MadSci Network: Physics |
Short answer: There are correlations between certain physical aspects of a sound (wavelength) and the physical size of auditory receptor organs, yielding differences in best frequency or ranges, but not quite the way you describe... so... The Long Answer: The modulation "height" of a sound is not measured in inches - amplitude modulation is measured in sound pressure level (usually expressed in decibels) so there is no physical dimensional correlate with the size of a listener. A sound with a higher amplitude has the psychological corelate of being "louder." However, there is a physical dimension which is critical to auditory perception - the wavelength of a sound which is the inverse of it's frequency. And there are correlations between the size of a receiver and its auditory range and best frequencies. Think about how a sound is generated for a moment (this may seem like a digression but I promise it's not). Sound is generated when some surface (a string on an instrument, a membrane on a speaker, a fold of tissue in the larynx) vibrates at a specific rate. If this vibration takes place in a medium in which sound can propagate (such as air or water), this vibration will travel through the medium at a specific velocity (the speed of sound - in air (at about room temperature at 50% humidity) will be about 344.62 m/sec (about 1131 feet per second). If your sound source is vibrating at 1000 cycles per sec, that means it vibrates 1000 times every 344.62 meters or 1131 feet, so the wavelength of a 1000 Hz sound is about 1.1 feet long. The wavelength of a sound at 100 Hz would be 11 feet long and of a sound of 10,000 Hz would be 0.11 feet long. Here's where the correlation occurs between sound wavelength and a receiver's size comes in to play. Sound is a mechanically coupled sense - it depends on a mechanical system to respond to the vibrational rate and amplitude of the incoming signal in such a way as to faithfully translate those signals into neural signals inside the ear. While the stimulus seems simple, hearing mechanisms having undergone some pretty stunning evolutionary adaptations. But all of them must transduce both the frequency (or it's inverse correlate, wavelength) and amplitude of a signal, whether you are dealing with a small underwater hagfish or a terrestrial elephant. Organisms hear across a variety of frequencies (their auditory range). The range of detectable sound will vary based on a number of features, but mostly it is limited by the vibrational sensitivity or damping of their receiving organs. (I am just going to discuss terrestrial vertebrates here, but invertebrates and aquatic organisms also have wide array of hearing sensitivities, and many of the same principles apply, although the anatomy is often very very different due to the differences in the way sound propagates in air or water). Many amphibians and some reptiles hear through vibrations transmitted from their forelimbs to their shoulder girdles to their inner ear - this is called an opercularis pathway. Amphibians (frogs in particular) who rely on this pathway have a strong correlation between frequency range and most sensitive frequency and the size of their body. Once you start dealing with organisms which have specific vibrational pathways for dealing with air-borne sounds, you have to start factoring in other stuctures rather than body size. Amphibians with eardrums (such as modern ranid frogs such as the bullfrog) have a correlation between tympanic size and stiffness and most sensitive frequency and hearing range. The reason for this is that if you have a receiver which is a vibratable membrane, a larger membrane will be able to respond to lower frequencies because it will have zones of lower stiffness - very small receivers tend to be very stiff and hence only respond to higher frequencies. If you want to think of it in day to day application, think of your stereo speakers - the tweeters put out the highest frequencies and they are the smallest The woofers or subwoofers put out the lowest frequencies and they are the largest. Given a fixed power output, it is easy to move a small structures faster (higher frequency), whereas with the same power you can moved a larger structure slower (lower frequency). Once you get to an organism like mammals, you are dealing not only with eardrums, but a specialized membrane called the basilar membrane, which exists in the coiled cochlea or the inner ear. If you unrolled this membrane, you'd find that the basal end, near the eardrum, is quite narrow and very stiff, whereas the apical end, furthest from the eardrum, is quite wide and not nearly as stiff. Because of this, different regions of width and hence stiffness, will respond best to certain frequencies (or wavelengths). To avoid getting into all the mechanics of neural auditory transduction, you can check out this very good overview page on basilar membrane function: http://www.asu.edu/clas/shs/sinex/shs311/2- ear/basilar.htm So there are correlates between a sound's wavelength and physical size (and hence stiffness) of parts of the receiver's auditory system. So how does the physical size of the receiver factor in to this? Well, in general a larger animal will have a larger vibrational receptive region than a smaller animal - a mouse hears well into the ultrasonic region (>20,000 Hz), whereas adult humans are pretty much limited to 20 Hz to 18,000 Hz), and elephants actually use infrasonic (<20 Hz) for long distance communication. There is a nice table showing differences in the audiograms of different animals here: http://www.lsu.edu/de afness/HearingRange.html However; there are a lot of individual differences within a species - remember, although mechanical transduction is the basis of auditory input, processing is a neuroelectrical and neurochemical one, so there are a lot of factors which fine tune an individual's degree of auditory function. And also bear in mind that although such a correlation can describe a best frequency or best auditory range, you do not just hear your most sensitive frequencies - punch enough power into an ultrasonic or infrasonic signal and even humans will hear it (although it's mostly very very irritating harmonics or subharmonics). Believe it or not, that was a VERY short version of one part of the answer - I hope it clarified things...
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