[haysdb]

I have spent the last couple of nights calibrating my new dedicated theater room. The speaker system is composed of 8 Triad Gold speakers - 2 in-wall LCR's, 1 in-room Center, 2 on-wall surrounds, 2 in-wall surrounds, and 1 Powersub. The audio system is composed of, among other things, a Lexicon MC-1 surround processor, 7 channels of Adcom amps, and an Audio Control Bijou 5.1 channel EQ. I also have an Audio Control R-130 Real Time Analyzer.


EQ'ing the system without the RTA would be next to impossible, and even WITH the RTA, it's non-trivial. First off, pink noise is not continuous but has a randomness to it. Rather than seeing a nice stable set of 30 LED's, there is a lot of flickering, even at the "slow" setting. Missing from the R-130 is the ability to average over a longer period of time such as 20 seconds. This makes it necessary to "manually average" by watching the LED's and noting what percentage of the time each one is on.


Second, moving the microphone a small distance can result in different results. If you measure from just one position, you can potentially make all other positions worse, attempting to improve one. The "Home THX Equalization Manual" published by Lucasfilm recommends averaging the measurements from four locations. That sounds good except that the equalization curves from these four locations can all be different, which can result in some frequencies being made better but some being made WORSE at each position. In my case most of the changes cancelled each other out, leaving only the worst and most consistant problems to be equalized out. My room seems to have about a 6dB peak somewhere in the 160-200Hz region, although the center channel seems to peak at a somewhat lower frequency, which leaves me scratching my head. The low bass region has been "interesting" to EQ. The RTA is a 1/3 octave RTA, so there are only 6 LED's covering 80Hz and below whereas there are 11 sliders on the EQ each covering 1/6 octave. Either my peaks and nulls are falling in between these 1/3 octaves or I am blessed with a room which has few problems in the lower bass region. The only LED's that consistantly stray from a flat line are those at 32.5 and 25 Hz, and I am having a hard time bringing myself to cutting frequencies that are normally hard to come by!


The attached picture is a shot of the RTA display for my front left channel only - (no subwoofer and no EQ. Of the 30 LED's, the lower 6 are the region below 100Hz so the rolloff is normal, and the top 12 LED's are the region above 1KHz, which is "dominated by the direct field", so it can also be ignored. The 11 LED's in between, covering the third octaves between 80 Hz and 800 Hz inclusive, is what is being EQ'd. It shows about a 4 dB peak, really more a HUMP than a peak, at 200 Hz. I am wondering if this is the region addressed by the accoustic treatments, of which I do not have enough. If wonder if I could cover another 50 square feet with acoustic insulation if this hump might be lower? That's academic however since I don't have another 50 sq ft of wall available short of covering my fireplace and some raised panel oak doors!


I wasn't able to achieve a perfectly flat response from 20 Hz to 1 KHz, nor did I really try that hard to do so, but I did end up with the RTA showing the response being +/- 2dB over most of this range, with a couple of flickering lights outside that range.


I haven't done any serious AB comparisons yet since I haven't had a switcher monkey to push the bypass switch on the Bijou for me, but I did watch the "Corrs: Live in London" DVD tonight at reference level (i.e. LOUD) and found nothing to complain about. It sounded pretty fantastic. The Triad Gold's can handle the full output from the 150w/ch Adcom amp (the clipping LED's were flickering) without audible distress and the 15" Gold Powersub just totally cranks. Someday I will add a second sub, but for now the single sub equits itself QUITE well.


Given the ultimately minor EQ I ended up dialing in, I have to say Dennis' theater design is quite excellent. His construction techniques are also proving themselves as the room makes narry a buzz or rattle. As for the EQ, I could probably replace the Bijou with a Helmholtz resonator since almost all the EQ is focused around the 160-200Hz bump. Aside from that bump I don't think I needed any more than 2dB of adjustment anywhere else.


