Question about Set Target Level As... option

[lovingdvd]

I'm a little confused about the "Set Target Level As..." option. In the target text field I have 75dB which is my target. Yet when I use the Set Target Level As option, it changes this to like 78dB. Why is it doing this? Don't I want to use 75hz? Or is the case that the level doesn't really matter and its all relative and post calibration I just turn the sub down 3dB to get from 78dB to 75dB?

The other challenge I hit is that apparently it wants to use a different target when I do my sub alone compared to when I do my mains. For instance when I tell it to Set Target As and I tell it to use the Sub for the level, it sets it to 74.88hz. If I tell it to calibrate based on my mains, it wants to set it to 78hz.

So what happens then when I measure my sub and mains together? Each has its own desirable target and they are 3dB apart. Does this matter?

[JohnM]

Setting the target level is used to find out where the speaker response would ideally lie so that REW can work out what is above that level (and might be a resonant peak). This is fairly easy for a main speaker, a pink noise signal is generated centred on 1kHz and spanning the range from 500Hz to 2kHz (same as a normal HT level calibration signal). Measuring with that wide signal averages out the peaks and troughs of the response, which are narrow compared to the span of the test signal, to give a mean level, which becomes the target.

For the sub it is not so easy, as the sub has a limited frequency range. The test signal covers the range from 30 to 80Hz (again, same as an HT sub level calibration signal) so gives an average of the response across that range, however peaks and troughs in the sub response can be pretty wide and so it is more difficult to find an average that is representative of the level the sub would be producing if it were not affected by the room's behaviour. For sub measurements you can often make a better estimate by looking at the response and setting the target level manually. As you apply filters and correct the various peaks, the results of the Set Target Level measurement get closer to the true average.

There are complications to be aware of when comparing the sub measured target with that of one or both main speakers. If you have connected REW to one input on your processor (e.g. the left channel of an analog input) and measure the sub response, then measure the main response, they would ideally have the same level, so that the transition from sub to main is even (assuming we have an overall flat response as our desired result). If the main speaker has a higher measured target you should increase the level of your sub. If you have connected REW to BOTH channels of a processor input, the sub would usually be receiving redirected bass from both inputs, so you would expect that measuring the sub alone would give a result that is higher than that for a single main speaker, but ideally the same as you would get if measuring with both main speakers running.

Arguably the target level can be calculated from the frequency response rather than determined from the pink noise measurement, but the measurement tends to give a better result. It's an area I'll look at further.

[cyberbri]

When I use it, and I'm no expert, I use the "set target level" adjustment to set what SPL level I want the "find peaks" and "apply filters" to aim for. If I set it to 70dB, Find Peaks finds peaks that are over 70dB, and Apply Filters gives me recommended filters to get the response to 70dB.

[lovingdvd]

cyberbri wrote:

When I use it, and I'm no expert, I use the "set target level" adjustment to set what SPL level I want the "find peaks" and "apply filters" to aim for. If I set it to 70dB, Find Peaks finds peaks that are over 70dB, and Apply Filters gives me recommended filters to get the response to 70dB.

Oh wait a sec - this is very interesting and it may answer another question I have going in a different thread. So let's say I have a fairly flat range except for a DIP at 60hz of -5dB. So instead of using a +5dB gain, I can tell REW to set the target of 70hz. Then it'll flatten everything out, applying about an extra -5dB to all other frequencies? Basically I'm trying to figure out how to avoid adding gains and instead lowering everything else accordingly in order to account for any dips.

[cyberbri]

If it's fairly flat except for one small dip, personally I'd experiment to see if the dip goes away and if the rest of the graph changes if I move the mic forward/backward, or move the sub slightly.

A 5dB dip is noticeable if the rest is pretty flat, so personally I would try adding 2~3dB to that and see how it acts at that volume and at higher and lower volumes. That would be much easier than trying to cut everything else down and keep it smooth.

[lovingdvd]

cyberbri wrote:

If it's fairly flat except for one small dip, personally I'd experiment to see if the dip goes away and if the rest of the graph changes if I move the mic forward/backward, or move the sub slightly.

A 5dB dip is noticeable if the rest is pretty flat, so personally I would try adding 2~3dB to that and see how it acts at that volume and at higher and lower volumes. That would be much easier than trying to cut everything else down and keep it smooth.

OK, will give it a try - thanks!