2nd time around..measurements

[Mattlock]

The room doesn't cause cancellations below the lowest modal frequency of the room...and you'd need a rather large room to have modal issues down to 20Hz (most rooms end around 30Hz, but there's no need for generalities when the specifics can be measured)

Quote:
SteveCallas wrote:
The valley in response in the low end was clearly a room effect, not subwoofer response related, as can be seen from his new measurement with the seat moved.

Quote:
kbgl wrote:
Also added a little boost to flatten it out a little between 10 and 20 hz.

Just as expected

Actually the first filter was set at 21.25. See graph below without filter, filter and predicted curves...Nothing boosting below 20hz..

Matt

[DrWho]

I was originally referring to the boosting at 20Hz, which is apparently right at 21.5Hz. With better port loading, you would have less of a dip in that region. And by dip, I'm not referring to that narrow Q right at 21.5Hz, there is a wider Q which is also amplified by the influence of the room. After looking at literally thousands of these measurements, I have no doubt of what I'm seeing in the response...and the EQ curve just further confirms it.

If you're not convinced, take the sub outdoors and rule out the influences of the room. It'll be an eye opening experience.

Heck, the whole point of measuring is to see what's actually happening in the system. Is the goal here to get confirmation that your system sounds good, or are you trying to make things sound better?

[kbgl]

My mistake on the comment about flattening the response between 10 and 20. I was watching the graph while Matt made the adjustments. Without my glasses, I couldn't read the actual numbers off his laptop screen, and we were working fast trying to get done by 10:00pm. I was replying to Steve's question about the rising low end.

I think the tuning is fine. As with all things, there are trade-offs.

[Jman]

DrWho wrote:

If you're not convinced, take the sub outdoors and rule out the influences of the room. It'll be an eye opening experience.

He can measure nearfield to see the same thing. Personally I wouldn't make any boosts under ~25hz. Just asking for trouble when you crank it and the right (or wrong ) content hits the sub.

[SteveCallas]

DrWho wrote:

and you'd need a rather large room to have modal issues down to 20Hz (most rooms end around 30Hz, but there's no need for generalities when the specifics can be measured)

I could easily affect the measured <20hz response in both my old place and new place by opening/closing doors or measuring from different spots in the room. What the sub is outputting isn't changing, the variable being changed (one at a time) was volume being pressurized or location from which the measurement was taken. Heck, we can clearly see differences in the <20hz response from two of Mattlock's own measurements with no filters in play.

Ilkka's subwoofer is 3 ft tall, which pushes the first standing wave up to ~200Hz.

Perhaps I am confusing this statement of his (which shows up in the distortion measurement) with a standing wave?

Ilkka wrote:

DIY TC Sounds TC-2000 15" ported 270L
The very much discussed “LLT”. Very good extension and frequency response for larger rooms. Small rooms may have too much low end gain. Quite strong port resonance at ~160 Hz. High max SPL at all frequencies. Low compression up to 110 dB. The 115 dB was terminated due to amp clipping – not woofer bottoming. Low overall distortion, though the upper end could be cleaner. End cap to end cap resonance shows at ~75 Hz. Very low group delay for a ported subwoofer. Good decay too. The enclosure is naturally huge.

The port resonance isn't triggered by the active driver playing at the port resonance frequency...the fact that air moves through the port triggers the resonance. Try playing a 13Hz sine wave or whatever frequency the port is tuned at...you'll hear it singing the port resonance tone.

Interesting. Why then would the blip in the FR of Ilkka's sub show up at the first resonant frequency? At that high a frequency, there should be next to no air moving through the port, so what causes the blip? I can't say I've ever heard my port sing a 300+hz tone whenever low frequency sine waves have been played through it.

Although IMD doesn't directly show up in THD measurements, you still see an increase in higher orders of distortion when the bandwidth of the system is increased (ie, lower port tunings).

When comparing the harmonic distortion by component measurements between the low tuned TC2k and small sealed TC2k - while two different samples of the same driver were being used - I'm not seeing the fruition of what you are describing unless you are only referring to measurements below the tuning frequency.


I agree with Jman Mattlock, I don't see too much benefit in the low end boosting, just potential for trouble.

[Mattlock]

SteveCallas wrote:

I agree with Jman Mattlock, I don't see too much benefit in the low end boosting, just potential for trouble.

okay...I'll bring those down...

thanks

[Mattlock]

Question: I'd be curious to know if anyone sees any material benefit/enhancement to my responses in the low end from using bass traps? Just curious....

