14. Forry, wrong number! I
The frequency ranges of speech and hearing

     Have you ever noticed how hard it is to hear names, letters and numbers clearly over the phone, for example when someone spells a name so you can be sure you've got it right? Most other words we hear in a specific context, and we can easily guess them even when they aren't perfectly clear. For example, if someone says, 'Do you know the time? I forgot to wear my ________ today.', you will probably assume they said 'watch', and be right, even if none of the sounds were clear. But context often offers little or no help in recognizing names, letters and numbers. Try to complete this sentence: 'My new phone number is: two two four _____________ seven ______________ nine one.'!

      One reason it is so difficult to hear some sounds clearly over the telephone is that only frequencies within a certain range are picked up and transmitted over the phone lines. The frequency range of good human hearing is about 20Hz to 18,000Hz (it goes up to 20,000Hz in young people). The sounds of human speech fall mostly in the range of about 100Hz to 8,000Hz, and seldom extend beyond 60Hz and 12,000Hz. Microcassette recorders and telephones, however, usually only pick up and transmit frequencies between 300 and 3,400Hz. While this covers the middle of the range used in human speech, there is a severe cutoff on both ends of the spectrum.

      On the previous page, we learned how we still perceive the fundamental frequency of a tone even when it is not physically present, as long as the harmonics are audible. That means that we can still identify a male voice as male, even without any frequencies under 300Hz (the average frequency of a male voice is around 120Hz). But what about the higher frequencies? We have learned that although fricatives consist of semi-random noise (i.e. all frequencies are present), they still fall into certain frequency ranges, e.g. a [s] sound has frequencies in the 4,000-5,500Hz range; the noise produced in making the first sound of think is centered around 8,000Hz; and a [f] sound is characterized by frequencies of around 10,000-12,000Hz. All of these frequencies are missing in telephone communications, so these sounds will be difficult to distinguish from each other. (Source)

     Here is a very useful and fun page from the University of New South Wales in Australia. There is an individual sound file for 19 different frequencies at 34 different decibel levels. Use headphones, or the tests won't be accurate. Start at 1000 Hz (1 kHz) about halfway down the column. Don't start in the upper third of the chart, so you don't hurt your ears, Move up to hear the tone at different dB levels, or down until you can no longer hear the tone to find the approximate outer limit of your hearing at that frequency. Then try the other frequencies. To draw an equal loudness curve, compare two different frequencies at different dB levels till you find two that seem to be equally loud, then repeat across the columns. More detailed instructions are available on the same page.


     When you talk over the phone it is as though both parties suffer from a mild hearing loss. The following page features a series of audio files that simulate various types and degrees of hearing loss, so you can get an even better idea of what it is like to listen to speech with some of the higher frequencies missing; one sample has added background noise:


      Here's another simulation of one kind of hearing loss (audio link is at the bottom of the page), in this case centered around 4,000Hz, due to exposure to noise:


     We have noted in passing that sex, age, health, lifestyle and environmental noise have a lot to do with the frequencies of speech and hearing. Learn more about sound and hearing on the next page, which features text and audio links to some interesting – and short! – related programs from the "A Moment of Science" site. There are lots of units on this page, so it is mainly for reference, to come back to when you have the time and inclination.

Forry, wrong number! II: Fun Links from "A Moment of Science"


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