Testing and Adjusting

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This is a lengthy report on testing and adjusting my system.

I have VTF-3R sub and Ventriloquist speakers connected to a Pioneer VSX-D912 receiver. The room is essentially 25' long by 15' wide, with a large stone corner fireplace on the end with the screen and front speakers. On the other end, there is a 6' wide foyer that extends another 12' to the front door. The foyer is off-center, about 3' from one side. The ceiling is 10' and 4"x12" wooden beams run across the short-way from on about 4' centers from the front to the back. My Ventriloquist speakers are mounted on the beams, angles down and toward the listening area. The listening area is about two-thirds of the way back from the front speakers. The sub was in a corner beside the front door.

I have a Behringer calibrated microphone and a Behringer Shark to provide phantom power and preamp for the microphone. These I connect to my laptop computer. I have TrueRTA running on the laptop. TrueRTA provides generation of various signals and both oscilloscope and spectrum displays of signals fed in by the microphone. The signals generated by TrueRTA are fed to the receiver.

I have a RANE PE 17 parametric equalizer between the LFE output and the sub. The PE 17 is single channel, has five filters, each independent, and each with separate gain, frequency, and bandwidth controls.

With the RANE in bypass mode, the receiver fed by pink noise, and TrueRTA in spectrum mode, I had a big hump (about 20 dB) at 30 Hz and a smaller one at 60 Hz. I had a sizeable hole at 40 Hz, 80 Hz, and 112 Hz. I tried tinkering with the various filters to take these anomolies out, but between the sensitivity of the frequency controls and refresh time of the spectrum display found it difficult to track what was going on before running out of time.

Next day I decided to take an orderly approach. With filters 2-5 in bypass, I set filter #1 to max gain and min bandwidth. Then I set TrueRTA to 30 Hz sine wave and oscilloscope mode. Then I gingerly adjusted the frequency control to maximize the scope display. Then I switched TrueRTA to pink noise and spectrum display and adjusted both gain and bandwidth to eliminate the hump at 30 Hz. That being a success, I did the same procedure to set the other four filters. The result was a flat respone (+/- 2 dB) from about 20 Hz to about 125 Hz.

Then I moved the microphone a couple of feet and discovered I now had about 30 dB between 30 Hz and 40 Hz. Totally unsatisfactory. So I moved the sub from beside the front door to the small corner niche where the foyer joins the main room and went through the whole procedure again. This resulted in my having a flat response (+/- 2 dB) from about 20 Hz to about 125 Hz over a significant amount of viewing floor space. There's about a 5 dB ragged trough between about 150 Hz and 250 Hz, but I can't do anything about that with an equalizer in the LFE path.

I haven't done any distortion measurements yet, but that's my next activity.

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This is a photo of the front speakers with the screen down. The screen is a 100" diagonal tab-tensioned Draper in high-def-gray. It has an additional 12" black drop so the speakers hide in the black.

You can see the home-made brackets used to mount the center speaker.

You can also tell I have a very tolerant wife. The copper will eventually go in the attic. The orange is the extension cord that powers the screen. I have been waiting until settling on the sub location before doing moving the wires to a final place.

The metal post and wires sticking down from the ceiling previously mounted a ceiling fan, which had to be sacrificed because it was in the way of the projector beam. Again, I have a very tolerant wife.

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This is a picture of the projector location. I got the mounting from Mounts and More on the web.

In the picture there are three cables to the projector, one for power, one carrying progressive scan component video from the DVD player in front, and one that goes to a computer in another room. There's actually now a fourth for composite video from the VHS player, but the picture is so crummy it isn't used much.

I have a wireless keyboard/mouse combo that talks to the computer in the other room, and I can switch between the computer and DVD player using the projector's remote. I plan to run an audio cable from the computer to the receiver so sound from the computer can be played over the speakers.

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This is a picture of the mounting of the rear speakers. Also another view of the projector.

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This picture is a view to the rear from the screen. All three of the rear speakers can be seen, and the sub in its previous position can be seen under the potted imitation yucca. The gun cabinet has now been displaced, and the sub is sitting where it was.

