Thursday, July 5, 2012

Optimum Clarity

As mentioned in my previous blog on Formula One Race Cars, there is a lesson to be learned about the well understood weight-to-horsepower ratio in making something go fast. If something is very light AND there is a lot of power behind making it move, then it will go fast. The lighter you can make something, the faster it will go with the same pushing force. This is a fine lesson to apply to audio speakers and amplifiers.

From this lesson, it appears that all you have to do is buy a really big power amplifier to create optimum clarity.  But there is a limit as to how much pushing force (power) you can use with your speakers. There is a physical constraint as to their power-handling ability and adding more amplifier power can literally blow them up. So what is an audiophile to do? Let's focus on what musical peaks are first and then look at what can be done about them.

Speaker Specifications
Two pieces of data are supplied by speaker manufacturers: the sensitivity of the speaker and the power handling capacity. This tells you two things: how loud they will go and how big of an amplifier you need to obtain optimum clarity. Let's first look at a typical acoustic suspension speaker with a 1-Watt sensitivity of 85dB (usually stated as 85dB/W/m) and you have purchased an amplifier rated at 128 Watts RMS.

At the 1-Watt level, the speakers deliver 85dBa of sound pressure (at one meter) into the room and at this level the amplifier has a power reserve capable of producing a maximum SPL of 109dB. This means that the dynamic range is the difference between the loudest it will go and the level at which you listen, here 109-85 or 24dB.  If you turn up the volume control for an average listening level of 91dB, your speakers are demanding 4 Watts of power from our amplifier and this translates into a dynamic range of 109-91 or 18dB. So here's the catch: people normally listening to music at loud levels require a system capable of handling and delivering a lot of power if you want to maintain a wide dynamic range (optimum clarity).

The amount of power demanded by your speaker doubles with every 3dB increase of sound pressure, so for a speaker with a 1-Watt sensitivity of 85dB, the power demands go approximately like this:


As you can see, power demands for low sensitivity speaker get pretty rediculous once extreme sound pressure levels are approached.

The Listening Position
Question: What audiophile listens to music at one meter from their speakers? Answer: None. So the 1-Watt SPL of 85dB is lower at the listening position depending upon how far away you are from the speakers. The SPL drops off exponentially according to the Inverse Square Law, meaning that the SPL drops off with the square of the distance. For example, if the 1-Watt sensitivity of your speakers is 85dB at one meter, at two meters it is 1/4 that SPL, three meters 1/9 , and four meters 1/16.

Putting it another way, if your speakers have a maximum SPL of 109dB at one meter, at two meters it will be about 103dB (-6dB down), at three meters about 99.5dB (-9.5dB down), and four meters about 97dB (-12dB down). The maximum SPL you can hear from your system at your listening position is lower the further from the speakers you listen.

If your listening position is four meters from your speakers, the loudest you can hear them go is a function of the [Manufacturer's Rated Maximum SPL] minus 12dB.

For example, if your speakers have a sensitivity of 92dB/W/m with a power handling capacity of 150 Watts, and your listening position is four meters from the speakers, then the following is true:
  • Maximum SPL @ 1 meter = 107dB
  • Maximum SPL @ 4 meters = 95dB
You may want your system to go louder but you have a dilemma: it is physically impossible to sustain levels much above the 95dB level at the listening position.

How to Resolve this Dilemma
As mentioned earlier, you could buy a bigger amplifier and hope that you do not toast your speakers one day in a breathtaking but brief experience.  I would not recommend doing this.  Despite your best intentions, someone will one day twist the volume knob on your preamp all the way up after which you will be searching for replacements.

Another way to do this is to bi-amplify your speakers. Now the sound pressure will not change but the clarity will improve giving you the impression that your system can go louder. What's happening is much like several people carrying a heavy chair instead of one person struggling with it alone: it makes carrying the chair easier when two people do it and so does multiple amplifiers attached to your speakers. It's the same chair (same weight) but you've added twice the people (more power) so the load is distributed making it is easier to carry.

Using multiple amplifiers also allow you to choose amplifiers better matched for the job. You have all heard an amplifier that sounds great everywhere except the bass so use such an amplifier for the higher frequencies and find another that better suits your taste at lower frequencies. When done properly - and with a quality electronic crossover to assist in maintaining clarity - your system will respond accordingly. With bi-amplification, you can also have your passive crossover network components removed and the internal wiring modified thereby eliminating the phase distortions and insertion losses introduced by these components.

So there you have it. Another mystery explained and debunked.  Stay tuned for more along this line of additions in the infinite search for high-end audio perfection.

Yours for higher fidelity,
Philip Rastocny

I do not use ads in this blog to help support my efforts. If you like what you are reading, please remember to reciprocate, My newest title is called Where, oh Where did the Star of Bethlehem Go? It’s an astronomer’s look at what this celestial object may have been, who the "Wise Men" were, and where they came from. Written in an investigative journalism style, it targets one star that has never been considered before and builds a solid case for its candidacy.

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