Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Path: utzoo!mnetor!uunet!seismo!kitty!larry From: larry@kitty.UUCP (Larry Lippman) Newsgroups: sci.electronics,sci.med Subject: Re: Injury via Phone Message-ID: <1952@kitty.UUCP> Date: Fri, 21-Aug-87 23:53:43 EDT Article-I.D.: kitty.1952 Posted: Fri Aug 21 23:53:43 1987 Date-Received: Sun, 23-Aug-87 06:58:25 EDT References: <414@gtx.com> Organization: Recognition Research Corp., Clarence, NY Lines: 86 Summary: Improbable, but not impossible... Xref: mnetor sci.electronics:1196 sci.med:2981 In article <414@gtx.com>, al@gtx.com (0732) writes: > One often hears about people blowing whistles or air horns into a > telephone to thwart obscene callers. Is a telephone capable of > transmitting enough sound amplitude to cause pain or damage to the ears? > If so, this seems to be a dangerous capability in the hands of some crank. The general answer to your question is that acoustic injury via telephone is improbable, but not impossible, depending upon specific circumstances which I will outline below. Bear in mind that there is a substantial difference in sound energy required to make a listener "uncomfortable" versus the energy level required to say, damage the tympanic membrane. First of all, consider the receiver element in a telephone handset. Almost all receiver elements in use today are electrodynamic transducers, not unlike that of a radio speaker. In a typical receiver element, the changing voltage as applied to an electromagnet coil creates a variable magnetic field which causes a magnetized, thin metal diaphragm to flex, thereby compressing and rarefying the air, thereby creating sound waves. Now the point is that this diaphragm can only move SO much, regardless of the magnitude of applied voltage, thereby _limiting_ the maximum sound level it can produce. A typical electrodynamic receiver element, such as the "U1", will "top out" at well under 120 dB SPL, even at its peak reproduction frequency (say, around 1 kHz). For what might be referred to as "casual" exposure, 120 db SPL is not going to cause any physiological injury. As a point of comparison, the threshhold of pain is generally considered to be 140 dB SPL. Second, there are limits to the amount of electrical energy which can be transmitted over a dialed telephone connection. Audio frequency energy transmitted over a telephone line will not be propagated beyond a certain level; the reason is that certain telephone network components such as transformers and inductors will magnetically saturate and thereby limit transmission level. Furthermore, active functional components of the telephone network such as amplifiers, repeaters, PCM codecs, etc. will also saturate and not pass audio energy beyond a certain level. In general terms, if one were to electrically connect an audio frequency generator to a dialed telephone connection, regardless of the magntiude of input energy, it is improbable that more than 20 dBm (100 mW) of energy could be delivered to the receiving telephone IF the call were switched through an electromechanical central office, and IF both the calling and called party were in the SAME central office. If the telephone central office were ESS and/or if the called party was served by a different central office, it is highly improbable that more than 10 dBm (10 mW) of energy could be delivered to the receiving telephone. Even using the worse-case figure of 100 mW as applied to say, a U1 receiver, it is improbable that even 110 dB SPL could be achieved - still a far cry below even the threshhold of pain. Furthermore, at this kind of energy level, the "click-suppression" varistor across the U1 handset would begin some serious conduction, thereby shunting energy away from the receiver. Third, a telephone transmitter - either traditional carbon or the newer reluctance variety - will saturate if a sound level (like a boat horn) is applied to it, thereby limiting the amount of any electrical signal it may generate. So, for any or all of the above reasons, it is improbable that sufficient acoustic energy (sound pressure level in dB) could be created such that physiological injury would occur. There is one factor that could contribute to possible physiological injury - but it is not commonly found. I will relate a little firsthand story... As a combination EE/biochemist with a multidisciplinary background, I have done some forensic science consulting for a number of years. About ten years ago, I was retained as a consultant in a workmen's compensation case. It seems that a middle-aged woman who had worked as a switchboard operator for 20-some years had applied for compensation resulting from partial hearing loss in one ear. Her allegation was that then presence of clicks and loud noises in her "headset ear" over many years caused her hearing loss. My function was to work with an otolaryngologist to disprove her claim. My first reaction was that there was insufficient sound pressure level to cause such injury. However, I learned that for the past five years the woman used a Plantronics miniature headset (the kind with a flexible plastic ear tube). Now with such a headset, there was an almost perfect sound conduction between the receiver element and the ear. So, we contacted Plantronics and got some engineering information. We got a sample headset and conducted our own sound pressure measurements using a Bruel & Kjaer "artificial ear". The conclusion was that the maximum sound pressure level which could be experienced through this headset in a switchboard environment was still insufficent (by generally accepted medical criteria) to cause hearing impairment. But the use of a receiver with a plastic ear tube did make me seriously consider the possibility of injury. <> Larry Lippman @ Recognition Research Corp., Clarence, New York <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231 {hplabs|ihnp4|mtune|seismo|utzoo}!/ <> FAX: 716/741-9635 {G1,G2,G3 modes} "Have you hugged your cat today?"