David

 

[haysdb]

I forgot to attach the picture showing the unequalized output for the left channel.

 

[haysdb]

This picture shows the equalized frequency response of the center channel and subwoofer from one microphone location.


1) The rolloff above 1KHz is expected since the omnidirectional microphone is reading the "room response" and frequencies above 1KHz are dominated by the direct field. I hope I'm saying that right. Anyway, according to the "Home THX Equalization Manual" published by Lucasfilm, this rolloff is expected and of no cause for concern. The Bijou only allows frequencies up to 1KHz to be adjusted anyway, avoiding any temptation on my part to "fix" these frequencies!

2) I could dial out the 2dB bumps but the other three microphone locations don't exhibit these bumps so I'd made those locations a bit worse by making this one a bit better.


David

 

[LarryChanin]

David:


Thanks for reporting your experiences with the Bijou. I recently purchased one on ebay and I'm trying to do my homework before it arrives. I have a home theater computer connected to my equipment so it will be convenient for me to use a software-based RTA to assist in the room equalization. I believe that some these programs offer some benefits over many standalone RTA's in that they are more accurate and permit plotting and saving frequency response curves. I plan on using Acoustisoft's ETF5 free demo program . I haven't started using it yet, but it permits the selection of 7 fractional octave displays starting at 1/2 octave and going up to 1/12 octave. (It has 1/3 and 1/6 octave displays to match the Bijou's scales.) It does file averaging for multiple microphone positions. It also has a Sequential Data Acquisition feature which simulates a real-time display by running a plot every couple of seconds or so. This permits you to see almost immediately the results of your changing equalizer settings.


One thing that I have noticed in reading both the Bijou "Owner's Enjoyment Manual" (as AudioControl likes to call it) as well as the THX Room Equalization Manual, is that neither mentions what to do with the side surround channels. How to you plan to approach equalizing the side surrounds? Will you run a test signal into all 6 channels and reset the equalizer?


Thanks.


I'd be very interested in reading further reports on how you are doing.


Larry

 

[Dennis Erskine]

Quote:

One thing that I have noticed in reading both the Bijou "Owner's Enjoyment Manual" (as AudioControl likes to call it) as well as the THX Room Equalization Manual, is that neither mentions what to do with the side surround channels. How to you plan to approach equalizing the side surrounds? Will you run a test signal into all 6 channels and reset the equalizer?

What do you mean ... side channels? The THX, 5.1 spec requires side channels and the Bijou does a right proper job with them. If you have a 7.1 system with two rear channels, you need an additional two channel EQ (if you want to EQ the rears).

 

[haysdb]

The side channels are EQ'd just like the front L, C, and R. For each channel, all channels except the one being calibrated are disconnected. In my case this was a simple matter of disconnecting the single DB25 cable between the Bijou and my 5-channel amp and then using one RCA cable to connect the channel I wanted to equalize.


I haven't gone back and tried to equalize my surrounds. My first attempt had me cutting and boosting every frequency. I ended up just setting the sliders back to the middle. Can anyone tell me if dipole surrounds are EQ'd just like direct surrounds? One would assume so but I wonder why the frequency response was so out of whack?


David

 

[LarryChanin]

Quote:

Originally posted by Dennis Erskine What do you mean ... side channels? The THX, 5.1 spec requires side channels and the Bijou does a right proper job with them. If you have a 7.1 system with two rear channels, you need an additional two channel EQ (if you want to EQ the rears).

Hi Dennis:


Thanks for the response. I was referring to the "side surround channels" in my posting to differentiate them from the surround back channels which I knew were not accommodated by the Bijou. I know that the Bijou handles the side surrounds, however it's manual doesn't instruct the owner how to set them up as it does with the other channels.


In particular, as David's posting suggests, there's no discussion of how to approach dipole side surrounds.