In case anyone wonders about the decor, we live in Cochise County in southeastern Arizona. My wife wanted a house "John Wayne would feel comfortable in." The ugly critter above the gun cabinet is a local javalina, a lot like a pig in appearance but more related to a bear. Big tushes, and mean in groups. We see them around the house, along with coatamundi, deer, foxes, coyotes, quail, rabbits, and road-runners. Oh, and the rattlesnake that was in my garage last fall. The bears and mountain lions stay mostly up the mountain a ways.

[Retread]

I ran distortion measurements this evening, using the same setup discussed in the previous post. I used my Radio Shack SPL meter to set the absolute sound level at about 80 on the SPL meter and then ran TrueRTA in sine wave and spectrum modes. This resulted in a display of the fundamental and harmonics in relative dB height on the screen. Since dB=20*log(In/Out) for voltage ratios, a 20 dB difference represents 10% 3rd harmonic distortion, a 40 dB difference represents 1% distortion, and a 60 dB difference represents 0.1% distortion.

It turns out that distortion measurements in a real room are difficult to make. There is a noise floor that would obscure distortion levels much below 1%, and there are miscellaneous buzzes and rattles from various objects around the room that show up as distortion sources. If I raised the sound pressure levels to get more separation from the noise floor, more and more buzzes and rattles showed up.

In any case, except for frequencies at which there were loud buzzes and rattles, 3rd harmonic distortion stayed at 1% or below down to 30 Hz. Even at 16 Hz, in scope mode, the signal stayed a nice-looking sine wave, and in spectrum mode the distortion at 16 Hz appeared to be less than 5%.

I don't know what the rest of you expect, but I suspect this is all fairly good.

[Pete]

Try looking at some articles at www.harman.com (http://www.harman.com), in the white papars section. The papers are very technical, but if you can get through them and apply what they teach, it could benefit you greatly.

They have a paper on: getting the bass right, and: subwoofers, optimum numbers and location. They have some good suggestions on speaker and sub placement to cancel out room modes (another link has the calculator which shows you the harmonics for your room), and smooth out (improve) response at more than one location in the room (i.e. across several seats).

Raising gains significanlty in an EQ 'to fill a low', is not a prefered solution because you will be working the sub harder at certain frequencies, in reality you are down on response in those frequencies because you are sitting at an acoustic node which cancels that frequency at your location. So at another location, you will now likely be even hot at that freq. Although chopping a hot frequency is ok, to get flatter response.

±2 db: if you can get that at all the seating locations that matter, I'd say you are doing pretty good. :)

Cheers
Pete

[Retread]

Try looking at some articles at www.harman.com (http://www.harman.com), in the white papars section. The papers are very technical, but if you can get through them and apply what they teach, it could benefit you greatly. Pete

Yes, thank you. I have those. I have a very tolerant wife, but she has her limits -- which means there are few places where I have the potential to put an object such as a VTF-3R.

[DNelms]

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It looks as if you have your project well in hand. I thought I had a tollerant wife until I moved our entertainment center in front of the living room window and that was the end of her tollerance. She then made me kick my daughter out of the her room and that became my theater room. We made a pact; She does not tell me what I can do in my theater room (aka Dave's Cave) and I cannot tell her what to do with the rest of the house.

I think I made a fair trade because my room is where everyone wants to be anyway.....:D

[cschang]

Dave, I have a friend that has a room like that. It is called the "Man's Room" and no women are allowed without permission.

Too bad he doesn't have a Hsu. :D

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Raising gains significanlty in an EQ 'to fill a low', is not a prefered solution because you will be working the sub harder at certain frequencies, in reality you are down on response in those frequencies because you are sitting at an acoustic node which cancels that frequency at your location. So at another location, you will now likely be even hot at that freq. Although chopping a hot frequency is ok, to get flatter response. Pete

Boosting those frequencies had destroyed the LFE transient response. Your comment started me thinking and led me to change the approach. All good now, as I posted on the setup page.

[Pete]

That's great! I guess the tips I read from Bob Hodas audiorevolution.com were true!

Cheers
Pete