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Spec Wars: Looking Inside Loudspeaker SPL Specifications

While it might be possible to attain stated max levels from that box, it'll probably only happen once... and it might catch fire.

By Christopher Grimshaw July 10, 2017

Image courtesy of JAKO5D/Pixabay.com

The past decade has seen a significant rise in the deployment of active loudspeakers, mostly 2-way designs housed in molded plastic cabinets. With modern processing and plenty of class D power, it’s possible to get remarkably good sound quality from these lightweight models.

They will also go surprisingly loud, with manufacturers routinely claiming in excess of 130 dB from a single loudspeaker! With that in mind, let’s take a closer look at some of these claims and examine whether they’re plausible or indeed possible.

There will also be some tech talk, mostly relating to loudspeaker cones and what happens when feeding several kilowatts into one.

But first, a couple of definitions.
Xmax: a loudspeaker cone’s linear travel. Different manufacturers define it in different ways, but it’s generally accepted that a given loudspeaker will start sounding bad when driven past Xmax.

Xmech: the mechanical limit of a cone’s travel. Permanent damage (torn suspension, smashed voice coil, folded cone, etc) is extremely likely if a loudspeaker sees enough power to hit Xmech.

Current State Of Affairs

There are currently a lot of manufacturers, and they’re all competing for your cash. If there’s a simple, easy-to-grasp number that makes their product look better than others, they’ll try to inflate that number as much as possible to increase sales.

Here’s a car analogy: the maximum sound pressure level (SPL) rating of a loudspeaker is similar to the maximum speed of a car. The problem is that it’s very easy to measure a car’s maximum speed – there’s a dial in front of the driver telling him how fast he’s going.

On the other hand, the maximum output of a loudspeaker is much more difficult to measure because it depends on a significant number of factors, including the test signal itself. Most of the time we tend to trust the manufacturers and don’t try to verify performance claims.

What if I told you that Car A has a maximum speed of 160 miles per hour (MPH), and Car B has a maximum speed of 170 MPH? Car B is faster, right? However, would you still want to buy Car B if the engine immediately caught fire when it reached 170 MPH?

This is the sort of thing we’re talking about when a standard 12-inch, 2-way loudspeaker has a stated SPL of 136 dB. Indeed, it might be possible to attain that level from that box, but it’ll only happen once, and it might catch fire.

In the interest of not naming names (since there are many loudspeaker companies that play this particular game), I’m going to set out an example system that utilizes quality off-the-shelf components, powered with sensibly-sized amplifiers, and then see how it might compare to the specifications produced by the industry at large.

Example System

To help save the environment, I’m not going to do the destructive testing myself. Instead, let’s run some simulations.

The 12-inch midbass cone driver I’ve chosen is a proven unit from a noted manufacturer. It has a 4-inch voice coil and comes with a stated 1,000-watt continuous power rating, 98.5 dB sensitivity, Xmax of 7.3 millimeters (mm) and Xmech of 26.5 mm. It’s a very good driver and can be found in top-end active loudspeakers.

The high-frequency portion is a 1.4-inch compression driver with a 3-inch pure titanium diaphragm. It’s rated for 110 watts of continuous power, and has 109 dB rated sensitivity when loaded on a 90- by 40-degree horn. We’ll choose our amplifier so there’s plenty of headroom for short-term peaks, delivering 2 kilowatts (KW) to the cone driver and 220 watts to the compression driver.

How does it stack up? Just based on the sensitivity of each driver and how much power is available for it, the cone driver will produce 131.5 dB and the compression driver will produce 132.5 dB. That’s a pretty good match. We could round it off and say the loudspeaker can do 132 dB across it’s bandwidth.

However, let’s take a closer look. When we load the cone driver into a ported box (which is what most active loudspeakers use) tuned to 50 Hz, the sensitivity drops to about 94 dB at low frequencies, which means a direct hit on our maximum SPL rating – it drops to 127 dB. That’s not good, particularly when competitors are saying their cabinets will do 10 dB more.

It gets worse when we put some hefty bass through the system.

Bass ports help drivers over a very narrow bandwidth, where they reduce cone excursion in exchange for air moving in and out of the cabinet. Above the tuning frequency, the driver has to do the work on its own. By 66 Hz, the port action has pretty much stopped, and with 2 KW input, the cone has to move 15 mm one way. While that won’t destroy the driver outright, it won’t sound good – we’re at twice the driver’s Xmax.

If we want to remain within the driver’s linear region, we can only hit 120 dB, which will present huge problems for the marketing team.

There will also be problems with the port itself. Even with generously-sized ports (a pair of triangular ports, 4 inches along the short edges), there will be a lot of port compression, where the driver is generating so much pressure that the air in port itself overloads and begins to experience turbulence.

To avoid port compression, we need to keep the speed of the air in the port below 34 meters per second as a maximum. Here, the air speed will be around three times that – the port will be seriously compressing and making a lot of extraneous noises. This results in the loss of even more SPL at the bottom end.

One last thing, and it’s big: all of these figures are derived in half-space, meaning that the loudspeaker is positioned on the floor or against a wall, which provides reinforcement for the lower midrange and bass frequencies.

Once the loudspeaker is placed on a stand, there will be even lower SPL at those frequencies. Exactly how much depends on the loudspeaker’s positioning, so for now it’s best to table it for now, to be considered another time.

Even without considering the impact at lower frequencies, our example system falls short of the claims made by a good many manufacturers, despite using quality components and plenty of power.


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About Christopher

Christopher Grimshaw
Christopher Grimshaw

Christopher Grimshaw is a long-time audio professional and the founder/owner of Grimshaw Audio, based in the U.K.
http://grimshawaudio.com

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