Hey all, I haven't been around in a while because of school - I am now a physics student at UMBC, and haven't had much time to come by between class and video games.
Background: I slapped some speakers together before school started so I could have something nice to listen to in the dorm. It's a pair .75 ft3 boxes each housing a hivi f8 and a vifa something or other tweeter. This box is on the small side for these speakers, and they arenn't tuned quite low enough (~45 Hz), so there is a bump in response of about 2dB at ~ 70Hz, according to winisd. I always have found the bass on these to be a little excessive.
What made my problem worse is that I just got some reference headphones (sennheiser hd-280's) and realized just how bad my problem is. These phones sound damn good - almost too good. I found that I can hear low bass better with my left ear because of these damn things. I can hardly listen to the speakers anymore...of course my friends think im nuts.
I have recently been working with a music making program called fruity loops(main reason i bought the headphones), which has a parametric eq on it. I made a test tone, and found exactly where the resonance was (second C, i think), and found that to be ~130 Hz. I put a notch filter there, and it sounds fine.
What could cause a nasty bump at 130 Hz? I don't think it's the tuning...I have an inkling that it is my crappy job of putting padding on the inside of the enclosure, but im not sure. Any suggestions?
Background: I slapped some speakers together before school started so I could have something nice to listen to in the dorm. It's a pair .75 ft3 boxes each housing a hivi f8 and a vifa something or other tweeter. This box is on the small side for these speakers, and they arenn't tuned quite low enough (~45 Hz), so there is a bump in response of about 2dB at ~ 70Hz, according to winisd. I always have found the bass on these to be a little excessive.
What made my problem worse is that I just got some reference headphones (sennheiser hd-280's) and realized just how bad my problem is. These phones sound damn good - almost too good. I found that I can hear low bass better with my left ear because of these damn things. I can hardly listen to the speakers anymore...of course my friends think im nuts.
I have recently been working with a music making program called fruity loops(main reason i bought the headphones), which has a parametric eq on it. I made a test tone, and found exactly where the resonance was (second C, i think), and found that to be ~130 Hz. I put a notch filter there, and it sounds fine.
What could cause a nasty bump at 130 Hz? I don't think it's the tuning...I have an inkling that it is my crappy job of putting padding on the inside of the enclosure, but im not sure. Any suggestions?
Maybe it's a panel resonance? Also, if you stuff excessively or your cabinet has leaks (other than the port), your tuning might not be what you think it is. Best thing to do when in doubt is to measure the impedance curve- especially if you can get both magnitude and phase.
BTW, I graduated UMBC in physics in '77. Congrats- it's a terrific department.
BTW, I graduated UMBC in physics in '77. Congrats- it's a terrific department.
bostarob said:
Along the lines of what Rino said, the bump at 135 Hz is approx 3 times your tuning frequency. And your tuning frequency was just approximate. I have to admit though, I have never seen a problem occur at 3 times the tuning frequency-I just thought I would point out the relationship.
Just for the heck of it, what are the inside dimensions of your box? Wondering if it is a standing wave.
9.5" W
7" D
19.5" Tall
The Fs of the woofers should be around 33Hz.
As far as I know, i don't have the means to measure impedance. Maybe the Physics dept. does? But i really don't have the time for that. That could be a summer project. What about standing waves? There really aren't any holes or leaks.
What would measuring the impedance curve give me?
Thanks a lot, guys,
andy
7" D
19.5" Tall
The Fs of the woofers should be around 33Hz.
As far as I know, i don't have the means to measure impedance. Maybe the Physics dept. does? But i really don't have the time for that. That could be a summer project. What about standing waves? There really aren't any holes or leaks.
What would measuring the impedance curve give me?
Thanks a lot, guys,
andy
If you measure the complex impedance and plot it on the complex plane (real part of impedance on the x axis, imaginary part on the y axis), resonances like leaks and panel rattles will show up as little loops. The main resonances (the ones you want in the bass) will be big loops. Plotting the magnitude of the impedance versus frequency will give you quantitative info on whether the box is tuned the way you think it is.
