Notices
Marine Electronics Forum

Transducer 'sensitivity'

Old 09-14-2005, 04:56 AM
  #1  
Member
Thread Starter
 
Join Date: Jul 2003
Location: Auckland, New Zealand
Posts: 60
Likes: 0
Received 0 Likes on 0 Posts
Default Transducer 'sensitivity'

What does tranducer sensitivity mean?

I am looking at getting a 1kw Airmar transducer with transom mount to hook up to a Raymarine C120 and DSM300. Airmar say their M260 transducer, with its extra 50kHz elements, is '10 times more sesitive' than the M256. What does this actually mean in the real world? Better target separation? Surely not better depth - doesn't make any sense? Can someone please explain how, in reasonably simple terms, these multi-array transducers deliver the benefits we pay a 40% premium for. I'm probably going to go the M260 route but it would be nice to know what I'm getting for my cash.
Old 09-14-2005, 07:23 AM
  #2  
Senior MemberCaptains Club Member
 
Join Date: Jan 2003
Location: Houston,Tx,U.S.A
Posts: 11,777
Received 860 Likes on 462 Posts
Default Re: Transducer

It all relates to target seperation ..
the more power your transducer has to better it can seperate small fish in a pod or school and not show it as one fish ....

Often low power units will show what appears as one XXlarge fish when it really is 5-10 smaller schoolie size ones...


John
Old 09-14-2005, 08:08 AM
  #3  
Member
 
Join Date: Mar 2004
Location:
Posts: 65
Likes: 0
Received 0 Likes on 0 Posts
Default Re: Transducer

Transmit sensitivity relates to how many dB of acoustic energy are generated by the transducer per volt. The units are dB re 1micro Pascal per volt at 1 meter distance. Receive sensitivity tell how many dB are required to generate 1V per micro Pascal. The units are very important if you are making comparisons! For most echosounder applications (fishfinding, hydrographic survey, etc.) most folks look at the transmit sensitivity as a guideline. Also it is good to note that the dB scale is logarithmic and that a 3dB gain is approximatly 2x the sound intensity. That is why a transducer with a transmit sensitivity of 178dB has 16x the power of a transducer with a transmit sensitivity of 166dB. Therefore, more acoustic energy into the water means more range (plain and simple). Finally, I will mention figure of merit - FOM is the sum of the absolute values of TVR and RVR and the resulting number tells you how well the transducer operates as a reciprocating device (transmitting and receiving) as well as at what the best operating frequency will be. For transducers built with piezoceramics the resonant frequency is the best transmitting frequency and the anti-resonance is the best receive frequency - the resonance frequency is ALWAYS lower than the anti-resonance frequency. When you look at figure of metrit you will determine the best operating frequency (and it as a rare occasion that the best operating freqency is lies somewhere between the resonance and anti-resonance frequencies.) I hope this information is useful. FYI - target separation has to do with having short and crisp signals, this relates to the Q of the transducer, the lower the Q the better!
Old 09-14-2005, 10:44 AM
  #4  
Senior MemberCaptains Club Member
 
Join Date: May 2005
Location: Eastern N.C.
Posts: 179
Likes: 0
Received 0 Likes on 0 Posts
Default Re: Transducer

ummm.....yep, exactly what I was thinking, but I wont add anything since I am not as good at putting things in laymen's terms and mine would most likely sound too technical.
Old 09-15-2005, 03:27 AM
  #5  
Member
Thread Starter
 
Join Date: Jul 2003
Location: Auckland, New Zealand
Posts: 60
Likes: 0
Received 0 Likes on 0 Posts
Default Re: Transducer

Mr Fitzgerald, you are a star - the mists are clearing. The logarithmic scale is always a trap for young players. Right, if I may pick your brains a little further, can we move onto impedance and 'the transformer'.
The Airmar data sheet compaes the M256 with varous flavours of M260. Using the information I now have thanks to your tutorial the M260 is head and shoulders 'better' than the M256. They list a comparison with a M260 fitted with a transformer. What's this? It leaves the Q value the same but dramatically decreases the impedance and improves the TVR, FVR and FOM. What is going on here? As an aside, if FOM is' figure of merit', what is TVR and RVR - presumably transmit something and receive something.
Thanks again for your help - really is much appreciated.
Old 09-15-2005, 08:39 AM
  #6  
Member
 
Join Date: Mar 2004
Location:
Posts: 65
Likes: 0
Received 0 Likes on 0 Posts
Default Re: Transducer

Obald, I am glad I could help out.

