Dj'ing Convergence

SAE Institute

Liverpool

Written Assignment

How loudspeaker cabinet design affects sound quality.

Student details

Andrew Greslow

12647

AED0907P

15th May 2009

word count: 3327

Introduction

Loudspeakers are used daily in many different sectors of the music and entertainment industry. For those working in live sound, cinemas,radio stations, film studios and of corse recording studios and For musicians who use electrical instruments, the quality of the sound produced by their loudspeakers is of great importance.

If we where to make a perfect recording and listen, back through a standard pair of loudspeakers, what we would be hearing would be a less than perfect representation. This is due to numerous physical and mechanic factors that colour the sound as it is reproduced by the loudspeaker. The design of the loudspeaker cabinet can play a major roll in the quality of the sound produced by the drivers.

This paper seeks to explain how design, size, shape and other factors of a loudspeaker cabinet can effect sound quality.

At this point, it is worth asking the question what makes as a good loud speaker?

Current research

Tests have been undertaken to compare relationship between loudspeaker measurements and listener preference. In these tests, different candidates ( trained listeners and untrained listeners) were required to listen to 4 different types of loudspeaker in a controlled environment. (http://seanolive.blogspot.com/2008/12/part-3-relationship-between-loudspeaker.html - accessed web page on 25/05/09)

Appendix 1: shows the frequency response of the four different loudspeakers, P, I, B, M, used in the testing.

"There was clear visual correlations between listeners loudspeaker preference and the set of frequency graphs," (Floyd E. Toole, 1986.)

The loudspeaker with the flattest, smoothest and most extended frequency response curves ( P and I ) where preferred and the loudspeaker ( M ) having a bumpy bass and holes around 3kHz least preferred. (http://seanolive.blogspot.com/2008/12/part-3-relationship-between-loudspeaker.html - accessed web page 25/05/09)

Appendix 2: shows the listener preference to loudspeaker P,I,M,B.

'The most important thing about any speaker is its frequency response. It is here that designers spend most of their effort, for the wider the frequency response (the higher and lower the musical tones it can reproduce), the more you

hear.' (http://directacoustics.com/wordpress/?page_id=56 - accessed web page on 26/05/09)

The true infinite baffle concept

In his book, Neil Newell explains the concept of the infinite baffle design. He firstly goes on to how a loudspeaker would be placed in an invert flat plane which would stretch out to infinity. Forward radiation is completely isolated from the rear radiation so pressure equalization can not exist. There is also no baffle edges to cause diffraction, nor is there an enclosure walls to generate acoustic or mechanic resonances. If you had the right driver ( drivers ) then the sound reproduced by this system would be excellent. Unfortunately this is a rather unpractical concept. (Newell & Holland, page 64)

There are two practical versions of the true infinite baffle design being used today. The first design is called the 'Finite baffle' where usually a meter squared board is used to mount the driver in. The down fall to this design is that response nulls are created, due to cancelation around the sides of the plane. This creates a third dimensional figure of eight like pattern in free air. The low frequency cut off rate is determined by the size of the baffle. The larger the baffle, the better the low end response will be. This type of loudspeaker isn't used in many recording studios as the rear radiation is incredibly hard to control due to it's open back design. Finite baffle loudspeaker are usually free of resonances making their time domain response limited only by the room that they are placed in and driver used.

(Newell & Holland, page 66)

The second design is called the ' Infinite baffle ' or more commonly known as the sealed box, closed box and acoustic suspension. It is the nearest practical embodiment of the true infinite baffle design, but because of it's Finite nature it's design is flawed. It gives rise to diffraction effects and resonances. Solid angle of radiation is no longer clearly defined loudspeaker. (J.M.Woodgate,Page 304)

As this is design is a completely sealed box, it completely isolates the rear radiation. As the open back deign sounded 'open' this design tends to sound quite 'boxy', but if the correct sized box/driver is used and they are well matched, along with correct dampening the system can be very accurate. This is one of the main reasons why closed boxes have such a strong following. Large closed boxes can be the bases of an excellent loudspeaker system.

The air trapped inside the box reacts against the diaphragm in both directions. This acts like a spring, stiffening the suspension of the driver unit raising the resonant frequency. The smaller the box the stiffer the spring gets, this intern raising the box/driver's resonant frequency. As the resonance of a system defines the frequency at which the roll off begins, low frequency roll off will increase up into the frequency spectrum the smaller the box size used.

This is usually at a 12dB rate below the resonant frequency but depending on the systems Q value the roll off rate could start much sooner, even making it's way in to the musical spectrum.(Newell & Holland, page 67)

To get around this problem a heavier cone can be used or a more flexible suspension. This can also have it's