Box size and sound charateristics

**ThomasW** · 15 January 2005, 12:51 Saturday

It's not like the wife is willing to let you cut a hole in the floor/wall/ceiling
and mount a 'true' IB design .... :B

4- 18" BluePrint BP1803's in a common manifold

Edit added the pic below to provide some perspecitve as to how BIG 18" woofers are .....

**JonMarsh** · 15 January 2005, 12:54 Saturday

Hello Trond-Eirik,

The plots show the predicted transfer function based on T/S analysis of the main driver parameters.

Additionally, in a room, there is the interaction of boundary loading, which modifies the LF response further below 100 Hz; depending on the distances from boundaries (say, floor, rear wall, and nearest side wall), AND the ratio of these distances (if you don't have the sub in the corner, then it's best IME to have a golden mean ratio), the LF response can be modified by 6-9 dB in various fashions- peaking and dips. Tis is what most refer to as low frequency room gain, and it's one reason why you don't usually want a dead flat driver response, becuase then with many room positions, the bass will be too heavy in some frequency ranges. Of course, that can be fixed with an equalizer, too, but being the kind of guy I am, I like to keep the extra stuff in the signal path to a minimum, and find acostical solutions to acoustical problems. Just an old fashioned guy, I guess.

Then they'res the driver voice coil inductance and how that modifies the upper bass response (above 75 Hz). Low Le is ideal; higher Le can be compensated to a degree for frequency resonse roll off with EQ; but that won't help the higher upper bass distortion which usually accompanies high Le designs. It's like the difference in upper bass distortion between a TC2+ and DPL12; they're both very good below 50 Hz, but the TC2+ is considerably cleaner in the upper bass range, to 200 Hz.

Now, considering just the curves you're showing, for a sealed box, normally an LT (Linkwitz Transform) equalizer is used to control the Q and extend the low end. In principle, with the correct LT transform (which complements the box curve in a very specific mathematical way), the final transfer function for a wide range of box sizes can be made to look very similar. The main difference is that for the smaller boxes, you need more EQ and power to hit the same low frequency extension - so thermal capability of the motor starts to become fairly important. Power requirements can double or quadruple wtih the smaller box sizes. But box size shrinks, and we know a lot of folks, especially wives, really like that.

Motor linearity and thermal power handling become very important.

If you don't use an LT equalizer, then IMO you should work with a much narrower range of box sizes- this is because of both the roll off characteristic and the Q/transient response. I prefer very tight, well damped bass, which for me translates into two principles: having an LF resonance/Q in the range of 0.4 (overdamped) to 0.566 (Bessel minimum delay, just slightly under damped); 0.5 is probably the "sweet spot". Then you have a gradual slow roll off, which complements a room curve pretty well. IB's usually work in the low end of this range, depending on the driver T/S parameters. If a sealed box is smaller than will support a Q in the range of 0.5 to 0.566, then I use an LT transform (modified) to tune it to this characteristic. That's actually a dip near the LF region and boost below that in a shelving EQ.

Tuning any of these designs optimally requires some knowledge of the room size and planned placement. I use at times either MathCAD cals, or RPG Acoustics Room Optimizer. Cara is another good tool, and can setup with great flexiblity and excuriating detail. You don't need these to get good sound, but you might need them to tune the sound to the optimum point.

~Jon

**TEK** · 15 January 2005, 16:13 Saturday

Thomas:
Hmm, don't tempt me :twisted:
My wife is a wonderful girl, but I do not think se will allow me to start cutting holes in the floor or sealing in our livingroom!
I really can't understand that :roll:

John:
Thanks for the very good description and info. I think I'm starting to get some of the basic stoff now.
I totally agree that the best way to solve the problems is to do it with a correct design, instead of trying to adjust for the errors afterwards.

Is there any good description "out there" that really describes what a Linkwitz Transform equalizer actually does, and what's the different between it and a regular, parametarized equalizer?
I'm finding a LOT of stuff about it, but nothing this basic.

