HUGE update! The second subwoofer is finished!
A detailed shot showing the system I used to keep the T-nuts in permanently.
The inside end of the access panel.
The bracing system for the second sub. This one should be more mechanically secure in that the vertical braces were cut so that they would go inbetween the horizontal braces.
A closer shot of the internal bracing.
The "Third Row Riser". Two Salerno Subwoofers nestled side by side.
These measurements were taken using a Radio Shack SPL meter as a microphone. No microphone calibration file was set in REW, per the advice of some in this forum. A sound card cal file was in place. The output of the meter was fed into the line input of the sound card integrated into my HTPC. (VIA VT1708S, which is the sound integrated onto my Asus P8B75-M LX Plus motherboard)
The below sweeps were taken with my subs powered by a Behringer iNuke 3000 DSP amplifier. Both drivers are Stereo Integrity HT18 D4 drivers, meaning they have dual 4 ohm coils. The coils are wired in parallel, resulting in a 2 ohm load seen by the iNuke. Also, I have replaced the stock fan in my iNuke with an 80mm Noctua 1800rpm fan. After installing the fan and turning on the amp, I thought I did something wrong because I didn't hear anything. But the fan was indeed on and moving air. Huge difference!
Configuration: Bi-Amp 1
Peak Limiter: -1.4dBfs Hold 200ms Release 200ms
Filter/Crossover (same settings for both channels)
High Pass 1 & 3: 20Hz, Butterworth, 12db/oct
Low Pass 2 & 4: OFF
PEQ (same settings for both channels):
Filter 1: -12db, 20Hz, High Shelf 12db/oct
Filter 2-8: Off at this time
Dynamic EQ: OFF
iNuke settings discussion:
Configuration: Bi-Amp 1
I had initially set the amplifier to "Stereo", thinking that both inputs were summed and sent to both outputs. However, this is not true. In this mode, Input A and B are indeed separate. However, the EQ settings are linked so that both channels have identical EQ settings. I found in Bi-Amp, the one input is indeed fed to both outputs. Normally this configuration would power a woofer and a tweeter. However, since I set the filters identically for both channels, both A and B outputs were identical. And so it would seem that "Bi-Amp 1" is the same as using a Y splitter to connect the LFE output to both inputs on the iNuke. Only I don't have to buy a splitter. Theoretically, "Bi-Amp 2" should work the same as "Bi-Amp 1" However, I haven't tested this.
Peak Limiter: -1.4dBfs Hold 200ms Release 200ms
Admittedly, I didn't realize I had this enabled. Looking at the measurements, it doesn't look like it affected anything. According to the iNuke software, this limit would kick in at roughly 1800W. When I do further sweeps tomorrow, I'll have to do some while watching my output levels to see if I am actually anywhere near the limits. I doubt I am, however. I'll have to play around with this one and see how it affects things.
For those of you just joining us, I have created a "Phantom 10Hz High Pass filter" by combining a 20Hz 12db/oct High Pass filter and a -12db 12db/oct High Shelf filter. These filters sum to create the "Phantom" High Pass filter at 10Hz. As seen in the measurements below, this method indeed does work on the iNuke.
On to the measurements! All measurements are at the prime listening position unless otherwise noted.
Front row sweeps with just the subs, and with integration of the fronts. The higher level sweep is just the subs - sound card output fed directly to the iNike 3000.. The sweep which is lower in level is with the front speakers engaged - HDMI Stereo output to my receiver, with the fronts (Klipsch Rf-7 IIs) crossed over at 40Hz.. I didn't level match between going directly to the sub, and going through the receiver, so disregard the level difference and rather examine the room response. The measurements were taken from the front row left, center (prime) and right seats. This was done to see how positioning affected the response.
Looking at just the sweeps done with the fronts engaged, the frequency response actually looks pretty good, with the exception of the hole at 70Hz. However, notice in the above chart that with just the subwoofers, the 70Hz hole is not there. This tells me that it is not a room issue, and is rather the subs interacting negatively with the fronts. Or, it could be the fronts interacting with each other. I'll have to try measuring again with the subs off to see if this is an issue with the way the front speakers are interacting with each other.
