Room Gain II - Mythbusters - Test Data

[JohnA]

Don'cha just love test data!?

Procedure:

Rat Shack analog meter on the floor 39 3/8" from front of subwoofer motorboard. HP 204C signal generator driving input of Acurus A250, right channel only. Adjust 204C to achieve 92 dB at RS meter. Measure AC out at amplifier terminals with my Wavetek 27XT. Both subs measure 3.5 ohms DCR at amp ends of speaker cables. Apply Response correction to RS meter from widely published table. Corrected power reading from actual to 1 watt using Crown Audio online calculator (it's Saturday, I'm too lazy to fire up OO Calc). This is all on sine waves and played for 5+ seconds while taking readings. So it is continuous output.

http://www.crownaudio.com/apps_htm/designtools/db-power.htm

I'm not sure this should be considered a response curve because it is
based on flat power input not flat voltage input to the sub. If you see an error in my procedure, please speak up.

Results: output in dB at 1 watt at 1 meter

20Hz 78.4 dB

30Hz 90

40 96.4

50 94.7

60 95

80 92.2

100 95.9

So, 92 dB/w/m is certainly plausible (but in 1/4 space) and 96 dB/w/m is accurate enough for in room in corner sensitivity. It is clear the new, handsome, smaller cabinets killed my bottom end, or I do not have enough tuning weight on my PR. I was able to easily hit 118.5 dB at 40 Hz at 1 meter without audible distress or distortion (the whole house was buzzing!). That calculated 84 watts, so I have another 7 dB of available power, for 125+ dB max at 1 meter per sub. The subs are rated at 600 watts continuous and the designer, Mr. Cheney, has told me they cool themselves pretty well and don't suffer unduly from power compression. They do not have dust caps and I can see the voice coils.

So, I conclude I don't need more subs OR power, but that I either have some compression in the DVD soundtrack (like The Matrix), or I still have compression turned on somewhere in my DVD or BluRay players. I can get almost the desired effect by turning up the sub level 7 to 10 dB. But that's way off calibration and stinks on music.

[michael hurd]

John, can you do an outside 1 meter and 2 meter ground plane test and compare your results to your indoor measurment?

For instance, I used RoomEQ wizard ( free download from the HT Shack ) Behringer UCA-202 USB soundcard ( plug & play with Vista ), Behringer Xenyx 802 mixer, Behringer ECM 8000 measurement mic with calibration file ( available at the HT Shack ) to generate these measurements. ( about $ 150 USD in supplies )

Red= in-car measurement with mic clamped in headrest at head level on driver's side, mixer powered off 120VAC inverter, laptop on battery. Subwoofer was an Acoustic Elegance AV12X in a sealed 1.3 cu ft sonotube cabinet. Subwoofer in trunk, oriented side to side, seatback up ( as normal position ) and all windows and sun roof closed.

Blue = 1m ground plane measurement of the same device without any room gain, it starts to roll off pretty high. 4 sweeps averaged.

I will say that passive radiators need a bit of power to get moving, especially those with a bit stiffer suspensions. Do you have the T/S parameters on the drivers, as well as the parameters of the passives, and the size ( internal ) of the enclosures?

[michael hurd]

Also,if you are not aware, depending on if you were to measure the voltage and calculate for 3.5ohm DCR, the actual impedance can spike as high as 30-40 ohms or more.

Case in point... 40 volts across 4 ohms = 400 watts across the coil. Now, if you were to apply that same 40 volts across a 40 ohm impedance, ( spike on either side of the impedance minima ) you have a mere 40 watts being delivered to the coil. This would calculate into 10 amps for the first case scenario, and 1 amp across the coil for the second.

My Marantz AV receiver has a -3db point of 8 hz in the sub output, I measured it with my test rig.

Is there a possibility that you can measure the tuning point with single frequency test tones? The duration need not be long at all, check for highest PR movement, and lowest active driver movement ( this will be tuning ) You can download NCH tone generator for free if needed.

[robertcharles]

Mike,

Do you have any measures of your BEAST. I was wondering what the graph looks like for that system and how it looks in room.

Thanks,

Robert

[michael hurd]

I have some data on the smaller half sized box, good half space and 1/8th space outside measurement, but I don't have any inside measurement of the big cabinet. It sounds awesome, but right now it's taken apart to be moved. When I get some more time I will work on that.

[michael hurd]

John, I think I may have some insight into your mystery loss of bass. Please read this post on the AVS forum:

http://www.avsforum.com/avs-vb/showpost.php?p=8855640&postcount=1

The LFE channel needs boosting by 10 db in the AVR, it is recorded -10 db down in DD and DTS. Some receivers don't get this correct, it also depends on your interconnections ( how you are connected from player to receiver ) as well as if the player is doing a decoding or the receiver is.

So you may find while it is adequate for music, if your AVR / connection etc is not applying the analog 10db boost for movies, you will have a perception of low bass.

[JohnA]

Interesting, thanks!

This is not a new problem, I've noticed it from the beginning. Adding 8 to 10dB above the calibrated level does make the impact closer to what a theater sounds like.

I run both disc players through my upgraded ACT-3 Pre/Pro (almost Stage One equivalent) via digital interconnect, Coax for the Denon and Toslink for the Blu-Ray. The TV feeds the Pre/Pro via digital coax. The ACT-3 does all decoding and bass management. The exception is the 6.1 analog outputs from the Denon for SACD and DVD-A. I use the ACT-3 calibration tones to set all levels (and the subs' levels) and time delays. The analog outputs from my Denon are set to +10 dB in the player. It takes that to make the sound meter read equal.

Originally, the ACT-3 applied a +15 dB boost to the subwoofer out when all channels were crossed to a subwoofer, as I still do. I don't know what it does now, after the upgrade, but the sub levels sound the same.

I played with my phasing first by reversing the subwoofers' polarity and later by adjusting the time delay (distance) from the subs to my position to maximize bass output and have almost solved my bass "problem". It does sound deeper and has anoticably sharper punch. Turning up the sub levels that 8 to 10 dB, manually, makes the gunfire pretty impresive.