Strange Frequency Response in Bass... 40Hz-104,4dB ; 85Hz:82,3dB ??? HELP

II recently made a 3-way speak with the Dayton 385 series II 15 inch woofer.

It's specs are QTS .32 VAS 301 and Fs 18,8Hz.

I made a bass reflex with 160 liters and the duct tunes do 23Hz as suggested by mh-audio.nl but t's sounds a little strange in the frequency response, there's a huge gap in the between the 40-55Hz and 70-90Hz with almost -13dB lower with the 85Hz frequency, while the 60Hz is +6,8dB louder??? what went wrong?

I meausured with a dB meter some frequencies between 20 and 100Hz and came wth the following numbers:

Hz dB
20 85,6
25 86,1
30 93,6
35 97,6
40 104,4
45 103,2
50 97,9
55 97,7
60 104,3
65 109,9
70 100,9
75 98,5
80 95,1
85 82,3
90 87,1
95 91,9
100 95,5

I would like to hear some suggestion of where did I went wrong. I'm using an active crossover crossed at 200Hz to my scan speak 8545 midbass.

Thx

Murillo
 
Picture of the speaker?

What's the height of the woofer above the floor?

A casual look at what you measured (not knowing how or where the measurements were taken) indicates that the actual tuning of the woofer is higher than you intended. Did you simulate the response using a computer program?

Otoh, the 104 at 60/65 may indicate a room peak.

Take the unit outside and face it UP and measure 3+ feet away, above the speaker... as far from any adjacent wall as you can.

Also consider using a freeware FFT program to generate some graphs.

Don't forget to check the active xover to see that you don't have it set incorrectly...
 
Yes, you are measuring the room. Take it outside on a large paved surface and to a single boundary measurement. ( D'Appolitto will show you how ). Or stick it up on a 20 foot pole and do near field measurements. That will tell you what the speaker is doing. But then again, who cares? It IS what the speaker does in the room that matters. You don't listen to the sub outside now do you?

Inches can make a difference. Put the mic as close to where you will sit as you can. Move the sub all around and see what you can do. Real soon you will see why we use multiple smaller subs. :) With a single sub, you will find it will integrate easier if it is exactly between the two mains. If not, it will cause nulls. The steeper the crossover the easier. ( I use LR4 subs to main, LR6 would be better if you have to be off-center)

Do not think you are going to get anything close to a flat response. Dips are far better than humps. If you insist on adding eq, worry about the peaks and don't think you can force out the dips. Just a little work and you can get it sounding quite good.
 
bear, tvrgeek, Skiivari, you are the guys!
I just measured now with a FFT software and the graphic is almost flat between 20-100Hz, I think I did something wrong with the other measurement. I think I measured the room. My room is very small for these big speakers, 15 inch woofers are not easy to adjust.
My speaker is a clone of the JBL L300, but with a different configuration of the drivers.
I use a 15 inch dayton crossed at 200Hz, a SS 8545 and a SS illuminator tweeter crossed at 2K with my 6 mono powers and a LR4 active crossover.
The result is very acceptable, but I think they need a bigger room to breath, my room is just 150 sq.ft.
I also tested with a JBL 2215H 15 inch driver but the dayton goes lower, the negative side is that it has 4dB less sensitivity than the JBL.
 

AllenB

Moderator
Paid Member
2008-10-18 11:31 am
It sounds more like the FFT has smoothing which is hiding the peaks. Single tones will expose them.

A smaller room may bring the room issues higher in frequency. Using smaller woofers might seem to be a way to avoid the issue but maybe placement would be the way to start.
 
It sounds more like the FFT has smoothing which is hiding the peaks. Single tones will expose them.

A smaller room may bring the room issues higher in frequency. Using smaller woofers might seem to be a way to avoid the issue but maybe placement would be the way to start.

Thx Allen for the suggestion. But I measured the FFT without smoothing, I used 1/24 octave and 1/12 octave also, the results where similar. The only thing I´m noticing is that does not make difference when I remove the duct in the 23Hz Fo frequency that is originally calculated. As I was told, if the duct is correct, the cone will not move at all at this frequency 23Hz, but it looks like that´s is moving.
 
But I measured the FFT without smoothing, I used 1/24 octave and 1/12 octave also, the results where similar. The only thing I´m noticing is that does not make difference when I remove the duct in the 23Hz Fo frequency that is originally calculated. As I was told, if the duct is correct, the cone will not move at all at this frequency 23Hz, but it looks like that´s is moving.
Pink or white noise sources are random and don't excite room modes as much as pure tones.

Although excursion will be least at tuning, some alignments still have a fair amount of cone movement visible at Fb.
Using a white dot on the cone makes it easier to compare excursion at different frequencies to see if the box actually is tuned to 23 Hz.

23 Hz also may be so low that the difference between it and sealed are very little.
 
As you can see by these tips, speaker building is one place where creativity still counts. I used a laser pointer bouncing off a piece of foil tape once to measure movement. I wanted to use this for feedback of servo, but it got way too complicated. Any good DSP guys looking for a project?

At resonance, the driver will still move. If it did not, you would have no sound. What the box does is force it to move only about as much as it should, not massively flopping in the breeze. The mass of the air in the port winds up working exactly against the driver at resonance. Nifty. Simplistic description of course.
 
yep rooms do that to speakers....

my speakers in my old room had a huge kick to the bass and lots of output around 50-60hz and 25hz and below with a huge null at 40-30

my new room in in phase at 25-45 but has a null at 50-70ish and a flatter bass out put above that

basically. try relocating the sub, for example if the 30 40hz are in phase with the room and your sub is in a corner.... you will get a huge gain at those frequency. so you move it out to diminish that.

to get a feel for what i mean. play different sine waves and walk around the room. in some places the frequency is strong and at others you can barely hear it.

so its always a compromise, best to just have a play and get it the best you can for your room


bass response in a room is like a expanding wave. the peeks and nulls be come longer and cover an incresingly wider band of frequency's as you go down the scale
 

geminni

Member
2006-12-26 2:05 pm
This is how I have found the best place for subwoofer.
Put the woofer in room where is your listening place. Play some low frequencies. I played some sine tones from 25 hz to 150hz. Go arround the room and try to locate the spot where are low frequencies even, or where you don't hear drastic dips or drops.
When you find that space, move the woofer on that spot and then you'll have good bass at your listenign spot.
I have two identical subwoofers so I have managed to make even much better response :)