Matt

[kbgl]

SteveCallas wrote:

I agree with Jman Mattlock, I don't see too much benefit in the low end boosting, just potential for trouble.

I would at least halfway disagree Steve. The EQ added resulted in more output. This was a low spot in the response, not a null. We played WOTW with good strong LFE effects, but the sub level could have easily been reduced about 3 dB and it would have been fine. I've seen people crank the overall level up till the actors sound like they are shouting in order to get the effects to sound strong. Turning up the volume level 6 or 7 dB would not only boost the 21 Hz frequency, it would boost them all.

On the other hand, I monitored the excursion during one of the demanding scenes, and it was getting a little close to what I would assume were its x-max figures. It might be a better compromise to move the couch back some and live with a slight notch at 50 hz, and have the additional headroom.

What are the x-max and x-mech figures for this driver?

I think I saw about 1 3/4" peak to peak excursion for a second or so. Couldn't see it real well, so I may be wrong.

[Mattlock]

Well, I made more measurements and wanted to post for feedback...

Graph 1: Couch moved 2ft forward with no filters (sub in rear)

Graph 2: Couch moved 2ft forward with filters (sub in rear)

(I pulled back on the gain for filters 1 and 2 from +5 and +7 to +3s on both)

Graph 3: Sub moved to front of room laying down with NO FILTERS (this is the only possible option and quite frankly it lwill most likely will get spousal disapproval in a big way) Actually I would have to remove the baseplate in order for it to fit between the walls and fireplace base and also be forced to reduce my screensize slightly....this thing is just huge

Practically speaking is the differnce between 2&3 that big enough to warrant moving upfront and dealing with the aesthetic challenges????
How about I just work some more with my filters ??????????

The other option would be to put a smaller second sub up front but I assume that would have to be tuned pretty close to my current one??

I could probably get a 500 liter 24 inch sonosub (or box) put up front that would look much better..

[kbgl]

Try laying it down in the back of the room. I think the second curve looks pretty good as it is.

You might try reducing the effect of the 604hz filter and see how it looks. Maybe -20 dB, or reduce the bandwidth of the 604 hz filter a little

What are the limits on excursion with this driver?

[Mattlock]

xmax on the Q18 is 27mm

[kbgl]

Mattlock wrote:

xmax on the Q18 is 27mm

So 2" peak to peak excursion should be no problem for this driver!

[SteveCallas]

Graph 2 is good enough I'd say, I wouldn't worry too much about trying toget any flatter than that in your room. What I would do though is take that measurement again, this time with the mains in play. You'll want to make sure nothing changes dramaically down lo when the mains are in play, otherwise you'll want to change the filtering again.

As for bass traps, in order to affect frequencies below 80hz, the traps would have to be extremely thick. I doubt that is a realistic option. However, adding some absorption to the room in the form of pannels or corner traps should help the overall sound quality, even if it isn't really affecting the low end bass.

[tnargs]

Mattlock wrote:

...The other option would be to put a smaller second sub up front but I assume that would have to be tuned pretty close to my current one??

I answered that in post 37.

[kbgl]

You might want to make a target rolloff of 24 dB / oct and see if you like it better than the 12 dB.

[kbgl]

JohnM wrote:

From the impulse response the distortion looks very high, getting on for 10% 2nd harmonic and several % 3rd harmonic. May want to check the sub isn't being driven too hard.

How do you read that off of the impulse response?

[kbgl]

SteveCallas wrote:

Again, not the case, that dip is merely another room related or phase issue. If you take a look at Ilkka's testing of a large and low tuned sub, you'll see that the FR is not affected in the least by any type of standing wave created inside a large enclosure. You do benefit from decreased harmonic distortion at that frequency, but it doesn't affect FR.


Port resonances will show up as a blip in the FR, but with his first resonance being ~250hz and a 24db/octave 80hz crossover in play, it's a non issue.

I was wondering if a circular baffle inside the sub made from foam, or maybe peg board or MDF would be beneficial in breaking up some resonances.

[JohnM]

kbgl wrote:

How do you read that off of the impulse response?

The scaled down copies of the main impulse to the left of it are distortion artefacts. The first one (going left) is the 2nd harmonic, the next one is the third harmonic etc.

[kbgl]

JohnM wrote:

The scaled down copies of the main impulse to the left of it are distortion artefacts. The first one (going left) is the 2nd harmonic, the next one is the third harmonic etc.

Like This?

[JohnM]

That's them.