The equipment to do this is pretty elementary- if you don't have an FFT tester, you can do it with a signal generator and a 'scope.
Standing waves at 130 Hz are highly unlikely for your enclosure.
The equipment to do this is pretty elementary- if you don't have an FFT tester, you can do it with a signal generator and a 'scope.
Standing waves at 130 Hz are highly unlikely for your enclosure.
What are your enclosures made from. How thick is the cabinet wall. It could be panel resonances. If they are at around 130 Hz you can actaully feel this with your fingers if you hold them in light contact with the box while slowly sweeping a sine wave into the speaker. Alternatively, sweep while putting your ear in close contact with the panel. If this is the cause, then build the box stronger by adding internal bracing.
Andrew
Andrew
The two boxes are certainly not in an ideal set up. they are close to the wall, and too close to eachother. They are hugging my computer monitor on either side of my small desk. Additionally, I live in a dorm room with cinder block walls. When I sit right at my computer the ~130Hz peak is most pronounced. Across the room, the lower bass is amplified more.
I really think I should move them around, but I'd have to reorganize and move everything else around too.
Thanks,
andy
I really think I should move them around, but I'd have to reorganize and move everything else around too.
Thanks,
andy
No need to go into details of Z measurement- it's covered in detail in Dickason's "Loudspeaker Design Cookbook" and d'Appolito's book on loudspeaker measurement. Anyone playing with speakers should have both, Dickason at a minimum.
The basic setup is using an amp with a resistor in series with its output. The signal generator is hooked to the amp's input and the voltage across the resistor and the speaker series combination and the voltage across just the speaker are measured (I use a two-channel scope for this). The former measurement ought to be constant if the sig generator and the amp have flat frequency response, but it's nice to verify this experimentally. Using the voltage divider equations, one can plot point-by-point the magnitude of the impedance. Use the scope to measure the frequency at each point if you don't have a good frequency counter (I don't).
The phase of the impedance can be derived by doing a Hilbert Transform to the magnitude data. Spreadsheet software can normally do this- it's a simple differentiation with respect to frequency. With this data in hand, plotting the real and imaginary parts of Z is pretty simple and will give the sort of plot that can be used to determine what's going on with a speaker's resonances. If, in this case, there's no little bumps and loops at 130 Hz, the problem is the room, not the speaker.
Impedance minima and maxima can be verified and gotten precisely by using the scope in X-Y mode and using Lissajous figures (explained in gory detail in the ARRL Radio Amateur's Handbook, and in sufficient detail in Dickason). But normally, the Hilbert Transformed data will be good enough. FFT/MLS methods are even better and darned cheap these days. But for those with limited equipment, a scope and generator will get perfectly good data.
The basic setup is using an amp with a resistor in series with its output. The signal generator is hooked to the amp's input and the voltage across the resistor and the speaker series combination and the voltage across just the speaker are measured (I use a two-channel scope for this). The former measurement ought to be constant if the sig generator and the amp have flat frequency response, but it's nice to verify this experimentally. Using the voltage divider equations, one can plot point-by-point the magnitude of the impedance. Use the scope to measure the frequency at each point if you don't have a good frequency counter (I don't).
The phase of the impedance can be derived by doing a Hilbert Transform to the magnitude data. Spreadsheet software can normally do this- it's a simple differentiation with respect to frequency. With this data in hand, plotting the real and imaginary parts of Z is pretty simple and will give the sort of plot that can be used to determine what's going on with a speaker's resonances. If, in this case, there's no little bumps and loops at 130 Hz, the problem is the room, not the speaker.
Impedance minima and maxima can be verified and gotten precisely by using the scope in X-Y mode and using Lissajous figures (explained in gory detail in the ARRL Radio Amateur's Handbook, and in sufficient detail in Dickason). But normally, the Hilbert Transformed data will be good enough. FFT/MLS methods are even better and darned cheap these days. But for those with limited equipment, a scope and generator will get perfectly good data.
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