Loudspeaker Myths: Anechoic Chambers , the NRC and Flat Frequency Response
Myth #3: An Anechoic Chamber is a Must for Designing and Measuring Loudspeaker Performance
Everybody would love to have an anechoic chamber at their disposal. They are extremely useful in allowing a designer to accurately and consistently run a series of measurements very quickly, without having to worry about room effects. This not only reduces design time but also measurement error.
However, with recent advances in measurement software, a competent designer or loudspeaker reviewer can remove the room effects of a measurement by gating the measurement. This gives you a true anechoic representation of frequency response above the room transition frequency (usually 200-300Hz). In order to gain high enough resolution (1/20th octave or greater) to see loudspeaker resonances, the measurement should be done outdoors or in a very LARGE room to minimize truncation in the time domain caused by gating.
For lower frequencies you can use a technique called “ground plane” which is a VERY accurate method for measuring bass frequencies. You can then splice the measurements together to form a complete acoustical response of the speaker system.
Ground plane is a tried and true methodology for accurately measuring subwoofers. If anyone tells you otherwise, realize they are likely doing so to create exclusivity of their product that only they can accurately measure its true performance using their methodology. Ground-plane or tall-tower measurements are both used as the basis for calibrating anechoic chambers at low frequencies, so there is no dispute about their accuracy if well done – except among the biased and ignorant.
Now if one is purely engaging in scientific research to develop a correlation between how loudspeakers measure and what listeners prefer, then access to an anechoic chamber is paramount. It is what allowed Dr. Floyd Toole and Sean Olive to develop their groundbreaking research back at the NRC decades ago that is still an evolving practice at Harman today. It doesn't mean you can't design a great performing loudspeaker system without one however. Many DIY'ers are producing excellent loudspeakers using state of the art parts with a solid understanding of loudspeaker mechanics and how to carefully measure and analyze them.
Myth #4: A Speaker Should Measure like a Flat Line from 20Hz to 20kHz
The ideal loudspeaker would measure like a straight line from 20Hz to 20kHz right? Well maybe, but how is the measurement being taken? At what loudness and what measurement resolution?
The Ideal Loudspeaker Measurement?
Most speakers roll off before 20 Hz and are compromised in reproducing fundamental tones relative to the harmonic relatives of those tones, so the relative harmonic distortion is going to increase by the same ratio that the system favors the harmonics relative to the fundamental bass frequencies.
If the manufacturer decides to make a speaker flat to 20 Hz by making its mass very high to artificially reach down that low in a small box, or equalizes the system to artificially boost the low end rather than by creating a system where the native efficiency of the driver is kept in full down to the lowest frequency needed to reproduce all the sound we can hear (arguably down to 16 Hz), then he or she has made a serious compromise in the reproduction of the lowest frequencies we can hear. The reality is that more and more synthesizers are reaching down that low, and more and more musical artists (rap artists too) are creating super-low frequency content that is played at very high sound pressure levels (SPLs) at live events. This trend is being augmented by the demand for more and more subwoofers of increasing output and power handling by artists performing at live events. While 40 years ago bass below 40 Hz was an oddity, today very high output bass in the upper 20 Hz region is not at all uncommon at SPLs at or above 120 db.
However, this doesn’t stop manufacturers from manipulating their measurement graphs either by employing a large vertical axis or by averaging or smoothing the measurements.
What about compression?
How many times have you seen a shoebox-size subwoofer with an 8 or 10" driver claiming below 20Hz extension? The manufacturer will almost never tell you at what SPL and distance that measurement was taken. Sure it can likely hit 20Hz at say 80dB at 1 meter but what does it do at meaningful output levels? 80dB at 20Hz is barely audible (see chart). When looking at manufacturers' loudspeaker measurements, pay close attention to the test conditions, output levels, measurement distance, vertical axis and if smoothing was applied to the graph.
How Many Measurements Are Needed?
Some companies claim it takes thousands of frequency response measurements to put together a true response of the product in order to predict how it will perform in a real room. While it’s extremely important to understand how a speaker radiates sound both on-axis and off-axis to formulate a family of curves, the reality is it DOESN'T take thousands of measurements. However if you mislead the consumer to believe it does, and it has to be done anechoically, well then you’ve just convinced them how your product is different and exclusive from their competitors.
You can get a very good idea of how a speaker radiates by measuring on-axis and off-axis in 15 degree increments horizontally and vertically until you hit 90 degrees off-axis. That’s six measurements vertically and six measurements horizontally if the driver topology is symmetrical on the front baffle or 12 measurements if it isn’t. So you are looking at a total of 12 or up to 24 measurements to get a really good idea of on and off axis performance and any anomalies that need to be corrected either in the crossover integration or diffraction off the baffle.
If you want to get a full 360 degree view of performance to develop a sound power response it would still take at most 36 vertical and 36 horizontal measurements at 10 degree increments totaling 72 measurements, not thousands. Harman actually runs a total of 70 measurements via a spinorama to collect both horizontal and vertical data of a speaker at a distance of 2 meters with 1/20th octave resolution. From that they can generate all of the on-axis and off-axis information needed to develop a directivity index and sound power response.
