Prefab vs Custom sub box vs OEM enclosure
Is it worth it to make your own subwoofer box?
Let’s start by defining prefab vs custom sub box. Prefab box (short for prefabricated) means that the enclosure is made as “mediocre” as possible, to accommodate a large number of possible speakers. Since the builder doesn’t know what exact speaker you will place in the box, it is made with normal guidelines in mind. Basically, you are sacrificing sound quality for convenience. When you make your own custom box, you specifically tailor it to your speaker and to your own needs.
The prefab box will work well with sealed enclosures, because volume is the only factor that influences the overall result. Choose a box that is of similar volume and you are good to go. A small variation of few liters won’t affect sound quality that much for sealed enclosures. When it comes to bass reflex, things start to get a little more complicated. You should always go for a custom box, if possible, when making a bass reflex enclosure. While it is common sense that a prefab box will be inferior to a custom one, lets go a step further and compare manufacturer made / OEM boxes with custom ones.
What exactly are we comparing?
I have an old Hertz subwoofer lying around, so I decided to put it on the test bench. It is a Hertz EBX 200r. Please take note that this model is around 10 years old, and if I am not wrong, 2 or 3 newer generations have been released since then. The contemporary equivalent is this new model. When I bought this subwoofer, back in the day, I was thinking that if I buy it with the enclosure already made by the manufacturer, it is the best box ever. Furthermore, I don’t have to bother making one myself. After I measured the box and speaker, in the present day, I have found out all kinds of curious stuff, that will make you think twice when deciding between prefab vs custom sub box.
To woofer used is the ES200 (of that time), which is an 8″ woofer. It was the most entry level solution Hertz was offering at that time. They were selling either just the speaker (ES200), or with the enclosure made by them (EBX200r).
Here is the actual spreadsheet of the ES200 : HERTZ_ES_200 (pdf file). An let’s take out the useful information :
- fs = 47 Hz.
- Qts = 0.52.
- Vas = 7.86 L.
They are recommending an enclosure with the following specs :
- fb = 37 Hz (tuning frequency of the box).
- Vb = 19.5 L (volume of the box).
- Dv = 62 mm (diameter of the vent).
- Lv = 260 mm (length of the vent).
Right away there is a problem. The dimensions of the port and volume of the box don’t match with the tuning frequency of the box. For the dimensions quoted by them, the tuning frequency is exactly 39 Hz. Probably this was a typing error. Either the tuning frequency is wrong or the dimensions. But judging by how things fit for 39 Hz, and looking at the response graph they give on the spreadsheet, most likely the tuning frequency is wrong. It should be 39 Hz instead of 37 Hz. I’m going to forgive them for this error. If you are following their instructions and make the box by the dimensions they provide, you will get the 39 Hz tuning frequency, regardless if they wrote 37 Hz on their spec sheet.
The OEM enclosure
Enough with the speakers, lets talk about enclosures. Here are some pictures with the OEM enclosure (the EBX200r) :
As you can see, the enclosure is nicely made, with a trapezoidal shape, 2 flared rectangular ports and some handy indents on the sides. These you can use to firmly grip the box while carrying it. The grille looks flimsy in the pictures, but I assure you it is solid.
After removing the grille and speaker, we can see that the inside walls are lined with approximately 1″ of sound dampening foam. The visual aspects at the outside and inside are all fine and dandy, but let’s get to the measurements to see what’s really happening.
Measuring the speaker
First, let’s start by measuring the speaker. We are going to measure the thielle / small parameters of the speaker. It is almost certain that you will get different values compared to those quoted by the manufacturer. Because of the manufacturer process of the speaker, not all of them are created identical. If the glues sticks or dries in a different way, than the compliance is modified, the resonant frequency is modified as a result, and a whole lot of parameters in consequence. However, after measuring the speaker, the numbers are quite a bit off.
Parameters of the speaker : quoted figure first / measured figure second :
- fs = 47 Hz / 41 Hz.
- Qts = 0.52 / 0.45.
- Vas = 7.86 L / 15.98 L.
Observing the impedance chart of the speaker, there are no suspicious peaks or dips in the graph, to indicate that the speaker is not working correctly. Even though the speaker is quite old, it is in perfect working condition.
