"All of the CDs we've seen suffer from the same fatal problem."
"Note the peak/dip pair at 110 and 122 Hz where the response varies a staggering 32 dB across a range smaller than one musical whole step."
"To solve this problem we created a series of MP3 files you can download and burn."
"Be careful not to make any noise yourself, or tap your foot, or breath loudly near the microphone capsule."
|Download the RealTraps Test Tone CD Zip file HERE (3.2 MB).
Note that these tones measure raw low frequency response only, and are meant for use only when it's not practical to connect a computer to your audio system. These tones do not identify modal ringing or other time-based errors which are just as important. But they will give you an accurate assessment of the low frequency response of your loudspeakers in your room. For more thorough room testing we recommend the freeware Room EQ Wizard, or ETF and R+D programs for Windows from Acoustisoft. For Mac users we suggest FuzzMeasure. Our Room Measuring Primer explains how to use room measuring software.
There are a lot of commercial test tone CDs available, but all of the CDs we've seen suffer from the same fatal problem: The tones are spaced at either 1/3 or 1/6 octave intervals, which is far too coarse to assess the low frequency response in rooms the size you'll find in most homes.
The graph below shows the response at the listening position in a typical 16 by 10 by 7-1/2 foot room. Note the peak/dip pair at 110 and 122 Hz where the response varies a staggering 32 dB across a range smaller than one musical whole step. This behavior is completely hidden when measured at 1/3 or even 1/6 octave spacing.
To solve this problem we created a series of MP3 files you can download and burn to a CD. Our intent is not to duplicate all of the features of commercial test CDs. Therefore, we supply only low frequency sine waves, in 1 Hz increments, with a track organization that's easy to understand. We also created a blank sheet of logarithmic graph paper you can print to plot the results. Top
You can use any inexpensive Sound Pressure Level (SPL) meter such as those sold by Radio Shack. Simply place the meter where your ears are when seated in the listening position, play the CD, and write down each number displayed by the meter. Each tone is ten seconds long, which gives you enough time to switch the meter range when needed. Yes, it will take the better part of an hour to run these tests, but when you're done you'll know your room's true low frequency response.
Some test CDs "calibrate" the level of each tone to match the known inaccuracies of the Radio Shack SPL meter, but we didn't do that for two reasons. First, meter calibration offsets are not that useful because all Radio Shack meters are not the same. More to the point, low frequencies are usually accurate enough even with inexpensive microphones. We compared our Radio Shack SPL meter to our own expensive AKG calibrated microphone, and they were within 1 dB of each other below 600 Hz. The main point of these tones is to find the major peaks and nulls at low frequencies, and any inexpensive meter is fine for that, calibrated or not. Top
Note that the playback volume must be set high enough for the tones to be well above the ambient noise in your room. If you have nulls that are 30 dB deep - which is common - the tone must be at least 30 dB louder than the ambient noise in order to measure the full extent of the nulls. Track 30 contains 15 seconds of pink noise which you'll play before running the sine wave tests to ensure an adequate signal level. Play Track 30 and set your receiver's volume so the SPL meter reads about 70 dB before running the tests. Also be sure the meter is set to use "C" weighting (the default), which is needed when measuring low frequency response.
If the level at low frequencies is so high you hear obvious distortion, then run the tests again at a lower level. Likewise, if all of the tones seem fairly soft, you'll need to raise the overall level. Common sense will tell you when you're within a useable range. You may find that some low frequency tones cause windows and furnishings in the room to vibrate and rattle, so you'll need to fix those before you can measure the low frequency response accurately. Also, be sure there are no other noises present when running the test, such as the rumbling of a furnace or air conditioner. Likewise, be careful not to make any noise yourself, or tap your foot, or breath loudly near the microphone capsule, while reading the meter levels.
The tones are spaced at 1 Hz intervals, with ten tones per CD track, and the layout is shown below under Track Contents. At most frequencies you can hear the transition from one tone to the next, but you can also use the CD player's time readout if needed. For example, Track 10 plays 100 through 109 Hz, so if the time readout says 23 seconds that means you're 3 seconds into the 102 Hz tone. If you get lost, just rewind to the start of the current track and resume from there.
The close 1 Hz spacing also makes it difficult to squeeze that many tiny dots onto the graph paper at the upper end of the range, above about 100 Hz. So just plot all ten tones for each track in the same space, one on top of the other, and the peaks and dips will still reveal themselves clearly. Top
USING THE FILES
All of the files needed are in the test-cd.zip file you can download at the top of this page, and they are named numerically to end up on the CD in the proper order automatically. Simply extract all of the files to a folder on your hard drive, then drag the files as a group to your CD burning program to create an audio (not data) CD. You can sort the list of file names alphabetically, if needed, before dragging the files.
You can also print the accompanying Test Graph.gif file to plot the results. If you have a graphics program, print the image from there and tell the program to fill the page when printing.
(All tracks are recorded at -1 dB.)
Track 1: 10-19 Hz in 1 Hz increments.
Track 2: 20-29 Hz in 1 Hz increments.
Track 3: 30-39 Hz in 1 Hz increments.
Track 4: 40-49 Hz in 1 Hz increments.
Track 29: 290-300 Hz in 1 Hz increments.
Track 30: Wide band pink noise Wave file, 15 seconds, for setting the level to about 70 dB for the subsequent tests. Top
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