Another thing I notice is that all the sweeps are very similar up to about 80Hz. Past that, the left seat is hearing less. This seat is right next to some 2" mineral wool panels, and is also pretty far off axis with the right speaker. I may have to experiment with toe-in to see if I can get that seat sounding better. However, this seat is the chase lounge seat, and therefore the wife seat. Maybe I should leave it as is so she will be less likely to ask me to turn it down?
Rear with and without fronts.
Rear row with the fronts. Immediately I notice that the hole at 70Hz is gone, and now we have holes at roughly 25 and 45Hz. Now, let's see what is shared between the two rows.
This sweep is the combined average of the front and rear rows. There's a nasty peak at 28Hz which is common to both the rows. This I should be able to tame with EQ. However, I'm going to experiment with some mineral wool panels I acquired today. They're 80mm thick, and just three of them are pretty darn heavy. I'll have pictures of the stuff tomorrow.
The lower sweeps are subs A and B individually. The highest sweep is both subs working together. All three sweeps were done with the sound card output fed directly to the iNuke input. The only thing I changed between the three sweeps was which subwoofer was plugged in. Other than that, the output level and microphone position were identical.
Effects of the "Phantom 10Hz HP filter". The filter was crated by combining a 20Hz -12db/oct HP filter and a -12db HS filter set at 20Hz. Both sweeps were done with the same output level from Room EQ Wizard. At the amplifier, however, the input gain was set to -9db. When the phantom 10Hz HP is in effect, I have the input gain set to +3db to compensate for the 12db of loss from the negative gain high shelf filter. When I remove that filter, I have to adjust my input gain to -9db to keep the same overall level.
We see at 30Hz, the sweeps are identical. At 20Hz, I'm down by about .5db. By 12Hz, I've lost 2db. By 8Hz, I've lost 3db, and by 6Hz I'm down by 4db. And so, while I am losing a little by using a high pass, I'm not losing much. I figure a few db is an acceptable trade for a little peace of mind that I can play up to reference without breaking anything.
Phase comparison. Setting either sub to 180 degrees out of phase completely trashes the LF response. I may experiment with some delay if treatments don't help fix some of my issues.
Two sweeps at -5 and -10dbFS. These were done with the sound card output connected directly to the iNuke 3000 input. Interestingly, the gaping hole from 30 to 55Hz isn't there when the fronts are in the mix.
This chart shows my room gain. The blue curve is a near field measurement taken from about 1 foot from the driver. The gold curve is at the listening position. Levels were adjusted between tests to compensate for the db loss associated with moving away from the subwoofer.
I'm not sure where exactly to line the two sweeps up since they don't resemble each other very much. But it looks like I'm getting anywhere from 13 to 20db of room gain by the time I'm at 10Hz.
These sweeps were generated by telling REW to divide the above LP curve by the NF curve. The resulting curves show a room gain of 13 to 20db at 10Hz. The higher curve was generated using the above curves as is. The lower curve was generated by offsetting the Near Field curve by approximately +7db to get it to more closely match the LP curve. According to my room dimensions, my room gain should be starting at about 30Hz. So my true curve is somewhere inbetween these two. So worst case, 13db of room gain at 10Hz, best case 20db.Conclusions
First and foremost, LTD02's idea of summing a 20Hz HP and -12db HS filter indeed does work in practice. Kudos!
Second, the Behringer iNuke 3000DSP doesn't seem to have any trouble pushing gobs of power into two 2 ohm loads, even way down at 10Hz. From some excursion tests I've done, I am definitely not amp limited. The Behringer is enough to push me beyond XMax even above tuning. Your mileage may vary if you are pushing a high wattage driver in a sealed configuration, but as far as LLT goes, the iNuke 3000 is good to go!The Way Ahead
Tomorrow, I will be doing more measurements with bass traps in place. Hopefully these will tame down some of my issues so that I won't have to do a lot of equalizing.Edited by DanLW - 4/5/13 at 3:28pm