Room Measurements

Hmm…. :rds2:

 

You can add additional delay in 0.02ms increments to the other processing delay.

 

For the moment, I'm just going to tell you that you are wrong

 

I would keep the DEQ in the digital domain and definitely before the preamp or you will be subject to quantisation distortion at lower listening levels.

If you keep it in the digital domain it will only affect the frequencies you are adjusting anyway so there is nothing to be gained be having it after the crossover and it avoids any latency issues

 

Just one little point which may already have been made,although I didnt see it.....your mic either has to have a suitably flat frequency response(and very few do at low frequencies)or you have to add in a suitable correction curve to your FFT software to compensate for this.

Most of the decent mic makers publish the frequency responses of their gear,and it's easy to compensate from that.

 

True, didn't really think about that one much but the delay really does seem small. Must be under 3ms.



.... I just measured it and the delay is 0.8ms - That’s with both the graphic and parametric EQ active in dual mono mode using analogue inputs and analogue aux outputs.

The MB2 Xovers from woofer to midrange at 380Hz. So at 380Hz this would (I think, had a few beers!) produce a phase difference of 109.5 degrees. Thus if you add an additional delay of 1.83ms to the DEQ it will be in phase again at the crossover point.

Of course the crossover isn’t just at an exact frequency but I would think this is accurate enough to try it and see what happens to the frequency response, which would be a good indication of whats going on with the phase higher up the crossover slope.

 

The ECM8000 is not particularly expensive but it has a very flat frequency response I believe. I would not want to use it for distortion measurements or anything but for this use it is fine.

Although CoolEdit does not, many software will take a input from the 'system' and one from the mic so that it can compare the two and compensate for it as it knows what it sent out in the first place.

 

Tenson,

I only want deq between bass amp and xover. I also want the behringer to sit between the transport and dac to convert from optical to electrical (without deq) and also to add as a dac until I get my decapo back. I dont want deq at both points else digital sources will get deqed twice! In the digital domain it cant improve my turntable bass so there is definately something to be gained from putting it in the analog domain instead - it can then affect all my sources! As it is just being used in the bass then I should just put it before the bass amps to keep it clear of the treble and mid. I dont play it quiet

I had considered latency and the need to quantify it. I assumed it was very small from the buffer. The ECM8000 mic has pretty flat response - such that its likely to be lost in the speaker variation.

 

Sorry it is not possible to have it in digital thru mode AND take analogue inputs and outputs with processing.

I would think it pretty damn transparent with the mods - don't write it off being used full range in the analogue domain just yet. Unless you play it at full volume, its quiet. Especially playing the TT this would probably show itself.

 

I could just buy an optical to electrical digital converter...

 

So, assume an anechoic chamber and imagine what happens if you put ONE cycle of a 380Hz sinewave into that combined system. Which leads to the following three related questions: What do you get out? And what are the amplitudes(*) if you are "on axis"? Can you place the microphone somewhere such that you can record a single 380Hz sinewave?

[(*) hint: defintion of the crossover point..]

 

By combined system do you mean the MB2 with the delay in the bass crossover in the chamber?

If so then you would get out two cycles, one from the mid dome and then one from the bass driver. The amplitudes would be half of whatever they would be if there was no delay and both drivers sounded simultaneously. I don't think you can record just one sign wave if two are being produced, no matter where the mic is.

Obviously you could if you use a gate or if there was beaming with the woofer and you placed it off-axis or something but I don’t think you wanted a silly answer!

What are you getting at? Bear in mind that TL's work by delaying the woofers rear output by a quarter cycle or so at the tuned freqency.

 

Yep.

That doesn't seem like a silly answer.

The point is this, if you take a speaker with multiple drivers (including ports), then measure its output in three dimensional space, you'll measure a variety of interference effects in the crossover region of the frequency response as you move around the chamber. When you put together a speaker, you diddle with the design as a whole to get the on-axis response "right".

Now, if you start fiddling around with the delay to one or more of those "outputs", then the interference pattern moves around in 3d space, and depending on the relative listening position, possibly introduces a big suckout there.

Getting the on-axis response "right" matters (especially with room treatment) despite the nuisance of having a real room and listening in the reverberant (rather than free) field.

[So, Markus, this isn't just group delay, but (potentially) a change in the amplitude response too..]

 

Ahh I see what you mean. As I said before, I think its the sort of thing you have to try and see what happens. You must realise such an effect could easily happen if there was a reflection from the floor 0.45m away causing 'destructive interference'. Its simply a suck it and see exercise. Maybe one could improve the off-axis response, I mean it couldn't get any worse right?!

I should have seen what you were getting at before actually as I was dealing with sound steering most of today in another situation. Funny how things link up.

By the way, I think the main thing you would get is distortions in the vertical axis polar response, but not so much in the horizontal which is not as bad as it could be as most people always listen at the same level.

 

I think I made precisely this point in a discussion somewhere, when someone (not you) wanted to convince us that phase was inaudible, no matter what the circumstances.

 

Oedipus - interesting points!

I have now heard back from Bryston. Because I have gone from the stacked cab to the single cab (I took out the xbd as the stack is 12 feet tall with it!!!) the crossover will need adjustment. Currently the bass will be 3db too low (the 7bsst doubles power into half the load when two bass drivers are being driven) so I will need to take the mid and treble down by 3db as I thought, on top of my adjustment to match the treble and mid. Again this 3db bass cut that is in place currently will also make the system sound a little forward.

 

Tenson, have you told BBV how to measure the on axis response accurately without access to an anechoic chamber?

 

Not explicitly as such - perhaps you can say something about this SM?

Feel free to bring your mk 7 lithos Groove + and vibe +!

 

No, lets get the mic recording something first!

 

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