Getting Bass from TD15M in a 5.5 ft³ Cabinet

[djnagle]

Djnagle, I think I'm getting closer to -6dB at 50Hz, like shown in the first measurement. I'm not trying to get by without subs here guys, just trying to get a full, emphatic sound above the primary subwoofer range.

In a sealed cab, stuffing makes the driver think it is in a larger box, but in a ported cab it is just to stop reflections. If you took all the stuffing you have in the cab and compressed the air out of it, you might be looking at .5 to 1 cu ft of space being taken up.

So that plus the driver, horn, CD, and bracing; you might be looking at a 4.5 cu ft cab.

Also, with those short tubes you are tuned to more like 55-60hz.

 

[audiozaz]

Simi, I think you may have misread my last post.


My measurements above are 1cm from the cone, except the first one, but both show around -15dB at tuning. The noise floor is relatively high around there and may limit that notch effect somewhat in both/either measurement.

Lol, I had the wrong graph in my head. That's more like it on the nearfield trace.

az

 

[dumptruck]

So that plus the driver, horn, CD, and bracing; you might be looking at a 4.5 cu ft cab.

I agree, but that makes the sims even farther from what seems to be happening.

Also, with those short tubes you are tuned to more like 55-60hz.

The measurements show otherwise.

 

[RayW]

BB6 gives about the same result as WinISD (imagine that!)...

Without doing the full near field measurement we can't compare to that though.

 

[dumptruck]

I think I can do more measuring at lunch. Do you want to see the port measurement with 5" tubes or no tubes?

 

[RayW]

'Nother question...what kind of space are you measuring in? Could we be looking at a room response question instead of the speaker?

 

[djnagle]

If the room is having some affect, you could measure at 1 foot increments away from the driver and see if there is a significant difference.

 

[dumptruck]

Well, right now, one end of a room about 16'x18'x9' which opens into the rest of my house and also has an 8x5 offshoot on one side, like this:

The woofer center is 42" above the floor in the previous measurements posted.
I had pretty much the same results in the basement space, which is about 16'x30'x8', except I tend to get a 100Hz null down there with a single speaker. In both cases, subjective bass response is a bit weak.

 

[RayW]

I think I can do more measuring at lunch. Do you want to see the port measurement with 5" tubes or no tubes?

Doesn't really matter as long as it's matched up to a woofer nearfield measurement. However, I won't be able to add the responses together here, you'd have to do that inside WinISD.

What I would probably do, if you have the parts, is throw together a quick BSC circuit and throw it inline and listen. Should be an easy check but will probably be a big inductor for a baffle that wide.

 

[dumptruck]

Doesn't really matter as long as it's matched up to a woofer nearfield measurement. However, I won't be able to add the responses together here, you'd have to do that inside WinISD.

I was going to post the overlayed port/woofer measurements from ARTA first, and then I'll export them and see what I can figure out for merging them properly. I have a handful of sim programs (full ones, not just bass stuff) and I think some have some features for doing fancy merge sims. I'm not sure if the dual ports will confuse things...

What I would probably do, if you have the parts, is throw together a quick BSC circuit and throw it inline and listen. Should be an easy check but will probably be a big inductor for a baffle that wide.

I guess that's something I could have put in the first post. I've been there already, but I haven't tried to measure the results much. A digital +2-4dB shelf starting at 120Hz was a big improvement and really got it sounding more like I had hoped, but I don't think that exactly proves that the cabinets are otherwise working close to the ideal. I'd like to feel confident that I'm not crippling the bass response unnecessarily and then compromising performance to fix it, ya know?

 

[RayW]

I was going to post the overlayed port/woofer measurements from ARTA first, and then I'll export them and see what I can figure out for merging them properly. I have a handful of sim programs (full ones, not just bass stuff) and I think some have some features for doing fancy merge sims. I'm not sure if the dual ports will confuse things...

My bad. I didn't mean WinISD, I meant whatever you're using for measurement (apparently ARTA). Most of the measurement suites I've used had the ability to add curves.

The dual ports won't be an issue. You measure at one and then use the ratio of total port area to woofer Sd to adjust. That takes the number of ports out of the equation.

I guess that's something I could have put in the first post. I've been there already, but I haven't tried to measure the results much. A digital +2-4dB shelf starting at 120Hz was a big improvement and really got it sounding more like I had hoped, but I don't think that exactly proves that the cabinets are otherwise working close to the ideal. I'd like to feel confident that I'm not crippling the bass response unnecessarily and then compromising performance to fix it, ya know?

I understand. You have to know where you're starting from in order to make meaningful changes moving forward.

 

[dumptruck]

How are relative levels supposed to be accurate with this, though? Seems like measuring from such small distances, relative SPL error would tend to be huge? I did 1cm from woofer cone (about halfway in from surround), and flush with the rear baffle plane in the port hole. Without doing anything to compensate for the other port and just averaging the two plots, I get this, which looks... kind of great. Oh wait... averaging doesn't make sense does it. Hold on...

 

[djnagle]

From this it looks like you are getting an f3 of about 30hz....that ain't bad! I thought measurements where taken from 1 meter out from the driver?

 

[dumptruck]

Not these kind of measurements. My software doesn't want to cooperate, but here's the sum in PCD for now (nothing wrong with it, just can't adjust the graph scale appropriately), no adjustment made for the second port and it's still completely unrealistic.

 

[RayW]

Dennis, you can do far field measurements at 1m but for bass the room is going to rule so you measure very close to the driver so that the direct sound swamps the room and takes it out of consideration.

 

[dumptruck]

Hmm, here's what happens when I put 2ms simulated delay on the port in PCD. I wonder if this could actually be happening...

 

[RayW]

A 5" port has a cross sectional area of 126.7cm^2 so 2 would be 253.4cm^2. Divide that by the Sd of the woofer (855) and we get .296. Take the log and multiply by 20 and we get -10.6dB. So adjust your port response down by 10.6dB and then add to the driver.

It looks like you have the phase reversed since the sum around the handoff point from woofer to port is less than either single response.

 

[dumptruck]

A 5" port has a cross sectional area of 126.7cm^2 so 2 would be 253.4cm^2. Divide that by the Sd of the woofer (855) and we get .296. Take the log and multiply by 20 and we get -10.6dB. So adjust your port response down by 10.6dB and then add to the driver.

Ohh.. okay here's that:

It looks like you have the phase reversed since the sum around the handoff point from woofer to port is less than either single response.

You're right it does, but phase is as measured (exported min. phase), and now shown above (without delay added). :scratch2:

 

[audiozaz]

A 5" port has a cross sectional area of 126.7cm^2 so 2 would be 253.4cm^2. Divide that by the Sd of the woofer (855) and we get .296. Take the log and multiply by 20 and we get -10.6dB. So adjust your port response down by 10.6dB and then add to the driver.

It looks like you have the phase reversed since the sum around the handoff point from woofer to port is less than either single response.

RayW,

Thanks for posting this, I'll try to measure mine that way to compare with DT's data.

You seem to be one of the real knowledgeable speaker guys around here, what do you think of a vent located at the bottom of the cab, right at the floor boundary versus one of the same size located most anywhere else, does it really have higher output efficiency as I remember reading? And for the above measurement technique, is it valid for me to measure the vent located right next to the floor and would there be anything to modify in the calculation?

az

 

[dumptruck]

Port phase should be wrapping at tuning frequency where the driver phase zigs, right? Maybe I need to try that again.