PROFESSOR POE'S ALMANAC

ON YET another wet and brisk February morning, Professor Poe was to be found in his kitchen, a cup of coffee in one hand and his definitive books on speakers in the other. Around him lay a long line of notes. Poe made another cup of coffee and started to write the title Understanding Studio Monitors Of The Dynamic Type. Under this, he wrote as follows . . .

Essentially Studio monitors try to give you the best of both worlds, sound something like your average home hi-fi, have the strength to cope with high level signals for maybe 15 hours a day and yet have the delicacy to respond to all the subtle nuances that the music has to offer. Most good hi-fi speakers will offer the latter but the constant high levels would soon put them into early retirement. So let us now look at the designs behind them.

Unfortunately this is all a very inexact science, partly because there are so many designers out there with their own idea of what reality sounds like, even to the degree that different cultures have a bias towards certain sounds. Secondly the interaction with the mixer, the room and the amplifier all play an incredibly important role in the whole process. That is one of the reasons why a recording in the studio sounded fine but then when you finally get the tape home it sounded like it was buried under 15 duvets. A studio that has sorted out its monitoring system is well on the way to having something useful to offer.

But on to the speakers themselves. One of the first big problems with studio monitors is that to hear the bass properly the speakers themselves have to be big. This creates its own problem as the sheer size loads up the room; then if you get a bigger room you then need bigger speakers. So let us ask the first question, why have such big boxes, or any boxes at all.

1. There are two main reasons why loudspeakers are put into boxes. The first is because the speaker has to be supported by something and a baffle or a box is as good as any. Secondly the sound which is coming from the back of the speaker is totally out of phase with the sound from the front. If the two meet up then you get severe cancellation of sound. So the reason that the bass disappears when you take a speaker out of the box is not because the box creates the bass, it is because the box stops the front sound from hearing the back.

2. Designers have learnt how to harness the out-of-phase energy which is trapped in the box. First you have to get around this bit of information. All speakers have a resonant frequency. This means that if you ran a waveform up the range you would find one particular frequency at which the speaker will start to resonate. So if you make a hole in the box and tune it like an organ pipe to the resonant frequency of the speaker you will create a situation where the rear sound will travel out of the tuned pipe and come back in phase and augment the front. This design can offer deeper bass but at the expense of being a little uncontrollable at the bottom end as at certain frequencies there is insufficient loading of the speaker. (Good designers have gone a long way to overcome this problem.) Incidentally, the drivers for this type of design tend to be of a stiffer tension than infinite baffle designs. Infinite baffle systems use a long throw driver (large back and front movements) that is kept taut by the fact that it is in an air tight box. This design offers a clearer, tighter bass end but has a weaker bass end than tuned port systems. Designers of small speakers usually use the turned pipe method as a way of getting more deep bass sound for less enclosure.

3. Most of the first two chapters are relevant to the bass side of the sound. In two-way systems the movement of the upper speakers is so negligible that they hardly need to be in a box at all and in some designs the upper mid and tweeters are just sealed off at the back and suspended over the bass driver. Also, because the highs are hitting ears sooner than the bass they sometimes stack the drivers slightly offset so as to hit the ears in the same time array. Most hi-fi speakers use passive or fairly simple crossovers to separate the bass middle and top. In Studio Monitors, it is fairly common to do this with an active crossover. The price for this is that you need three stereo amps as well as the crossover.

In a large scale studio system you might see 2 x 15 inch for bass, 2 x 5 inch for mid and 1 x 1 inch for high. For Nearfield the 6.5 inch or 8 inch for bass mid and a 1 inch for highs is the norm. Amplifiers tend to be in the 100 watt plus variety. The theory here is that it is kinder on the speakers to feed them a nice clean signal from a high-powered amp than a distorted one from a small amp working overtime and possibly going into clipping. It is the clipping that generally takes out the tweeters. There was a time when horn drivers were popular in the studio but in recent years soft domed drivers have taken over. These offer lower distortion and sweeter sound than their horn loaded counterparts.

There are also other types of speakers that work on a completely different system called Electrostatic designs. These can be quite beautiful in sound, but unfortunately don’t move too much air and don’t handle high volumes for too long and so, if used at all, stay mostly in the classical arena. The only other thing that comes to mind is the Q factor, but we will cover that again. n

• Next issue: Placement, Soundproofing and getting rid of muddy bass with studio monitors.