An
Introductory Guide to Subwoofers
Installation/placement
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Testing that location, first the easy (but not accurate) way.....
Chose a CD or DVD you know well that has lots of low bass, and put it in your player. Next place your subwoofer in the listening position. If it's small enough you can just put it in your chair. Go to the spot place where you'd like place the sub. Now sit down in that place and listen to the recording. If it sounds reasonably good in at that position, go a head and put your sub there. If the bass doesn't sound good, try other available locations. Chose the location that provides the best sounding bass.
Note: moving the sub as little as
6"-12" one direction or another, can have a noticeable impact on it's
performance. Don't be afraid to experiment!
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Hey, just how accurate are my
ears?
Unfortunately human hearing isn't accurate for gauging sound pressure levels at
a given frequency. Depending on the output level and frequency, human hearing
rolls off the top and bottom of the spectrum. Below is a Fletcher-Munson curve.
From the curve it's easy to see why the human ear isn't good for determining
frequency response.
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Since you can't trust your ears, you need to get something you can trust.
For ~$40 you can pickup a reasonably accurate SPL meter at Radio Shack. Now these aren't nearly as accurate as a calibrated microphone, but they're significantly better than most people's ears.
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NOTE; if one has a PC with a duplex
soundcard, and either a SPL meter or test microphone with appropriate preamp,
then manually using test tones isn't necessary. Download a copy of the freeware
program Room EQ Wizard HERE.
(download password is the number 42 ) To run on a PC REQW requires the free version of the Java Runtime program, that
can be downloaded HERE
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Testing that subwoofer location, get some test tones......
If you have a PC you can plug into the audio playback system in your listening room, this testing is very simple to do. Download the NCH Tone generator. If you don't have a PC close to your audio system, the trial version will allow you to copy tones to your hard drive and you can them burn them to a CD.
Although it doesn't support saving test tones to your hard drive, the older simpler freeware version of the NCH tone generator available HERE never expires.
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A nifty contraption is the NTI Minirator 1.4MB download. This is a Macromedia Flash software version of the device seen in the graphic. The virtual version operates like the real thing. This means the controls on the graphic unit operate a signal generator. Click the mode button until you see "Signal:Sine" in the text window then use the up/down buttons to change frequency. The control on the lower right corner 'plugs' and 'unplugs' the device from your soundcard. Unfortunately the limiting factor for this is the frequencies are only in 1/3rd octave increments, and the lowest frequency it plays is 20Hz. So consider one of the less 'cute' programs for your real testing.
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Also there are many freeware signal/function
generator programs available.
Here are a few more
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NOTE: USE ONLY SINE WAVES, run them at LOW volume levels for short periods of time, 5-10 seconds per frequency is more than adequate for what you're doing.
When testing subwoofers one should use tones that are 1/6th octave apart (note 1/3rd octave spacing on some test CDs isn't accurate enough for what we're doing.)
Here's a list of 1/6th octave frequencies useful for testing subwoofers.
16Hz, 18Hz, 20Hz, 22Hz, 25Hz, 28Hz, 31.5Hz,36Hz, 40Hz, 45Hz, 50Hz, 56Hz,63Hz, 80Hz, 89Hz,100Hz, 125Hz
NOTE: If you decide to test lower than 16Hz, this should be done at VERY low output levels.
If using a computer program to
create your own tones too difficult Secrets
of Home Theater has test tones available for you to download, or you can
download a 3.2 MB zip file from the RealTraps website by clicking HERE
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The correct way to make your measurements......
Taking measurements while holding the meter or microphone is not recommended. Placing the meter or microphone on a table, chair or other large object also isn't recommended. The proximity of your body or other large object to the measuring device will have a adverse impact on the measurements.
To make accurate measurements mount your mic or meter on a stand. The best stand for this purpose is a boom microphone stand. These range in price from $25 up.
Using a boom allows accurate
placement of the mic/meter, and avoids the issues with handheld
measurements. If you don't want to purchase a boom mic stand, fabricate
something similar using wood or PVC tubing.
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Testing that location, now that you have test tones......
As with the listening test start with the sub located in the listening position and like our first test, set the meter or mic where you want to put the sub. Play your test tones and write down the output levels. Start by setting your meter to the 70dB setting. Set the output level of the 50Hz signal so the meter is reading 75dB. Now play the rest of the test tones and write down the output levels.
When using a Radio Shack SPL meter, you need to adjust for the fact that it's not accurate at all frequencies. Add or subtract according to the numbers listed below. These frequencies are 1/6th octave apart.
16Hz add
11.5dB
18Hz add 8.0dB
20Hz add 7.5dB
22Hz add 6.5dB
25Hz add 5.0dB
31.5Hz add 3.0dB
36Hz add 2.5dB
40Hz add 2.5dB
45Hz add 2.0dB
50Hz add 1.5dB
56Hz add 1.5dB
63Hz add 1.5dB
80Hz add 1.5dB
89Hz add 1.5dB
100Hz add 2.0dB
111Hz add 1.0dB
125Hz add 0.5dB
Note that different spreadsheets and websites have different corrections for the RS meter. I don't know which corrections are the most accurate ( the unit to unit variability between the meters maybe a bigger issue). The ones listed above are some of the more commonly used corrections.
If you don't mind using a pencil here's a blank plot you can printout and use.
If writing down and hand plotting
the numbers is too
labor intensive, there are Excel spreadsheets that will create the frequency
response plot for you.
This is my version of the most commonly used Excel spreadsheet. Click
HERE to open it.
This spreadsheet has the Radio Shack meter corrections automatically added to
whatever frequency you type in. Note, there are more spreadsheets on the next
page....
Use your test tones take
measurements of the possible locations for your subwoofer to find the best
possible placement. Of course this means having the flattest response (the
various frequencies have the closest match in output levels.
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