How to test your subwoofer?

[PiNPOiNT]

Is there some software out there that will allow you to test the capabilities of a subwoffer in terms how low it will go on the frequency chart?

or any other way?

[crazyoldfart]

Is this improtant? If you like the way is sounds, should it matter whether it can hit 20Hz or not?

You can buy a calibrated mic, mic preamp, and some good software for your PC and measure the response at various SPLs and get a good idea of how low it goes, but a good mic for those low frequencies is expensive and hard to find. I have ten calibration microphones that range in cost from $20 to $3,000, and only one is close to being accurate below 30Hz (and that one is not worthy above 5,000Hz). The same goes for sound cards. Most home grade PC soundcards are not very accurate below 40Hz, so you would have to buy a professional card or dedicated SPL Meter.

When I was younger, and not as finalcially stable, I would use my Oscilliscope with a good microphone and preamp to measure SPL. This method worked well, but was extremely time consuming as I had to calculate dBv from the linear Voltage on my display, and I had no reference as to relative SPL, so I couldn't measure accurate SPL levels.

Back to my point, it shouldn't really matter how low your sub will go if think it goes low enough. If you fear you are missing something down in the sub-20Hz region, don't worry, there is very little information down there that isn't merely impact or room vibration. And, most soundtracks and albums have very little information beow 30Hz.

Unless you are using a Bose Acoustimass Sub, it is likely you are getting everything down to 35Hz or 30Hz. Very few subs fail to reach 35Hz.

Just enjoy.

[rkkwan]

With a test disc like AVIA, you can get a decreasing tone from 200Hz down to 20Hz. Using a SPL meter, you can grossly figure out the response curve.

But at least you can get a feel what 40Hz note or 30Hz note sounds like. For example, my VTF-2 plays mostly flat to about 30Hz (according to the RS meter), and then starts dropping, but 20Hz note is still "feelable".

There are some organ recordings with documented 16Hz bass notes from the 32'' pipes.

If I could jump into this thread and ask a question about sub calibrations? I did a search and there is a lot of info but I could not find this question, although it must have been asked before:

I know to set the crossover frequency to its max on the sub and use the receiver crossover frequency but what about the volume knob on the sub? Do I also set that to max and adjust the volume using the receiver settings? Running a test tone through the sub when set to its max volume is very loud and I have been concerned about damaging the sub. Currently I have the sub volume control set to about half and then adjust the level with the receiver when I run a test tone.

And if I could ask another question - my old receiver did not have digital inputs so I had my dvd player hooked with the analog connections. My new receiver has digitial inputs so I have my dvd player hooked up with that. Before I could use the dvd player''s test tones but now hooked up digitially, there is no sound when I use the dvd player''s internal test tones. I don''t really want to adjust my speakers using the dvd player (will use the receiever) but was curious why this is?

[crazyoldfart]

To answer your questions as best I can:

1. There is a limit to how loud your receiver can drive the sub output. I would set the receiver''s sub level to "0", or the middle setting, and then adjust the level control on the subwoofer to the desired oudness for accurate sound (or whatever rocks your boat). Then you can do minor adjustments with the receiver to tweak the sound and will likely not run into problems of overloading the sub output on the receiver and getting tons of fuzzy distortion.

2. The DVD player cannot adjust the individual speaker levels going out via the digital connection. The goal is to send the digital audio stream straight from the DVD to the digital output with no impeding processing. If you are using the player to decode the multi-channel audio, then it must provide the level control via the analog outputs.

[rkkwan]

muckyd -

1. You should use a combination of the the sub's volume knob and the receiver's LFE setting to set the "correct" level. Keeping the sub's knob at between 10' to 12' o'clock is a good idea, but it also depends on the LFE output from the receiver. As to what's the "correct" level, I think one good idea is to use the test tone in AVIA or similar test disc. Some people like to run the sub a few dB "hot", and that's okay too if that fits one's taste.

2. The test tone from the DVD player are for the analog output only. When you use a digital connection, you calibrate the speaker levels via the receiver's setting, not the DVD player. Just use the receiver's own test tone, if you don't want to buy a test disc.

Edit: I think I said the exactly the same thing as COF. Which is good for me. [IMG]'/idealbb/images/smilies/3.gif'[/IMG]

[KlineKR]

Pinpoint

It''s fun and enlightning to test the sub and not that difficult if you can beg/borrow an analog signal generator. They are fairly cheap and anyone in the electronics biz will have access to one. Connect the sig gen output to the sub external input on the receiver or directly to the line level input on the sub. Start at the lowest output amplitude level and lowest volume level to avoid damage. Establish a reference level at about 120 Hz. Read the frequency directly from the sig gen and plot the response using a SPL meter. The radio shack meter reads about 6.2 dB low at 20 Hz and 3 dB low at 30 Hz.

I have also heard of people doing this with downloaded wave files of individual sine wave frequencies through their CD players instead of a sig gen but I have not tried this.

[HAL 9000]

I have used a Panasonic condenser microphone capsule available from Digikey electronics (www.digikey.com) part # P9959ND. In the low end and midrange, it is flat within 1db, even down to below 20Hz. Of course you will need to power it but this involves nothing more than a 9 volt battery, a 2k2 ohm resistor and a 220uf capacitor (to isolate the DC from the sound card input). You will also need a 100k ohm resistor after the capacitor to discharge the DC, so the input of your soundcard doesn't have to do this!

Add some good RTA software such as "Real RTA" (www.trueaudio.com), and you're good to go. The RTA software has a calibration function that cancels out the response deficiencies of your sound card, yielding flat response.

Building the microphone power supply involves soldering, but this is a good excuse to learn to do it. This would make an excellent DIY project.

[crazyoldfart]

If you are going to use the Radio Shack analog SPL meter, adjust the levels when you write them down by the setting below:

20Hz = +6.2dB
25Hz = +4.4dB
31.5Hz = +3dB
40Hz = +2dB
50Hz = +1.3dB
63Hz = +0.8dB
80Hz = +0.5dB
125Hz = +0.2dB
160Hz = +0.1dB

Also, to test the response of the sub in the room without the affects of the room:
1. Place the sub in the room, 1/3 the distance from each wall (ie. of the room is 18ft x 24ft, place the sub 6ft from the long wall and 8ft from the short wall)
2. Place the SPL Meter as close to the woofer cone as possible on a camera tripod stand.
3. Set the level on the sub as low as possible where the sub sound is slightly louder than the room noise.
4. Record the response.
5. If you have a tuned port or a passive radiator on the sub repeat the measurements with the SPL meter just outside the port or as close to the passive radiator as possible.

If you can go outside:
1. Place the subwoofers active driver as close to the ground in your driveway as far from the house as possible.
2. Set the level so the meter reads the signal instead of any noise in the area.
3. Place the SPL meter in the ground 1 meter from the subwoofer directed at the driver.
4. After recording the levels, adjust them by reducing the recorded SPL by 6dB.
5. If there is a port or passive radiator, repeat above with the port/radiator facing the SPL meter.

If you want to see how the sub works in the room (with the added room reinforcement), keep the sub where you like it and measure it from your listening position, then measure it three more times with the SPL meter in another location. Average all the curves together.