TCA high output subwoofer discussion Thread. What are our performance goals?

[craigsub]

Admin note - this page will be edited frequently.

Hi all, as we get closer to release of our subwoofer line, I thought everyone might appreciate some information regarding what makes for good subwoofer performance. We have all seen the tests of subwoofers done in which a sine wave and/or a short burst is asked of the subwoofer at particular frequencies, then posted.

We have also seen where people go to the max output at 20 Hz and declare that the subwoofer which is the loudest there "wins".

Yet, in real world testing, subwoofers which were not the loudest at 20 Hz are found to deliver a much more satisfying home theater and music performance overall.

Over the past few years, while testing over 100 subwoofers and also listening to some of those made by A-line Acoustics, I learned a lot about what people like, and don't like, in terms of a subwoofer's performance.

I fully intend on writing this in plain language, so it does not require one to be an engineer to understand it.

Parameter 1: Driver Efficiency/Sensitivity.

While these two terms mean different things, they also mean the same thing, as high efficiency and sensitivity go hand in hand. Efficiency is stated as a percentage: what percentage of the electrical "watts" are converted into acoustic "watts".

Sensitivity is stated as 2.83 Volts delivered into the loudspeaker, and the output that is derived from this power being delivered. 2.83 volts into an 8 ohm load is also 1 watt delivered into an 8 ohm load.

We will use sensitivity, as it is much easier to follow. If we have a single driver which is 95 dB sensitive, it will require 1 watt to deliver 95 dB.

If we compare this to an 85 dB sensitive driver, it will require 10 watts to deliver the same 95 dB. Each time we require 10 more dB from the driver, it will require 10 times the power to achieve this 10 dB. It is also generally recognized that 10 dB higher SPL "sounds twice as loud". For example: 100 dB will sound twice as loud as 90 dB.

There are exceptions to this, however. We often will find that, at lower frequencies, once we get above about 90 dB, it requires only appx. 6 dB higher SPL to sound twice as loud.

The important point is this: In a subwoofer, driver sensitivity is quite important to overall performance.

The reason for this is, in a subwoofer, "watts" can be used up fairly quickly. In a normal home, using a 15 amp circuit and a 120 volt rating, we have 1800 watts available to us from the typical 15 amp circuit.

This is also why it is important to try to put one's subwoofer on a dedicated circuit, unless one is using a subwoofer amp of less than 1000 watts.

Why is sensitivity so important ? Simple - and this is important:

A subwoofer is rarely delivering any single frequency during a movie soundtrack. It is usually being asked to deliver several frequencies at once.

Let's take, for example, 2 different driver arrangements from two well respected companies, and look at their computer models for output.

These are for 2 subwoofers requiring 1500 watts, so we can use a single 15 amp circuit. One subwoofer is in a 12.5 cubic foot cabinet and the other in a 10 Cubic foot cabinet.

Frequency ........ Subwoofer A ......... Subwoofer B
15 ........................110 dB ..................108 dB
20 ........................114 dB ..................113 dB
25 ........................117 dB ..................117 dB
30 ........................119 dB ..................120 dB
40.........................120 dB ..................124 dB
50.........................121 dB ..................127 dB
63.........................121 dB ..................129 dB
80.........................121 dB ..................130 dB

As anyone can see, both of these subwoofers deliver excellent performance. We would expect both to be used in a room in which room gain starts at 25-40 Hz, and average 6-9 dB per octave of room gain. A room in which the largest dimension is 22.6 feet will see room gain start at 25 Hz, where a room that has as its largest dimension of 14 feet will see room gain begin at 40 Hz. We can also expect to see additional output from boundary gain. In the typical room, both subs will have no problem delivering reference bass from below 20 Hz up to an through the crossover.

Let's look at the differences in the 50 Hz "slam" range. Subwoofer "B" will deliver 115 dB with only 100 watts of input while subwoofer "A" will require 400 watts at 50 Hz to deliver the same SPL.

By having a more efficient driver, subwoofer "B" will have more amp power remaining to deliver the infrasonic (15-25) bass at the same time it is delivering slam bass (40-80).

