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 link to a commercial hearing aid site
with sound samples of both pure tones of different frequencies (these are part
of a 'hearing test'); and other sounds in low, middle and high frequency ranges,
plus a bit of music with a combination of these, so you can get a feel for the
frequency ranges we are discussing here:
http://www.worldhearing.com/worldhearing_015.htm
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:
http://facstaff.uww.edu/bradleys/radio/hlsimulation/
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:
http://www.sfu.ca/sonic-studio/handbook/Audiogram.html
The preceding pages mention the affect of aging
on our hearing. The following site further explains this. You need only read
paragraph five:
http://www.learn.co.uk/edenproject/articles/article1.htm
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.
Next: Forry,
wrong number! II: Fun
Links from "A Moment of Science"
on to next page back index I index II home