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Why Passive Radiators?

By jbagby

05
Feb
2013

The Salk SoundScape speakers use dual passive radiators. These are obviously more expensive than port tubes, more complex to tune and require more cabinet work to implement. So why use them?

As the designer of the bass section of these two new speakers, I thought I would explain why passive radiators are perfect for this application, and why SoundScape series speakers will out-perform most other high-end speaker systems, and even many subwoofers, in low bass extension and output while, at the same time, providing extremely low distortion bass.

Achieving excellent low frequency performance is much more complex than simply using a speaker design program and arriving at a nice woofer/box alignment curve on the computer screen. This is a good place to start, but on its own it fails to address some critical aspects of performance. Unfortunately, many speakers are designed this way and that’s where it ends. As a result, the speaker’s low frequency performance meets specifications at low input levels of just a few watts, but when the volume is turned up, sometimes at levels as low as 50 Watts of input power, both low end extension and distortion are quickly compromised. This happens more often than you may know, even among high-end speaker systems. Let me explain why this is, and how we resolve this issue in the SoundScape series.

How it works

Low bass performance is all about pressurization. Since the speed of sound and density of the air are stable enough in our listening environments to be treated as constants, we find that low frequency SPL (Sound Pressure Level) becomes directly proportional to a speaker’s volumetric displacement (how much air it can move) at a given frequency.

Simply put, the sound volume a woofer can generate is directly related to the amount of air it can move at a given frequency. And the lower the frequency, the more air it is required to move in order to create that volume level.

This is where the problem lies with many speakers. It models exceptionally well at 1 Watt per Meter, but at just over a few Watts it can become limited in its ability to move enough air to maintain higher SPL’s at low frequencies. Consequently, as the volume goes up, the effective bass cut-off frequency (its -3dB point) rises too, and usually so does the driver’s distortion because the voice coil begins to leave the magnetic gap, thus increasing the distortion. Often these speakers are ported to increase efficiency in the low bass, especially near the port tuning frequency. Unfortunately, this quickly becomes a source of limitations as well.

The first thing that must be considered in resolving the restrictions is the design of the woofer itself. Not only does the woofer need to be optimized for bass extension in its parameters, but it must be capable of moving a lot of air with very low distortion. The woofers used in the SoundScape series are custom made drivers with aluminum cones, very long linear strokes, and have specially designed motors that significantly reduce electrical distortion. These woofers are capable of a linear stroke of +/- 24mm (Xmax one-way), or approximately 2” linear peak to peak excursion.

The Soundscape 12 woofer is optimized for a “vented” enclosure and the alignment chosen has an F3 (-3dB bass cut-off frequency) of 18 Hz. As you know, most vented woofers use ports to tune the box resonance and augment the low frequency response. However, a woofer of this design forces us in a new direction for solving the limitations of simple ports. Here’s why – It moves a lot of air, and thus is capable of a much higher SPL at very low frequencies. But, this means that the port must be able to displace high volumes of air as well without creating noise or inducing flow restrictions that reduce the bass output, and what it takes to do this correctly may surprise you.

The Port Problem

Let me explain with some numbers. It is not unusual, even on many high-end speakers, when using a single 12” woofer, to use a 3” diameter port. In some cases this may be an acceptable port size if the woofer itself is not capable of displacing a large volume of air, or if the system cut-off is fairly high in frequency. However, neither of these two restrictions sounds much like a true “high-end” speaker system and, in the case of the SoundScape 12, the goal is very clean bass to below 20hz (18Hz, actually) and the capability of achieving this at a fairly high SPL if required (110+ dB at 20Hz).

So, how will the 3” diameter port perform? Let’s take a look. In order to tune the enclosure to 20Hz the 3” port needs to be a little over 15” long. That appears doable. So, what’s the problem?

The general rule of thumb is for port air velocity to be kept below 5% of the speed of sound (approximately 17.3 Meters/Second) in order to ensure that audible “chuffing” noise does not develop along with restrictions that could limit bass output. Unfortunately, as typical as a 3” x 15” port is, in this application it reaches 17.3 M/S of air speed at only 20 Watts of input. Suffice it say, I am sure Jim and SoundScape owners would expect better performance than this.

