Bass reflex alignments explained – Step by step
What are the ported box alignments ?
The bass reflex alignments are like a description on how the box will sound like. A good analogy would be with the sealed boxes. If you haven’t read the article, I encourage you to do so, as it will make more sense. But I will briefly explain it anyway. To calculate the volume of a sealed box, you first need to make up your mind which Qtc you want. Qtc is a factor of Qes, Qms and the Q of the box. Since Qes and Qms are speaker qualities, and are fixed, only the Q of the box can be altered, by modifying the volume of the box. In a reverse fashion, by knowing which Qtc you want, and considering that Qes and Qms are given by the speaker manufacturer, you can calculate the volume of the box.
Sealed vs bass reflex alignments
Here are the sound characteristics corresponding to different values of Qtc for sealed boxes :
- Qtc = 0.5 : Perfect transients, but low efficiency.
- Qtc = 0.707 : This is the number most people try to reach for, as it gives good transients and flat response with minimum cutoff.
- 0.7 < Qtc < 1.2 : Better efficiency, somewhat degraded transients, steeper roll off.
- Qtc > 1.2 : High efficiency, bad transients, bad frequency response.
Since, the ported enclosure is more complex than the sealed box, it is not as easy as choosing a value of Qtc. Instead, there are several well known bass reflex alignments, from which you can choose from. I made the analogy with the sealed enclosure, because it is easier to understand, but for bass reflex, the process is much more complex. Making slight variations for Q in a sealed box, will result in a minor alteration of the response. However, for bass reflex, can cause notable increases or decreases in the bass region, which are referred to as “misalignments”. Ported boxes have much steeper roll-off, and if confronted with misalignment issues, can cause serious transient ringing.
So, to determine to volume of the box and the size of the vent, you first need to choose a certain alignment. Because different drivers have different parameters, and therefore different drawbacks, you need to choose an alignment that compliments your driver, so that you will get, more or less, a flat response.
Types of bass reflex alignments
There are two basic categories for bass reflex alignments, one of which extends into two other categories :
involve an active electronic filter equalization, to achieve the desired response. This type of alignment is not particularly popular and we shall not focus on it. Instead, we shall concentrate our attention to the unassisted bass reflex alignments, which do not need extra electronic devices to achieve a predicted response. Because of this simpler design, it is more widespread.
Unassisted flat alignments
generally requires values of Qts lower than 0.4, and are divided into 6 categories :
- SBB4 or Super Forth-Order Boom Box – is characterized by a large box, low tuning frequency (longer vent) and good transient response (which puts the term boom-box out of place).
- SC4 or Forth-Order Sub-Chebyshev – about the same enclosure size and f3 as the SBB4, but with different tuning frequency. Somewhat degraded transients compared to SBB4.
- QB3 or Third-Order Quasi-Butterworth – is the most popular vented alignment, because it yields a smaller box and a lower f3. However, the transient response is not as good as SBB4 or SC4.
- B4 or Forth-Order Butterworth.
- BE4 or Forth-Order Bessel.
- IB4 or Butterworth Inter-Order.
The last three bass reflex alignments are called discrete alignments, because they exist for only one single value of Qts. These are quite difficult to obtain, because the box losses affects the value of the alignment. Out of the three discrete alignments, BE4 has the best transient response.
Unassisted non-flat alignments
are generated using a higher value for Qts. These bass reflex alignments tend to have an inferior transient response and frequency response. For this reason, they are not suited for high-end audio applications. However, if the negative parts are not an issue, the non-flat alignments can reach lower values of f3. These alignments are split into 3 categories :
- C4 or Forth-Order Chebyshev – can be useful for low values of ripple (less than 1 db).
- BB4 or Forth-Order Boom Box – has a peak in response close to roll-off, similar to high Qtc sealed boxes (1.2 or higher).
- SQB3 or Super Third-Order Quasi-Butterworth – is a high value Qts extension of the QB3 alignment.
Before we explain the bass reflex alignments, we need to talk about box losses first. When you are manufacturing a box, you have to take into account that there will some air leakage, or other factors that will change the result. All of these factors combined describe the box losses. You will encounter three main types of losses :
- Air leakage – QL.
- Absorption from damping material – QA.
- Vent losses – QP.
Total losses (QB) is the sum of all 3, which add together in the following fashion :
1/QB = 1/QL + 1/QA + 1/QP
In a real world scenario, where the sound dampening material would be absent, or 1″ of wall lining, QA would be minimal. Also, considering that the port is not obstructed, QP is negligible as well. So, when we are talking about box losses, we are talking about box leakage, which is QL. As a result, different amounts of QL, will affect the frequency response in a different way.
