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  1. #1
    DIYaudiophilehack
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    Actual db levels for sub when setting up HT

    I've been futzing around with Radio Shack spl meter and the correction factor table provided by a member of this site. When I set the level of the sub so that it is playing at the same level as the mains, the sub is waaaaaaaaaaay too loud. Is there a rule of thumb about how flat and how low the sub should play?

    Here is what I get for readings using 70db for reference @1khz. All correction factors have already been applied.

    20hz= didn't try as it is below the tuning frequency of the port and I've heard bad things can happen
    25hz=69.4db
    31hz=68db
    40hz=68db
    50hz=73.3db (room issue)
    63hz=66.8db (room issue)
    80hz=70.5db

    Sub is crossed at 40hz and provides the most seamles transition from mains at this setting. When I back it off about 7-10db it sounds much more normal for HT movie use. I don't use the sub for music.

  2. #2
    M.P.S.E /AES/SMPTE member Sir Terrence the Terrible's Avatar
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    Quote Originally Posted by toenail
    I've been futzing around with Radio Shack spl meter and the correction factor table provided by a member of this site. When I set the level of the sub so that it is playing at the same level as the mains, the sub is waaaaaaaaaaay too loud. Is there a rule of thumb about how flat and how low the sub should play?

    When you say waaaaaaay to loud, it is based on what? Measurements? Or personal taste? Just trying to get a perspective on where you are coming from. If it is taste, then the subwoofer should be calibrated using standard methods, but readjusted after that to taste(you may not like the industry standard level)

    Here is what I get for readings using 70db for reference @1khz. All correction factors have already been applied.
    Bang!! Here is the problem. 1khz is a frequency that lies in the main speakers. You are trying to balance the subwoofer(who's frequency balance lies in the least efficient part of our hearing) to a frequency that lies in part of the most sensitive part of our hearing. That WILL make the sub's level too high. The majority of the energy in soundtracks is under 1khz anyway.

    You should always use pink noise for balancing/setting you speaker levels. Pink noise is wideband, and equal at all frequencies. It also better represent's program material. Using a single tone as reference willI(especially if its out of the subwoofer bandwidth) will skew the results of the calibration. Using pink noise, the overall levels of the calibration should be 75db's. All main speakers should be calibrated to that level. The subwoofer(which does carry near the bandwidth of the mains) should be set 4db higher. After this calibration level has obtain, you should adjust the level to YOUR taste. The level may be okay for you, or it may not.

    20hz= didn't try as it is below the tuning frequency of the port and I've heard bad things can happen
    25hz=69.4db
    31hz=68db
    40hz=68db
    50hz=73.3db (room issue)
    63hz=66.8db (room issue)
    80hz=70.5db

    Sub is crossed at 40hz and provides the most seamles transition from mains at this setting. When I back it off about 7-10db it sounds much more normal for HT movie use. I don't use the sub for music.
    This IMO is not actually a bad measurement for 80hz and under. However it is a crude measurement in that it doesn't have enough resolution to tell the whole story. Also if you set you crossover point at 40hz, then your 63 and 80hz measurement would be out of the subs operating range(and into the mains) which negates its inclusion in the measurement. So in actuality your sub only varies 1.4dbs from 40-25hz. Not bad!! But that is based on the crude measurement. What you need to do without the assistance of a RTA is just calibrate using a SPL meter, and the average overall frequency measurement obtained from it based on pink noise. For this method of measuring, this is the most accurate way.

    Without a more flexible tone generator, a RTA, or the ability to measure from more than one point, using individual tones will give you pretty spotty results.
    Last edited by Sir Terrence the Terrible; 10-11-2004 at 07:56 AM.
    Sir Terrence

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  3. #3
    DIYaudiophilehack
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    Thanks for taking the time to respond in detail. I need to better explain how I took these measurements. I can see by part of your answer that I left quite a bit out from my initial post. I have also since realized that I made a few major mistakes during my set up and testing procedure.

    I was curious to see how flat and low I could get the system to play in PLII movie. The focus was on frequencies from 80hz down to 20hz, but I had the whole 5.1 system active so that I could see how things were transitioning from mains/center/surround down to subwoofer. My reasoning (rational or not) was that people seem to strive for 20hz-20khz with little variance and I was curious to see how close I was.

    Using the dvd player as a source, I ran pure test tones at the intervals specified above. The intervals were supplied by a member of this board and are considered good to use when applying correction factors for the inaccuracies of the Radio Shack SPL meter. The meter was set up in the listening position and I took repeated measurements at those intervals while making adjustments to the output level of the sub, the tuning frequency of the sub, the crossover frequency, and it's position in the room . When I was finished, I had the results listed above, which looked great on paper but sounded lousy by ear when playing actual source material instead of test tones. The 1khz reference was largely played through the mains/cneter/surrounds as dictated by the 5.1 processor set to PLII, not just the sub. I can see how trying to play a 1khz tone at the same db as a 30hz tone through only the sub would create quite a problem.

    Where I erred greatly was two-fold. 1) I never went back and tried to level match the sub with the pink noise generator function of the HT receiver. 2) I needed a better sampling of frequencies than what I used initially (as you suggested) to get the whole picture.

    When I started looking further into the problem I found two big issues. 1) After finally going back and level matching with pink noise function I found I had the sub at ungodly high levels in comparison to the rest of the sysytem. 2) I ran each tone from 20hz to 80hz instead of just the intervals listed above and found a gain of 19 db@46hz! I can only guess that the huge gain at 46hz was contributing to the overall increase in level for the sub.

    I've yet to get a chance to look further into this, but am hopeful that eliminating the big bump will allow me to have pretty flat response down to 25hz AND allow me to play the sub at a level that doesn't ruin the sound quality.

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