What if we use two 3” ports? OK, let’s see. Two 3” ports need to be almost 33 inches long each in order to tune to our 20Hz frequency. However, they still reach 17.3 M/S of air speed at 80 Watts. Hmmmmm…. How about using two 4” diameter ports? Well, these won’t hit 17.3 M/S until 250 Watts of input. That’s more like it, but on the other hand they need to be almost 59” long each. Now, I need to fit two 4” diameter, 5 foot long tubes in the cabinet. If that wasn’t enough, this length results in a strong port resonance at 87 Hz, and that should be a no-no too. But, is 250 Watts enough for a state of the art speaker? Should port design be robust enough that we never really approach their limitations on the system? If so, then two 5” diameter ports will take over 500 Watts before hitting our 17.3 M/S air speed output. But again, they will also be almost 8 feet long. Are we getting a feel for the limitations here? In order for the port to be capable of output without air speed and possible noise issues it must be large in diameter, but if it is large in diameter then it must be very long – too long to fit in a reasonably sized cabinet. (Keep in mind that the internal volume of the cabinet must be increased to take the volume of the port tubes into consideration.)

The answer? Enter the Passive Radiator

Passive radiators work on the same principle as a port, even though that may not seem intuitive. The air inside a sealed box is compressible, but then “springs” back. This is called “compliance”, and mechanically we treat it as a spring. Any spring will have a resonance frequency. If you hang a weight on the end of the spring it will be oscillate slower, or at a lower frequency. Increase the weight further and you lower the frequency further.

In our vented box, the resistance of the “slug” of air in the port acts like a weight attached to our box compliance “spring”. The greater the resistance, the heavier the weight, the lower the resonance frequency – and you can do the same thing with a passive radiator (basically a non-driven cone) by attaching actual weight to it until the system resonance reaches your target tuning frequency. Makes better sense now, doesn’t it?

Now, I know what some of you are thinking: “I’ve heard some passive radiator systems in the past and they sounded boomy or muddy in the bass, and I didn’t like the passive radiator sound.” Hey, I hear ya, but things have changed. Early passive radiators were simply driver cones and suspensions without motors attached. In fact, most still are today. The problem is that these are difficult to tune low enough to reach the proper tuning frequency and they often are very limited in their mechanical parameters as well. In addition to these restrictions many designers opted to leave the system underdamped with a peak in the midBass so you could “”hear” their passive radiator and its “robust” bass.

But the passive radiators we use are different. They were designed from the ground up to be passive radiators and not just woofer cones. They are capable of handling large amounts of mass while their suspensions remain linear and centered. They also have very large linear excursions of almost +/- 30mm. To tune these radiators to 20Hz results in over 1 kg, or almost 2.5 pounds, of moving mass on the sides of the enclosure.

Two radiators, rather than one as you often see, are used for two very important reasons. First, two provides twice the surface area as a single radiator, thereby increasing maximum output before restrictions set in. And second, with this amount of moving mass, if it was on only one side of the enclosure it could create a “rocking” issue. However, if one is placed on each side of the enclosure then they will move out together and in together and all rocking inertia will be cancelled out.

Comparisons

So how do the two passive radiators compare to the ports used in the examples above? The two 12 cones have an effective moving diameter of about 11”. With the mass added they behave the same as two 11” diameter ports with an air mass equivalent to a tube 37.9 feet long for each – air velocity is not an issue. The limitation here is the mechanical excursion of the radiator at +/- 30mm each. Calculations reveal that these two radiators reach their linear excursion limits at the 20Hz resonance frequency with a little over 500 Watts of input power. This is essentially the same performance shown above for the two 5” diameter 8 foot long ports, but with no wind noise or line resonance, or the problem of trying to find a place to fit two 8 foot long pipes.

At the resonance frequency, in this case 20Hz, nearly all of the output comes from the port or the passive radiator. If the port cannot move enough air effectively at high SPL’s then their output, and consequently the overall system’s bass output, will be compromised.

Ever hear a speaker that sounds full at lower volumes, but when you turn it up the bass just can’t keep up? We all have. In the SoundScape 12, however, at 20Hz the cone’s motion is critically controlled by the air spring of the compliance of the air in the enclosure; nearly all output is coming from the two 12” passive radiators with their large excursion capability, and these are capable of providing over 110 dB output at 20Hz (116 dB with two speakers) with very low distortion before reaching their mechanical limitations. All this while the cone is barely moving. And this is without any room gain!

Most subwoofers on the market cannot match this level of performance in the low bass. But then again, the drivers and alignments used in the SoundScape 10 and 12 are as good or better then nearly all subwoofers already, and in this case you get two of them.

The SoundScape 8, SoundScape 10 and SoundScape 12 represent a no compromise loudspeaker design. Their performance in the low bass is only one example of this “no holds barred” approach.

Jeff Bagby

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