First speaker project...help with low frequency fall off

[Cannikin]

Hey guys,

I'm in the middle of my first speaker project. I'm using some Dayton Audio ND65-8's:

http://www.parts-express.com/pe/show...number=290-206

And a DTA-2 amplifier:

http://www.parts-express.com/pe/show...number=300-385

According to the specs on the speaker, and inputting the parameters into BassBox 6, I came up with a test enclosure made from 1/2" MDF. All joints are glued and pin-nailed except for the front so I can detach and make modifications inside:



I started everything up and immediately noticed that it didn't have nearly the amount of bass I was expecting. I pulled my Radio Shack SPL meter (C weighting, fast response) and tested the frequency response between 40Hz - 300Hz:



One problem is the noticeable dip between 150-190Hz...any ideas on this one?

The rolloff below 120Hz pretty much mirrors the official spec sheet from Dayton. But according to Bass Box the F3 for this enclosure is right around 59Hz so I expected this thing to hold steady down to that frequency...maybe my expectations were wrong? Could my port be too close to the back wall (it's about 2.5 inches away) and I should be adding a 90 degree elbow in there? Could I have just enough tiny air gaps that the port can't perform it's duty correctly? Am I just expecting way too much from a 2.5" speaker?

Here's what BassBox came up with for the box and port configuration:



Another weird phenomenon was that stuffing the box with Acousta-stuf actually made the bass response much worse...it was noticeable by ear right away and the SPL meter confirmed: 50-60Hz dropped by 4dB. I had to just add a minimal amount at the very back of the box, and that was almost the identical response as having no padding whatsoever. Again, my expectations about the magic properties of Acousta-stuf were incorrect to begin with.

On the bright side, I listed to "No Surprises" by Radiohead and heard details I've never heard before. I hooked up a couple other pairs of speakers to make sure I wasn't crazy and sure enough, they didn't reproduce that detail (you could hear the guitarist's fingers on the strings on the repeating four note intro). I just wish I could get the bass out of this thing that I was (perhaps over-enthusiastically) expecting!

Any help would be greatly appreciated!

Thanks,
Rob

[isaeagle4031]

Re: First speaker project...help with low frequency fall off

Baffle step, leaks, suffing, expecting too much, etc, etc. Try running your sim through WBCD or Winisd and see what you get. Is the port blocked by the stuffing? The port has plenty of room to the wall, no issue there.

[JasonP]

Re: First speaker project...help with low frequency fall off

One problem is the noticeable dip between 150-190Hz...any ideas on this one?

Educated guess based on experience: Nearby boundary effect, causing a droop in response.

Another weird phenomenon was that stuffing the box with Acousta-stuf actually made the bass response much worse

I think that is a hint that the T/S params of your driver do not match spec, and the box you are using is likely too large for its actual spec.

[billfitzmaurice]

Re: First speaker project...help with low frequency fall off

One problem is the noticeable dip between 150-190Hz...any ideas on this one?

The first thing I'd consider is the measuring protocol. Chances are that's room related.

according to Bass Box the F3 for this enclosure is right around 59Hz so I expected this thing to hold steady down to that frequency..

I get an f3 of 45hz with 52Hz tuning in 2.4L, so either something's wrong with the build or that's a measurement protocol issue too. But don't expect a lot of low end out of it in any event, displacement limits it to 90dB maximum output.

[JasonP]

Re: First speaker project...help with low frequency fall off

I get an f3 of 45hz with 52Hz tuning in 2.4L, so either something's wrong with the build or that's a measurement protocol issue too.

Oh measurement! Could it be affected by the weighting on the meter? He said he is using an RS SPL meter.

[billfitzmaurice]

Re: First speaker project...help with low frequency fall off

Oh measurement! Could it be affected by the weighting on the meter?

Not above 50Hz.

[tyger23]

Re: First speaker project...help with low frequency fall off

The RS SPL meters only allow the use of A or C weighting. The A-weighting curve looks like this:


The C-weighting curve looks like this:


So, if the RS meter is set to A-weighting, then the bass response curve will be severely affected.

Personally, I think the issue is likely related to A-weighting setting along with room reflections affecting the 150Hz region.

[greywarden]

Re: First speaker project...help with low frequency fall off

Anything under 300Hz in a room of reasonable size is absolutely moot, unless it's from a nearfield measurement.

[Cannikin]

Re: First speaker project...help with low frequency fall off

The RS SPL meters only allow the use of A or C weighting. The A-weighting curve looks like this:


The C-weighting curve looks like this:


So, if the RS meter is set to A-weighting, then the bass response curve will be severely affected.

Personally, I think the issue is likely related to A-weighting setting along with room reflections affecting the 150Hz region.

I'm using C weighting, fast response.

[Cannikin]

Re: First speaker project...help with low frequency fall off

Thanks for the suggestions guys! I'm going to do a little more testing in a better environment (everything was simply resting on my desk for this test) and I'll update this post with some more details. I'll also model the speaker in WinISD and see how that compares to BassBox and maybe make some adjustments.

[Chris Roemer]

Re: First speaker project...help with low frequency fall off

The "rolloff" below about 300Hz should follow your box sim output, NOT Dayton's measured response (unless they specifically tell you test conditions, and that those were in a box pretty much like yours).

WinISD ( www.linearteam.org ) likes a box about double the size of yours, with about the same tuning. It yields about the same output down to 100Hz or so, and at 40 and below, but there's a slight lift at 80Hz, over +2dB at 60, and +3 at 50. As has also been mentioned, your SPL measurements might be suspect. Do you know what "baffle step" is? Also, MOST folks stuff closed boxes, but line vented boxes with convoluted (egg carton) foam. You can stuff a vented box to death.

That little guy is probably Xmax liimited with only 2 or 3 watts by the time you get down to 80Hz.