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Need speaker advice for 3.5mm jack

 

New member
Username: Mike696

Post Number: 1
Registered: Oct-12
Hi,

I'm just looking for some basic speaker guidance.

I would like to buy a single speaker to plug into the 3.5mm jack output of a mini amp, but I'm a bit clueless as to what spec the speaker would need to be.

I've listed below the specs from the mini Amplifier/Speaker:

INPUT SENSITIVITY: 1mV
INPUT IMPEDANCE: 5K OHMS
POWER OUTPUT(1KHz): 200mW (16 OHM LOAD)
DISTORTION (1KHZ): < 2% THD @ 200mW
FREQUENCY RESPONSE: 100Hz - 10KHz
POWER SOURCE: 9V ADAPTER

From what I understand I think I'd need a single 16 ohm speaker, but I don't really understand what wattage I'd need? Also, the speaker would need to be quite loud, used for playing high frequency sounds (bird calls to be exact). Would there be a certain type of speaker better suited for this than others - which wouldn't distort the sound too much at high volume?

Any advice would be greatly appreciated

Thanks a lot

Mike
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 17479
Registered: May-04
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The amplifier specs would suggest this is meant to drive a headphone - not a speaker. You can argue a headphone is a type of speaker, but it is a speaker intended to be heard from a very short distance away from the driver. I'm guessing you would prefer the birds hear the output of the speaker from more than a 1/4" away from the driver. (Though the thought of someone fitting the birds with tiny earbuds is amusing.)

Why this amplifier? If you are after something portable, there are battery driven amps with much more realistic wattage output. Try one of these; http://www.parts-express.com/cat/amplifiers/106 Most can be driven from a 12VDC battery pack.

Beyond having an amplifier with greater output capacity, you are fighting the laws of physics as size is part of a speaker which exhibits high efficiency - the ability to turn electrical watts into acoustic watts. What that means to you is a speaker able to make any volume with the output of your stated amplifier would need to be the size of a 36" range's shipping carton. Considering you are not in need of low frequencies, that would still translate into a horn loaded driver of considerable dimensions.

Taking into consideration you are probably going to be using this speaker outdoors where you have no reinforcing walls to add to the efficiency of the system, you are in effect positioned with one foot in a hole.

Given the amount of information you have provided, I would suggest a horn loaded PA speaker would be your best option. Horn loaded drivers are, however, highly directional and this design will result in sound being projected more or less in one direction only with minimal dispersion to the sides and rear. That is though where I would start, looking at speakers intended for use with a relatively low powered PA amp. Try J W Davis; http://www.jwd.com/index2.html

The problem here would be most PA speakers are intended for use with a 70 Volt output system and even a modest PA amp is likely to have a minimum of 5-10 watts. The botom line here remains 200mW is still not sufficient output to drive any speaker to any volume level which can be heard from more than a very short distance.




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New member
Username: Mike696

Post Number: 2
Registered: Oct-12
Thanks a lot for your reply Jan, it's a real help to me.

The amp doesn't need to be portable, it will just be plugged into the mains. I just wanted a small amp so it didn't take up much space.

If I was to buy another small amp which was more suitable, could you suggest some rough specs to guide me? It doesn't need to be amazing, just so the calls being played could be heard from a minimum distance of 50 meters and ideally with little sound distortion.

Would you still suggest a horn loaded PA speaker if I buy a different amp?

Thanks for your help

Mike
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 17482
Registered: May-04
.

I guess I would. With the information you've provided it still seems as though you would want a horn loaded system. Horns are directional, that's what gives them their higher efficiency. By directing sound in a specific, rather narrow beam, less acoustic wattage is wasted by a wide dispersion pattern into areas that aren't effective. Using horns outdoors means you have no reflecting walls and while sound is not completely absent at the sides or to the rear of a horn, there isn't much there. Add to that the fact higher frequencies are more and more directional as the frequency rises and you'll have a system that can achieve relatively high output from a few watts but only in a rather narrow beam of sound. You haven't stated whether this will work for your needs or not.

But I know of no speaker system capable of producing perceptible sound levels at 150-180 feet away from the driver when it has no more than 200mWatts to work with. Sound pressure levels drop off at a constant rate of -6dB with every doubling of the distance away from the driver. SPL measurements of a driver are taken at one meter away from the driver. At two meters the SPL has already dropped by -6dB and at four meters it has dropped the same amount again. You can do the math to determine just how loud the sound needs to be at the driver for your needs. Figure average conversational levels to be in the 80-some dB range. 90 dB or higher (at that 50 meters distance) would be awfully loud for a bird call unless you are wishing to attract extremely large carion.

