ago, I was asked to be an expert witness by the Canadian Civil Liberties Association, which was contemplating an action against the local police services board regarding an upcoming G20 meeting in Toronto. The police services board was considering using Long Range Acoustic Devices, or LRADs, as a crowd-control device. LRADs are loudspeaker systems on steroids and are easily capable of generating outputs on the order of 120 dBA at 10 meters. Although we don’t have very good models for noise exposure over 115 dBA, we do know that levels of 120 dBA (with peaks being up to 15 dB higher) can create acoustic trauma. I was called in to help convince the courts that the use of such excessive levels could potentially pose a threat to one’s hearing status.
Looking at the specification sheet of the various models of LRADs, I was struck by the similarity between the polar pattern of the directionality of these devices and that of music loudspeakers. Figure 1 shows a directional polar plot for several frequencies. Like loudspeakers, LRADs were only “directional” away from the police officers for certain frequency regions—specifically the higher frequency region.
The specification sheets for the LRADs indicated that it would be quite safe for the police officers who were using them, although “hearing protection may want to be considered.” LRADs can generate intense sound blasts, as well as amplified speech, and although I was initially concerned only about the use on unsuspecting citizens who were exercising their civil rights, I also became concerned about the hearing well-being of the police officers who stood to the rear of these devices.
The manufacturers claimed that, because the generated noise and amplified speech was aimed away from the police officers, there would be no danger. Indeed, the specification sheets described the expected attenuation at the level of the police officer. Upon examination of these specification sheets, however, I noted that the attenuations at the level of the police officer were all stated at 3000 Hz, and not for lower frequency sounds, which may be quite intense for amplified speech.
Like many musical instruments, the higher frequency (treble) notes tend to be aimed away from the musician (and sometimes directly at another musician downwind). This has nothing to do with the legal status of LRADs, but has everything to do with the laws of physics. One cannot simply legislate acoustic behavior (although the state of Indiana did try to legislate the exact value of ? in 1897). Trumpets have most of their intense (mid and high frequency) energy aimed, like a laser beam, away from the musician. Similarly, LRADs have most of their intense energy aimed away from the police officer but only if the energy is very high in frequency.
Looking at it from another point of view, many of us have 5.1 surround-sound systems in our home—at least the nerds do. Have you ever wondered why they call it 5.1 and not 5.0 or 6.0? After all, there are typically 6 loudspeakers in this arrangement. In a typical home surround-sound system, there are indeed 6 loudspeakers: 5 of them being treble, and one a woofer (sub-bass). Because of the directionality of the 5 treble loudspeakers coming from 5 different locations in the room, there can be a sensation of sound from 5 directions; they are all quite directional and sounds that emanate from them are broadcast much like a laser beam in a line-of-site fashion. The 6th one, however, can be placed behind the couch, facing into a wall, or under some other piece of furniture. The bass sounds that are broadcast from this 6th loudspeaker are non-directional, and it doesn’t matter where the loudspeaker is situated. Hence, despite having 6 loudspeakers, they only generate a 5 dimensional listening space—or a 5.1 theater system and not a 6.0 theater system.
LRADs can teach us much about musical instruments, home stereo systems, and even the acoustics of our own voice. But there is no saving grace for the directional characteristics of LRADs for the hearing of a police officer or an unsuspecting protester downwind.
It was interesting how the lawyer for the police services board was trying to argue that the laws of acoustics didn’t apply (it may have been a Canadian thing?). The acoustic laws governing the behavior of LRADs and the acoustic laws governing how trumpets, hearing aids, and loudspeakers work are the same—regardless of what the judiciary rules.
Acknowledgement. Parts of this article were previously published at HearingHealthMatters.org/HearTheMusic. Used with permission.