I agree, the M260 performs better than the B256. About transformers, transformers will be used in a transducer when it is necessary to match the transducer impedance to the echosounder. Some echosounders are designed to drive a high impedance (say 250 to 400 ohms) and others (like some simrad or furuno units) work best with a lower impedance transducers (70 and 90 ohms respectively). It's all in the design of the output stage of the echosounder transceiver - no two echosounder manufacturers are exactly alike.

Typically, the piezoceramics used in transducers have high impedance so in order to get the very best performance it is crucial that the impedance of the transducer match (or nearly match) the load the echosounder "expects". When you are comparing transducer configurations (as you describe, different impedances and different transmit sensitivities - TVR = Transmit Voltage Response) you will see that these two seemingly different configurations are actually stating the same values if you know how to convert the apparent impedance reduction into dB gain. Here is a sample calculation - say you have some transducer configuration with no transformer and the measured impedance is 335 ohms and the measured transmit sensitivity is 169dB and you want to predict the transmit sensitivity of that same transducer but with a 75 ohm matching transformer in the transducer. The transmit gain is equal to 20 x log (base 10) of the square root of 335/75 (ratio of the impedances) which would be (roughly) 6.6dB. Add this number to 169dB (the measured sensitivity of the 335 ohm configuration) and you have "predicted" the transmit sensitivity (TVR) for the 75 ohm configuration to be 175.6dB. Also, you can add this gain value to the figure of merit (FOM) to predict the FOM for the 75 ohm configuration. Note that RVR (receive voltage response) does not change. You can use this equation with the specifications you have for the various configurations and you should see that they are all equal. All the best!



Old 09-15-2005, 09:53 AM
  #7  
Senior Member
 
Join Date: May 2004
Location: Down Under
Posts: 410
Likes: 0
Received 1 Like on 1 Post
Default Re: Transducer

Mr E Fitzgerald, thanks for the explanation, now I'm really confused! Jokes aside, what does the diplexer in a transducer do?

thanks John
Old 09-15-2005, 10:20 AM
  #8  
Member
 
Join Date: Mar 2004
Location:
Posts: 65
Likes: 0
Received 0 Likes on 0 Posts
Default Re: Transducer

Wildfish - the diplexer is a signal splitter installed inside the transducer. Basically it is a circuit (typically a set of coils, inductors, and diodes) with two wires input and four wires output. The input signal is from the echosounder and is typically one frequency or two frequencies alternating (a high frequency and a low frequency - most likely 200kHz and 50kHz). The diplexer acts like a traffic cop and directs the high frequency signal to the high frequency ceramic (or array) and directs the low frequency signal to the low frequency ceramic (or array). A well designed diplexer should not cause any signal losses at either frequency.

The proper application of a diplexer is when you have a dual frequency sounder that has a single pair of output taps or a single wire pair to the transducer and you want to use a transducer that has a dedicated high frequency ceramic (or array) and a dedicated low frequency ceramic (or array.)

The alternative is to use a single ceramic (or array of ceramics) that resonate at both the high and low frequency - which is a very useful and popular lower cost approach. Note however that, while dual resonating ceramics can operate reasonably well at both frequencies, they will not be as efficient or have as good sensitivity as dedicated (single frequency) elements. I hope this information is useful. E Fitz.
Old 09-15-2005, 08:09 PM
  #9  
Senior Member
 
Join Date: May 2004
Location: Down Under
Posts: 410
Likes: 0
Received 1 Like on 1 Post
Default Re: Transducer

Thanks E Fitzgerald, that makes perfect sense. Another question regarding output power and echosounders. If a sounder is rated at 600WRMS and it is a dual frequency (50 & 200kHz). Does this mean that it transmits 600Watts into each element of the transducer or is it split 300Watts each giving a combined (total) output of 600Watts?

Thanks in advance

John
Old 09-16-2005, 07:32 AM
  #10  
Member
 
Join Date: Mar 2004
Location:
Posts: 65
Likes: 0
Received 0 Likes on 0 Posts
Default Re: Transducer

Wildfish - the 600wrms, dual frequency transducers I am familiar with have a single ceramic that can operate at both frequencies - though not simultaneously. The thickness of the ceramic is such that it resonates at 200kHz (aka "thickness mode") and has a specific diameter such that it can also resonate at 50kHz (aka "radial mode"). I will also point out that it is the geometry of the ceramic that determines the frequency (-ies) and beam pattern. So to answer your question, this type of transducer can handle alternate 600wrms pulses at both 50kHz and 200kHz. You are welcome! Have a great weekend!

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are Off


Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright © 2021 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.