Designing an Audiophile Loudspeaker Part III: From Concept to Design

by Audiophilia on December 6, 2012 · 0 comments

in Loudspeakers

by Michael Levy

Part 1
Part 2

So, now I have a concept of what I want an audiophile speaker to do. To put it in absolute terms I want it to recreate the sound field recorded by the microphones as accurately as possible. To do this a speaker must simulate a coherent full frequency point source to the ear. How do you do that?

Listening to many loudspeaker designs through the years has confirmed the theory that the shape and structure of the cabinet is very important for the quality of imaging and the smoothness of the bass. The cabinet must correctly tune the drivers and let them meld acoustically while not getting in its own way by creating any audible artifacts internally, externally, or with the room.

In order to understand what is happening, I think of the cabinet interacting with the sound waves it produces in the air as if the sound waves were waves in water. I try to see them as they move across boundaries or reflect off of walls. Of course, I use the speed of sound in air (approximately 1125 feet per second) to calculate the size of the waves at the frequencies I am dealing with. This concept guides me in both the external and internal shape of the cabinet, which are dealing with different parameters that must converge in one design. It also guides me in the choice of drivers.

There are several points at which the speaker cabinet can introduce distortion.

1. Diffraction distortion:
As sound moves along the surface of a speaker cabinet it is turned by edges it encounters. The sharper the edge, the greater the distance to that edge, the greater the effect. This can cause some sounds to pull on to the speaker instead of imaging. It also narrows the width of the imaging field making the center seating position the only place where imaging is good.

2. Panel resonances:
The drivers in a loudspeaker not only move the air, they also move the panels that make up the cabinet. (To every action there is an equal and opposite reaction) This may cause them to resonate at certain frequencies, fogging the image and adding a ‘woofy’ or ‘wooly’ sound. The size, composition, shape, and reinforcement of those panels controls their resonances.

3. Room/Speaker resonances:
Room resonances are caused when the dimensions of a room reinforce a frequency or certain frequencies of sound. Those frequencies will predominate, fogging the image, and often creating a boomy sound. There can also be resonances at a lower level created by reflections off of the speaker cabinet and the walls. All of these will tend to diminish the quality of the imaging and alter the timbre. This can be addressed in the shape and placement of the speakers and shape and damping of the room, and, finally, you can employ equalization.

4. Internal resonances:
The shape of the cabinet should avoid parallel panels internally to control standing wave resonances which could cause some nasty peaks in the response. The cabinet also must be tuned for the driver to smoothly extend the frequency response while maintaining a high speed.

5. Driver melding:
The positioning of the drivers, their bandwidth, the shape and composition of the baffles, and crossover slope and point all effect the quality of the driver melding.

6. Doppler distortion:
Doppler distortion can make a smooth note on massed violins sound slightly sour or cause a vocal to become slightly hard sounding. It can also cause a slight lack of focus of the images. Doppler distortion is created when high frequencies are being output from the same driver as bass frequencies. If a driver is moving at a low frequency while producing a high frequency, the higher frequency is modulated by the lower frequency, causing a constant pitch to go up and down with the lower frequency produced. The distortion can be minimized both by cabinet tuning and crossover design.

By minimizing the size of the cabinet, and using a shape that minimizes diffraction distortion and the interaction of the cabinet with the room, the speaker cabinet can disappear acoustically. By using a wide bandwidth midrange, carefully choosing crossover slopes and points, and closely coupling and matching the speed of the drivers, the ear can be fooled into hearing a point source.

So, it seems that one of our goals should be maximizing performance while minimizing size. But before we work on minimizing the effective size, we need to know what our performance parameters are.

1. Frequency response
I define full frequency output from a loudspeaker as from infrasonic to hypersonic, it must have a usable bandwidth from below 20 to above 21,000 cycles. These are the minimums, the wider the bandwidth the better.

2. Efficiency and Dynamic Range:
Dynamic range is a product of the efficiency and power handling. For example you might have an efficiency of 86 decibels at one meter with one watt of input. If you have a power handling of 100 watts that gives you an output of 106 decibels, and two speakers each at 100 watts would make that 112 decibels. That is quite loud enough for most situations. (ear damage and pain occur at 120 decibels).

2. The Cabinet — Creating the form:
Form follows function, but one more thing, it must look good standing on the floor in a modern listening room. I chose a three driver configuration because it increases power handling while lowering Doppler distortion, and allows response from well below 20Hz to well above 21kHz.

Sorry for the delay with part III. We were recovering from Sandy.

Next: Designing an Audiophile Loudspeaker Part IV: The Final Form: Choosing the Components

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