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Loudspeaker Measurements Standard: Equipment & Calibration

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Obtaining useful loudspeaker measurements has never been easier than it is today.  Measurement results can be obtained using computer-based measurement software, a quality two-channel recording audio interface, a calibrated measurement microphone and a little patience.  We use the following equipment and software to conduct loudspeaker measurements:

  • SoundEasy V18 by Bodzio Software 
  • ARTA/STEPS by Ivo Mateljan 
  • RME Fireface 800 Firewire Audio Interface
  • ATI AT6012 Audio Amplifier
  • Earthworks M30 Omnidirectional Measurement Microphone
  • Sper Scientific 2 Point Acoustical Calibrator (850016)
  • Dayton Audio OmniMic V2 Precision Measurement System
  • Hand Built Impedance Measurement Probes
  • Radio Shack Digital Multimeter with RS-232 output

SoundEasy is a very powerful loudspeaker modeling, design, and measurement package. This package includes components for obtaining frequency response, phase, impedance and distortion measurements, to name a few.  For those interested in loudspeaker design, this package has an excellent price-to-performance ratio and covers all of the bases imaginable, including finite element method, non-linear driver analysis, T/S parameter extraction, analog crossover design, digital crossover design, cabinet modeling and much more. The manual is actually a great resource for those interested in learning more.  Just keep in mind it is not for the faint of heart, the manual is 525 pages and covers some topics that are heavy in mathematics. 

ARTA software is a very easy to use impulse response measurement package that provides a cost effective method to obtain loudspeaker frequency response, cumulative spectral decay and other frequency domain information.  STEPS is bundled with ARTA and provides swept sine measurements for generating the distortion measurements used here.  While SoundEasy also provides this functionality, it is much more involved to obtain measurements and the output graphs are not as attractive for publishing purposes. 

The RME Fireface 800 is a high-quality 56-channel audio interface including analog preamplifiers,fireface800.jpg phantom microphone power, an excellent computer mixing interface and industry leading device drivers.  This equipment is used to reliably convert the analog measurement information from the impedance probes and measurement microphone into the format needed by the measurement software.  Additionally, it serves as the audio source for excitation signals required for measurement.

The ATI AT6012 is a 12-channel power amplifier used for measurements.  The amplifier is used because it is flexible, reliable and has excellent performance.  The AT6012 is capable of driving very low impedance speakers and provides very good voltage stability.  In cases where speakers must be bi-amplified or tri-amplified for testing, the AT6012 amplifier has sufficient current to drive all channels simultaneously at the voltage levels required for testing.

ATI AT6012.jpg

                                        ATI AT6012

The Earthworks M30 omnidirectional measurement microphone was selected for all acoustical measurements.  This decision was made based on the M30’s solid reputation and specifications such as ruler flat and extended frequency response.  Although the microphone is +1/-3dB from 5Hz to 30kHz without a calibration file, the included calibration file will be used for all loudspeaker measurements.  If you find yourself needing to measure a jet engine, the 142dB maximum input might come in handy.  If your measuring something like loudspeakers, the low self-noise and excellent distortion performance may be more useful.

Even though the Earthworks M30 is a calibrated measurement microphone, there is no way to determine absolute sound pressure levels without a point of reference.  This is because the RME Fireface does not have a reliable zero gain setting on the preamplifier.  Thus, the simplest solution to determining absolute SPL is use of an acoustical calibrator.  Leveraging the expertise of Herb Singleton at Cross Spectrum Labs, the Sper Scientific acoustical calibrator was selected. Cross Spectrum will independently validate the accuracy of the acoustical calibrator and the M30 calibration as needed.

OmniMic V2 .jpg  Earthworks M30 .jpg

Dayton Audio OmniMic System (left pic) ; Earthworks M30 Microphone (right pic)

 

probes.jpgDayton Audio OmniMic V2 Precision Measurement System is quite a mouthful.  This is a bread and butter no-nonsense package that provides just what most folks need for measurement without all of the cost of a software package, audio interface and a calibrated microphone.  It’s simply a calibrated USB measurement microphone with a software package and test tone disc that is exceedingly simple to use.  It gives access to frequency response measurements, SPL measurements, distortion analysis, room analysis, bass decay and the very nice polar response graphs used in our loudspeaker reviews.  This package is not for a professional loudspeaker designer, but is definitely useful for someone trying to tune a system or analyze speakers.

To perform impedance measurements on loudspeakers, a pair of probes is required.  The probes connect to the line input of the soundcard and measure the current through a known resistor by measuring the voltage drop across the resistor (if you’re confused, see Understanding Ohm's Law, Impedance and Electrical Phase 101 ).  The probes are very simple, consisting of ¼” mono phone connector to RCA jack.  The end of the RCA jack has a 47k ohm resistor from center to ground and a 22k ohm resistor in the signal path.  The probes provide a method to safely determine voltage differences using a standard sound card.

