In the last few months, I’ve been lucky enough to try all three of the new Core series of monitors from Dynaudio. The Core 7 proved to be a very capable 2 way and the Core 59 thoroughly spoiled me, reminding me that big and 3 way outperforms smaller and two way, though of course, you have to pay for it. The Core 47 really hit the sweet spot for me, both in terms of performance and size and during the test article I referred to my expectation that the Core 47s and a pair of the new Core Subs would make a really interesting setup. I did audition a very impressive installation 6 Core Subs in a 5.1 system with Core 59s and Core 47s at NAMM and the performance was truly trouser flapping! I was intrigued by the specifics of the design and in conversation with designer Stephen Entwistle, I dug a little deeper into what lay behind the Core Sub.

Low frequencies aren’t complicated to produce, the problem is that the best way to create bass is to use large drivers in large cabinets and for most users that’s impractical. What the majority of users want, or are prepared to accommodate is extended low frequency performance at reasonably high SPLs from a reasonably small cabinet, the 2x18” cabinets typical of touring PA systems are bigger than most can use. To create loud, deep bass from a small cabinet is possible but very difficult to do well. Why is that?

Small Drivers

The Core Sub uses four 9” drivers in a very sturdy sealed box measuring 390 x 365 x 514 mm (W x D x H), that’s roughly 15”x15”x20” if you don’t speak metric. A 9” driver is relatively small for a driver handling sub.

I’m sure we’re all aware that in order to create sound a driver has to displace the air in front of it. it moves out and compresses a volume of air equivalent to the area of the driver and the distance the driver moves. The opposite is of course also true as the driver moves inwards to rarify the air. This in and out motion creates sound. In the same way, as in an engine cylinder, the displacement is a function of the bore (driver diameter) times the stroke (driver displacement). To displace the same amount of air from a smaller driver the driver needs to move further. As Stephen says:

“Bass performance is typically dependent upon volume velocity; so you can have a large driver with low excursion and a small driver with high excursion and achieve the same volume velocity. As you’ve seen, the 9” Core Sub driver has quite a high excursion capability.”

To move at the same frequency the smaller driver has to move faster. This introduces problems which don’t exist for larger, slower drivers. To see this as a bad thing would be to miss the point, it is a consequence of making a sub of the desired size. It isn’t a fault but it is a challenge. If you are prepared to make a sub the size of a van you can make many of these issues go away but you are still left with the issue of having a sub which would be unacceptably big.

Small Cabinets

The bass performance of any driver/cabinet combination is largely dictated by the resonant frequency of the cabinet and the driver. I was interested in the choice of a sealed box for the sub considering the Core 7, 47 and 59 all use ported cabinets. I’m on record as being a big fan of sealed boxes as their time performance is better but ported designs can extend the low frequency response below the resonant frequency of the driver as the tuning port begins to radiate sound just where the driver begins to lose output. This can improve both efficiency and low end frequency response. So why not use a reflex cabinet on the Core Sub? Stephen says:

“We chose to port the Core ‘main speakers’ to maximise SPL performance and frequency response. We typically use sealed enclosures for our subwoofers to avoid port noise which can be very problematic in subwoofers, especially if you want to play ‘low and loud’. “

So by using a sealed box cabinet instead of a ported cabinet, you lose out on some low end and some level but the transient performance is improved and port noise is eliminated. However, the smaller the cabinet the more difficult things become. An ideal speaker behaves identically during both the positive (forwards) and negative (backwards) halves of the duty cycle. The problem is that the air in front of the speaker is free to move but the air behind the speaker is contained in a cabinet and acts like a spring. The smaller the cabinet the more effect the compliance or “springiness” of the air has on the driver. if the driver moves differently backwards compared to forwards, that will be a source of distortion.

 “High excursion diaphragms and small boxes mean we need a robust driver, the compression and rarefaction of the air inside the cabinet can be quite significant and strains the driver differently to a traditional system. This is the reason for choosing an aluminium cone for the basis of our ‘hybrid drive’ technology. The diaphragm is a sandwich construction of; MSP (the large dust cap), aluminium and paper.

“MSP is Magnesium Silicate Polymer and is our proprietary cone material. We use it as the dust cap and extra stiffness/damping control for the diaphragm, which contributes to its size and shape.

“This follows our mantra for using MSP as our woofer/mid-woofer cone material, it gives us a very robust diaphragm with a good balance of stiffness and damping for our renowned sound quality.”

So the driver needs to be particularly stiff because of the pressure difference created by the springiness of the air. Stiffness can be engineered in but usually introduces more mass to the driver which already has to move further and faster than a large driver in a large cabinet because of the long excursion. In a less efficient sealed box design, this means you need more power to get the same output. A light, stiff and well-damped material is the ideal.

Small Magnets

Because we are putting smaller drivers into a smaller cabinet, that is sealed, we are sacrificing efficiency in order to make a smaller sub which isn’t ported. As long as there is sufficient power available to drive the system, then getting a suitable output should be just a matter of bigger amplifiers, shouldn’t it? Well, power is easier than it used to be because modern class D amplifiers are so powerful and so good. The Core Sub carries two 500W Pascal amplifiers. Does throwing power at the problem make it go away?