Measuring the OEM box
After measuring the resonant frequency of the box, instantly, there was another red flag. The tuning frequency of the box is 52 Hz. And the first question that pops into mind is : why would you recommend to someone else to tune the box at 39 Hz, but if you do it yourself, you tune it to 52 Hz? The answer to that question is really a mystery to me. The reason you would want to tune the box that high, is to get a Chebyshev frequency response. A high peak at resonance with a lower volume for the enclosure. By doing so, you sacrifice frequency response bandwidth and non-linearity in the response. A pretty high price to pay.
I thought that I made errors when measuring, but I used two methods to calculate the resonant frequency of the box. One is the nearfield frequency response measurement of the speaker. At resonance, the port emits most of the energy and the speaker barely moves. This will translate in a large dip in the frequency response chart, which corresponds to the resonant frequency of the box. This is a very accurate measurement, because it is indifferent to the inductance of the coil, and was seen at the 52 Hz mark.
For good measure I used another classic method, from the impedance chart. The bass-reflex impedance chart will show 2 peaks. The lowest point between the 2 peaks corresponds to the resonant frequency of the box, which is 50.47 Hz. The 2 numbers are very close to each other, which indicates that the measurement was done right. Since the nearfield measurement method is more accurate, I will use that number : fb = 52 Hz.
I’m showing the impedance chart for the OEM enclosure, but I will show the nearfield response for the custom box. This way, you can see both methods of measuring the tuning frequency of an enclosure.
Taken into consideration all of the above, there are 3 different types of frequency responses that we need to compare :
- The box that they recommend you to make.
- The enclosure that they think they are making.
- What they actually make.
Why is there a difference between these 3? Well, we already saw that there is different fB between the box they make and the box that they recommend. And the difference between the box they think they are making and the one they are actually making, comes from the fact that the speaker has different specs. At least my specific speaker.
Judging from the graph we can draw the following conclusions :
- The recommended box :
- Has the widest frequency response. (33 Hz – 250 Hz).
- Has the lowest amount of ripple (1.3 dB).
- The designed OEM box :
- Has narrower frequency response bandwidth. Because of the large peak, the 3 dB point is not in its usual place. (44 Hz – 250 Hz)
- Has the highest amount of ripple (5.5 dB).
- The actual box :
- Has the narrowest frequency response bandwidth. (45 – 250 Hz)
- Has a high amount of ripple (4 dB).
For a bass-reflex box, 3 dB is the highest you can go in terms of ripple, without compromising linearity too much. Considering that they were aiming for a 5.5 dB peak at resonance, completely baffles me. It will make the subwoofer sound more boomy, but no one will thank you for that. Not with that amount of ripple.
Designing our custom box
The first advantage that we have in a prefab vs custom sub box battle, is that we get to use the measured thiele / small parameters. The measured values are quite different from the quoted values and the result should be advantageous to our cause. When we are designing our box, we have to take into consideration Hoffman’s Iron Law, which states that : between low frequency extension, efficiency and small enclosure we get to choose only 2. Or to be more accurate, if you increase the value of one attribute, you will have to decrease one or both of the other two.
Taking into consideration Hoffman’s law, I am going to prioritize the following :
- Low-end extension with high priority.
- Efficiency with medium priority.
- Enclosure volume with low priority.
Considering the above guidelines the enclosure has the following specs :
- fb = 36 Hz.
- Vb = 44 L.
- Dv = 66 mm.
- Lv = 130 mm.
Taking a look at the response curve, we can describe the box :
- Frequency response bandwidth of 31 Hz – 250 Hz (+/- 3 dB)
- 2 dB of ripple for an improved efficiency, but not too much that it sounds boomy.
- As a drawback, the box is double in size and that might be a deal-breaker for some people. You can adjust accordingly, depending on you needs.
