Mark, you are correct only when panels resonate. The mass of even drywall is such that low frequencies are reflected rather well. So yeh, room gain, always there.
Edit: even in resonance a fair bit will be reflected. And it will generate broad spectrum distortion.
Edit: even in resonance a fair bit will be reflected. And it will generate broad spectrum distortion.
I wish you were correct, but no. Bass passes through alot. Just go outside your house/structure and listen...or better, measure and you will appreciate.
I think Mark ist right on this one. You have to distinguish between roomgain and construktive Interferance. I belive what Mark means with roomgain is a pressurisation of the whole room via the displaced airvolume of the moving cone.
That is only possible in airtight and stiff rooms.
That is only possible in airtight and stiff rooms.
For the same reason you have to make enclosures stiff and seal them airtight, as the pressure escapes easily
There is no significant advantage of an anechoic chamber over properly conducted MLS measurements as long as the the three dimensional character is properly defined in the design of the loudspeaker. Separating the the crossover design from the cabinet / driver design is meaningless. I often think an echo chamber is an important tool that should be used to characterise the room behaviour of loudspeaker systems; it's the fastest way to measure power response, and the fastest way to find anomolies in the power response which will make the loudspeaker sensitive to differing environments, AKA room and placement sensitive. Loudspeakers that are not particularly fussed by different rooms and placements are much easier to live with.
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Yes, thx for continued clarification. pressure gain is hugely depend on rigidity of construction.
Even the strength and shape of room modes are too, as rigidity effects their Q.
I can't see how that is true. MLS is just another statistical technique to isolate direct sound from reflexions. MLS doesn't fully accomplish that, nor does gating, nor does any other technique, other fully than eliminating reflected sounds into the measurement.
i guess it mainly come down to defining "significant advantage" 🙂
How about when the leaky room reaches pressure equalization? Loudspeaker putting out enough SPL which offsets the leaks. The result is higher pressure in the room ... no?
Yeah maybe some pressure gain, on some kind of freq dependant curve,
but not really enough to factor into thinking.
On one hand you have a totally sealed room with rigid walls and the other, the great outdoors. Room gain lives in between those two. Probably better to say how much room gain will depend on the rooms construction/openings but not that there won't be any gain at all.
When MLS is properly used it is the the gating of the microphone is what removes reflections, not statistical processing. Only the part of the signal that arrives before any reflections is processed. The low frequency limit of MLS measurements is set by the free path length before the first reflection.
I was once able to do measurements on a large theatre stage (the largest in Australia) and by suspending the loudspeaker under test I was easily able to have the free path length around 9 metres, equal to about 25mS of free path length, thus giving valid measurements to 40 Hz.
To obey the inverse square law test for anechoicity (which is to necessary to achieve "eliminating reflected sounds into the measurement") down to the same cutoff frequency an anechoic chamber has to have the same minimum free path length so to measure to 40 Hz requires greater than 9 x 9 x 9 metres in boundary dimensions, or for 20Hz 18 metres cubed.
Yes, this is my understanding as well. However, as the frequency being evaluated goes down, and approaches 1/(t=gate window), the resolution goes down as well, doesn't it? I thought at f=1/(t=gate window), the resolution was something like 1/2 octave... the equivalent of half octave smoothing. An anechoic chamber does not have this limitation.
I think that for most typical monopole speakers, this is no problem. Bass drivers are almost always pistonic at these frequencies, and vents / PR's usually follow the theoretical curve. in other words, narrow band peaks and valleys are rare. Except for cabinet resonances, and there are other ways to detect those.
Can someone remind us what the measurement resolution of a gated impulse response when the frequency being evaluated is 1/(gate window)?
j.
There's a guide to MLSSA here: https://audioxpress.com/article/measuring-loudspeaker-low-frequency-response
The problem of low frequency resolution with an anechoic chamber is similar to measuring with MLS. An anechoic chamber can only absorb wavelengths that are shorter than 4 x the length of the wedges, but as wavelengths approach approach this frequency absorption becomes less efficient. The result is that the bottom octave above cutoff loses resolution much the same as an MLS measurement. I think the distance between reflective boundaries is about the same for both MLS and anechoic chambers for the same cutoff and resolution.
Advantages of anechoic chambers are a low noise floor and freedom from drafts, though neither is usually a dealbreaker for loudspeaker designing and testing.
The problem of low frequency resolution with an anechoic chamber is similar to measuring with MLS. An anechoic chamber can only absorb wavelengths that are shorter than 4 x the length of the wedges, but as wavelengths approach approach this frequency absorption becomes less efficient. The result is that the bottom octave above cutoff loses resolution much the same as an MLS measurement. I think the distance between reflective boundaries is about the same for both MLS and anechoic chambers for the same cutoff and resolution.
Advantages of anechoic chambers are a low noise floor and freedom from drafts, though neither is usually a dealbreaker for loudspeaker designing and testing.
Is not the aim of the initial design to place a stake in the ground as to a consistent and accurate representation of the sound as initially recorded? Tone controls, room treatments and corrections can then take account of 'local' conditions in that specific placement. This allows for movement of equipment to other rooms and/or locations. To design equipment to work 'correctly' only in a specific location seems restrictive to me. (Though I suppose it depends on personal circumstances and plans).
Of course, and to do that you have to understand and take into account a whole lot of stuff about the perception of sound by the human ear/brain auditory system and the relationship of loudspeaker/crossover design and the interaction of that loudspeaker and the space it is in has on perception. If a loudspeaker is designed well, it does the job without being overly sensitivity to room acoustics and placement. This is the opposite of designing "equipment to work 'correctly' only in a specific location".
Hi,
by MLS statistical processing, i meant the cross correlation of the captured DUT response vs the MLS signal. I think that's something every technique has to do, whether FFT sine sweeps, period noise, or random noise. That's all i meant by statistical....
And sure, gating can be applied with any of those techniques to remove low frequency reflections.....(albeit at the cost of frequency resolution).
ARTA handles those techniques along with external excitation (like a balloon pop etc) if desired.
Here's a snip from it with some recommendations for what to use when:
Big rooms are nice for quasi-anechoic speaker measurements huh? 🙂
I take it outdoors, especially with subs. Poor man's chamber Lol.
I've measured subwoofers outdoors. A mate of mine (actually a manufacturer of large THX approved commercial cinema systems) constructed a pit in a field that allowed for true 1/2 space loading, i.e. baffle flush with the ground surface. There's huge problems with infrasonic interference AKA air movement, atmospheric conditions, traffic, etc. The advantages of an 110,000 cubic metre quasi airtight chamber are huge. But as has been pointed out by others for very low frequency measurements, once the wavelength as several times the largest dimension of the loudspeaker system free field measurement offer no meaningful advantage.
One over-riding engineering principle I have lived by since enrolling in tertiary engineering in the 1970s is that to be confident with any complex measurement demands three independent means that do not depend on the same assumptions/principles to make the measurement. More often than not this turns out to be futile at the first round, because humans are so inclined to take things at face value. However with persistence this approach reveals the mis-applications, omissions, simplifications and false assumptions inherent in each approach. Once resolved sucessfully the three independent measurement methods will give the same result; only then can one be confident in a measurement. This is especially true for measuring loudspeakers.
The point I have tried to stree several times is that the dimensions of the measurement volume required for an anechoic chamber for a specific cutoff frequency are the same, or greater than, the dimensions of free space for a gated measurement system with the same cutoff frequency.
Sure. It's just wavelength periods that give us distances needed and gating times. ...i think/hope most everyone sees that.
(personally, i'm not a fan of gated measurements, but that's a discussion that's clearly OT.)