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For beginners - Why do your loudspeakers have holes in them?

Take off the grille cloth of your loudspeakers and you will see that they have holes, with tubes extending inside the cabinet. Why? What would happen if you blocked them up?


It is more likely than not that your loudspeaker has a 'port', which clearly is there for a purpose and not for decoration. But first, why are the drive units mounted in a cabinet?

The answer to this question is that the sound radiation from the rear of the drive unit would otherwise cancel out the sound from the front, at low frequencies. So although the drive unit is producing a lot of bass energy, much of it is canceled out and wasted. The function of the cabinet therefore is to contain the rear radiation from the drive unit, and preferably dispose of it (easier said than done). A fully closed cabinet is known as a closed box, acoustic suspension or sometimes infinite baffle, although some people use the term infinite baffle in a different sense.

The advantage of the closed box is its relative lack of resonance - when the signal stops, the sound stops. But as we know, most loudspeakers are not of the closed box type because they have a port and are not completely sealed. Adding a port to the cabinet turns it into a Helmholtz resonator, named for German scientist Hermann von Helmholtz.

A Helmholtz resonator consists of an enclosed volume of air connected to the outside world by a narrow tube. A beer bottle has this shape. And, as you know, you can blow across the neck of the bottle, when it is empty, and produce a musical note. This demonstrates the property of resonance, that the air inside the bottle will vibrate easily at a certain frequency, given an energy input.

It is the same with the bass reflex loudspeaker cabinet, which has a port. The air inside the cabinet will vibrate readily at a certain frequency. The frequency at which the air inside the cabinet vibrates is determined by the volume of the cabinet and the dimensions of the port. So what is the benefit of this?

The answer is that small and medium sized loudspeakers have an inadequate bass response. But if the cabinet is tuned so that the air inside vibrates at a frequency where the natural response of the low frequency drive unit is just starting to diminish, then the cabinet can 'help out' and extend the bass response down to a lower frequency than the closed box. So a bass reflex speaker has a better bass response than a closed box of the same size.

The disadvantage of the bass reflex is that it is resonant. It has to be to work. And not only does that make frequencies around the resonant frequency louder, it prolongs such sounds in time. So for example, a kick drum heard through a closed box loudspeaker will be nice and tight. But heard through a bass reflex it will trigger the resonance and there will be significant output at the resonant frequency, which will continue for a fraction of a second even after the kick drum beat has gone. The sound of the bass reflex loudspeaker is often described as 'boomy', and poorly designed examples can display a significant 'one-note bass' effect.

It is an interesting experiment to block the port and compare the sound. This doesn't turn the bass reflex into an ideal closed box because the parameters of the cabinet will not be optimized for the low frequency drive unit. Even so, the reduction in bass output is interesting to hear. It may seem therefore that bass reflex loudspeakers are not suitable as monitors because they color the sound in the bass and do not accurately reproduce the signal. However, most of the world listens to music on bass reflex loudspeakers, so it is useful to monitor on a similar type so that the engineer gets a flavor for what the listener will eventually hear.

In short, the closed box loudspeaker is more accurate, but the bass reflex has a better low frequency response, at the expensive of some boominess.

By David Mellor Thursday June 28, 2012

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