Adjusting box losses
When constructing a box, you will have to take account the box losses, or more to the point, the leakage (QL). So, before you start your build, consider that the box will have a normal amount of leakage, which is QL = 7. After you have completed the box, you will have to measure the box losses and see if the value is close to 7. If indeed QL is around 7, then you can congratulate yourself. Otherwise, you will have to take corrective measures.
Discovering that QL is lower, means you will have to make the box bigger. This usually means that you will have to make the box all over again. If QL is higher, then you will have to make the box smaller. This is done pretty easy, by placing solid objects (rectangular pieces of wood) inside the box, to make the net volume lower. If you are afraid of the low QL result, you can over-volume the box, and after you measured the box losses, you can reduce the volume by the appropriate amount.
How to measure box losses?
You cannot predict how much losses you will have for a predefined enclosure. As a result, only after the box is complete, you can see how lossy it is. To measure the box losses, you will have to make some measurements for the speaker and the enclosure. After you get those values, there are several calculations that you will need to do. Since it is pretty difficult to write all of those formulas, I prefer to add an excel spreadsheet, where you input the needed values and QL is calculated for you.
Excel spreadsheet for calculating QL : Calculate QL
Explanation of terms inside the spreadsheet regarding the enclosure :
- fb – resonant frequency of the box.
- fH and fL – The impedance curve of the enclosure will show 2 peaks. Note the frequencies that correspond to those 2 peaks. fH represents the value for the bigger peak and fL the value for the smaller peak.
- R0 – the impedance at fb.
How to calculate box size using bass reflex alignments ?
First of all, get acquainted with this table in excel format : alignments table. Now follow these steps :
- Choose the alignment you would like, and go to the appropriate table.
- Then go to the QL = 7 column, since it’s the typical loss figure.
- Find out the Qts of your driver, by measuring it, or look for it in the tech sheet.
- If Qts is lower than 0.4, you will end up with a flat alignment. If it’s higher than 0.4, the alignment will be non-flat. You can also tell by the peak-db or ripple-db from the table (if it’s 0, it is flat). On the same row with your Qts value, you will have : H (tuning ratio) , α (the system compliance or box volume ration) , f3/fs (the f3 ratio).
- Find out the Vas and fs of your speaker, by either measuring it or by looking it up in the tech sheet.
- Calculate the volume of the box : Vb = Vas / α .
- Calculate the tuning frequency : fb = H * fs .
- If you want to calculate the f3, you can do that by taking the f3 ratio value from the table and multiplying it by fs. Calculate f3 = (f3/fs) * fs .
- Choose a diameter for the port. The bigger the better. Half of the diameter is the radius (R).
- Calculate the length of the port : Lv = [(14630000 * R2) / (fb2 * Vb)] – (1.463 * R) . Length and radius are in inches and volume in cubic inches.
Let’s do a real world example, for an 8″ woofer with the following specs :
Qts = 0.52 ; fs = 47 Hz ; Vas = 7.86 L
- Alignment chosen : SQB3 (since Qts has a high value, it’s a non-flat alignment).
- Looking in the middle column where QL = 7 .
- Qts = 0.52 .
- H = 0.8116 ; α = 0.1971 ; f3/fs = 0.6835 .
- fs = 47 Hz ; Vas = 7.86 L .
- Vb = 7.86 / 0.1971 = 39.88 L (2433.63 cubic inches) .
- fb = 0.8116 * 47 = 38.14 Hz .
- f3 = 0.6835 * 47 = 32.12 Hz .
- Choosing the diameter to be 4″. This means R = 2″
- Lv = [(14630000 * 4) / (1454.66 * 2433.63)] – (1.463 * 2) = (58520000 / 3540103.24) – 2.93 = 16.53 – 2.93 = 13.6″
So for our 8″ woofer we got :
- SQB3 alignment.
- Volume of the box 39.88 L .
- Tuning frequency 38.14 Hz .
- f3 = 32.12 Hz .
- Port diameter 4″ .
- Port length 13.6″ .
Similar to Qtc from sealed boxes, the bass reflex alignments work in a similar fashion. It’s like choosing your response curve before you even made the box. Unlike the closed enclosure, bass reflex has an additional element (the vent) and it’s not as simple as choosing a Qtc value. Driver Qts is an important variable in this equation and will determine whether the alignment will be flat or non-flat.
- Loudspeaker Design Cookbook 7th Edition by Vance Dickason (Audio Amateur Pubns, 2005).
- Image source : link.