What makes a speaker play loud

[Vladi]

I was wondering what makes a speaker more sensitive and play louder than another?

I am looking at my RF-3 IIs and they play so loud and I can barely see the woofers move.

I was at Best Buy yesterday playing with some JBL speakers, and I could see the woofers moving so much, yet they were not as loud as my Klipsch.

I feel this is a dumb question, but why do the woofers move differently, and clearly my RF-3s move less, but they are louder.

Can someone point me to a read?

Vladi

[Ou8thisSN]

I personally would love to have this question answered by the pros here, but I suspect it has something to do with the Horn-loaded design.

[cyclonecj]

Here are a few good articles from the 1950's that will get you started. Happy Reading!

http://www.hifilit.com/hifilit/Klipsch/Klipsch.htm

[jzoz01]

I was at Best Buy yesterday playing with some JBL speakers.....

Vladi

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Well your first problem is you're listening to the cheapo jbl's at best buy. I'm assuming they were the Northbridge series. Those are only rated at 91db 1watt/1meter. Your RF3 speakers are rated at 98, so at the same wattage yours will be 7db louder, at least at 1000htz they will be. Secondly, you were listening at best buy, which is not know for being set up to house the best accoustics. I assume your speakers at home are in a much smaller room, that would make then sound louder. Lastly, it depends how each was powered, as every time you double the wattage, your total spl increases by 3 db.

[Vladi]

jzoz, thanks for your input. I do realise all you said.

I have actually cranked up my RF3s really loud, but have not see large motion on the woofers.

That is what interests me most, how is it possible to be so loud with not much (visually noticeable) movement of the woofers.

[minn_male42]

for the frequencies being reproduced by the rf3's, you SHOULD NOT see any movement....speakers that exhibit visible woofer movement are distorting the signal!!!!

[tpg]

So, are you saying they shouldn't move, or that they should move fast enough that you don't see them?

[prodj101]

driver movement should only be visible at extreme volumes. I have to take my RF-5's to about 100 db before I see anything at all, and even than, it's hardly any movement.

[WMcD]

I'll try to keep it short. Always a problem. I've failed again.

You may be seeing the difference between a woofer in a sealed box and a woofer in a ported box.

One overall problem is that direct radiators, these woofers, are not able to radiate low frequencies. Low frequencies have long wavelenghts. Looking at the woofer size (say fixed at 12 inches in diameter) it is getting relatively smaller in terms of wavelengths it is being asked to radiate, as the frequency goes down. This means that radiation resistance, its load, the ability to transmit, is decreasing.

At the midbass and above, this problem is solved by the mass of the diaphragm and the weakness of the motor. The motor is so weak that the diaphragm goes through less excursion as frequency goes up. This is called mass loading. It is a type of self equalizing where high frequency response is killed off to match the poor bass response.

At the lower end of the bass there are two ways of making more sound. So maybe we don't have to kill off the high frequencies with mass loading quite as much. The tricks work over a limited range and we have to be careful to make things come out right.

1) In a sealed box system, the mechanics are set up to that the resonant frequency is placed where response would otherwise be really falling. The sound pressure off the back of the diaphragm just compresses the air in the box. But going no where.

As the driving signal is going down in frequency, the diaphragm is approaching resonance. The diaphragm moves more freely undergoing greater excursion. The movement you see is the diaphragm moving at or near the resonant frequency. Here all the peaking result is coming off the front of the diaphragm.

There is a parameter called Q, which is a function of damping, to control just how much the diaphragm bounces around. When there is too little damping, you get an over emphasised bass at resonance. You hear car systems with this from down the block. It gives the impression of powerfull bass, but only at one, narrow, frequency.

2) We have as an alternative, a ported box. Here there is tube or port positioned in the box and open on one end to the air. This creates a trapped mass of air in the tube. It does not act like a resonant pipe, like a pipe organ, if designed correctly. Rather it is just a mass. A passive radiator is another form of mass. But, good for us, the end of the tube does radiate sound pressure.

Again, still, we have the problem that at low bass, the response off the front of the diaphram is otherwise going to heck from lack of radiation resistance. Is there some way we can take advantage of the pressure off the back of the diaphragm. Yes, this is it.

The the air in the box forms a spring. It is connected to the mass of air in the port. That is another form of resonator. It has some output at the open end of the port, over a narrow range. With proper design that is very near the resonance of the diaphragm.