If the speaker has any frequency response anomalies, you will find them within the first 90 degrees. There is NO magic here. When looking at loudspeaker measurements, Dr. Toole’s research indicates that a single spatially averaged response curve is useful in helping to separate speaker resonances (bad) from acoustical interference (not so bad, or irrelevant depending on what causes it). However, you still need to see each individual on-axis and off-axis measurement to truly understand how the speaker is radiating in the room.
It reminds me of a familiar quote I once heard from an enlightened audio colleague:
Not everything that matters can be measured and not everything that can be measured matters.
It is my belief that some speakers that audiophiles often consider to be bright are in fact bright because the manufacturer placed too much emphasis of flat anechoic response power response without thoroughly testing the product in real-world room environments or carefully optimizing driver integration during the design phase.
Measurements are a great starting point in understanding how a speaker will perform in a room, and how the drivers integrate into the system. But, in my opinion, nothing replaces controlled listening tests with the target goal of reproducing a live unamplified performance as the reference. People will also react strongly to a speaker that is up 3 dB at 5kHz and down 3 dB at 500 Hz vs. a speaker that is exactly the opposite. One will be ‘bright and thin,’ while the other will be ‘dark and heavy,’ but both are ± 3 dB—a great response. This is very similar to how two sports cars can produce similar track results but offer completely different driving experiences and preferences. It is also why specifications and even measurements don’t tell us the whole story.
Myth #5: The National Research Council – the Buck Stops Here
The National Research Council (NRC) is a Canadian government laboratory, operating in many scientific disciplines, which exists to perform some basic research, but mainly to provide a scientific resource for governmental departments, services and technical facilities for the benefit of industry and the public. From the 1970s to the early 90s a program of audio research, created and managed by Dr. Floyd Toole, contributed usefully to the understanding of listener preferences in sound quality and how these were represented in technical measurements of loudspeakers and in rooms. Despite some manufacturers claiming they were intimately involved in the audio research conducted at the NRC, it was actually initiated and supervised by Dr. Floyd Toole. No manufacturers were there contributing to the science.
In that process much was learned about how to conduct meaningful double-blind subjective evaluations, as well as how to collect comprehensive anechoic data on loudspeakers that could predict sound fields in listening rooms. Much was learned about the psychoacoustics relating what was heard and what was measured, providing a basis for subsequent research leading to reliable correlations with subjective ratings of sound quality. After 1991, the research begun at NRC continued in the research group at Harman International, which is currently under the supervision of Dr. Sean Olive, who began working with Dr. Toole at the NRC in the mid-80s. The measurement method created at the NRC evolved into what is called the spinorama, and it is embodied in the recently issued ANSI/CEA-2034 "Standard Method of Measurement for In-Home Loudspeakers". More work remains to be done, but much progress has been made.
Listening Window Response, Spinorama and Sound Power by Dr. Floyd Toole
listening window is a combination of 0, ± 15 degree vertical, and ± 15 and
30 degree horizontal measurements (the NRC measurements used 15 degree increments, Harman uses
10 degrees). This describes the average direct sound arriving at a group of
listeners. This is one component of the full set of measurements, called the spinorama. However the listening window response does NOT include the first reflections.
The spinorama is a 360 degree set of 70 frequency response measurements (10 degree increments on horizontal and vertical axes) intended to capture the complete sound that is radiated into a room - and therefore which will arrive at a listener in that room. They are intended to allow us to estimate what happens in rooms. THESE measurements DO take first reflections into consideration which are the second-loudest sounds to arrive at a listener.
We start with the on-axis frequency response which will be the first sound to arrive at a single listener in the sweet spot, at which the speaker should be aimed. The listening window is intended to be an estimate of the first sound to arrive at a group of listeners - in a good loudspeaker there is little difference between this and the on-axis response. Next, we look at the combined energy of the early reflections, estimated by knowing the angular ranges over which wall, ceiling and floor reflections occur - this curve is called the "early reflections" curve.
Then we estimate the total sound power, the energy radiated through a sphere surrounding the loudspeaker, by weighting the individual frequency responses and combining them (this is NOT the simple average of all the frequency responses that is often mistakenly claimed).
So, out of it all we get estimates of the three principal classes of sounds arriving at listeners: direct, early reflected, and late reflected or reverberant. With this data it is possible to predict with good accuracy (a) the average steady-state room curve (above the transition frequency of course) measured in the listening area and (b) the subjective preference rating as determined in double-blind, positional-substitution listening tests. This assumes that the room is "typical" - not having aberrant acoustical treatment.
But what does this mean exactly?
This testing practice is certainly a good way (not with nearly 100% certainty mind you) of predicting subjective speaker preferences when placed in ordinary untreated listening rooms. However, we can't help but wonder how these results change in rooms that have more controlled acoustical properties that don't mask subtle details with too many reflections. Most living rooms in Florida, for example, have vaulted ceilings and tiled floors opening to larger rooms which makes them somewhat unpalatable for critical listening.
