CODE Acoustics – Why don’t you just make a passive bookshelf speaker, like everyone else?

You know the type; a little rectangular box with a 6.5″ mid/bass driver and 1″ tweeter, ported to extend the bass response and using a 2nd order (12dB) passive crossover.

Good question! It certainly makes the most financial sense; It’s easy to design, produce and transport, and we suspect this is why a lot of our peers go down this route.

However, for us the answer is simple; there are far better technical solution to allow a truer representation of recorded music. Surely this is what it’s all about?

<A quick caveat!

In the article below we outline our proposed solution to better a 2-way compact passive speaker; namely a 3-way DSP active speaker. However, we fully accept there are many ways to ‘skin a cat’ in audio, and there are many fine speakers that have been produced (and will be produced) that follow a different path. That’s cool, and that’s what makes hifi so interesting!>

That’s not to say the little passive bookshelf is all bad.

What are the advantages:

1) Their size means they are easy to place in a wide variety of rooms.

2) They often have prominent (artificial) upper bass, but very little low bass. This is helpful in the sense it doesn’t excite the room modes (to the same degree as a large ported speaker). So you don’t get boomy bass, which will annoy you, and your neighbours.

Sample bookshelf speaker frequency plot:

3) The dimensions are similar to a human head, and a number of studies have suggested, this makes music reproduction sound more natural (we agree!).

4) A passive speaker allows ‘system tweaking’, whereby you can adjust source, amplifier and cables, to obtain that elusive, ‘perfect sound’.

5) Due to it’s simplicity, it’s harder to ‘mess up’ a 2-way design, than it is a 3-way for instance.

So what are the disadvantages?

1) Having a Mid / Bass driver is a compromise, as the driver is required to make delicate precise movements required for midrange, whilst also making large excursions to reproduce bass transients, such as a kick drum. At low volumes where excursions are lower the drivers is performing okay doing both jobs.

However, as soon as you raise the volume, things go downhill rapidly and you run into something called intermodular distortion. Essentially the driver is simply not capable of making these precise movements for the midrange while it’s thrashing around trying to reproduce bass.

Consequently the midrange becomes harsh and ill defined. Your brain struggles to reproduce the signal due to the distortion and the whole listening experience becomes fatiguing fast. Your only solution is to turn down the volume, or pick some different music with little bass. We have certainly come across people who have effectively tailored their musical choices around the limitations of their speakers. For us this is an unacceptable compromise.

2) There is no real low bass due to the 24dB roll off below the port tuning frequency.

3) A front firing port can emit significant amounts of delayed mid-range sound, emanating from the rear face of the driver, causing phase shift, known as time or group delay.

Front ported bookshelf speaker:

4) Cabinets are often thin and poorly braced resulting in the cabinet ‘playing along’. This will give the impression of bass notes having significant overhang (where they should start and stop, they sort of slowly fade away). The sound has been described as ‘warm’ by some manufacturers. We would call it distortion!

5) To produce realistic bass, you need to move a lot of air gently. A pair of 6.5″ drivers will struggle to do this (unless you are in a very small room), even very good ones. Unfortunately you can’t cheat the laws of physics.

6) The crossover point is right in the middle of the most sensitive part of your hearing (1-5kHz, with the crossover at 2.5kHz). There is also no time delay filter on the tweeter, so it’s sound reaches you before that of the mid / bass. Again this is group delay and we have a particularly very low tolerance in this frequency band.

7) Gentle crossover slopes (2nd order or 12dB per octave) are not steep enough, so the mi/bass driver is producing significant levels of treble, when it’s distorting heavily due to cone break-up, and the tweeter is producing too much mid, causing it to make significant cone movements, compromising the upper treble.

8) Passive speakers allow the amplifier poor control over the drive units. This is simply because there are a number of components between the two, be it inductors, resistors or capacitors. In an active speaker the amplifier is connected directly to the drive unit.

Summary

So whilst there are some advantages of a passive, compact 2-way design, I hope we’ve shown that there are also some significant technical deficiencies.

So is there a better alternative?

Yes, most definitely. We would advocate a 3-way active design, which overcomes a large number of the deficiencies of a passive 2-way.

At Code we make a DSP controlled, active 3-way speaker called SYSTEM-1.

