DIY Speaker Measurement Part 1.
Setting up an old laptop for speaker measurement resulted in a lot of research and findings. I am summing it up in a series of blog posts for the ones looking for a getting started with speaker measurements for dummies kind of guide. Please keep in mind that it might not be 100% accurate from a strict academic point of view. But it will hopefully be accurate enough to get you going and figure out the rest along the way.
The basic idea is to measure speakers yourself with the help of a measurement microphone and a computer running some measurement software. I am using an old laptop with Windows 7 since a majority of the software I am planning to use are written for Windows. There are many commercial applications but I will focus on free alternatives like HOLMImpulse, Sirp and REW. But first an introduction to the additional hardware needed.
I will use the following lines and colors based on the connection type:
Red is for analog signals, green for digital signals and blue for power. The analog signals are of different levels:
Microphone level is the voltage of signal generated by a microphone. This is the lowest, or weakest, level signal of the four and requires a preamplifier to bring it up to line level.
Instrument level signals fall between mic level (lower) and line level (higher) signals. These signals refer to any level put out by an instrument, commonly from an electric guitar or bass. A preamplifier is required to bring the signal up to line level.
Line level signals are the highest level signals before amplification. This is the type of signal that typically flows through your recording system after the preamplifier stage and before the amplifier that powers your speakers. The two types of line levels are consumer and professional.
- Consumer line level is rated around -10 dBV and is what you’ll find in products like a CD player.
- Professional line level is rated around +4 dBu and can be found in equipment like mixing desks, preamplifiers, and signal processing equipment.
Speaker level signals are post amplification. After a line level signal enters an amplifier, it exists to the speakers at what is called speaker level. These signals are much higher in voltage than line level and require speaker cables for safe signal transfer.
Warning: You should never plug a speaker level signal into a source expecting anything less than a speaker level signal.
The speaker in the picture below is labeled DUT and it stands for Device Under Test. It takes a speaker level signal, radiates sound waves recorded by the measurement microphone. The output signal from the microphone is microphone level.
In the next picture a computer is introduced. It is using an internal sound card that accepts line level input and output. This would work but is far from ideal. A regular sound card is not built for professional sound equipment like balanced XLR and 6.3 mm jack connectors. The environment inside a computer can also be a quite hostile environment for low level analog signals.
