hyperplanar
Senior Member
@hyperplanar and @Maiky76 , I'm quite excited about this, managed to work out exactly the filter that is used for the HF Trim -2dB switch on the back of the speaker, and I mean exactly!Long story short is that the exact filter used is a 6dB per octave High Shelf Filter at 3000Hz for -2.0dB gain exactly! I'll show my workings in the following, and also find attached a converted SPL Trace for the Listening Window which includes the -2.0dB High Shelf, so this is effectively what the Listening Window measurement would be if Amir flipped the JBL 308p Mkii to HF Trim -2dB position & measured the speaker, I'll attach the text file here:
Effect of Trim Switch -2dB on Listening Window.txt
So the data in the above file can be used for folks who would have wanted an HF Trim -2dB measurement from Amir for EQ purposes - use that file above as the measurement if you want to EQ with the Trim Switch in -2dB position.
OK, so here are my workings on how I derived all this.
Step 1: REW measurements to derive effect of HF Trim Switch at -2.0dB position
I basically did measurements in the exact same 6 places close around my listening position in a 40cm x 25cm rectangle around my listening position, and flipped the HF Trim Switch to either the 0dB or -2dB position for each measurement, so 12 measurements overall. I noticed immediately that the Trim Switch produces a nice smooth offset of -2dB for each measurement I took, so it was very repeatable. However, I still did an average of each of the two sets of measurements (one set being HF Trim switch at 0dB, and the other set at HF Trim Switch -2dB position)....and it is these 2 averages compared that you can see in the pic below. You can see it starts to deviate from 1kHz.
View attachment 93899
Step 2: Working out the Exact Parametric Filter that is being used by the HF Trim -2dB Switch
See that green line in the above pic, that's the -2dB line obviously, I took that Green Line graph and used VirtuixCAD to SPL Trace that line so I could turn it into a text file which I could then import into REW as a Target File for EQ purposes.....so that's what I did. Imported that text file into REW as a Target Curve and then EQ'd the Trim Switch 0dB line (blue line in above pic) to the Green Line if you like. Here's the pic of that, and this was the nuts & bolts of working out the exact parametric filter that is being used by this switch:
View attachment 93900
Note in the above graph you can't see the Target Curve line because it is perfectly obscured by the highlighted green line which is the EQ result which proves how accurate this filter is to what is actually happening in the JBL 308p speaker when you flip that HF Trim switch to -2dB. Yes, so you can see that I'm using "HS 6dB at 3000Hz with -2.0 Gain" in the list of filters in that pic.
Step 3: Applying the above Determined Parametric Filter to Amir's Listening Window Anechoic Data
Right, so this is the last step, and was just a means of creating a text file showing the measurement data if Amir had indeed measured this speaker with the HF Trim Switch in the -2dB position. I simply took Amir's Listening Window data from his file and imported that into REW as a measurement, and I then applied that "HS 6dB at 3000Hz with -2.0 Gain" Filter that we determined in Step #2 above:
View attachment 93901
So above you can see the Effect of the -2dB Trim Switch. I then did an SPL Trace of that EQ'd curve in VirtuixCAD to turn it back into text file data for use by folks for EQ purposes, and following you can see the sucessful SPL Trace I did in VirtuixCAD (zoomed in for better accuracy of data capture):
View attachment 93902
Yes, so find attached the text file which would be effectively the same as if Amir had measured the speaker with HF Trim Switch in -2dB position, this is for the Listening Window only:
Effect of Trim Switch -2dB on Listening Window.txt
In-Room Measurements shown against Harman Curve - Effect of Trim Switch at -2dB position:
Right, so you can see from the following pics that HF Trim Switch at -2dB position is a better fit for the Harman Curve. First pic is HF Trim at 0dB position, and 2nd pic is with HF Trim Switch at -2dB position:
View attachment 93903
View attachment 93904
I hope you folks find this all useful, and I'm certainly gonna revisit my EQ using the Trim Switch at -2dB position. In fact, I'll redo my Listening Window Anechoic EQ using the text file I linked you.....basically I'll be taking that modified measurement and EQ'ing it flat.....although thinking about it I'll just need to take a look at all the graphs & see which makes most sense.....but that's neither here nor there for now. The main point is we now know exactly what the HF Trim Switch does in it's -2dB position, and now you've got the data file upon which you base EQ attempts or comfort for folks who can't or don't EQ and instead want to see what the HF Trim Switch at -2dB position is really doing, which is important because pretty much everyone on the internet says these JBL 308p Mkii speakers are a tad bright.
@amirm , can you envisage in your experience or knowledge any negative issues associated with using the processing inside the speaker (the HF Trim Switch) as a means of influencing the frequency response....any possible degredation of sound fidelity not related to the frequency response change? EDIT: on this point in this paragraph, just did some listening tests comparing a "software HF Trim Switch -2dB" vs using the actual HF Trim Switch -2dB and tonality is exactly the same as expected, and if anything the actual real HF Trim Switch -2dB sounds better, but in reality probably the same.
(@BYRTT , thanks for telling me about VirtuixCAD a few weeks ago re SPL Trace, wouldn't have know about this fantastic program, wouldn't have been able to do without for this little project).
Thanks for taking the time to do this my dude!
For future reference, when you want to get the difference of two measurements in REW, you can simply use the "A / B" function to accomplish that (you'll have to scroll towards the 0 dB line to see the resulting graph). Maybe that would have saved some time instead of tracing the graph in Vituix!
The results are very interesting, looks like the -2 dB switch is slightly too much of a cut to perfectly compensate for the baseline treble rise, but it does increase the predicted preference score in the end.
