Topic: Speed of sound in water

[sonar2000]

Found on the internet and now trying to figure this out...


Side-scan and its scanning counterpart are active sonars (where the instrument transmits acoustic energy and listens for its echoed return).  The acoustic energy travels through the water column at a speed determined by three variables:
• water temperature
• salinity
• pressure

Of these three, temperature has the greatest influence, followed by salinity and pressure respectively. The sonar operator needs to remember that the speed of sound increases when the water gets warmer, the salinity gets higher and the depth (pressure) increases.

Why is this important to know?  When the wrong speed of sound is used, the measured distance between objects (or from the sonar to an object) is not correct.  Most sonar manufacturers allow the operator to input a speed of sound value in the sonar software and have a default setting somewhere between 1475-1500m/sec (4839’- 4921’/sec) which covers a “typical” range of water conditions.  When a different speed of sound is applied, every 15m/sec (49’/sec) difference from the correct value introduces a 1% error (approximate) to a distance measurement.  This is not a huge amount, but it needs consideration when the question of sonar “accuracy” rears its interesting head.

Chuck

[sonar2000]

wonder if this is some of the issue with GPS location being off.  Given a transducer located at the back of the boat, the GPS puck midway on the roof and now this effect of temperature  etc on sound speed, How does a program take all this into account and give an accurate reading...
Chuck

[ITGEEK]

Chuck,
With the type of waters and depths that Humminbird
units are typically used for, I don't think the speed of sound
in the water will vary that much.
 Water temperature will be from above freezing to maybe 90.
 Salinity will be zero in most cases.
 Pressure is closely related to depth, so most will be using in
 shallow water, so there will be low pressure.

[sonar2000]

Thanks, i think the article was more for deeper depths but any variance will affect the algorythms a bit.
Depth pressure increases .441 PSI per foot in fresh water so pressure can be an influence over temp.
Not a big deal but something else to consider if trying to get accuracy..
I really think for us that the difference is where the transducer and the gps antenna is in relation to each other..
Chuck

[ITGEEK]

Well, the Humminbird units do have the current water temperature and
depth.
No measurement of salinity, though.

Maybe the Humminbird gurus could use the temperature and depth
information to fine tune their programming agorithms, as far as adjusting
for how long it takes for the pings to return.

If we are all striving for more accuracy (depth and distance), I don't think it's unreasonable
for the Humminbird engineers to look at this.

Hopefully Greg will pass this on.

This is some good information that most people would never think of.
Thanks for posting this Chuck.

[Bob B]

Chuck, The other complication would be that for most of the year.....excluding winter....there will a significant deviation in water temp from shallow to deep water.  This would make it almost impossible to compensate.
 Hopefully the deviation isn't that great at normal fresh water fishing depths.