Tact question

Cool, where have you been? I'm helping out a band at the moment by playing bass and we are looking at recording a few hits at www.ten21.biz I know the guy (a bit) as I nearly got a job there once lol!

 

The last one was Windmill Lane Studios, Dublin.

The head engineer their (Brian Masterton) is involved with another small studio job in Dublin we are working on. When I asked him to show me an example space he likes to work in, he took me to their 5.1 editing suite (full of Genelecs). Didn't get into the main studio though (or see U2) but still a big buzz for me.

I loved the Genelecs too by the way.

 

Cool studio, nice to see they have actually made some factual statements about their room acoustics (most studio websites are full of BS).

Don't know about the M&K monitors though, not a big player here, are they any good?

 

systems and signals 101

You are both wrong. Now, I could just tell you why, but where would the fun be in that

Tenson, time for you to try an experiment... Hook ETF up to your DEQ and set up a single parametric EQ with Freq 100 BW 1/10th GAIN -10db. Do a low freq measurement with ETF.

Now, look at the low frequency cumulative spectral decay

What does this tell you?

[I'll assign more homework tomorrow]

 

Its quite simple, changing the amount of energy you pump into the box, does not change how lossy the box is.

Think of it like having two slides, a steep one and a shallow one. You can start one kid sliding down the shallow one earlier so that both kids cross at a certain point at the same time, but you can not have them keep pace with each other the whole way down - one has a faster rate of decay.

I won't deny that the situation can be improved greatly, but it can't be 100% fixed. If it worked as you suggest, then using something like the TacT that can get the frequency response perfectly flat would also give you a 100% perfectly smooth waterfall plot across all bass frequencies, and it doesn't, it just gets a lot closer.

Thats why I suggest using both an EQ and bass traps.. because the EQ can get the waterfall plot to look pretty smooth and then the bass traps can give a much faster decay rate. If the decay is fast enough I find that the remaining discrepancies are not really noticed.

Please look at the pictures below. The first is an un-EQ’d response of someone’s system, the second is after a bit of EQ. Look at the large peak about 70Hz or so in the first, now look in the second. The second had the response pulled down at that point so it was in-line with the rest (some pretty server suck-outs in that room) and then the over-all response boosted to keep the bass level at a reasonable volume.

Notice that where the peak was, there is still a longer decay relative to the other frequencies.

Equally look at around 100Hz. There is a suck-out here. Boost it and the rate of decay is still faster than the other frequencies.

However, look around 40Hz (just before the room is dominated by pressure mode), there is a suck-out. Boost at these frequencies and the decay gets longer too. I think this is because the suck-out was caused by a reflection from the back wall rather than a room mode. The reflection cancels out at that frequency but then you still have some of the sound that went out in different directions that is not cancelled out and the decay of this sound is still seen at a lower level.




Now if the entire decay rate is improved enough using room treatments, then the differences become less obvious to the ear. You get a flat frequency response with a decay rate that is so fast over the entire range that it all just sounds ‘fast’.

As a side note, look at the differences in LF spectral decay for different woofers, they are nowhere near that large, which is why I believe the room is more important than the speaker for getting tight, fast bass.

Your posting style reminds me a lot of S&M's.

 

Seriously, forget the measurents and the text you posted, and try measuring what the Equalizer is doing. You're missing something important and I expect that that CSD plot will stop you in your tracks... it holds the clue to how the other kid catches up!

There's no need for insults

 

Now I have to admit I feel kinda shit right now (think I got something) but I don't see how that helps the other kid catch up. The initial response is improved but then it just rings on for longer afterwards.

I also admit that I don't know everything there is to know about acoustics, and digital signal processing especially, but I think the guys at the likes of RPG know rather a lot so if the problems can be completely solved with EQ why do they continue to make room treatments?

Edit: Ahh unless the phase is just so, that it cancels out the decay... but it obviously doesn't work perfectly in the real world if you go back and look at the graphs I posted Perhaps this is due to using two speakers. You can EQ each separately and thats good but then when you play them together they interact with each other and the EQ won't be right anymore either. So I suppose the best way to make this work is to use a single point source for the bass - not something many would like to do. Of course I could be completely on the wrong track, do tell.