CODE SYSTEM-1

Using a subwoofer with your 2-way book-shelf speakers will solve a couple of the issues raised above. We’ll do a separate article comparing this setup with our 3-way DSP active speakers. We still think the 3-way’s are the way to go!

So below we’ll discuss in more detail the relative merits of a ‘standard’ passive 2-way vs. our DSP active 3-way design.

We’ll split the discussion into the following sections:

• Passive or Active
• Crossovers
• Mid / Bass or Mid & Bass
• Port vs. Sealed
• Cabinet Construction
• Driver time alignment
• System Tweaking
• Conclusion

Active or Passive?

Just in case you’re confused by the terms ‘passive’ and ‘active’: An active speaker connects an amplifier channel directly to a drive unit, with the crossover placed before the amplifier. Whereas a passive speaker sits the crossover between the drive unit and the amplifier channel.

Active speaker:

Passive Speaker:

This means that for a 3-way active stereo speaker, you’ll need six channels of amplification. Passive speakers whether it be a 2-way, 3-way or how every many ‘ways’ will generally (unless you’re bi or tri amping) use a single amplifier channel per speaker.

This is the cheaper solution, but has some major disadvantages, which I’ll describe in a minute.

The term amplifier ‘channel’ can be confusing. A stereo amplifier for instance has two channels; one for the left hand side, and one for the right hand side. A monoblock has one channel and a 5.1 AV receiver has five (with a line level output for a sub).

A passive crossover is non powered and uses resistors, capacitors and inductors, to separate the input signal into a high frequency signal for the tweeter and a low frequency signal for a mid / bass driver (in a 2-way).

One big issue is component tolerancing. Passive components are all produced to a tolerance (often +/- 5% or worse). So if a crossover contains 10 elements, it’s very possible, and in fact quite likely that the crossover in you left hand speaker is quite different from the crossover in your right hand speaker! This will mean it’s almost impossible to produce a coherent stereo image, where the sound appears three dimensionally, between the speakers. This is the cornerstone of high end hi-fi.

Passive crossover:

Super hi-end passive speakers use very tightly toleranced components that are then pair matched at the factory! This is a very expensive and time consuming process.

An active speaker uses a powered crossover. This can be analogue or digital. Traditionally you used analogue active, based around op-amps. This solution is very component heavy (see pic below), but does have the advantage that the components are far better toleranced than their passive counterparts, so stereo image is less affected.

Active Analogue PCB:

The newer solution, which we use in our systems is digital based, known as Digital Signal Processing or DSP for short. It’s essentially using a computer to manipulate the digital stream from the music, to give the output you desire. An obvious advantage is that tolerancing is a thing of the past. You can make as many changed to the signal, and the left hand processing will be identical to the right hand processing. So stereo imaging will be a lot sharper.

DSP Module:

However, it’s not a completely free lunch. Too much processing can cause it’s own issues, and has blighted early room correction systems. We adopt a ‘just enough’ philosophy!

Directly connecting a drive unit to an amplifier channel (active) of sufficient power gives fantastic driver control, or damping. This is far harder to achieve with a passive speaker, because the amplifier has to control the drive unit indirectly, as the passive crossover is in the way. The active solution results in a more articulate midrange and far snappier bass.

Crossovers

A 2-way speaker usually has a tweeter covering the high frequencies and a mid / bass driver covering the mid and bass frequencies. The ‘crossover point’ is often at 2.5kHz.

Conventional 2-way crossover (2nd order 12dB per octave filters):

A 3-way speaker has a tweeter covering high frequencies, a midrange driver covering the midrange only, and a bass driver covering the bass range only. Our crossover frequencies are 4.5kHz and 250Hz.

Code SYSTEM-1 crossover (4th order 24dB per Octave L-R filters):

The most sensitive part of our hearing is 1-5kHz, so you need to be VERY careful about what your crossover is doing in this range. So putting your crossover slap bang in the middle of this band, as in a 2-way is far from ideal.

The designer of a 2-way speaker is forced to do this however. The 6.5″ mid / bass driver is not acting as a piston in this region, and has real problems with ‘break up modes’. This means it’s impossible for the designer to move the crossover up to the 4-5kHz range. The other solution is to push the crossover down to 1kHz, but this isn’t an option either as this will put too much excursion through the tweeter. So 2.5kHz it is, even though this is far from ideal!