[craigsub]

15 inch ported vs 18 inch sealed

In the roughly 3 plus months since we decided to take all design work internally, we have looked at a lot of different design ideas for our subwoofers. One of those is this: which will provide for a better overall music and home theater experience, a 15 inch ported subwoofer or an 18 inch sealed subwoofer.

One item of importance, in working with our OEM's engineers, the drivers for each was carefully designed to be a ported subwoofer driver or a sealed subwoofer driver. You won't see TCA taking identical drivers and just putting them into a ported or sealed cabinet for marketing and convenience purposes.

Here are the output levels from the ported and sealed designs, both from comparable size boxes in external dimensions, with the ported being a bit larger. The ported sub would have a 500 watt amp and the sealed sub a 650 watt amp. Pricing to the end user would be the same.

Again, the figures are in 1 meter GP measurements...

Frequency ........ TCA 15 Ported ...... TCA 18 Sealed
15 ........................103 dB .................102 dB
20 ........................109 dB .................107 dB
25 ........................111 dB .................113 dB
30 ........................115 dB .................113 dB
40 ........................116 dB .................118 dB
50 ........................116 dB .................120 dB
63 ........................116 dB .................122 dB
80 ........................116 dB .................123 dB

If we made decisions based on how loud each sub was on a single, 20 Hz sine wave, the 15 inch design would win. Let's say, though, that we need to deliver a 110 dB signal at 50 Hz.

The 18 inch sub would require less power to deliver the 50 Hz signal than would the 15 inch sub (less than half the power), leaving more headroom for delivering simultaneous power at lower frequencies. In our tests here, using program material, the 18 inch sealed subwoofer sounded much more powerful on every scene than did the 15 inch ported subwoofer. This is why we elected to put the sealed 18 into production over the ported 15.

Our design goal for the 18 inch subwoofer was to match the output and sound quality of the JL Audio Fathom 113. If one checks out the test results on Home Theater Shack, one will see we did a pretty solid job on the output.

The sound quality issue will be answered when people get the chance to hear this sub, starting at our GTG in November, 2009. I can say I am enjoying this single 18 immensely, even though our 4500 cubic foot room really needs 2 of them, or the dual 18 which is due out early 2010.

[craigsub]

The Dual 10 Inch sub (CS-210) ...

The dual 10 Inch sub is not designed to be an output monster. It's designed to give those looking for a great "all use" sub a large piece of high end sound with a solid home theater ability. Even so, it's still pretty potent. Here are the numbers on it (again at 1 meter) ...

20 Hz - 97 dB
25 Hz - 101 dB
30 Hz - 104 dB
40 Hz - 110 dB
50 Hz - 114 dB
63 Hz - 116 dB
80 Hz - 117 dB

When one considers the room and boundary gain in a smaller room, this sub is going to answer a lot of questions for those needing compact and potent.

[craigsub]

Thread is now open for discussion. With 3 more posts reserved, this should be enough room for my blathering. :biglaugh:

If anyone wishes to post, for example, his own ideas regarding what is important in a subwoofer, please feel free.

[craigsub]

Also reserved.

[Kevin_Wadsworth]

This is an good thread, Craig. Thanks for posting your thoughts.

I think your noting the importance of the 50 Hz "slam" range is important. I hadn't been quite satisfied wiht the sound of my DIY sub in my theater, in spite of very good measured performance and a ton of output at 20Hz and below. When I did a frequency sweep with the mains (an embarrasing omission), I found an interaction that was causing a ~10 dB drop from 50 -60 Hz. I haven't managed to fix it yet (adding 10 dB of EQ boost doesn't sound like a good idea), bu tI suspect that improving this regios will go a long way towards improving my subjective subwoofer quality.

[craigsub]

Here is a model comparing an 11 Cubic foot sealed project that will be our 3rd subwoofer vs. a 14.35 cubic foot ported sonotube project using the Creative Sounds SDX-15 driver ...



