What is Acoustic Power?

The concept of acoustic power is not readily considered by most people, due in part to the industry's preoccupation with sound pressure. To put it simply, sound pressure is the result of sound power. Sound power is the cause, sound pressure is the effect. The goal in driver design is to maximize the amount of acoustic power available from the device, given the constraints and trade-offs between the available parameters. Once the acoustic power has been realized, we then turn our focus to pattern control devices (horns) that will channel this power to a smaller unit area, increasing the sound intensity for that area, increasing the sound pressure (at a given listening position) beyond what it would have been if no pattern control was used.

In many conventional cone-type transducers, it can take as much as 100 electrical watts to produce one acoustical watt, yielding an efficiency of about 1%. At the other extreme, a perfectly efficient transducer (that unfortunately does not and cannot exist) would produce one acoustic watt with the application of one electrical watt. The M4 comes closer to this ideal than any other driver available today, producing nearly one-half watt of acoustic energy from a single watt of electrical energy.

One acoustic watt is equivalent to 107.5 dBSPL at four feet from an omnidirectional source. A device's directivity can be described with a term called "Q" which is an indication of its ability to confine the applied energy to a smaller unit area. Q, therefore, is a parameter of the horn, not the driver. If we take one acoustic watt and couple it to a horn with a Q of 2 (hemispherical), the intensity and therefore the pressure will increase by a factor of 2 to 1 in the area covered by the device. This is quite useful, since it allows more energy on the audience and less on the walls and ceiling. It is common practice to convert the Q rating into decibels by the formula:

How much sound can you get from one electrical watt?
By considering Q in terms of the Directivity Index, the increased sound pressure provided by increasing Q beyond unity (Q = 1) is readily apparent. Let's look at the numbers and get a feel for what a large source of acoustic power and a high Q horn can do for a system design (see figure).

When you consider that a 3dB difference in efficiency represents a two-to-one power ratio, the benefits of the M4 become very apparent. Also consider that the M4 is more efficient in its passband than some conventional mid/hi frequency drivers are in their passband. This means that it can require several high-frequency drivers for each M4 in the system, for flat power response.


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