What About Distortion?
In speaking with Dr. Toole, we discussed how the NRC measures loudspeaker distortion. The NRC uses stepped tones for measuring distortion while the current (proprietary) Harman system uses tones/chirps. With tones one can also measure harmonic distortion, which is almost useless, but better than nothing because if it is very low, things might be tolerable and if it is very high, things are likely to be intolerable.
Harmonic distortion measurements also tell us if the system is well designed in general from a basic standpoint. While it may be true that the absolute total harmonic distortion (THD) number is a useless metric to determine the human annoyance value of any given level of distortion, it is true that a system which is well made is usually going to have a lower THD than a system which is poorly made and/or designed. While THD in itself is a poor metric, it is a relatively good measure of how well the product is made in general. It’s harder to find a system with a high THD that sounds clean than a system with a low THD that sounds clean.
Distortion remains one of the unanswered puzzles in loudspeakers because the signal that generates the distortion determines the audible effects of the distortion-properties of the listener need to be incorporated into of the "measurement" system. Pure tones are virtually unknown in music, as are multi-tone test signals. Music is the test signal that matters and it is indeterminate. In the end, a workable distortion measurement scheme must incorporate a perceptual model of simultaneous masking, wherein the signal that generates the distortion also causes portions of the distortion products to be inaudible. Any measurement that does not include the concept of masking is destined not to correlate well with audibility. It is a topic that was not addressed at NRC, and because of its complexity not everybody is equipped to embark on this kind of research. It is worth doing though.
Bottom Line about the NRC Research
I believe the NRC audio research offers some great target goals for loudspeakers that should be considered when designing a speaker system. In fact, I would go so far as to say it is the research done by Dr. Toole and his staff that is largely responsible for the modernization of loudspeakers today. Before Dr. Toole's research, speaker design was considered more of an art form than a science. His research made listening tests more objective, bringing real science to the field for understanding perceptual preferences that was once lacking.
Most manufacturers now recognize the importance of minimizing diffraction by better integrating the drivers and crossover networks and narrowing the baffle area. It is rare today to find a legitimately high fidelity consumer loudspeaker in a big wide 1970’s style speaker cabinet with tweeters firing off opposite ends of the baffle. Most competent loudspeaker designers don't do this anymore, especially in a horizontal placement of drivers.
It's important to realize that not everyone agrees with the primary design goals of how a speaker should radiate in a room. Some prefer to design a product to be omni-polar where it radiates similarly around the speaker while others focus their efforts on designing a speaker that controls dispersion in efforts to get more direct sound energy at the listener. The latter tends to give you better, more focused imaging and clarity at the primary listening seat while the former tends to give everyone good sound but not as highly focused. Listening preferences and design goals vary, which is why there is still a viable market for the 400+ speaker brands out there. People should evaluate a speaker’s radiation pattern in context with how closely they’ll sit to the speakers. Imaging isn’t particularly as relevant in the absolute farfield (i.e. sitting very far from the speakers making you hear more of the reflected sound than the direct sound).
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Recent Forum Posts:
listener training can help immensely like having the potential listener/buyer learn what real instruments sound like in live music situations of all types. i was able to make my own decisions after i acquired these skills and relating the sound of speakers and electronics to those learned real sounds.
i still strive to hone my perceptive and correlative skills. do i believe in speaker break-in? well, yes. i may not subscribe to the multi THOUSAND hour period but perhaps greater than one hundred.
wire? yes, they are different but miniscule in difference compared to speakers or electronics. the better (cleaner, that is lower distortion) the electronics and speakers, the more you can hear. big bucks on wire isn't an option for me and i have never purchased any for more than $100. i do own a couple of $1k interconnects that were won at raffles and those ARE better than most.
that will be enough from me for now.
wlmmn, post: 1052898
Gene, I love these videos that offer a refreshing break from the intellectual dishonesty of marketing brochures. I love hearing marketing BS get a kick in the teeth with cold hard facts! As someone with extensive schooling and field experience as a live sound and recording engineer, I feel like I don't hear or read enough “reality check” time in home-theater-oriented websites. I would've gone on an angry 10-minute rant on the “digital” speaker part. Like you said in another video, you don't hear a lot of this stuff in the pro audio market because it's not tolerated among technical people who have studied signal flow and the physics of sound.
Thank you very much for your compliments and insightful feedback. Unfortunately its a challenge to speak your mind these days without offending people (either manufacturers or fanboys of the products you are referring to). We try to remain brand neutral in these videos but there always seems to be offended parties nonetheless thinking we are specifically talking about them more so than a trend we may be observing. Hugo is pretty new to this industry and he couldn't help notice all the marketing fluff around speaker cables and speakers. So we just let the camera roll and I spoke my thoughts. My apologies to anyone that suffered hurtfeelioma