Edit again: Having said that, if it works that way one could make a system which measures the room and then adds in the correct sound out of phase at the right time to improve the decay rate over the entire frequency range, not just at that point. As this has not been done I guess that is not how it works after-all.

You have me intrigued now, what is the point you are driving at?

 

An "A" grade so far

More homework. here's another experiment... I'm not sure you have the tools for this one, but I think you might...

Create a 1 second "tone burst" of 100Hz sinewaves at say -20dBFS (make the waves start and end at zero, it's easier to see what happens). Create the file with say 1s of silence at the beginning and at the end...

Now, push that through your DEQ... with the same PEQ (Freq 100Hz, BW 1/10 oct, -10dB gain).

Now look at the output waveform - pay particular attention to what happens at the end of the "steady state" behaviour...

 

Watching with interest guys. Been to a wedding tonight and have way too many pints on board ATM so will have a better look in the morning.

Good work but stay clear of the insults, lets get to the bottom of this.

 

OK, a bit pissed but still couldn't resist. Are you saying that the Tact system is modifying the signal phase AS WELL AS level at the room modes?

If so, I am really intrigued now.

 

Come on ......can wait for the next graph !

 

Okay here you go, I think this is what you meant.



Not anything I didn't expect really, just the fact it looks like a resonance?

Here is another picture of an actual helmholtz resonator (taken from the web) being struck to excite it (ooh-err!).



Andy, unless using FIR filters all equalisers alter phase as well as amplitude. I only ever thought of it in regards to the relative phase of sound arriving at the listener and not the effect that interaction would have on the decay rate, so I owe you an apology Oedipus it does try to alter the decay rate. I have still to see it work ideally in real world set-ups though and certainly no reason not to improve things further with physical bass treatments. It also makes me realise even more why people often claim EQ makes things sound funny, because it is actively trying to counteract the room mode. Interestingly, I have heard very similar descriptions of what active bass traps do to the music!

 

Yep, know that. I was referring to the fact that maybe the Tact system being a bit "clever" about it?

 

As far as I know, no, its just the way it works out The TacT has a finer resolution of adjustment than the Behringer though so it will do a slightly better job of it (if you monitor the LF waterfall and set it up carefully). If you are still using a stereo bass source though I'm doubtful whether the improvements in adjustment resolution are worth much. Am I right in thinking the system that comes with the TacT does not actually have the ability to show the decay rate?

 

Andy (T)

Looks like your new pupil may yet obtain an As grade yet.
How is the state side master of blind test terror these days?
Still playing with those average atc's & benchmarks

 

It is what I meant. You can see the waveform clearly changes direction.

FIR filters can have arbitrary phase.. I cover this in my class "Digital Signal Processing 101"

There are limits to filtering, or at least (mostly!) minimum phase filtering like EQ units do. The reason is that, even though the modes are, the complete room response is not, minimum phase..

However, physical devices like bass traps and helmholtz resonators (and this will really twist your noodle) are minimum phase This is why the concept shows up in "unexpected" places in engineering.

Here's where you get a failing grade. You cannot link a result like this with peoples subjective impression without supporting statisical data from proper subjective testing.

 

Yeah, what I meant is that AFAIK unless using FIR filters you can not escape the minimum phase behaviour of an EQ, right?

Yes but I have yet to see it work perfectly even in the bass range under real world conditions. I have not had a play with a DEQX though so it may get (probably does) closer.

I see what Tony means by a 'blind test terror'! It was merely a thought, not a factual statement :MILD:

 

The complete answer to that question is quite long

How far "de-reverberation" can be pushed is still an open research problem - people are actively working on the topic...

FWIW: it's a good piece of hardware. At the time I owned mine, the software for "room correction" was a joke! That said, if you're prepared to jump through some hoops it can be more flexible than the Behringer.

Oh please, I am a "blind test terrorist"--- I am not the one living in fear - he is

 

I am pretty good and using (awesome) Quad 989's at the moment (with the Benchmark). I am also bucking convention and using Bryston PowerPak amps rather than tubes

[I still have the ATC 50's which are on "summer vacation"...]

 

Andy,
Yes the quads can make some nice sounds for sure.
Do you venture back to the UK at all these days?