Mid / Woofer breakup:

In contrast, our mid-range driver is 4″ diameter. This means it’s ‘break up nodes’ are far higher up the frequency range than the 6.5″ mid /bass, which is why we can comfortable shift our crossover point to 4.5kHz. Another advantage is our tweeter is working FAR less hard than a tweeter in a 2-way design, partially because of the higher crossover point, and partially because of our steep crossover slopes.

To keep costs down, passive 2-way’s often use 2nd order crossover slopes, which drop at 12bB per octave. Our speakers use 4th order slopes, which drop at 24dB per octave (far steeper).

Filters:

The problem with shallower slopes is similar to that of shifting the crossover point too high or too low. If the slopes are shallow the tweeter ends up producing too much midrange and the mid / bass ends up producing too much treble.

Just as a side note, some designers insist first order (6dB per octave) is the only way to go as phase response is better. True, but when you’ve got significant energy going through your tweeter at it’s resonant frequency (due to the extremely shallow slope), the sound will be horrible at anything other than low volume, regardless of how ‘phase correct’ the signal is! The same goes for the mid / bass. If you take it up to 6kHz and beyond, the cone and surround will be severely breaking up and the sound beaming due to a large diameter cone relative to the wavelengths it’s producing. Again, unpleasant.

The 4th order slopes in contrast mean the tweeter produces very little midrange and the mid-range produces very little treble, which is as it should be.

Mid / Bass or Mid & Bass

Another big issue is with the mid / bass driver in the 2-way. It’s required to produce both mid-range and bass frequencies, which have very different requirements. Mid range frequencies requires very fast and precise SMALL movements of the driver. Bass frequencies require very fast and precise LARGE movements of the driver (think of a kick-drum for instance). The problem is that while a drive unit is working hard to reproduce these bass transients, it’s not very well placed to make the delicate small movements of the mid-range. This is known as intermodulation distortion.

You’ll notice this if you play some bass heavy music on your 2-way and turn the volume up. The vocals will start to lose clarity. Effectively they are distorting.

2-way vs. 3-way:

By contrast, the 3-way has a mid-range driver and a bass driver. So even if there is heavy bass content in your music, the bass driver(s) will be making these large movements, whilst the midrange driver is virtually still, going about it’s duty of reproducing that delicate mid-range. So a 3-way will play significantly louder for the same level of distortion as a 2-way.

As I mentioned earlier, to accurately reproduce bass, you need to move a lot of air gently. A pair of 2-way speakers, with two 6.5″ drivers have a combined moving area of 308cm2. Whereas our 3-way speaker have four 8″ drivers has 848cm2 of area, or 2.75 times as much. Therefore, to move the same volume of air, where our bass drivers would say move 3mm on a bass transient, the 6.5″ mid bass drivers would need to move 8.3mm, which would exceed the linear travel (x-max) of almost all 6.5″ mid-bass drivers. Not only that, the bass wouldn’t sound great (due to the thrashing cone movement) and the midrange would be pretty horrible also.

Port vs. Sealed

Another feature of the bookshelf speaker is it tends to ported.

Ported speakers have a couple of advantages:

1) They aid efficiency, which is desirable when using low powered amplifiers.

2) They reduce driver excursion where the port takes over from the driver.

Ported and sealed cabinets:

However, there are some significant disadvantages:

1) The ports often face forward (on a 2-way book shelf) and a fair chunk of the rear radiating mid-range sound has a habit of exiting through the port. This sound is delayed compared to the direct radiating sound and will cause group delay.

2) Ports have a habit of exciting room nodes, producing bass boom. This is perhaps impressive for 30 seconds, but then becomes quite annoying as a particular bass frequency dominates the music. Ports are often fired forwards to reduce this effect, but then you run into point (1)!

3) The port doesn’t provide very effective damping for the drive unit and gives a speaker with a relatively poor transient response.

4) Bass drops at 24dB per octave after the tuning frequency, which is quite steep. This can produce the effect of a bookshelf appearing to have good upper bass, but virtually no lower bass.

5) Unless the port has a large cross sectional area, the airflow can reach a velocity where it becomes audible, which is called ‘chuffing’. This is a very noticeable type of distortion and highly undesirable.