When comparing these two subwoofers, here are the numbers:

Frequency .......... SDX SonoTube ........ TCA Sealed
15 ...........................112 dB .................. 108 dB
20 ...........................112 dB .................. 113 dB
25 ...........................112 dB .................. 117 dB
30 ...........................113 dB .................. 120 dB
40 ...........................114 dB .................. 124 dB
50 ...........................114 dB .................. 127 dB
63 ...........................114 dB .................. 129 dB
80 ...........................115 dB .................. 130 dB

Let's say, for example, we needed to hit 110 dB at 50 Hz. The large, single driver ported subwoofer will require 200 watts to deliver 110 dB @ 1 meter GP. The sealed sub design from TCA, even with a smaller footprint, will only require 35 watts. The ported sub uses a 500 watt amp (it could handle more - about 1000 watts, which I would recommend the individual use, as the extra 3 dB of headroom would come in handy, but I wanted to use a real world build that was being done, based on graphs from Home Theater Shack.), which means 40 % of the available amp power is used delivering the 110 dB (20% if the larger, 100 watt amp was used) vs. 2.3 % of the amp power in the sealed design.

Taking this a step further, room gain rarely happens at 50 Hz, but DOES happen at 15 Hz.

Let's assume the subwoofer has to deliver 115 dB at 50 Hz, 115 dB at 35 Hz and then attempt to deliver 105 dB at 15 Hz, all simultaneously. The Sonotube will require 650 watts at 50 Hz and 750 watts at 35 Hz. The sealed sub will need 100 watts at 50 Hz and 300 watts at 35 Hz.

Even if the Sonotube sub had the 1000 watt amp, which subwoofer will have more headroom for the 15 Hz signal ?

Of course, in the real world, both subs are going to try to hit all three levels ... the sealed sub would be able to deliver, simultaneously, over 105 dB at 15 Hz, over 115 dB at 35 Hz and over 115 dB at 50 Hz.

The Sonotube would not be able to do so.

Before anyone thinks I am picking on the idea of a DIY project, I am not. In the DIY realm, if one wanted to use the SDX-15's, I would put a pair of them in a sealed cabinet (10 cubic feet internal) and use a solid 2000 watt amp. This yields an excursion of 30 mm @ 20 Hz and a much better overall performance picture.

Why Room and Boundary Gain Is Important To Consider when Looking At Overall Performance.

Room gain and boundary gain are not easy to predict. Each room is going to be different. Kevin Haskins of Exodus Audio and DIY Cable fame has posted some graphs of the predicted output of his new 21 inch driver in several different configurations along with his "best guess" regarding what one can expect in a typical room. Another way of putting this is it would be an "average" of several rooms.

The purpose of this is to show why it is so important to be able to deliver linear output IN ROOM rather than in free space. Linear output is not a frequency response curve, but rather total SPL one can expect. Below is a simple intrepretation of the in room graphs Kevin posted vs. the modeled graphs of the anechoic measurment. What we will show is anticiapted room/boundary gain starting at 80 Hz and ending at 10 Hz.

80 Hz: 4 dB
70 Hz: 4 dB
60 Hz: 4 dB
50 Hz: 5 dB
40 Hz: 7 dB
30 Hz: 9 dB
25 Hz: 11 dB
20 Hz: 12 dB
15 Hz: 15 dB
10 Hz: 18 dB

Next, let's take the SonoTube project and bump its amplifier to 1000 Watts, and see what the expected in room output will be for each subwoofer, using the room gain listed above ...

Frequency ............. SDX Sono 1000 watts ............ TCA Sealed
10 ............................. 119 dB ............................. 120 dB
15 ............................. 130 dB ............................. 123 dB
20 ............................. 127 dB ............................. 125 dB
25 ............................. 126 dB ............................. 128 dB
30 ............................. 125 dB ............................. 129 dB
40 ............................. 124 dB ............................. 131 dB
50 ............................. 122 dB ............................. 132 dB
60 ............................. 121 dB ............................. 133 dB
70 ............................. 121 dB ............................. 133 dB
80 ............................. 122 dB ............................. 134 dB

Using these "typical" (and again, I cannot stress enough each room will be different) variations, when considering the somewhat smaller sealed subwoofer will still require 1/10th the power in the "slam region" from 40 to 80 Hz, which subwoofer is going to deliver a more satisfying experience in one's room ?

