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low frequency driver (size and quantity) vs. room (size and volume)
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July 29, 2017 - 5:42 pm
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Hi Dennis and everyone!

I took the liberty of starting this thread as a place where we can hopefully go into depths of this very important topic that has a lot of people confused. I would like to keep the focus of the discussion in low frequency transducer size and quantity and how they relate with the size and volume of the room that they are placed in.

The speaker size vs room size debate has been going on for years and you have covered this in your article and video here: https://www.acousticfields.com…..room-size/

You have also covered a topic called “Bigger Is Not Better” in your video about how bigger size subs or more subs is not necessary the way to go here:

To summarize shortly your videos and article about the topic in question you have stated the following:
– Rooms are boxes of pressure and they are pressure sensitive. They can hold only so much low frequency energy due to their size and volume. The goal is for the correct amount of energy to fill the room while at the same time using the correct transducer size to accomplish this. You must match the size of the low frequency drivers used to the room size and volume so you can hear more sound and less room.
– The difference between a 10” and a 12” subwoofer is about 2dB in output. From a bigger size driver one gets more output and energy. If you have a peak in a response curve at a certain low frequency, the added energy of the driver size increase will excite it and the harmonics of it.
– We want definition from our low frequency drivers and we don’t want our room to participate in the lifespan of the bass notes. We want the room to let the notes live and die on their own. We do that by managing the attack and decay rates.

My opening question for the discussion is related to the much seen argument that a larger low frequency driver is superior to the smaller one. I would like to use this argument found on diyaudio.com (source: https://goo.gl/pWHrd5):

When asked if larger drivers are better and if there is any harm/benefits to have more bass capability than is needed for your room and dial it back and if there Is any sort of bass driver/multiple driver total radiating area vs room size correlation, an answer was given as follows:

“When you come from the perspective of minimizing distortion, larger woofer areas move less for a given SPL at low frequencies. The types of distortion that are prevalent include the familiar harmonic distortion, of which it’s been shown that only the higher harmonics are typically audible (i.e., 4th-10th harmonics, etc.). But those harmonics also turn into higher frequencies that modulate with the fundamental frequencies, producing non-harmonic sideband peaks. For woofers, the predominant type of modulation distortion has shown to be AM distortion, i.e., not Doppler–otherwise called FM distortion–which is the predominant modulation distortion type for higher frequency drivers: midrange, tweeters (see Klippel’s paper on modulation distortion measurements, also Nelson Pass’s article on distortion in amplifiers, pg. 6ff). AM distortion is due to nonlinear properties of the much greater woofer diaphragm motions than midrange/tweeter diaphragms.

So larger is better in order to reduce distortion during music peaks and low frequency impulses, like kick and bass drums. The penalty of using smaller woofer areas is not being able to turn the SPL up to anything much above fairly low listening levels, and having to ignore those modulation distortion bursts during LF transients (which people that haven’t been exposed to live instrument sounds have learned quite well to ignore).

The best kind of bass, IMO, is horn-loaded, because the AM and FM distortion products move about 15-20 dB lower than the same drivers used in direct radiating mode. That’s why horn-loaded bass sounds as good as it does (when done well).

I once had the opportunity to listen in-room to direct radiating ported cabinets having 1, 2, and 4 (15″ diameter) woofers per side, and also a very good corner-loaded fold horn cabinet. The single and dual woofer cabinets had a definite “edginess” and artificial quality to their sound, while the quad woofer cabinet had a much more natural sound, albeit with the sound of “splintering wood” when driven hard. The horn-loaded cabinet sounded like the real thing.

So to address your question: there are no limits to woofer area/size of room, although at some multiple of woofer area size, the effects of direct radiation begins to disappear, except for the impulsive transients. Larger total woofer areas are better.

The tradeoff, of course, is a lower high-frequency roll off of larger diameter woofers, and the need to cross to the midrange drivers at a lower frequency. One way to get around this is to use multiple smaller woofer drivers, but then the 1/4 wavelength separation distance between the drivers playing at their highest frequencies start to create polar lobing issues. The other part of the tradeoff is size.

So the discussions about room coupling are mostly related to the small size of the diffuse region of the room (which is at a quite high frequency in a small listening room ). I look at this situation differently: at what frequency do you need to start adding extra surround channels to add apparent spatial width/depth, and how many absorption panels to control early reflections around the loudspeakers themselves? The type of acoustic absorption panels used by virtually everyone however don’t have much effectiveness below 100 Hz (and about 70 Hz for bass traps). This is the real limitation of smaller listening rooms–controlling early reflections, and usually not at the lowest woofer frequencies, i.e., usually above ~100-200 Hz for small rooms practically.

So my experience is to use as much “woofer area” as is tolerable in-room. For horn-loaded woofers that means horn mouth size: the larger, the better to control AM distortion and to lower the effective “Fc” of the room-loudspeaker array.”

And now to my opening question: How is it that say a 15” low frequency driver will generate more energy in the room than a 10” low frequency driver when they are both outputting the same amount of SPL (in dB) – let’s say for the sake of the argument at a modest 75dB at the listening position? Could you please explain how the mechanical wave of the sound produced is different from one and the other when the SPL (dB) is equal at the listening position? Is the intensity of the sound different (watts per square meter (W/m2))?

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July 30, 2017 - 10:27 am
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T, So to address your question: there are no limits to woofer area/size of room, although at some multiple of woofer area size, the effects of direct radiation begins to disappear, except for the impulsive transients.
Larger total woofer areas are better.

There is no “best” of anything for everything. You must always consider the room size/volume/amplitude issue. You must consider the space availability for low frequency producing devices and you must always consider the amount, type, and position of all lower frequency absorption . Distortion is of secondary concern.

So the discussions about room coupling are mostly related to the small size of the diffuse region of the room (which is at a quite high frequency in a small listening room ). I look at this situation differently: at what frequency do you need to start adding extra surround channels to add apparent spatial width/depth, and how many absorption panels to control early reflections around the loudspeakers themselves? The type of acoustic absorption panels used by virtually everyone however don’t have much effectiveness below 100 Hz (and about 70 Hz for bass traps). This is the real limitation of smaller listening rooms–controlling early reflections, and usually not at the lowest woofer frequencies, i.e., usually above ~100-200 Hz for small rooms practically

You can manage the low frequency energy in a room with proper rates and levels of absorption . There is no such thing as a “bass trap”. Low frequency energy is not trapped by anything. Our ACDA series of low frequency absorption technology has absorption rates and levels starting at 30 Hz. and going through 200 Hz. Low frequency energy is not about managing reflections, it is about unwanted modal pressure. Reflection management through diffusion or absorption is all about the time signature of that reflection. Low frequency pressure issues are another completely different domain.

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