Radio Shack SPL meter and sub levels [Archive] - Audio & Video Forums

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toenail
09-26-2004, 03:02 PM
I've seen a few posts about using the Rat Shack SPL meter to match db levels for all the speakers in a surround system. I've used one in the past for help tuning DIY speaker projects and setting up HT systems, but have a question about it's accuracy.

The owner's manual clearly states that it is not accurate below 500hz or above 10,000hz. If this is the case, how on earth could the reading for the sub frequencies be used to level match with the rest of the system? I've consistently found that if I set the level according to the SPL meter, sub frequencies are far too loud.

Anyone else had this issue?

Pat D
09-29-2004, 04:29 PM
I've seen a few posts about using the Rat Shack SPL meter to match db levels for all the speakers in a surround system. I've used one in the past for help tuning DIY speaker projects and setting up HT systems, but have a question about it's accuracy.

The owner's manual clearly states that it is not accurate below 500hz or above 10,000hz. If this is the case, how on earth could the reading for the sub frequencies be used to level match with the rest of the system? I've consistently found that if I set the level according to the SPL meter, sub frequencies are far too loud.

Anyone else had this issue?
Actually, the meter is fairly accurate from 50 to 5000 Hz or so and rolls off above and below that. That is with "C" weighting, which is what you want to use. It is down about 6 dB at 20 Hz and about 11 dB at 20 kHz. Anyway, with correction factors applied, it can do quite well for purposes for which it really was not designed. It is very useful for reading SPL levels to see if the levels are safe and for setting up an HT system.


With "A" weighting, there is a roll off below about 500 Hz of about 6 dB per octave, as I remember. You don't want to use "A" weighting to set subwoofer levels! That is an audibility weighting to reflect the lower sensitivity of human hearing at lower frequencies. Anyway, your reference to 500 Hz suggests to me that you might have been using "A" weighting.

Here is a link I can find right away:

http://www.audioasylum.com/audio/general/messages/49147.html

toenail
09-29-2004, 05:10 PM
Pat - You're the man. It would be nice if they included this information in the owner's manual. I do use the C weighting (slow) when setting levels. When I tried to achieve 25hz at the same level as reference for higher frequencies, the end result was a pretty thick bottom end. The sub is designed to be down 3db at 22.5hz so I didn't think it's that.

Woochifer
09-29-2004, 05:22 PM
Start with what Pat suggested. If you're not using the C weighting, then you're getting VERY erroneous readings.

If you apply the C weighting and the correction values for specific frequencies, and STILL have levels that subjectively sound too high, then I suggest that you get a test CD with low frequency test tones and do some measuring. Most small to medium sized rooms will have at least one or two problem frequencies where the room-induced wave interactions create a huge peak. This will make the bass subjectively sound overly boomy, and it will cause erroneous SPL readings when you use the wideband test tone.

The SPL meter is an absolutely essential tool in any audio system IMO.

Pat D
09-29-2004, 05:58 PM
Pat - You're the man. It would be nice if they included this information in the owner's manual. I do use the C weighting (slow) when setting levels. When I tried to achieve 25hz at the same level as reference for higher frequencies, the end result was a pretty thick bottom end. The sub is designed to be down 3db at 22.5hz so I didn't think it's that.
I haven't seen my owner's manual for a long time! ;) Where do you have your sub placed? In most rectangular rooms, subs have the flattest and deepest response in a corner. There is a more general method of placing a subwoofer if that doesn't work, as described by Alan Lofft:

http://www.axiomaudio.com/tips.html#place

As Woochie says, there are room resonances. If you divide 565 by the room dimensions in feet, you will get the frequencies of the primary resonances in the room--this may not work for peculiarly shaped rooms. An 8 foot ceiling gives us 565/8 = 70.6. So that would be one resonance at about 71 Hz.

Below the lowest resonance of the room, you get room gain at I forget just what slope, about 9 dB per octave, I think.

Using the low and high pass filters of the crossover, you can reduce the level of the effects of one or more of them, depending on how close they are together. Some people use an equalizer to even out the bass.

toenail
09-30-2004, 02:22 AM
Wooch-
As mentioned above, C weight and slow are the settings I use. I do have several reference test cd's (Autosound 2000 etc) that I use for level setting and troubleshooting. Everything from 20hz-20khz sweeps slow or fast, to individual tones from 20hz -100hz. The sweeps give an indication of how flat it plays through the range and expose room bumps/dips, the individual tones help to pinpoint the exact frequencies where the bump/dip is centered and where the bottom end rolls off. I need to apply the correction factor when taking these measurements. I was previously unaware of this.

Pat-
After months of experimentation I did settle on the front corner of the room. Had some WAF issues but was able to overcome. The improvement was pretty dramatic and eliminated a big gain at around 40hz and a big dip in the 70's.

I'll be interested in hearing the end result after applying correction factors.