There are a couple of good things. One is that the box-port system reverses the phase of the signal off the back of the woofer, so it is in phase with the front of the diaphragm, when the signal gets out of the port. I've compared this to a paddle ball game where moving the paddle makes the ball move out of phase.

It is also like pushing a kid on a swing. You intuitively get into the resonant frequency of the swing or pendulum. Visualizing. You push left, just as the swing is the farthest right. So the motion of the swing is always best when the input (you) is out of phase with how the swing is oscillating or, actually, resonating.

The other thing is that now, at resonance, the diaphragm has to, eventually, move the mass of air in the port. That is work because it is pumping air in and out of the port, plus its other work on the front of the diaphragm. As a result, the diaphragm doesn't move as much at system resonance. And we get sound pressure out of the port. Just like having a second woofer.

Okay, now to get technical.

We see the effects of the above in graphs in simulation programs of a) diaphragm excursion and electrical impedance.

In the sealed system. Diaphragm excursion (a) is high at resonance. Also, there is a corresponding hump in electrical impedance ( at resonance. This is because Ohms = Volts / Amps. The diaphragm at resonance is moving freely, less amps (current) and impedance (ohms) go up.

In the ported system. The diaphragm excursion (a) is low at the resonant frequency. This is because the diaphragm is pumping air through the port and is loaded, doing work. The impedance graph is a hump with a valley in the middle. It looks like there are two peaks but these are not really two resonances. It is just the diaphram resonance getting damped in the middle by doing work.

There is a bottom line to this. (You were waiting, I know.) The ported box system can have 3 dB better response (twice power) because we're getting the effect of the back of the diaphragm, and we have less excursion. As mentioned by others, excusion can cause distortion.

There is still a design choice of how to "spend" the 3 dB boost available in the ported system versus the closed box. You can juggle a lot of parameters and keep overall system effency where it is with the closed box, but spend it on extending the bass roll off 3 dB point, F3, down a bit. Or, you can juggle a lot of parameters, keep the F3 where it is, and increase overall efficency by 3 dB. This is because you don't have to knock down high frequency response with mass loading.

Gil

[pinipig523]

wow... gil lost me.

but that just shows i have a long way to go before i truly know what im talking about.

[DrWho]

so basically, the ported speaker should show less movement?

[JasN00b]

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On 3/29/2003 6:00:02 PM DrWho wrote:

so basically, the ported speaker should show less movement?

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more i think...

[elgrillo]

HOLY!!!! Im going to have to save gils post for when my brain is in prime condition to understand the technical. I too see how much I have to learn, to even begin to understand what sound systems are really about.

[formica]

Actually I think he was trying to say that the sealed speaker will have more movement only because it's natural frequency (the frequency at which it will oscillate wildly with almost no power) is often designed so that it'll near/in the audible range to help boost the speakers ability to produce the lowest frequencies.

The ported speaker's natural frequency will usually be designed lower as it isn't needed as much to boost the lower frequencies and without the "sealed box" to control it's movement, it is much easier for it to over extend.

Basically its' movements are dictated by tricks of the trade... that is where the manufacturer decides to place the Fs (natural frequency of the driver)

BTW: great post Gil !

[Vladi]

Gil, thanks for the great explanaition, although to be honest I don't understand everything... To visualize what you were saying, I did an experiment. I connected my RS-3 as main speakers, and turned up the volume. Yes, I see lots of movement of the woofers!

So this is because they are sealed box, and my RF-3 do not exhibit such movement because they are ported box?

[WMcD]

Yes, Vladi, that is the basic story.

I may have been explaining too much at one time. I'll commit the same sin.

If you are just looking at one side of the diaphragm motion in a sealed box . . . Then, resonance can be a good thing. At resonance the diaphragm is easier to move. Remember the voice coil and magnet is a type of electrical motor with an input from the amp. Therefore, all of a sudden, getting close to resonance, the diaphragm is easier to move.

Therefore, it undergoes a larger excursion and pumps more air just because it moves more. It is as if we are using a graphic equalizer. Just where the bass response is dropping off, we have a slide pot which we can bump up. It only works in a narrow range, but it is enough for where we need it.

