Subwoofer phase is a pretty simple concept in speaker/sub integration, but it is often overlooked by people starting out. Typically a subwoofer has either a phase switch (often labeled with either 0/180 or normal/reverse settings) or a phase knob (which allows more granular control of the phase).
This is sometimes misunderstood because the idea of a crossover point is viewed as a ‘brickwall’ or an immediate hand-off from the speakers to the subwoofer at that frequency. In reality, there is a fairly wide range where the speakers and sub overlap.
The basic concept is for the speakers and subwoofer(s) to be working together in this overlap area instead of fighting each other (constructive versus destructive interference to use physics terms). This is accomplished by adjusting the phase (and possibly position) of the subwoofer to get it in sync with the main speakers (in a purely home theater setup, an argument can be made that coordination with the center speaker is more important).
Below are two graphs showing the frequency response in my home theater with my Hsu VTF-2 sub and Ascend Acoustics CBM-170 mains using the 0 degree and 180 degree phase settings. In this example, the 180 degree setting results in a pretty flat response (close to +/-2db from 20Hz to 130Hz), but the 0 degree setting has a very large drop (greater than 15db) in the crossover range.
180 degree phase setting (click to enlarge):
0 degree phase setting (click to enlarge):
Measurements using standard setup at position 3 in room diagram.
I’m finally getting around to putting down some more notes on the AV123 get together earlier this month. Here is a summary of the equipment present and some comments:
Theater room:
Mains: Rocket RS1000
Center: Rocket RSC200
Surrounds: Rocket RS850
Rears: Rocket RS550
Subs: 2x Rocket UFW-12
Pre/pro: Emotiva LMC-1
Amp: Emotiva LPA-1
Sources: Denon 2910 and Onix CD-2 w and w/o Perpetual Technologies P-1A
Etc: R-DES parametric EQ on subs, plasma display, Panamax power
I assisted Dave Tremblay, the inventor of R-DES, with measurement during the sub EQ process using the setup listed earlier here.
Multichannel audio room:
Mains: Onix Reference 3
Center: Onix Reference 100
Surrounds: Onix Reference 1.5
Sub: Rocket UFW-12
Pre/pro: Emotiva DCM-1
Amp: Emotiva MPS-1
Source: Denon 2910
Etc: R-DES parametric EQ, Panamax power
Smaller theater room:
Mains: ELT LRS with PBS powered woofer stands
Center: ELT center SE
Rears: ELT DPA dipoles
Sub: ELT SW-10SE
Receiver/source: Pioneer combo
Etc: 30″ LCD TV
Two channel room:
Mains: Rocket RS760 and XLS (Onix Reference 1.5 for a period of time)
Integrated amp: Onix/Melody SP3 and Onix X-empower beta unit
Source: Onix XCD-88 and Onix XCD-99
This room got a LOT of attention because both speakers are new, the SP3 is a popular item at the moment, the X-empower is new, the XCD-99 is new, and the designer of both the RS760 and XLS (Danny Ritchie from GR Research) was in the room discussing the speakers.
Static display:
Rocket RS850 in piano gloss rosewood finish
Strata Mini speakers
Onix Reference 1 and Onix/Melody SP3
Rocket UFW-10
Onix CD-2
Emotiva DMC-1 based receiver (using D2Audio amp module)
Emotiva BPA-1 two-channel, bridgable amp
Audio Protien rack
One of the focuses of the GTG, SOCS and MMK for the Perpetual Technologies P1-A, was not really on display, but there were multiple sessions to demonstrate the measurement technique, discuss the SOCS corrections, and demonstrate the difference SOCS makes in playback.
Acoustisoft (aka Doug Plumb), makers of ETF, have released R+D (Resonance & Distortion) 1.0. It’s similar to ETF, but designed to do averaging of multiple curves for working with room problems. It also does DSP/EQ modeling of the Behringer DSP1124 Feedback Destroyer (BFD) in a way that is supposed to be much more accurate that typical EQ result modeling. The advanced version also has features to automatically pick EQ points for a give measurement or average of measurements. Here is an AVS thread on R+D
I have gone through Doug’s videos and started poking around with the application. I will post more when I have a chance to try it and compare to ETF. Doug says it will eventually have all of the capability of ETF and could be considered ETF v6.