The box design is complete, now let’s start building it. I’m using 3/4″ thick MDF with no double baffle for the front. The speaker is too small for justifying a double baffle, or any bracing for that matter. Before I placed the top on the box, i used silicone sealant for all the joints, inside and outside the box, for the terminal plug and for the bass reflex port. After that, I used carpet to wrap the box. Here are some pictures with the build :
Their box is trapezoidal and that helps with reducing standing waves, because it is more angular. Our enclosure is rectangular-shaped and has more parallel walls, which is not good when it comes to standing waves. To counter this, it is good to use a golden ratio when it comes to the exact size of the length, width and depth of the box. There are several golden ratios out there, and all of them are good, with minor differences between them. I’m using the one suggested by Ben Kok, which is : 1 : 1.12 : 1.41
Measuring the custom box
Pop the mic out, place it in the correct position so we can get our nearfield response of the speaker. Anxious to see how we’ve done with tuning our box.
After measuring our box, immediately there is a “problem”. According to out calculations, the resonant frequency should be 36 Hz. After the measurement, it’s 31 Hz. What ?? How is this possible ? Here is the nearfield response of the speaker :
The dip in the graph corresponds to the resonant frequency of the box, which is at 31 Hz. How did this happen ? Did we make bad calculations or building errors ? The answer is : no, something else happened. When you are designing a box, you have to take into consideration box losses (leakage). It is considered that there will be some kind of leakage (QL). Usually, the calculations are made with QL = 7. But after the box is finished, you can discover that you have high leakage (lower QL value) or very low leakage (higher QL value).
The value is inversely proportional with losses, because losses refer to 1/QL. You cannot estimate this value, and you can only measure it after you finish building the box. Our box has very low leakage (measured QL = 16), since I abused the silicone sealant on every seem of the box. To compensate for this, I need to reduce the volume of the box, by placing some solid wood blocks inside. But I won’t do that. I’m just thankful for the “free box volume”, and leave it as it is. Lower resonant frequency? Yes, please!
The specs of our custom box has changed, but we agreed it’s for the better and leave the box untouched. Let’s move on to comparing frequency response charts.
Prefab vs custom sub box comparison
Prefab vs custom sub box, let’s see how they line up. Let’s start with the frequency response of the OEM box :
It’s always a relief when the modeled response is on par with the measured response. Let’s take the 0 dB mark at 180 Hz. I’m choosing this point because it has the same magnitude as 180 Hz on the custom box chart, and because the response is somewhat linear at that point on both charts. This point on the graph is at -4.12 dB. The top of graph, near resonance is at 0.42 dB. So the response has 4.5 dB ripple. Very close to what we modeled (4 dB ripple).
Measured response :
- Frequency response : 47 Hz – 250 Hz.
- 4.5 dB ripple.
Let’s move on to the custom box :
As you can see from the graph, the response is much more linear. No big hump near resonance. Like I said, we are choosing the 0 dB mark at 180 Hz, which is actually -4.12 dB on the graph (same value as the OEM box chart). The peak near resonance is at -1.4 dB, which sets the ripple at 2.72 dB. Which is not far off the 2 dB of our original calculations. I’m glad it does not exceed 3 db, as it will make the box sound too boomy.
Measured response :
- Frequency response : 30 Hz – 250 Hz.
- 2.7 dB ripple.
- f3 point is very close to f0 since it has a very steep roll-off.
Overlapped charts :
You can clearly see the difference in linearity between the two graphs. Also the frequency response bandwidth is far greater for the custom box.
Pro’s for custom box :
- Broader frequency response bandwidth.
- Lower amount of ripple.
- Improved linearity.
- Decent efficiency.
Con’s for the custom box :
- Volume of the enclosure is doubled.
- The physical aspect of the box is not on par with the OEM one. However, if you have skills in this domain, you can have some impressive results.
- Very steep roll-off slope. However, this doesn’t really matter, since the frequency response goes very low in a linear fashion. f3 of custom box is way better than f3 of OEM anyway.
Choosing between prefab vs custom sub box vs OEM, really depends on your priorities and your needs. Of course, our OEM box had some issues, but that is not always the case with every ready-made box you will encounter. You might not have the necessary tools, or know-how to build you own enclosure and going OEM or prefab seems like the only solution. However, if you want the best and a response suited for you needs and preferences, you will have to go custom. Even if you don’t have the tools, you can ask someone that does this as profession, to make the box for you. A speaker enclosure will always make a compromise between bass extension, efficiency and volume. If you go for a custom box, you get to choose which one of these attributes means more to you and which one you wish to sacrifice.