Here is one thing I do to experiment. I stand at a bus stop in the Loop late at night. The bus is no where in sight. But there is this pole with the bus stop sign. I try to shake it. Like the bass driver it has some mass and a spring action because one end is bolted to the concrete. It is like half a tuning fork.

When I shake it at the correct frequency, the whole pole will move and shake. (My input is like that of the voice coil / magnet motor input.) No one would have the strenght to get that sort of movement except when the resonant frequency is excited. By analogy, there is no greater load, but the system moving it moves more.

In the ported speaker or one with a passive radiator, we have a slightly different story than the sealed box. When the box and port or box and passive radiator turn on at their own resonance, the motor is now driving the main diaphragm, plus the port or passive radiator. That is good for bass response because they are both causing a pressure increase, in tandem. On the other hand, the motor is loaded down doing more work.

This is not quite like bumping up the slide pot on the imagined graphic equalizer (resonance of the single diaphragm) to increase drive, or excursion. Rather, in a narrow frequency band, just where we need it, we have two diaphragms moving in tandem. (Phase reversal makes that possible. Which is why I went into it. Perhaps creating a confusing distraction.)

There is some mutual coupling. That is to say, the passive radiator and active radiator have both increased the pressure around the box. It seems like voodoo economics. None the less each has given the other more pressurized air to push against.

Very roughly, we turned on an extra woofer rather than bumping up excursion. Either one has less excursion than if the other was not there.

Most of the above is "explained" in T-S parameters of filter design. I come from a different vector of antennae design (radiation resistance) and mechanics of resonance.

It occurs to me that one area of confusion may be that we assume, incorrectly, that if a pumping system is moving slowly (less excursion in the ported/passive radiator) it is not moving air as well. Quite to the contrary. It depends on other conditions.

Thank goodness, the bus came by, the Chicago Police Department has not put me in the slammer for messing with bus signs, and now I got back to the apartment and have to tidy up the place. Specifics of the apartment, you don't "want to know", as Stevie Nicks says, in paraphrase. An attempt at humor.

The Sear Robuck vacuum cleaner is a type of air pump. Somewhat similar to our speakers in a rough analogy.

Gee, when the vacuum cleaner is pumping a lot of air the motor is running at normal speed. But when the air flow is blocked with that odd piece of debris, the motor speeds up.

Think about it. The vacuum cleaner motor runs faster when there is less air flow. Kind of counter intuitive. But it is like stepping on the gas when the car is in neutral. Lot of RPM and no power. So what the load is, make a difference.

This explains a bit about diaphragm excursion, in the ported/passive radiator system when there is a good load presented at the resonance. It moves less, because it is pumping more. It has more pressure to work against. Air flow.

I'm putting a lot of analogies in here, because that is how I eventually gained some personal understanding. There are other analogies in electric circuits. They may be more confusing.

Gil

[Vladi]

Gil, thanks again for the detailed explanation and analogies. I think I got the point this time

[TheEAR]

Gil,

Very good and detailed descriptions.Really impressive show of knowledge and good to see you took the time to post not one but two easy to follow guides.

You should write a book

[Deang]

I love it when Gil does that.

Gil, do you have any thoughts on optimal port function in relationship to the wall behind them? That is -- a formula or something for minimal distance so performance of the port(s) is not impeded?

[michael hurd]

In a sealed box the driver motion will be more than in a ported box, assuming that the frequencies being reproduced are near the " tuning point " of the ported box. What is meant by the tuning point is the mass of air in the port is in phase with the woofer motion and output ( sound level )is increased by up to 3 db in a perfect world. This means the port is " assisting " the reproduction of low frequencies at or near the tuning point.

The same principles can be found by a simple experiment. Take a pop bottle that is empty and blow across the top where the cap screws on. You will find that the bottle resonates and makes noise. The plastic has little strength and resonates readily. Now picture a speaker box as the bottle and the neck as the " port ". The diameter and length of the port, as well as the internal volume of the enclosure determine the frequency of the " assisting ". You can now see that porting a speaker will help to extend it's low frequency response, assuming it's electrical and mechanical specifications detemine that it can be used in a ported box.

The only problem is that when you feed a high power low frequency signal to a speaker in a ported box, that falls below the tuning frequency of the port and box, driver motion becomes uncontroled, and begins to vibrate rapidly out of control. In a sealed box system, the cushion of air in the box acts like a spring on the woofer, restoring it to it's at rest position.