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Which loudspeakers are best for accurate monitoring?

An Audio Masterclass website visitor has a dilemma, and a lot of money to spend. Which loudspeakers will serve him best as studio monitors?

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A question from an Audio Masterclass website visitor...

"In your opinion which system would be better for accurate mixing on, a PMC transmission line speaker or a Quested speaker, front ported? I am looking to spend up to $6000 on monitors." - Erik

If you have that much money to spend and you are going to choose from either the PMC or the Quested range, then you are going to have a pair of very high quality loudspeakers in your studio, whichever you choose.

There's an old saying, well actually I just made it up but it's going to be old eventually - the harder the decision is to make, the less it matters because the options are more nearly equally valuable.

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If I could rephrase that as a short, pithy sentence, maybe it could find a place in a dictionary of quotations?

So you might spend a long time fretting over whether you should buy PMC or Quested, but either would be an easy choice if your pockets are deep enough.

The choice is actually more down to the science of loudspeaker design rather than brand. You mention 'transmission line' and 'ported'. 'Ported' normally means 'bass reflex', so I'll use that slightly more common terminology.

The problem all loudspeaker designers have to surmount is what to do with the radiation from the rear of the low-frequency drive unit?

If it is allowed to escape into the air, then it will partially cancel the sound coming from the front. That would be a waste.

Ideally then, the output from the rear of the drive unit should be contained.

OK, so build a thick cabinet around it, thick enough so no sound can escape.

The problem now is that the air that is trapped inside acts as a spring, opposing the motion of the diaphragm.

Such a cabinet - the 'closed box' - can work well when properly designed. But still, it's working against the laws of physics, not with them.

The best way of getting rid of the rear radiation would be to have an infinitely long pipe leading to a parallel universe, to dump the unwanted energy.

OK, you don't need the parallel universe (and would you want to annoy the parallel inhabitants?), all you need is the infinitely long pipe.

Since this is impractical, a long pipe is used instead, usually folded up so that it fits neatly in a cabinet. It doesn't have to be cylindrical. The pipe is lined with damping material to progressively remove the energy. Also, the pipe is left open at the end so that the air inside does not act as a spring.

Clearly there is a lot of careful design and mathematics that goes into the precise dimensions of such a transmission line loudspeaker. But it can work very well indeed and offers a very natural sound, and in addition produces a good level of bass down to frequencies even below the range of human hearing.

The bass reflex has a rather different philosophy to deal with the rear radiation from the drive unit where there is a hole or 'port' connecting the inside to the outside via a short tube.

This tunes the cabinet to a certain range of frequencies and assists in producing more bass.

The problem is that the bass now becomes slightly but significantly boomy around the frequency to which the cabinet is tuned.

So the bass reflex cabinet is inherently less accurate than either the closed box or the transmission line, if all are properly designed of course.

So does this mean that it isn't as good when used as a studio monitor?

Far from it. Transmission line loudspeakers are in fact very rare in the wider world outside of the studio or high-end hi-fi, because traditionally they are big to work effectively. Very few people will ever listen to your recordings on transmission line loudspeakers.

Bass reflex loudspeakers, on the other hand, are very common - almost universal in fact. If you mix on bass reflex monitors, then you will get more of a flavor for what your typical listener will hear.

So, in conclusion...

I'm not going to conclude. Each and every engineer needs to make their own decision, and definitely listen before you buy. Secondly, these issues are extremely complex and you will need to make a choice whether to study for a PhD in electroacoustics if you want to understand them fully, or spend your time more usefully in your studio making music.

Oh yes, and don't forget that your listeners also use car audio, earbuds, headphones, smart speakers, radios, TV sets, portable stereos etc. etc. to listen to their favourite tunes. Your mix has to sound good on all of them.

By David Mellor Sunday September 22, 2019
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