[craigsub]

reserved

[a-rone]

This is the kind of stuff I really like about this audio community. I love learning about these kind of things from people far smarter than I am in regard to audio. Thanks for sharing your insights Craig. I look forward to learning more from each of you posts.

[Mac 11700]

Hey Craig..

What is important to me in a sub is accuracy..and speed..

Nothing more distracting listening to a sub that makes a near single explosion sound like rolling thunder..or muted like it happened miles away..

Same for music..I want a bass drum to sound like a bass drum..and not a snare..

How would you cope with these requirements.?

Mac

[craigsub]

Mac - The issue of speed in a subwoofer has been debated for years. Some will call it how "crisp" the subwoofer is. Rest assured, both our 10's and 18's will have no problem being as delicate in terms of speed as one could want while being able to "Bring the rain" when a movie sound track calls for it. There are reasons why our subs sound so good, but rather than post why, I will let your ears HEAR that it is true. :brainiac:

[Mike_TX]

Great thread!
Thanks.

Mike

[Mike_TX]

"Crisp" factor

Mac - The issue of speed in a subwoofer has been debated for years. Some will call it how "crisp" the subwoofer is. Rest assured, both our 10's and 18's will have no problem being as delicate in terms of speed as one could want while being able to "Bring the rain" when a movie sound track calls for it. There are reasons why our subs sound so good, but rather than post why, I will let your ears HEAR that it is true. :brainiac:

Craig -

I wonder if Mac was asking about any measurable value to describe that.
I think the ears tell it best, but to your knowledge, is there any surrogate marker for the "crisp" factor or a graph that displays it.

Thanks again,

Mike

[woofersus]

I always thought of that being a product of "control" of the driver, or the ability to not only move the driver's mass to excursion limits, but to stop it and return it to "home" position without any delay or overshot. I think sealed boxes tend to better in this regard because of the air pressure "cushion" that keeps the cone movement in check. The other side of that is that the resistance the cone has to overcome is variable depending on its position within its excursion range, which can lead to greater distortion. Ported boxes can do a good job with this, but proper tuning becomes paramount. I like the idea of an 18" sealed box (especially with output number like what are posted above) but I do wonder about the increased driver mass and its effect on control.

That said, I can't wait to hear it. I hope it's all that and a bag of chips.

[craigsub]

Mike (and Mac) - There are many factors which play into the "fast" and "slow" sound of subwoofers. The most common issue is the in room response curve. We will be discussing the response curve for our sealed subs in the near future. Kevin Haskins (DIY Cable and Exodus Audio) has some great quotes in a several page long document on Home Theater Shack.

The basics are this: In a typical room, the response is going to rise below a certain frequency. In a "perfect" or "lossless" room, this room gain will be 12 dB per octave. In a real world situation, look for 6 to 9 dB per octave in a closed room and 3 to 6 dB in a room open to the rest of the house.

Let's take a room where the largest dimension is 18 feet. Room gain will start at 32 Hz. We must also factor in some boundary gain in addition to room gain. If a subwoofer measures flat to 20 Hz outside, it will likely have an elevated response below 40 Hz, requiring the EQ of this curve down to eliminate a boomy sound.

We are deliberately setting the curve of the sealed subs to gently roll off (think about -6 or -7 dB at 20 Hz) in an anechoic (outside) setting in order to facilitate a flatter in room response.

As an aside, one of the reasons for doing this is we wanted to allow for the built in EQ system (ie Audessey, YPAO, etc ...) in today's receivers to be able to handle making for a flat response curve without requiring something like the many outboard units on the market today. The outboard units will be the "ultimate" in a flat, in room response (along with room treatments), but it was important to us that the "newbie" was able to take a TCA subwoofer, put it into his system, and get great sound using what he already owned.

Other areas which will make for a subwoofer sounding "crisp" will be items we have already touched on such as higher efficiency drivers with lots of headroom in response areas which are not aided by room gain.

The differences in "fast" or "slow" sounding subwoofers (from quality companies we all know) is far more determined by the in room response than any factor of a driver's being "faster'.