It’s an interesting deveopment of the product geared toward addressing room issues via EQ, which is a popular topic at the moment. Given the low cost of a the BFD ($100) and the relatively low cost of the software compared to some options, it seems like a useful tool for many people.
Of course, Room EQ Wizard is still free and overlaps in function, but lacks the important averaging of multiple measurements. Unless you listen alone in one place, this is very important.
Since the BFD isn’t designed with home entertainment use in mind, it does lack some function of its more expensive counterparts (Onix R-DES $400 and Velodyne SMS-1 was $500 now $700), particularly their ability to store mulitple EQ curves and easily switch between them. On the other hand, the BFD is designed for full-range EQ and could be used on main speakers as well as subwoofers.
I have started reading the Master Handbook of Acoustics to learn more about acoutics and room behavior. So far it seems like a pretty good resource, although the author makes some assumptions about the reader’s understanding of mathematics and ability to pick up on things quickly. To pick up on some of the topics hands-on, having a copy of ETF (the author uses ETF in examples, so having this particular program is useful, but other software could accomplish the same thing) and appropriate hardware to to basic measurements is helpful.
There is an excellent thread on using TrueRTA for audio measurement over on AVS - http://www.avsforum.com/avs-vb/showthread.php?t=572477
I highly recommend it if you’re looking to try out measurement for fun.
The past four days have been a bit of a blur and I’m paying the price (now home sick). This year’s AV123 Colorado GTG was great. There were a lot of great folks there: Danny Ritchie (GR Research), Ron Stimpson (SV Subwoofers), Keith Allsop (Perpetual Technologies, etc.), Phil Bamberg (BESL), Rich Hollis (Hollis Audio Labs), Dave Tremblay (RDES developer), the AV123 crew (Mark, Suzanne, Steve, Sean, Greg, Kyle, John, Wendy), Chris Cahill and lots of enthusiasts. I’m glad to have had the chance to hang out with these folks, even if some of the conversations were half (or more) over my head.
There was a lot of good discussion on many topics and some work done on measurement and number crunching. It was great to get a variety of perspectives and input on some topics.
Before I get around to posting measurements, here’s a quick breakdown of the equipment that I am measuring with. It’s nothing too fancy, but it seems to do a pretty good job:
Behringer ECM8000 mic
M-Audio MobilePre as both the mic pre-amp and the soundcard
Dell Inspiron D600 laptop on battery power
ETF 5.98x software
RadioShack SPL meter for setting the SPL calibration level in ETF
I have some additional software installed to play with like the free Room EQ Wizard and trial versions of various programs to try out (TruRTA, Spectra Plus, etc.).
I highly recommend the Room EQ Wizard if you have a soundcard with a stereo line-in (many notebooks only have a ‘mic’ in and not a stereo line in) and want to measure your in-room response. It’s an excellent program and can be used with just a Radio Shack SPL meter as the mic.
I’m going to spend some time talking about working on low frequency response in room. I’m just learning this stuff, so feel free to point out information of note (references would be great, I’ll be picking up some books on the topic soon).
The room itself in in a fully finished basement with pad and carpet on concrete and 7.5′ ceilings. Here is a diagram of the room, click to enlarge (pink - speakers, purple - sub, green - treatments):
The numbers along the sofa are reference points for the measurements I’ll be posting later. Each time I do measurements, I measure at all six points for each test. It takes some time, but gives a much more complete picture of how things sound for various positions. The sofa back and mic stand are also marked to try and keep measurement positions consistent between runs. I also run a new baseline for each test and the baselines have remained consistent.
The room dimensions noted here aren’t exactly correct. I remeasured and the length is just under 22′. That would give an axial mode of about 25Hz (I tend to use this page for calculating modes - >http://www.marktaw.com/recording/Acoustics/RoomModeStandingWaveCalcu.html ). Since the main sofa is right about the half way point in the room, that places it in prime first axial mode location.
Since the room is open to another area and a staircase, it’s more difficult to predict behavior. My measurements show that the expected 25Hz mode is closer to 22Hz, but that’s for another post. I’m hoping to pick up CARA sometime and model the full space.
Measurements to come soon.
I’m contemplating putting some of my audio postings/ponderings into a blog instead of forums. Might happen, might not.
