Understanding Specifications part 2

Frequency Response part 2
Last time I started to explain the frequency response graph.  Let’s pick up where we left off.   I had just shown a graph with a straight line and pointed out that this is where we get the concept of “flat” from.  A straight-line frequency response of a speaker means that what went into the speaker as electricity came out as sound and that transformation was perfect.  Of course, this is never the case, sadly not even with Danley speakers.  I do feel that we come very close to this elusive goal, but that’s another article.

This month I want to focus on some of the ways that a frequency response graph can be misleading or mis-read.  So here are two frequency response graphs.  Which one is the better speaker?
                                                                                 
A.    B. 

Well, if you said Graph A, you are sorta right!  At least you demonstrate that you understand that the straighter the line, the better!  But this is a trick question. Graphs  A and B are actually of the same exact speaker.  In fact, it is exactly the same data, graphed in 2 different ways.  Look closely at graph A.  The units on the Y or vertical axis are 10 units  – in this case decibels – apart.  We say that this graph is 10 dB per division.  Graph B is 3 dB per division.  Graph A will show a lot less detail because the Y axis which sets the resolution of the graph is too coarse.  Graph B is way less forgiving.  So which is right?  Well, they both are.  If you are in sales you might be likely to favor “A” because it glosses over the faults.  If you are in engineering you might favor “B” because it more clearly shows where the speaker deviates from the ideal.  The VIT here ( Very Important Takeaway – I just made that up ) is to always read the labels!   A graph with out a scale is of absolutely no value!  Unfortunately, manufacturers of speakers do not all use the same scale when showing their data.  I have even seen spec sheets where the scale changes from graph to graph depending on what they are trying to show to make sure the product is cast in the best possible light! SHAME!

Take another look at A and B.  You might be tempted to say that Graph A shows a speaker with better response in the bass frequencies, or as we say, the low end.  Examine the Y axis.  You will see that this axis is also not the same between the two.  Graph A shows the response between around 50 Hz up to around 10Khz.  Graph B shows the response from  around 35 Hz to 10 kHz.
So remember to read the labels!  Look at the scale ( the Y or vertical) and the range (the X or horizontal).  They both need labels, and they both need to be considered when comparing frequency response graphs.

Next time, we are going to discuss power ratings…

Prof. Doug Jones

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