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 Review, Logitech Z-540 computer speakers
Some thoughts and facts about sound
By: Sverre Sjøthun, November 7, 2002  Print this article


But before we return to the initial subject, the Logitech Z-540, I’d like to share some thoughts and facts about sound. Like I said, to get “proper” sound, you need volume and weight. Generally, the volume in the enclosure will make it possible for the woofer to reproduce the super-low frequencies, whilst the weight, in combination with very specific materials, will lower the resonance frequency in the enclosure.

Ideally, it’s only the driver that’s supposed to make the sound, but materials, enclosure design and damping of the enclosure plays a big role in what it sounds like in the end. With poor materials, little or no damping material and poorly designed enclosures, stationary waves and resonance occur, and it sounds dreadful.

There are many means to combat these problems, but the most common are an enclosure made of MDF(Medium Density Fibreboard) or NRFB(Non Resonant Fibreboard), bracing the inside of the enclosure, proper damping material and asymmetric design.

The adding of damping panels to the inside of the enclosure walls, usually in the form of bituminous pads, does much to reduce resonance hangover, but not much to reduce panel flexing while still being excited by the driver. The flexing is non-linear and the distorted harmonic structure of the total output from the speaker changes the timbre of the sound. Stiffening of the panels reduces the amplitude of flexing and this can be done by a combination of increasing the panel thickness and bracing.

Bracing taken to the extreme by Bowers&Wilkins

Bracing taken to the extreme by Bowers&Wilkins

You can get a pretty good idea about the enclosure quality by simply knocking on it – if it sounds hollow it’s usually not very spectacular. What you want is a dense, tight knocking sound.

The driver units obviously also plays a major role. One driver can not produce every frequency from 20Hz to 20KHz, which is what the human ear can perceive. Again, the keyword is physics. You need a big mass to produce low bass because big mass equals lower resonance frequency. And while adding mass to a driver, you make it slower, so you cannot reproduce the high frequencies. At this point, it doesn’t take much brain activity to figure out that what we need is a tweeter, or at least a midrange driver.

Seas Excel magnesium high-end woofer

Seas Excel magnesium high-end woofer

Now that we have two drivers in our system, we cannot let both play the same signal, as the woofer would play blurry and distorted midrange and the tweeter might end up being fried because of the high-powered midrange and bass signals. We need a crossover to distribute the correct signals to each of the units.

A cheap, very simple yet effective crossover consists of a capacitor and a coil – the capacitor act as a high-pass filter and is hooked up to the tweeter and the coil act as a low-pass filter and is hooked up to the woofer. This crossover will dampen the signal by 12dB per octave, and is a so-called 1. order crossover.

Finally, output is never going to get better than the input, which brings me to the amplifier. Next time you see a small 5Kg amplifier that the manufacturer claims to be 2x300W, laugh at it, cause it ain’t true. It might be true in a given period of time, in a certain frequency range, but it’s true RMS that counts -- true RMS at 8 ohm is the de-facto standard for measuring output power.

ZAPsolute Mk4, a 50W amplifier with a 650W PSU

ZAPsolute Mk4, a 50W amplifier with a 650W PSU

To get even close, you need a transformer, a PSU, that’s capable of delivering 600W minimum. With some yields we’re talking close to 8-900W, and a 900V/A toroidal core transformer tends to weigh 10-15Kg, which explains why good amplifiers easily hit 20Kg or more.

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