Yes and no. Lots of power is, of course, a good thing but smaller drivers inevitably have smaller magnets and voice coils than larger drivers. All that power produces heat and in a sealed box cooling the heat will ultimately affect performance. Larger systems not only produce less heat but they are better at dissipating it. While you could have extremely large magnets on the back of a smaller driver, there is a practical limit to how far this can go as the magnet will inevitably restrict airflow behind the driver.

In the Core Sub the need for powerful amplifiers and robust drivers is increased because the design uses the driver characteristics to extend performance below that which a cabinet of this size would naturally support:

   “Because we are using the drivers below the system resonance, it is important that they behave like ‘voltage followers’ so the amplifiers have control over the movement of the diaphragm. To achieve this, we use a relatively large magnet and a four layer, 2 ½”, copper voice coil in the construction of the motor system, which gives us a high BL (force factor). The drivers themselves are also relatively compliant to minimise loss when used in this fashion.

“A good high power amplifier enables you to control the drivers movement more effectively and push the excursion further in the small box. Class D simply makes these amplifiers more efficient at high power levels.”

Multiple Subwoofers

After something of a deep dive into the practicalities of making a small subwoofer, hopefully, you can see that making a small sub is difficult. If things are so much easier, if you just make your sub big, isn’t that the best way to go? Well, that does rather depend. If your room is too small to practically accommodate a large sub then that is one reason that is very difficult to get around. Of course, if the room is small, then the effect it has on the low-frequency performance of your monitors is likely to be significant in itself and this is one area where having multiple, smaller subwoofers might be an advantage.

A smaller sub also allows individual units to be used to complement the low-frequency performance of other Core speakers. The DSP built into the Core range makes good integration between for example a Core 47 and a Core Sub as simple as moving a switch.

The Setup At NAMM

The demonstration I had at NAMM illustrated the strengths of the system perfectly. The 6 Core Subs looked like overkill but they were all there for a reason. The drivers, being mounted on the side and having such long excursion, gave away some of the setup straight away. Running as a theatrical system the bottom three subs were running LFE. As Stephen says below, by using three subs, extra headroom is achieved and by spacing the subs the contribution of room modes can be minimised compared to using a single, larger sub in its place. The top three subs were extending the, already significant, low-frequency extension of the three Core 59s at L, C and R and the surrounds were handled by a pair of Core 47s.

“We wanted to make a compact subwoofer that on its own would work as a complement to a stereo pair of Core speakers, but when used in a cinema/immersive setup would be easily scalable to reach the required SPL, i.e. to use the Core Sub at 3 m and have the extra 10 dB LFE headroom compared to the centre channel, you simply use three Core Subs.”

“Using multiple subwoofers can also help control room modes by positioning the subwoofers to force the room to behave in a particular way, i.e. placing multiple subwoofers equally spaced across the front wall can help stop width modes from forming.”

How Does The Core Sub Sound?

I’ve explored the design decisions involved in making a high spec subwoofer, using the options taken by Dynaudio with the Core Sub to illustrate these choices but I haven’t said much about the experience of hearing the Core Sub. It’s kind of meaningless to assess only the bottom octave of a system without reference to the rest and while I didn’t spend a long time with the Core Sub, I was very familiar with the other elements of the system, having already tested the Core 59 and Core 47.

I principally look for two things in a subwoofer. The first is an effortless extension, it has to make it sound easy, to give the impression that it’s not about how low can it go but how low do you want? The other thing a sub needs, is something I got too used to working in live sound: Bass transient. The bass needs to hit hard and stop fast, without that there might be lots of low-end energy but it never has the sense of size, which make the real impression.

I’m happy to report that the Core Sub has both of these characteristics. The demo bordered on overwhelming at a couple of points but rather than just beating me up with bass in the way lots of cinema trailers seem to, the musical content has all the subtlety and agility I’d expect of a mastering setup. The Sub sounds great and more to the point when you consider how hard it is to make sub-bass from a compact box, it sounds better than it has any right to.

I said at the end of my test of the Core 47s that I thought the Core Sub would turn what was an impressive compact monitor into a really impressive main monitor. Having heard the Core Sub I’m now sure I’m right.

Core Sub Specs

• Woofer: 4 x 9in

• Inputs: Analogue and AES3 digital inputs (no sample-rate conversion)

• Resolution: Maximum 24-bit / 192 kHz, depending on input signal

• (32, 44.1, 48, 96 & 192 kHz sample rates supported, analogue runs at 192 kHz)

• Max SPL: 120 dB

• Specification sheet for Core part 1

• Frequency response (- 6 dB): 13.5 Hz – 200 Hz

• Frequency response (+/- 3 dB): 15 Hz – 165 Hz

• Box principle: Sealed enclosure

• Amplifiers: Pascal Class-D amplifiers: LF 2 x 500 W

• Dimensions: 390 mm, 365 mm, 514 mm (W x D x H)

• 15 3⁄8in, 14 3⁄8in, 20 1⁄4in (W x D x H)

• Weight: 48 kg (105.8 lb)

• Price $4000