Sound waves travel through solids,
liquids and gasses. Sound waves travel faster through solids and
liquids than gases because they're more dense. At sea level, the
speed of sound is 1130 feet per second with 50% humidity.
How does temperature have on the speed of sound?
The higher the temperature, the faster sound travels.
How does humidity affect the speed of sound?
H umidity is a measure of the amount of water vapor in the air;
the more humid the air is, the faster sound waves travel in it.**
How does the altitude affect the speed of sound?
Sound travels slower with an increased altitude (elevation if
you're on solid earth). This is primarily a function of temperature
and humidity changes.
A simple chart
the relationship between temperature, relative humidity, and the
speed of sound (in feet per second
These figures were calculated with an online calculator which
can be found
So What's this have to do with church audio? Plenty! How many
times have you adjusted the sound system during rehearsal and
everything sounded just right, but 15 minutes into the service,
you're having trouble with feedback and things don't sound very
good anymore? If the HVAC (heating, ventalation, air-conditioning)
system wasn't designed or installed correctly, it won't keep up
with the amount of humidity being added to the sanctuary during
congregational singing. In 20 minutes of congregational singing,
the humidity level can increase as much as 40%! This results in
the sound waves traveling faster and a whole new set of feedback
frequencies can appear to come from nowhere.
In winter, some churches sound very good, but when summer hits,
they sound much too bright. Once again, this is due to the increased
humidity in the room.
Here's an example: It's Sunday morning rehearsal, the temperature
is 70-degrees and the humidity is 40%. Everything is just fine,
though you needed to tweak your main parametric EQ by dropping
3616Hz 5dB to get rid of a feedback issue. Since you spotted it
as a very narrow freuquency, you also adjusted the width of the
filter so it was narrow. This, you figure, will ensure you're
not loosing frequencies on either side of the 3616 mark. Now it's
30 minutes later, and 400 people are now in the room. They begin
to sing. After about 10 minutes of singing, you're hearing a ringing
sound from the audio system. You reach over to your parametric
equalizer and begin to adjust the frequency once again. First
you adjust down in frequency, and the feedback only gets more
noticable. Then you adjust up, and at about 3640Hz, the feedback
is once again gone. What happened? Not only did the humidity increase,
but the temperature increased a few degrees as well. This is a
great sign that's screaming out saying "the HVAC system is
designed or installed wrong!" Rather than have adjusted the
parametric equalizer with an extremely narrow filter, it would
be better to make the filter a bit wider to "catch"
problems like this.
While these differences seem small and insignificant, they can
wreak a lot of havoc with you if you're not aware these issues
04-2003, Blake A. Engel
** I was corrected on a statement made in this article; I had
stated that the more humid air is, the more dense it is. Shawn
Roberts wrote and corrected me on this.
"Humidity is a measure of the amount of water vapor in the
air; the more water vapor, the less dense the air is. The less
dense the air is (more humidity), the faster the sound waves travel."
I know this may sound backwards but it is correct. The more humid
that an air mass is, the less dense the air is because there are
less molecules per cubic meter in that air mass. Sir Isaac Newton,
first stated that humid air is less dense than dry air in 1717
in his book, Optics. The reason is that a fixed volume of gas,
say one cubic meter, at the same temperature and pressure, would
always have the same number of molecules no matter what gas is
in the container. Now imagine a cubic foot of air that is at 0%
humidity, it would contain about 78% nitrogen molecules, which
each have an atomic weight of 28 and about 21% of the air is oxygen,
with each molecule having an atomic weight of 32. Remember that
molecules are free to move in and out of our cubic foot of air,
so if we add water vapor molecules to our cubic foot of air, some
of the nitrogen and oxygen molecules would leave because the total
number of molecules in our cubic foot of air stays the same. The
water molecules that replace nitrogen or oxygen have an atomic
weight of 18. This is lighter than both nitrogen and oxygen. In
other words, replacing nitrogen and oxygen with water vapor decreases
the weight of the air in the cubic foot; that is, it's density
decreases. These differences are small compared to the differences
made by temperature and air pressure, humidity has a small effect
on the air's density. But, humid air is lighter than dry air at
the same temperature and pressure.