The Radio Shack Digital Multimeter (DMM) with RS-232 serial output is a necessary tool for obtaining measurement and calibration parameters.  The RS-232 output links to computer software that provides the DMM output directly on the measurement computer screen.  This is extremely convenient while trying to set and test voltage levels at different loudspeaker playback levels.  As needed, the multimeter readings are compared to a Tektronix TDS3000 series oscilloscope with an input signal of 2.83VRMS at 60Hz, 120Hz and 500Hz to validate accuracy.

multimeter.jpg 

RadioShack Multimeter with RS-232

Calibration Routine

SoundEasy has a spectrum analyzer plug-in that has a built-in signal generator.  The signal generator can provide single/multi-tone sine signals, pink noise, white noise, square waves, tone bursts or custom sound files.  The first step in calibration is to determine the soundcard and SoundEasy volume level required to generate 2.83VRMS across the loudspeaker terminals.  Using the Radio Shack DMM display, each is adjusted until 2.83VRMS is displayed with no load attached with a 60Hz input signal.  The input signal is then increased to 120Hz and then to 500Hz to confirm the voltage output of the amplifier is stable versus frequency.  Next the loudspeaker is attached and the voltage stability is confirmed at 60Hz, 120Hz and 500Hz.  The values of the digital volume control in SoundEasy and the Fireface software are recorded.  This routine is then repeated for 8.944VRMS and 14.14VRMS across the loudspeaker terminals representing 10W and 25W into 8 purely resistive loads.

Since the RME Fireface and Earthworks M30 do not have a reference for absolute SPL measurements, the Sper Scientific acoustical calibrator is used to produce a reference 94dB and 114dB. The tip of the M30 measurement microphone is placed in the acoustical calibrator and exposed to a 94dB reference signal.  The spectrum analyzer plugin in SoundEasy is used to adjust the RME preamplifier gain until the reference level of 94dB is measured in SoundEasy.  The acoustical calibrator is changed to the 114dB setting and the input level is confirmed in SoundEasy.

Glossary of Terms

  • Analog Crossover – Set of active or passive filters used to split an analog audio signal into frequency bands appropriate for each driver of a loudspeaker.
  • Anechoic Chamber – A room in which reflections above a given cutoff frequency are attenuated to a level where they do not significantly affect the measurement of direct sound from a loudspeaker.  The cutoff frequency decreases as the thickness of the absorption material increases.
  • Baffle Step Diffraction – Inherent change in output of a loudspeaker as the wavelength of sound increases beyond the width of the loudspeaker baffle .
  • Bass Decay – Describes how long it takes bass frequencies to reduce in amplitude after being played into a room.  This is primarily determined by the Q of room resonance modes.
  • Cumulative Spectral Decay – A series of frequency response slices cascaded to show how sound reduces in amplitude with respect to time.  Cumulative spectral decay graph resolution is inversely related in the time and frequency domains.
  • Digital Crossover – Use of digital signal processing techniques to split a digital audio signal into frequency bands appropriate for each driver of a loudspeaker.  This technique allows more complex filter designs and better matching of transducer requirements than an analog crossover counterpart.  The split signals must be converted to analog and amplified before being transmitted to the loudspeaker.
  • Distortion (Loudspeaker) – Broad term describing both linear and non-linear change in the transmitted content of an electrical or acoustic signal during transmission.
  • Electrical Phase – A property of loudspeaker drivers in combination with a crossover that causes leading or lagging of current relative to voltage where high phase angles equate to less efficient delivery of power to a loudspeaker.
  • Finite Element Method (Acoustics) – Process used to solve complex equations relating to modeling the acoustics of a room to determine modes and sound pressure levels at various locations.
  • Frequency Response – A graph showing sound pressure level as a function of frequency
  • Group Delay - The time delay of amplitude envelopes of sinusoidal components through a loudspeaker.
  • Gating – A signal processing technique in which the duration of the impulse response used to calculate the amplitude response is limited in time.  This technique is widely used for acoustical measurements conducted in an environment that produces reflections to limit the impact of reflections in the measurement.  The trade-off is reduced frequency resolution causing loss of information at low frequencies.
  • Impedance – Electric or acoustic property that quantifies a loudspeaker’s frequency dependent resistance to an alternating effect such as an alternating voltage input signal.
  • Impulse Response – A time domain measurement of a system’s response to an impulse of very short duration.  In practice, the impulse is required to have a duration much smaller than the period of the highest frequency of interest.
  • Maximum Length Sequence (MLS) – A pseudo random binary sequence that is spectrally flat and is an ideal excitation signal for impulse response measurement.
  • Multimeter – A device that measures electrical properties of voltage, current and resistance using two probes.
  • Pink Noise – Broadband excitation signal that contains an equal amount of noise power per octave (per percentage bandwidth).
  • Polar Response – Frequency response of a loudspeaker measured at some number of frequencies, at many angles over a horizontal and/or vertical orbit.  The results may be plotted as a continuous polar graph of sound level versus angle for individual frequencies or bandwidths, or as a family of frequency responses plots at designated angles.
  • Quasi-Anechoic – Techniques used to approximate measurements made in an anechoic chamber while in a reflective or reverberant field.
  • Room Analysis – Consideration of a room’s acoustic properties in the design of a sound system.
  • Signal Generator – System that provides an output signal matching a user's desired input requirements including wave shape, frequency and amplitude.
  • Spectrum Analyzer – Tool used to analyze signals in their frequency domain components.
  • Sound Pressure Level – A standardized measure, in decibels (dB) in which a sound level is stated with relation to an internationally agreed upon reference sound pressure level.
  • Square Wave – A periodic signal that theoretically transitions from off to on and on to off instantly spending equal amounts of time off and on.
  • T/S Parameters – Thiel/Small parameters are loudspeaker driver parameters that quantitatively identify a loudspeaker’s small-signal performance and provide a mathematical method to estimate a driver’s performance in an enclosure.
  • Total Harmonic Distortion (THD) – Harmonic distortion is loudspeaker distortion having distortion products at integer multiples of the fundamental frequency.  Total harmonic distortion is a single number combining all harmonic distortion products.
  • Voltage Sensitivity – Describes the sound pressure level (SPL) at a standardized distance of 1 meter for a standardized input of 2.83VRMS.  Voltage sensitivity is used over efficiency because good power amplifiers behave as constant voltage sources.
  • Voltage Stability – Ability of an amplifier to maintain a constant voltage output regardless of the load impedance.
  • White Noise - Broadband excitation signal that contains an equal amount of noise power per Hertz (per fixed bandwidth)
  • Windowing - See gating.