But a horn loaded or, at the least, a very narrow dispersion driver system would appear to be what you need. I would think whatever project you intend this for would also have some guidelines for what equipment to use. No?



The amplifier power you need is relative to the speaker's efficiency. While not entirely interchangeable, "efficiency" and "electrical sensitivity" are more or less equivalent terms in audio. However, you'll see the term "sensitivity" used to state the SPL output of a speaker system when driven by 1 watt of input power.

After that, it's all a numbers and calculations game. As SPL drops with distance, you'll need higher wattage to accomplish the same volume level at any given distance. Every doubling of "watts" will result in +3 dB of additional peak volume. Since the speaker's sensitivity has been stated with one watt input, two watts would provide an additional three dB of SPL at the same distance of one meter. Four watts (doubling the two we already have) would provide another +3 dB at the same distance. From there power alone becomes less effective as that same doubling from, say, 25 to 50 watts only accomplishes an identical +3dB of volume at the same one meter distance.

This makes it more vauable to find the higher SPL values in the speaker system itself rather than trying to make up for low sensitivity in the speaker with high wattage in the amp. My guess would be, using a horn loaded speaker of average sensitivity, to project conversational levels at 50 meters, you would need no less than (approximately) 15 watts. You'll have to do the math to determine what any specific speaker will output at the stated distance and work backwards from there.





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New member
Username: Mike696

Post Number: 3
Registered: Oct-12
Thanks Jan.

The idea behind playing the calls is to lure colonial nesting birds (Swifts in this case) to new nesting sites. I intend to fit the speaker to a large colony box I'm designing. Creating the impression of an established colony encourages the birds to nest nearby. Here's a link to give you an idea of the sound they produce: http://www.youtube.com/watch?v=Rd3NlfbA7yQ
Currently most information out there is quite basic about playing the calls. To give you an idea, i've used 2 old Altec Lansing PC speakers before (I forget their spec). They worked ok except for distorting the sound at high volume

Realistically, the sound at say 75 meters only needs to be fairly faint, otherwise it's likely to annoy the neighbours! This is where the horn loaded PA speaker could work really well. Directing the sound away from the neighbours could be a big benefit.

I'll try and work out what I need from your information.

Thanks a lot for your input so far
 

New member
Username: Mike696

Post Number: 4
Registered: Oct-12
I've worked out from your figures that the second black horn speaker in the link below would offer an acceptable volume for what I need - 68dB at 64m, but I notice it's only 8w instead of the 15w you suggested, is this a problem?

http://www.henrys-electronics.co.uk/shop/8_Ohm_Round_PA_Horn_Speakers.html

Would this Amplifier work with that speaker type? I don't fully understand the Power RMS @ 8 ohms: 2 x 15.

http://www.henrys-electronics.co.uk/shop/stereo_hi_fi_amplifier.html
 

Platinum Member
Username: Jan_b_vigne

Dallas, TX

Post Number: 17485
Registered: May-04
.

Don't overly concern yourself with how much power the amp has. Having a bit more is fine, having too little can be a problem. For your useage, the exact number of watts are not so important since you won't be reproducing low frequencies.

The amp's specs are stating the amplifier will produce 15 watts into an 8 Ohm resistive load and do so in two channels simultaneously. If the speaker you pair with this amp is stated to be other than an "8 Ohm" load, then the amp will produce slightly more or slightly less wattage according to Ohm's Law. Once again, this is sort of getting into the weeds of audio specmanship since your use will not include low frequenices which are what eat up power from an amplifier.

The "suggested wattage" of a speaker is not an exact science by any means. Generally, you'll do best to ignore wattage recommendations on speakers and stick with the more hard and fast sensitivity spec. In reality, even it can be fudged but it is the best we have to work with. Figure you number from there, the baseline sensitivity of the speaker/driver, and not from a suggested minimum or maximum into the speaker itself.


Give the retailer a call and explain your plans to them. They should be able to provide detailed asistance for their own products.



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New member
Username: Mike696

Post Number: 5
Registered: Oct-12
Thanks for the information Jan. I'll give them a call and find out
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