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LAB MEASUREMENTS
Introduction.
Laboratory measurements of loudspeaker drivers are necessary for speaker system design. They involve theoretical background of humans engaged and measuring equipment. Thus many people believe it is a lost case for them.
However things are different. During the last years lab equipment is not as expensive as we think it is. With less than 500 USD one can get a quality measuring device that actually works as a computer peripheral; a PC 'addon' device. Surely if one intends to build a speaker system NOT just once, the price of this equipment will not discourage him or her.
In the past there has been a long discussion on the space requirements of an acoustics' lab environment, leading to the conclusion that quality measurements can only be carried out in a very large hall rather than in a conventional living room. The low frequency tail of acoustic measurements is jeopardized in typical 3m room height. Fortunately there are more than one alternatives to overcome this bad situation. In fact these alternatives have become mainstream practice even for industry engineers needing repetitive lab access and assessment of design progress.
-What about theoretical background ? Is there any off-the-shelf alternative too ?
No there isn't. Theoretical knowledge is a prerequisite that can not be exchanged with automatic setups of measuring devices. Measuring the gain response of a filter or the SPL response of a speaker system is not like taking pictures with a portable digital camera set to 'AUTO'. Fortunately there is a level of engineering math accessible by most high school graduates that can make things easy provided that one wishes to work for it.
Having spent years in the lab myself, taking electroacoustical measurements with different hardware (older Bruel & Kjaer gating system, MLSSA, PRAXIS) and for different reasons (speaker design, research on sound diffraction, audio amp power assessment, harmonic distortion), I will attempt to provide the required math substrate in a series of simplified articles and worked out examples.
I am confident that mixing several extensively illustrated examples with mathematical principles will successfully compensate for this site's visitors' lack of previous engineering background.
Material is divided in the following sections :
- Responses And Impedances As Complex Transfer Functions.
- Transfer Functions As Sine Sweeps in Reverberant Spaces.
- Transfer Functions And Impulse Responses.
- Impulse Responses As MLS Noise in Reverberant Space.
- Improving Low Frequency Accuracy in Reverberant Spaces
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