Sealed speakers also have advantages and disadvantages. What’s interesting though is that because our speakers are DSP controlled and active, it’s possible to negate almost all of the disadvantages:

Advantages:

1) Very good driver damping and transient response.

2) There is no recycled sound, so there are less phasing issues.

Disadvantages:

1) Bass naturally rolls off at 12dB per octave, so although you have more low bass, you give up some upper bass. Well this would be the case for a conventional passive system. However, because our speakers are DSP controlled, we simply apply some low level EQ, so our speaker run flat to 30Hz, without the need for recycled sound.

2) A sealed configuration is not as efficient. True, but we’re not using low powered amps. Our electronics box ‘CONTROL-1’ contains 1200 watts shared between 6 drive units, so a minor loss of sensitivity is not an issue! Our bass drivers also have a sensitivity of 91dB for 1 watt at 1 metre. This is significantly higher than a standard ported 6.5″ mid-bass driver.

CODE CONTROL-1:

Cabinet construction:

Most bookshelf speakers used non braced MDF cabinets with as low as 12mm thick walls. This is not great. You could even end up with a situation at certain frequencies where the box is actually making more sound that the drivers!

To get round this you need a heavily braced cabinet to push it’s resonant frequency well above the bass region. You then also need significant amounts of sometime like visco elastic sheets, which can absorb this waste high frequency energy. Many book shelf speakers simply have a single layer of egg-box foam, which will help a little, but not much.

Our mid / high cabinet uses 18mm thick MDF panels, while our bass cabinet uses 25mm thick panels, with significant bracing. Both cabinets are heavily lined with visco elastic damping pads as well as Melamine foam. Not to mention our bass cabinet also contains four sand voids to further reduce vibration.

Heavily braced cabinet:

Driver time alignment

In a passive 2-way speaker the ‘acoustic centre’ of the tweeter is significantly further forward than the mid / bass driver. So when you listen sitting on a sofa, the sound from the treble is hitting you milliseconds before the sound from the mid / bass driver. It may not sound like much, but our brains are highly susceptible to this type of distortion.

Note the differing distances to the listener:

Humans developed incredibly sensitive hearing to detect predators coming towards us thousands of years ago. But our brain didn’t develop to take a phase incoherent sound and re-process it so it sounds natural.

Our DSP software allows us to simply delay the tweeter and midrange drivers by varying degrees so all the signals arrive at precisely the same time, resulting in your brain having a far easier time. So you’ll be less fatigued whilst listening and ultimately enjoy the music more.

System tweaking

Audiophiles seem to be rather inquisitive by nature and will leave no stone unturned in the pursuit of that perfect sound. In the search for audio nirvana a tweaker can change source, amplifier, speaker and cables with alarming regularity, searching for that elusive perfect combination. Whilst this can be hugely enjoyable, it’s not necessarily the most effective way of achieving a cohesive sound.

How far do you go?

Have you heard the phrase ‘my system just sounds too bright’ before?! Brightness is an elevated treble, in relation to the mid / bass frequencies. Lots of manufacturers tune their equipment ‘bright’ so when you listen to their products at a show or demo, you are instantly impressed by the ‘zing’, ‘air’, detail’ or ‘crispness’. Granted, it sounds great for 30 seconds, but over longer listening periods it’s fatiguing. What’s worse is if you accidentally pair a bright CD player, with a bright amplifier and a bright pair of speakers. That will sound……… very bright!

Firstly, we don’t tune ‘bright’. Our sound system is more or less tuned flat i.e. our sound system is neutral, as it should be. This may not wow you over 30 seconds, but after your ears have readjusted to ‘real’, we think you’ll be thankful.

Going forward, we’ll make a number of different types of speaker, that all work with our control box, so you can experiment with different sounds, without running into the same old pitfalls of matching ill fitting equipment.

Conclusion

If you’ve made it to the end of the article, well done! Hopefully we’ve put a convincing case forward for why a DSP active 3-way speaker is a significant step forward over a passive 2-way. If you have any comments or questions, please get in touch (info@code-acoustics.com)

Thanks for reading.

Ceri Thomas
Managing Director
Code Acoustics

Speakers, Active Speakers, Audiophile, Code Acoustics, Hifi, High Fidelity