Regarding room tuning. I'm do not think that the ide of tuning a sub to a actual room and placement in a room sounds very appealing.
I agree about tuning to a given room size, but to tune the hardwiered stuff according to the actual room and room placement

When you get to that point, wouldn't a equalizer be a better solution?

IMHO: A womens mind is changed a bit to often for that :roll:

If I understood you correctly, what you say about the box size is basically:
- With a LT you can basically tune "any" box to sound as a wanted Q, given that you have enough power.

So to the interesting part:
A low q should give you tight, well damped bass.
A higher q would give you what? boomy bass that settels slowly?

One of the reasons I'm asking this is stuff noted from the review written by Brian:

Code:

For my musical tastes this box is just too large and I didn't have enough power. For a Tempest or Atlas 15, it is fine, but not for these drivers. The Avalanche started to sound more like the Tumult in the 2.1ft^3 box and the Tumult became even more of a low end animal. The low end is fun, but if it doesn't remain forceful with kick drums then I lose interest really quick. I lost interest very quickly with both of these. They sounded good and could shake the crap out the audio room, which is great if you want to demo for someone else, but not so peachy for everyday use in my opinion

The effect of a larger encloser (= lower Q) described here is almost the oposite of what my understanding was.
More SPL at the lower end? Yes that was expected. But what he also seems to describe is "slower" or "boomy" bass and bad behaviour in the upper bass range. So I'm still not sure if I have got a grasp of how the different graps will translate into actuall sound charateristics

**JonMarsh** · 15 January 2005, 18:49 Saturday

Originally posted by TEK

Thomas:
Hmm, don't tempt me :twisted:
My wife is a wonderful girl, but I do not think se will allow me to start cutting holes in the floor or sealing in our livingroom!
I really can't understand that :roll:

John:
Thanks for the very good description and info. I think I'm starting to get some of the basic stoff now.
I totally agree that the best way to solve the problems is to do it with a correct design, instead of trying to adjust for the errors afterwards.

Is there any good description "out there" that really describes what a Linkwitz Transform equalizer actually does, and what's the different between it and a regular, parametarized equalizer?
I'm finding a LOT of stuff about it, but nothing this basic.

Regarding room tuning. I'm do not think that the ide of tuning a sub to a actual room and placement in a room sounds very appealing.
I agree about tuning to a given room size, but to tune the hardwiered stuff according to the actual room and room placement

When you get to that point, wouldn't a equalizer be a better solution?

IMHO: A womens mind is changed a bit to often for that :roll:

If I understood you correctly, what you say about the box size is basically:
- With a LT you can basically tune "any" box to sound as a wanted Q, given that you have enough power.

So to the interesting part:
A low q should give you tight, well damped bass.
A higher q would give you what? boomy bass that settels slowly?

One of the reasons I'm asking this is stuff noted from the review written by Brian:

Code:

For my musical tastes this box is just too large and I didn't have enough power. For a Tempest or Atlas 15, it is fine, but not for these drivers. The Avalanche started to sound more like the Tumult in the 2.1ft^3 box and the Tumult became even more of a low end animal. The low end is fun, but if it doesn't remain forceful with kick drums then I lose interest really quick. I lost interest very quickly with both of these. They sounded good and could shake the crap out the audio room, which is great if you want to demo for someone else, but not so peachy for everyday use in my opinion

The effect of a larger encloser (= lower Q) described here is almost the oposite of what my understanding was.
More SPL at the lower end? Yes that was expected. But what he also seems to describe is "slower" or "boomy" bass and bad behaviour in the upper bass range. So I'm still not sure if I have got a grasp of how the different graps will translate into actuall sound charateristics

I think you have to be careful in interpreting anyone's subjective review, because it's hard to correlate what they are percieving and what they want with your own needs. There are two points he picked up on which I think qre probably quite valid regarding the Avalanche sub: lower Le, which translates into better mid bass and uppe midbass performance (many big sub drivers start rolling off at 75 Hz due to voice coil inductance), and about 2 dB or so higher efficiency, including at the top end.