Acknowledgements

We would like to personally thank the following people for their contributions and/or peer review of this article, all of whom are true experts in their respective fields. Their contributions enabled us to make the most comprehensive and accurate article possible on the very complex topic of loudspeakers cabinets dealt with herein.

 

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Recent Forum Posts:

Paul womble dung posts on September 13, 2016 17:45
i guess that some people don't understand why many of the technical technicalities can be very important to some other people.

not everyone only uses or needs good speakers at home to listen to music, or use a home theater/entertainment suite.

some people work with music, either in sound engineering, production, or are themselves musicians.
in which case they need speakers which are suited to them, and need to know WHY those particular speakers are suited to them, so they can recreate this wherever they are (obviously within the context of the average spaces they intend to do it in, even if they intend to use hired gear, it is wise to have even a basic understanding of all of this).

some musicians for example play acoustic instruments, and when they need amplified for some spaces, then generally certain set ups work much better for them.
other musicians may work purely electronically/digitally (like a techno music musician) so certain other set ups may suite them.

and you will find this will be similar for their home,perhaps, in that their domestic set up may reflect their work. although some of us like to leave work at work!

other folk need audio gear for research purposes and may be buying it on behalf of their university department so certain criteria may need to be fulfilled in various qualities.

i was quite impressed by how much detail the written stuff linked to from this page was, despite much of the language as yet being a bit over my head. but liked how it gave me a load of terms to look up and become familiar with.
although the algebra took a few reads through to grasp, it actually was not too difficult and was on a level with secondary education (middle school in U.S.?) and online calculators can help, or any basic calculator that has “scientific” settings on it.
my main interest at the moment is in deciding shapes and proportions with which to build cabinets for a slightly large sound system for mainly outdoor use, so i will need a little maths to work out the best proportions to get the best out of the materials used.

then that whole minefield of resistance, amps, etc etc etc, to get it all clean and efficient (with all of those taken into account and sorted, then even a 1500watt setup can rip the back-side off a much larger setup up, and save money, and weight.)



i think i will enjoy being a member of this forum!
DannyA posts on March 20, 2014 19:02
exlabdriver, post: 1024402
Danny:

If your system sounds good to you then don't worry about it. Why care about what other's subjective opinions are? Yours is the only one that counts.

It's best just to enjoy…

TAM

Agreed. Sometime that curiosity bug gets the better of me though.
exlabdriver posts on March 20, 2014 18:34
Danny:

If your system sounds good to you then don't worry about it. Why care about what other's subjective opinions are? Yours is the only one that counts.

It's best just to enjoy…

TAM
DannyA posts on March 20, 2014 16:26
At one point I was determined to know how my speakers stack up against other speakers in the same price/quality range. I eventually let the idea go. I like how my system sounds after all of the work I put in to placement and tuning. With that said, I've seen enough posts and comments about my speakers and their advertised specs to know that I would most likely be disappointed with the results. I could be wrong and I guess now I'm curious again but I don't want to ask “the question” if I can't live with the answer. What I don't know won't make my system sound any better or worse. But if I do know and the results stink, I'll be grinding my brain thinking about how to build a new system (that I can't afford right now).
Somehow I know my curiosity is going to get the best of me.
ira posts on March 20, 2014 13:10
Please keep in mind that if the multimeter you use is not a “true RMS” meter, it will have material errors in its RMS measurements. I could not tell the exact model number of the DMM you are using, so please make sure you look up its exact specs, and I recommend junking it if it's not a true RMS DMM - no need to include avoidable errors in your reviews.
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