While we're on the subject of surrounds, I was wondering whether you wouldn't mind providing us with your opinion regarding the need to equalize all four surrounds in a 7.1 configuration? In this case using a Bijou would you use an RTA to analyze the room's response to determine whether to buy an other two channel equalizer? Is the main reason for using an equalizer on the surrounds to match their timbre with the other speakers or are there other more important reasons?


Thanks again.


Larry

 

[Dennis Erskine]

You cannot timbre match with an equalizer. It's purpose is to achieve a flat(er) frequency response at the listening position for all speakers. In small residential spaces this is very hard to achieve. A 1/3 octave EQ or parametric EQ will get you very close. For considerably more money, you can use a Room Correction Processor such as the Audio Control Diva, Tact, JBL Synthesis, Snell (I don't know if this is still being made), or SigTech and get much closer. Further, since there's no such thing as a perfectly flat speaker, you're going to be limited by not only the speaker but the room as well. We do our best to make the room a contributor to better, not worse, sound.

 

[haysdb]

Quote:

Originally posted by Dennis Erskine You cannot timbre match with an equalizer. It's purpose is to achieve a flat(er) frequency response at the listening position for all speakers.

Dennis, is this why my center channel (Triad Gold In-Room Center) still sounds different from my main L&R (Triad Gold In-Wall LCR's) on calibration tones after equalization, or is it maybe that 65" TV it's sitting on top of?


David

 

[LarryChanin]

Quote:

Originally posted by Dennis Erskine You cannot timbre match with an equalizer. It's purpose is to achieve a flat(er) frequency response at the listening position for all speakers. In small residential spaces this is very hard to achieve. A 1/3 octave EQ or parametric EQ will get you very close. For considerably more money, you can use a Room Correction Processor such as the Audio Control Diva, Tact, JBL Synthesis, Snell (I don't know if this is still being made), or SigTech and get much closer. Further, since there's no such thing as a perfectly flat speaker, you're going to be limited by not only the speaker but the room as well. We do our best to make the room a contributor to better, not worse, sound.

Hi Dennis:


Isn't there some kind of equalization going on with THX's Timbre Matchingâ„¢?


Here's an excerpt from their web site:

Quote:

Timbre Matchingâ„¢matches the tone of your front speakers to your surrounds to compensate for the fact that only two surround speakers are used in a typical Home Theatre system instead of a full array as in a movie theatre.

Thanks.


Larry

 

[Dennis Erskine]

Larry---

Yes...it is done in the design and manufacturing of matched speakers...largely with matched drivers, careful cabinet design, etc. You cannot put new shocks on a Chevy and get an Audi ride.

 

[LarryChanin]

Quote:

Originally posted by Dennis Erskine Larry--- Yes...it is done in the design and manufacturing of matched speakers...largely with matched drivers, careful cabinet design, etc. You cannot put new shocks on a Chevy and get an Audi ride.

Hi Dennis:


I found additional information on THX's Timbre Matchingâ„¢ in the Owner's Manual for the Pioneer VSX49TX Ultra 2 receiver;

Quote:

Timbre Matchingâ„¢ : The human ear changes our perception of a sound depending on the direction from which the sound is coming. In a movie theatre, there is an array of surround speakers so that the surround information is all around you. In a home theatre, you use only two speakers located to the side of your head. The Timbre Matching feature filters the information going to the surround speakers so that they more closely match the tonal characteristics of the sound coming from the front speakers. This ensures seamless panning between the front and surround speakers.

Although this filtering may not be defined as equalization, I interpret this to mean that the receiver is doing some sort of electronic "Timbre Matching".


I think this is their electronic equivalent of putting new shocks on a Chevy.


