Measurement Systems

An integrated loudspeaker measurement system is one thing and measurement software is another. Companies that sell lab equipment for loudspeaker measurements offer a complete set of devices that -in most cases- do not cooperate with other -commercially available- hardware.

Such integrated measurement systems today are PC-based and apart from a calibrated microphone they include an audio interface, an amplification unit, cables, a software controlled rotating base (for off axis directivity measurements) and software for various OS (operating systems). In most cases several purchasing options are given allowing for different hardware configurations or units to be sold.

The advantages of such a measurement system is the increased quality in terms of measurement noise, system endurance, system upgrade-ability, repair-ability, technical support and printed documentation.

If a DIYer or any other speaker designer wants to purchase such an integrated measurement system, it would be a nice idea to check for purchasing options that allow for a separately bought microphone and amplification unit. This would lower the cost without compromising overall quality. Of course attention must be paid to the following two issues:

1. the microphone should be calibrated (stated sensitivity value, SPL response stored in a file) and its data should be easily imported to measurement system's software. Microphone's connection to the rest of the measurement system should be realized through very-low-noise cabling and plugs with proper ground shielding.
2. the amplification unit should meet the standards specified by the measurement system manufacturer in terms of delivered power, cutoff frequencies, minimum gain, gain flatness (sometimes not mandatory), maximum total harmonic distortion and noise level. For acoustical measurements other than those involved in loudspeaker projects, these standards may be prohibitive. For example measuring a DAC module involves very low noise signals and system manufacturer has validated the amplifier included in the system's configuration. Selecting a separately-sold amplifier for such applications is not an easy task and will definitely not lower the cost! However for ordinary loudspeaker measurements a quality low-cost amplifier can be easily found and purchased.

Current lab equipment manufacturers offer both types of measurement: MLS and SineSweeps (or Chirps).

• MLS uses a noise-like stimulus with energy equally spread within the measuring frequency range.
• SineSweeps use a tone stimulus with time-varying frequency that 'sweeps' the measuring frequency range. Its energy is concentrated at its instantaneous frequency.

Both techniques have their advantages and disadvantages in the field of acoustical (SPL and phase responses, harmonic distortion) and electrical measurements (impedance, harmonic distortion). They should both be available by such a system without direct or indirect limitation.

Another measurement operation offered is the 'SPL response splicing'. An SPL response acquired in the low-frequency range with increased echo suppression in a large space can be spliced together with another SPL response ranging above 300-400Hz, acquired under less echo-protected conditions. It is a common procedure either for a speaker company's design engineer or a DIYer (having access to a large space). For response splicing to be successfully applied it is important to have detailed documentation and practical software commands supporting it. It is not always the case so relative comments of other people in a discussion forum might prove helpful.

There are also another two very important issues about integrated measurement systems:

1) Operating systems evolve in time and within a few years could make the PC you use along with your measurement system become outdated. For example a measurement system based on Microsoft Windows XP may work fine but attaching a newly-bought printer to it may prove impossible: most currently available printers are not accompanied by Microsoft Windows XP drivers. Installing current software to such a PC could also prove impossible.

It would be nice to have your measurement system manufacturer clearly define a number of years for which upgrades would be available so that a modern PC could replace the outdated one. Unfortunately a manufacturer can not promise future OS compatibility with current hardware. It is the client's responsibility to plan future steps concerning the PC used along with a measurement system.

2) Lab equipment often operates under adverse conditions and damage could be brought to either audio interface or the amplification unit. Even if short-circuit or overload protection were guaranteed, repairing the faulty unit should be a clearly available option with turn-around-time and repair terms more-than-clearly defined.

Therefore when we want to buy an integrated measurement system we must first carry out a careful investigation of what exactly is offered and supported by lab equipment manufacturers. In such a comparative study lowering the overall cost should not be our first priority!