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Old 21st July 2004, 08:20 PM   #1
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Default bass guitar cab

Just before I start cutting the wood.....

I've got a Eminence Gamma 15" and I was wondering if there is any reason not to use a ported design for a 1x15 cabinet (versus sealed)?

I know the benefits of sealed versus ported in hifi systems, but how does this affect bass guitar?

I have read the threads and modeled both designs with Unibox, but I am still unsure. I have heard that most of the bass sound comes from the second harmonics and that some manufactures use sealed desings, but with 4/8x10 cabs.

Thanks
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Old 21st July 2004, 09:05 PM   #2
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In this case there's not much to recommend eitther from the standpoint of frequency response, as both a sealed box and an optimally vented box at 2 cubic feet will have an F3 at 65 Hz, along with a 6dB spike at 100 Hz. Sealed will work better as it will keep excursion under control below F3 whereas vented won't. This driver is not really suitable for a VB smaller than 20 cubic feet or so.

The bump at 100 Hz should give a decent tone, and the 65Hz F3 is adequate. A bigger box will gain some lower extension but it's true that the major component of electric bass lies from 60 to 100 Hz in the second harmonics.

[URL=www.billfitzmaurice.com]
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Old 22nd July 2004, 11:07 AM   #3
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So, would there be a benefit from using a sealed setup? By what you are saying, it seams that 4-6dB difference below 60Hz doesn't really matter. Would the transient response from a sealed 15" make a nice sounding punchy bass?
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Old 22nd July 2004, 12:21 PM   #4
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Realistically speaking the transient response is the least of your concerns; we're not talking hi-fi here. What matters is that below Fb the vented box is unloaded and the driver is free to potentially go beyond its excursion limits rather easily, while a sealed box controls that problem. You can use a vented box for bass if the Fb is low enough so that you're not gong to put much energy into it below the Fb; the driver you have doesn't allow for that option from a reasonably sized box. That, combined with the fact that your driver has roughly the same response in either a sealed or vented box, makes the sealed option the only logical one.

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Old 22nd July 2004, 05:57 PM   #5
TheoM is offline TheoM  United States
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Default bass guitar harmonics

Two points here on our cab:

Several years ago I plugged a bass guitar into my spectrum analyzer and did indeed see some interesting things.

1) The string attack is noise from DC to 20,000hz - that percussiveness is key to your sound - some like it some don't - but if you do then the sealed cab will let you go much lower more safely (ie driver protection). Point here is that having lotsa very low end may help regardless of the ratio of fundemental to harmonics. eq could fix up the difference in slopes (between sealed and ported), and only weight (speakers/power) can compensate for the effeciency loss. It might be worth it.

2) On checking open strings over time (from attack over 30 seconds of decay) I saw that the lower the string, the more prevalent the harmonics were relative to the fundemental. So on a low E (didn't have a 5 string!), for example, the first harmonic was like 30 db higher than the fundemental (30 db!). On the A, 10 db etc. This is a broad generalization, and I did the test several years ago so upon request I will repeat it and drop the spectra into this board.

Also, as the note decayed, the harmonics dropped away and the fundemental stayed - so after a few seconds of decay, the fundemental was equal or greater in power to the harmonics on the low strings.
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Old 22nd July 2004, 11:02 PM   #6
TheoM is offline TheoM  United States
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Default bass guitar spectragram

I attached a jpg - (hope its not too big) of a time series spectragram. This is frequency by time, with power (energy at that frequency) being color according to the scale on the right.

So, at time zero (left side), I hit the open E string on a Bass guitar, about 41 hz (guitar not in tune). You can see the attack covering all frequencies below about 8k. Note the fundememtal dies out, and the harmonic at 110 is stronger by about 20 db after a short time and but that all the high harmonics die out rather quickly.

Then I hit the open A (you see markers at 440,110,55 hz.) Note the harmonics are as loud as the fundemental, but die off from the top down as the note sustains.

Anyone want more tests while my signal path is set up?

(Passive Bass into Presonus MP20 instrument input into Yamaha AW4416, 24 bit digital out to computer, spectra plus)
Attached Images
File Type: jpg bassguitar.jpg (82.9 KB, 86 views)
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Old 22nd July 2004, 11:30 PM   #7
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Thanks for all the input.

I think I will go with the sealed design. It looks to be at max -5dB compaired to the VB (between about 40Hz and 58Hz). I like the idea of a smaller cabinet, which also means lighter.
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