Larry

 

[bart_75243]

Quote:

Originally posted by LarryChanin Hi Dennis: I found additional information on THX's Timbre Matchingâ„¢ in the Owner's Manual for the Pioneer VSX49TX Ultra 2 receiver; Although this filtering may not be defined as equalization, I interpret this to mean that the receiver is doing some sort of electronic "Timbre Matching". I think this is their electronic equivalent of putting new shocks on a Chevy. Larry

Isn't this dealing more with the head-related transfer coefficients as sounds actively pan rather than equalization? (So with non-panning sounds coming directly from the surround channels, there would be no correction at all due to this "timbre matching")?

 

[Dennis Erskine]

It's been a few years since I went through the full suite of THX seminars at Skywalker Ranch, so I'm not going to get into the nits of THX's Timbre Matching process without a serious review of the texts (as relates to front/side speakers). I'd suppose you could call it an equalization process and not to be confused with 're-equalization'... another LucasFilm THX process. I'd be more inclined to call it perceptual decoding; but, that is technically not correct either. Perhaps perceptual compensation would be a more acurate term recognizing that there are differences between theatrical settings and a residence due to room size, speaker array vs no array, speaker distance and height from the audience, indeed HRTF, and other factors (not to mention the side effect of 50 people chewing popcorn with their mouths open).


There is also, and it is mentioned in the LucasFilm literature, timbre matching with respect to the requirement of manufacturers matching the timbre of their speakers to each other...notably the left, center, right speaker set. A process that cannot be addressed by equalization...perhaps better stated as "by equalizers as known to the general consumer market". It is further important that the manufactured timbre of front/side/rear speakers also be matched. If the speakers themselves are mismatched, any predefined timbre matching process imbedded within a decoder could be for naught. Due to the way we perceive directionality of sound, mismatched speakers (front to side to back), could adversely affect those directional cues intended in the sound tracks.


The THX Timbre Matching process in the surround processor, may remove that singular difined aspect from the acoustic problem set. It does not remove the issues of room interaction, poor mixes, and mismatched speakers.


As a different observation on a slightly different front, notice speaker manufacturers list frequency reponse for their speakers as something on the order of 35Hz to 20kHz +/-3dB. Equalizing a speaker in a room to 2dB or better is a considerable challenge...much less building a room out of the box to something close to that.

 

[cgblack]

I think they just used a poor choice in words with Timbre matching. Timbre is what allows us to tell the difference between a trumpet and a violin playing exactly the same note at the same volume. And no amount of equalization, will make a violin sound like a trumpet It would take some significant DSP to make that happen.


Now one can argue that digital equalization is just one form of DSP, but so is digital reverb and echo, and no one mixes them up with equalization. To change the timbre, say from a trumpet to a violin, the DSP would need to isolate the base frequency of the trumpet, then modulate it with a frequency corrected sample of the desired timbre (violin). Let's not even get into bow sounds, etc. (I'm an amateur keyboardist, and have had a bit of experience creating digital sounds.)


To match the timbre of two different speakers, would be very difficult indeed. One would have to sample each speakers full frequency reproduction, and then do some very fancy analysis and correction of the signal differences to try to make them sound the same.


I would intepret it to mean that the timbre of already matched speakers, will change slightly due to the differences in positioning etc., and this equalization (processing) is to try to compensate for this.


These are just my opinions and should not be taken as fact!

 

[haysdb]

I'm still left wondering my my Triad Center sounds different than my Triad LCR's with the calibration tone. I guess I should have set them side-by-side and listened to the tones before I installed the speakers!


I suspect the difference is more due to speaker location (higher and on top of a TV) than any issue with the design, since the two speakers use the same drivers, albeit oriented differently (vertical vs horizonal), but I guess I expected them to sound more similar. Dennis has asked if they sound different with panned voices and that's hard to say. I would say that pans aren't as seamless as I had hoped, but my ears are not critically trained so I don't really know what to expect.


NEXT time, I'm doing three IDENTICAL speakers across the front. [Ultimately I hope to accomplish that in THIS theater, and have planned for such an upgrade down the road when I can afford a nice digital projector and get replace the RPTV with a proper vertical LCR center channel placed at 54".]


David