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|17th July 2009, 01:52 PM||#1|
Join Date: Jul 2009
The Physics of Speakers
Hi I was refered here from Perry Babin very insightful guy but couldn't help me out so anyone that can it will be great appreciated.
Just had a few questions regarding speakers:
1. How do I calculate minimum output of a speaker (I know max is
10 x log10(Power) + Efficiency = Maximum Output)?
2. What is the relationship between Watt RMS and Efficiency(Sensitivity) I note the equation above but do not understand how/why? Also, must a speaker be operated at RMS Watt value? If I want to make a speaker emit a 70 dB continuous tone followed by a 1 ms change to 85 dB for 40ms then 60ms later a 120 dB pulse tone how would I?
3. How do I calculate/find out the minimum power for a certain speaker to start moving? And how can I relate that to the sensitivity rule of doubleing the power increases the SPL by 3 dB so that I can know how much an increase of power actually goes into the speaker rather since just according to the equation there isn't a direct relationship between Eff and how much power you are supplying.
I work in a research lab and decided to build a startle reflex apparatus such as this one for prepulse inhibition:
It has been a project within itself but everything has been going smooth not thinking the speaker would be an issue until now I want to order parts but I can't find what I need mainly because I had no idea speakers were so complicated plus I need a 120 dB pulse. After a while of searching I came across this site:
I am very flexible on changing the design which looks almost exactly like this:
Picture of mouse inside app.
Since upon my readings distance playes a big role the current length is about 18 cm can be changed to 50 cm to make it easy for calculations.
Some other questions regarding my situation:
1.Other then the speaker I showed you what would be best fitting for this?
2. For the speaker above can it handle the conditions in the previous question 3? If not what speakers should I look for that can handle multiple changes over a small time period?
3. How will this "inclosed system" affect the sound?
4. What type of power do I need to supply this speaker?
|17th July 2009, 07:07 PM||#3|
Join Date: Dec 2002
Location: Calgary, Alberta
Well, I read through your post, and the 2 most obvious points, which I am not 100% sure I understand:
1) Why this concern over the minimum output? It seems completely irrelevent to me.
2) You seem to be approaching the whole thing from the wrong direction. In other words, you are trying to pick a speaker driver with a measured sensitivity, then drive it with a calibrated number of watts, to achive your target SPL in dB's. Forget all of that, it'll never work. Pick a speaker driver you like, get an amp with a volume knob, buy an SPL meter, and then use the SPL meter to measure the SPL and calibrate your amp output with it. That will establish your maximum. For the lower setting, you will have to adjust your sound source. Again, you will have to use the SPL meter for this.
Aerodynamics are for people who can't build engines. Enzo Ferrari
|17th July 2009, 07:40 PM||#4|
Join Date: Jul 2004
Location: Northern Colorado
In the absolute sense of conversion efficiency from electrical watts to acoustical watts, speaker efficiency is very low. As a rough rule of thumb, a Theile/Small "normalized efficiency" of 1% into a half-space (the inverse-phase rear radiation is absorbed) is about equal to 92 dB/metre/watt. So nearly all of the energy is wasted heating the voice-coil.
In addition, since speakers are required to cover an extremely wide frequency approaching nearly three decades (which is wide even for an RF antenna), even the most advanced and high-performance speakers have variations in response of at least 3 dB. A 3 dB response window is considered nearly unattainable without line-level electronic equalization, and a 5 to 10 dB overall variation is far more typical of commercial, off-the-shelf speakers.
As a result, real-world speakers are not close to flat at all. And that's merely on-axis. The magnitudes of the response variations get much worse off the design axis.
Speakers have no minimum drive power. Even microwatts will produce soft but audible sound (hearing is logarithmic, as is vision). There are conjectured "stiction" mechanisms in the spider, but by and large, distortion falls with level, so the lowest levels have the least distortion, unlike Class AB amplifiers, where there is typically a "knee" around the level of least distortion.
Maximum drive power at the low-frequency end is set by maximum excursion. Maximum power in the midrange and high fequencies is a more complex function of excess voice-coil heating and cone or diaphragm break-up - the latter typically has a sudden onset and extremely distorted sound. If very high SPL's are required, professional sound-reinforcement loudspeakers with high efficiencies are the most robust solution.
|17th July 2009, 07:42 PM||#5|
Join Date: Aug 2002
Location: kansas city mo, and on occasion, around the world ...
Blog Entries: 15
Re: The Physics of Speakers
perhaps you should expose youself, to a '120 db' pulse ,
so you would get an idea of what you want to do.
and how that would feel.
anything that is alive, has feelings.
even a mouse ...
db Environmental noise
0 Threshold of hearing
10 Normal breathing
20 Rusting leaves
30 Whisper at 30 feet
40 Quiet street
50 Interior home noise
70 Crowded restaurant
75 Kitchen appliances
80 City traffic
85 Hearing damage possible
90 Lawn mower
100 Chain shaw
120 Threshold of pain
134 .22 LR rifle
140 Jet engine at take-off
150 .410 shotgun
152 .22 LR pistol
153 20 gauge shotgun
155 .223 rifle
155 .25 pistol
156 12 gauge shotgun
156 .30-.30 rifle
156 .308 rifle
156 .44 Special revolver
157 .22 Magnum pistol
157 .45 ACP pistol
158 .380 ACP pistol
158 .38 Special revolver
160 9mm Para pistol
163 .41 Magnum revolver
164 .357 Magnum revolver
164 .44 Magnum revolver
|18th July 2009, 03:34 AM||#8|
Join Date: Jan 2008
Re: The Physics of Speakers
Also the Efficiency is stonily frequency dependent as is all of the above.
TO get what you want you will have to measure at the subject's location and simply adjust a volume control while watching a meter. Calculation can be off easy by 3, 6 or 12 dB.
I think this might explain why headphones are popular for experiments and hearing tests.
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