CHOOSING THE RIGHT SOFTWARE FOR SPEAKER MEASUREMENTS
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 current 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.
On the other hand companies that offer a loudspeaker measurement software allow for:
- the use of any type of microphone provided that its sensitivity is accurately stated and its SPL response is available in an electronic file,
- the use of any type of PC provided it meets some hardware requirements,
- any type of amplifier used for driving the speaker-under-test during lab measurements (provided it is powerful enough and that it's cutoff frequencies lie outside the required measuring frequency range,
- the use of one among several sound cards (usually externally connected through USB or installed in a PCI bus slot of a desktop PC) that have all been successfully tested for the required measuring frequency range and the suggested OS.
Obviously the overall quality of such a measurement system can easily be compromised by the following factors:
- a poor amplifier,
- an unsuitable microphone,
- noisy ground loops,
- sometimes the suggested sound cards confine the measuring frequency range to 20kHz while loudspeaker impedance measurements often need to extend far beyond that limit,
- software can also confront serious problems in OSs other than tested (and/or suggested) by its manufacturer.
If a DIYer or any other speaker designer wants to built his/her own measurement system with a separately purchased measurement software, it would be a nice idea to check for a low-cost microphone and amplification unit, paying attention to the following six issues:
- 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.
- The amplification unit should meet the standards specified by the software manufacturer in terms of delivered power, cutoff frequencies, minimum gain, gain flatness (sometimes not mandatory), maximum total harmonic distortion and noise level.
- The best sound cards are those that operate under most OSs available today and deliver the maximum number of accuracy bits and frequency range. The number of 'measurement accuracy bits' actually determines the 'digital' noise level expected to contaminate measurements. Therefore it is desired to have 24 or 32bits of accuracy and 48kHz true-measurement frequency range.
- Loudpeaker measurements simultaneously engage one output and two input channels:
- one output channel for the original stimulus delivered to the amplification unit,
- one input (recording) channel for stimulus reference and
- one input (recording) channel for DUT's (ie. device-under-test) response.
- The sound card, its software driver and the OS must all ensure the continuous operation of these three channels. Current OSs often re-schedule priorities of running processes raising significant problems to measurement integrity.
Current speaker measurement software 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.
6. Operating systems evolve in time and within a few years could make the PC you use along with your measurement software become outdated. For example a measurement software based on Microsoft Windows XP may work fine but attaching a newly-bought printer to the associated PC 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 software 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 or software. It is the client's responsibility to plan future steps concerning the PC used along with a measurement system.
For all these reasons a speaker designer should carefully investigate what exactly is offered by a software manufacturer. Other hardware availability and pricing should also be checked before any other step.