Now, there are guys that like that "kick" in the bass drum- but that comes in the 55 Hz area, not low bass. So, putting a driver in a smaller box, will give it a higher Q, more "puch" in this range, but with a good sub driver with Low Fs, it will still work pretty well down to 40 and below.

These same guys don't like dipole bass because it doens't have the punch they're looking for on rock and roll.

My interpretation of this (considering that I'm an ex rock and roll musician as well as classically trained) is that there is a coloration which they prefer; a specific tuning in their bass set. IF your preferences coincide with theirs, than the same setup may please you. I prefer something more neutral.

OK, re the LT transform concept, see this figure:

Curve A is a typical curve for the response of a driver in a box which is quite small, with a higher net Q than desirable- see the hump, and the early roll off.

Curve B is the LT transform equalization curve, which is a function complementary to the driver curve, to produce the desired acoustic transfer function "C", with a target roll off characterisitic, and hence, target Q. This is a bit exaggerated over what one might actually do in reality. Practical considerations include the power handling limitations of the driver (thermal voice coil capability) as well as the fundamental limits of excursion of the driver, which sets a practical limit out LF output. Generally, you want to set a target for output at a given frequency, and then make sure that you're not using such a small box that the power required exceeds the thermal capability of the voic coil.

BTW, you're still on my list of to-do's while I'm in Denver for a tumult LT Transform design. I finished a dual 18" Avalanche LT design that was previously requested today. There is FRD consortium workshop XL sheet, but it's not exactly user friendly. Try taking a look at the Marchand site, you can download a copy of the manual for the Bassis equalizer, which is an LT transform unit. That may help your understanding.

Regards,

Jon

**Spearmint** · 15 January 2005, 19:35 Saturday

Originally posted by JonMarsh

Now, there are guys that like that "kick" in the bass drum- but that comes in the 55 Hz area, not low bass. So, putting a driver in a smaller box, will give it a higher Q, more "puch" in this range, but with a good sub driver with Low Fs, it will still work pretty well down to 40 and below.

These same guys don't like dipole bass because it doens't have the punch they're looking for on rock and roll.

My interpretation of this (considering that I'm an ex rock and roll musician as well as classically trained) is that there is a coloration which they prefer; a specific tuning in their bass set. IF your preferences coincide with theirs, than the same setup may please you. I prefer something more neutral.

Regards,

Jon

Jon,

You maybe just the man I need to talk to.

A friend of mine plays bass in a band. Anyway last night he was filling in for another band, so I went along for a listen, especially since they were playing 5mins down the road from home.

To get to the point when I walked into the room, the sound was good and the Bass and kick drum were about normal to what I hear with other bands etc. Now here is the twist I got to sit along side the band on stage which was also behind the kick drum.

Well what can I say, the “Thwack” of that kick drum was nothing short of fantastic, I went out in front and the “Thwack” was no longer audible, during one of the breaks I was talking to some of the band members talking about “things”, and I mentioned what I had heard with regards to the kick drum, the sound guy told me he EQ’s it out for better sound in the hall, and this is standard practice, mind you the drummer was on my side and said it should be left in…

Now thinking about the EQ’d sound of the kick drum, and this is probably the norm of what I hear at most places, also the sound I get from my DVD’s and Cd’s is also very similar i.e. minimal to no “Thwack” based on the recording, and yet a lot reviewers of Subs talk about this “Thwack” from the kick drums when they do their testing, and until last night I had never REALLY heard it.

I am wondering if this is indeed standard practice to EQ this sound out, also do you know of any cd’s or Hi-res discs that may have a good playback of this “Thwack”

Box size and sound charateristics

Box size and sound charateristics

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