Rane AD22S "Environmental Effects on the Speed of Sound" Den
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- Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 1
* DENNIS A. BOHN Rane Corporation, Mukilteo, WA that society. This paper is for the members of the Audio Engineering Society who are in the trenches every day and need have a whole new army attacking room problems with a vengeance. Racks of equalizers and delay units arm these combatants as they wage - Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 2
ft (21 m) back to the listener's ear. Ignore all other delayed arrivals. The reflected wave arrives with some sort of phase relationship to only a difference of 2.19%, so there is no problem. The casual observer is wrong. Recalculation gives the following: Audio Eng. Soc., Vol. 36, No. 4, 1988 April - Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 3
is The velocity of sound c in dry air has the following experimentally verified values: c = 331.45 ± 0.05 m/s or c = 1087.42 ± 0.16 ft/s for audio frequencies, at 0°C and 1 atm (760 mm Hg) with 0.03 mol-% of carbon dioxide. 2.3 Temperature Dependence Substituting the equation of state of air of an - Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 4
BOHN Therefore there is no change in velocity due to a change in pressure. But this is true only if the temperature remains constant. Temperature changes cause density changes which do not affect pressure. Thus density is not a two-way street. Changes in pressure affect density but not vice versa. - Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 5
used in thermodynamics.] The average molecular weight of air decreases with added moisture. To see this, M is calculated first for dry air specifying the relative humidity. Table 2 gives calculated results for Eq. (14). J. Audio Eng. Soc., Vol. 36, No. 4, 1988 April Fig. 3. Temperature versus - Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 6
The results graphed in Figs. 3 and 4, and also tabulated in Tables 1and 2, can be added together to show the combined effects of temperature and relative humidity on the speed of sound. 1.02 1.97 2.93 3.90 5.90 8.03 100 1.05 2.01 2.98 3.98 6.03 8.27 J. Audio Eng. Soc., Vol. 36, No. 4, 1988 April - Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 7
/km versus relative humidity as a function of frequency at 20°C (68°F). Frequency Relative humidity (%) (kHz) 0 10 20 30 40 50 60 70 80 90 100 J. Audio Eng. Soc. Vol. 36, No. 4, 1988 April - Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 8
due to relative humidity as a function of frequency at 20°C (68°F). Frequency Relative humidity (%) (kHz) 5 10 20 30 40 50 60 70 80 90 100 J. Audio Eng. Soc., vol. 36. No. 4, 1988 April - Rane AD22S | "Environmental Effects on the Speed of Sound" Den - Page 9
listening problems in of time-delay tools is 68°F (20°C)," AD-738576, National Technical Information Service, U.S. Department of audio products prompted him to leave Phase Linear and accept the position of vice president of engineering for Rane Corporation. In 1984, he became a principal of Rane
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Environmental Effects on the Speed of Sound*
DENNIS A. BOHN
Rane Corporation, Mukilteo, WA 98275 USA
A detailed analysis of the environmental effects of temperature and humidity on the
speed of sound is presented. An overview of the available literature reveals serious
shortcomings for practical applications. New graphs, tables, and equations present the
findings in a more useful manner for sound reinforcement “se. The results show that
tight control of temperature and humidity must accompany the popular trend of splitting
microseconds when time correcting sound systems. Failure to do so makes precise time
correction a” exercise in futility.
0 INTRODUCTION
This paper presents, expands, and clarifies the en-
vironmental effects of temperature and humidity on the
speed of sound. These effects increase the speed of
sound and complicate the task of room equalization im-
mensely— much more so than previously thought.
The dramatic effect of relative humidity on sound
absorption appears as a separate section and helps ex-
plain many mysteries involving startling changes in
room response from day to day. Even a modest change
in relative humidity of only 10% can cause an additional
35 dB per 1000 ft (300 m) of absorption.
In one sense, nothing new appears in this paper. The
major effects described and the equations presented all
exist within published books on acoustics. Some from
the
Journal of the Acoustical Society of America
are
45 years old. However, this does not reduce the im-
portance of this paper. It is assumed that members of
the Acoustical Society of America are familiar with
this material. Unfortunately, very few people equalizing
rooms for permanent sound systems belong to that so-
ciety. This paper is for the members of the Audio En-
gineering Society who are in the trenches every day
and need all the assistance they can get.
What
is
new is the table and graphic treatment of
the material. Everything known regarding the effects
of temperature and humidity on the speed of sound
appears in this new form, as does the material on sound
absorption. Experience shows tabulated and graphed
data to he more useful than equations. Practical ap-
* Presented at the 83rd Convention of the Audio Engineering
Society, New York, 1987 October 16-19.
plications require concise look-up facts.
Before presenting the detailed analyses, a question
should be answered: why bother?
This is not a facetious question. Many people realize
that sound velocity depends upon temperature, baro-
metric pressure, relative humidity, altitude, air com-
position, and so on. Only somewhere they learned that
they may ignore these effects, that they are not signif-
icant. Well, 30 years ago the author may have agreed
with you. Then we were just beginning to understand
what room response meant, much less were we able to
do anything about it. We then developed ways to view
and alter room responses. Graphic equalizers and real-
time analyzers opened up a whole new window of op-
portunity for improving playback audio.
Progress continued slowly until Richard Heyser gave
us time-delay spectrometry (TDS). Then we experienced
one of those step function jumps in our ability to view
our acoustic environment. For the first time we could
actually see what we had been dealing with all along.
Today we have a whole new army attacking room
problems with a vengeance. Racks of equalizers and
delay units arm these combatants as they wage war on
all those response peaks and valleys. Each year they
demand finer equalization tools and smaller delay in-
crements with which to continue the fight. All this is
fine. Only we must not forget mother nature. TDS-
based test equipment allows us to see far more than is
probably good for us. And there is a natural tendency
to fix something if we can see it-without regard to
relevancy.
The thesis of this paper is that tight control of tem-
perature and relative humidity
must
accompany the use
of very small time-delay increments to fix room response
J. Audio Eng. Soc., Vol. 36, No. 4, 1988 April