How Can I Measure Speaker Impedance & Resonance ?

Thread Starter

jaack65

Joined Aug 8, 2020
1
I have some speakers and want to match then to my Yamaha receiver best power transfer @ 8 ohms according to manual. How to measure a speaker's resonance?
 

Audioguru again

Joined Oct 21, 2019
1,768
The impedance of a speaker is about 5 times its "nominal" impedance. A woofer that resonates below 80Hz has its impedance measured at about 400Hz so that the resonance and inductance have no effect. A tweeter also has its impedance measured at about 5 times its resonant frequency.

The enclosure for a woofer affects its resonant frequency. A sealed enclisure raoises the resonant frequency. A ported enclisure has two low frequency resonances, one for the box and another for the port.

I measure the resonant frequency of a speaker by using a series resistor (maybe 100 ohms) from my amplified signal generator and listen for the peak output from the speaker or see it on my oscilloscope.
 

crutschow

Joined Mar 14, 2008
25,267
want to match then to my Yamaha receiver best power transfer @ 8 ohms according to manual.
Unless you have a tube-amp, the speaker impedance is not matched to the amplifier impedance.
The output impedance of a typical solid-state amplifier is a fraction of an ohm so it is not possible (or desirable) to match the speaker to the source.
 

Audioguru again

Joined Oct 21, 2019
1,768
The owners Manual for the low cost Yamaha R-S202D stereo receiver says "Minimum RMS output power 40Hz to 20kHz at 0.2% distortion 8 ohms= 100W + 100W".
Then it says "Dynamic Power per channel 8/6/4/2 ohms 125W/150W/165W/180W IHF". (IHF power is momentary at fairly high distortion.)
Then it says "Total Harmonic Distortion to Speakers (20Hz to 20kHz 50W per channel 8 ohms)= 0.2% or less". Therefore it cannot produce 100W per channel at 20Hz at low distortion.

Therefore it produces the lowest distortion for all audio frequencies at 50W per channel into 8 ohm speakers. But it produces much more power to lower impedance speakers but with higher distortion.
 

KeithWalker

Joined Jul 10, 2017
1,147
The impedance of a speaker is close to its DC resistance so you can measure it with your multimeter. If the speaker is in an enclosure there may be equalizing components connected between the input and the speaker so make the measurement right at the terminals on the speaker. An 8 Ohm speaker measures approximately 8 Ohms. A 4 Ohm speaker measures approximately 4 Ohms, etc.


Keith
 

ci139

Joined Jul 11, 2016
1,677
el-dynamic speaker being an atmospheric flap + a 3D flexible membrane , a spring , a solenoid and an inductor has likely many...many resonances (shut up me stupid cat)
i don't like to dig the web right now (found something) http://education.lenardaudio.com/en/05_speakers_2.html
(( i haven't licked the valerian . . . if i'm tired or frustrated my brain starts to induce stupid jokes - likely to cheer/wake me up ... funny? ))
 

MisterBill2

Joined Jan 23, 2018
6,736
The solution for matching the speakers is to read the label and connect the speakers to the appropriate tap on the amplifier. If you use an impedance meter and find that the speaker impedance is 6.738 ohms, then what? Probably the amplifier has an 8 ohm output, and the adjustable matching transformer is expensive. I have used a variac for impedance matching, and it does work. and it made no audible difference at all.
 

Audioguru again

Joined Oct 21, 2019
1,768
The solution for matching the speakers is to read the label and connect the speakers to the appropriate tap on the amplifier. If you use an impedance meter and find that the speaker impedance is 6.738 ohms, then what? Probably the amplifier has an 8 ohm output, and the adjustable matching transformer is expensive. I have used a variac for impedance matching, and it does work. and it made no audible difference at all.
Yamaha amplifiers are solid state with transistors, not with vacuum tubes and a tapped output transformer.
They can drive 2 ohm speakers or 8 ohm speakers as shown in the owner's manual.
The output impedance of a modern amplifier is at the most 0.04 ohms so it is an AC voltage source for a speaker. You do not match the speaker impedance to the output impedance of the amplifier.
 

MisterBill2

Joined Jan 23, 2018
6,736
Yamaha amplifiers are solid state with transistors, not with vacuum tubes and a tapped output transformer.
They can drive 2 ohm speakers or 8 ohm speakers as shown in the owner's manual.
The output impedance of a modern amplifier is at the most 0.04 ohms so it is an AC voltage source for a speaker. You do not match the speaker impedance to the output impedance of the amplifier.
My matching arrangement was rather unique, matching up to a permanently installed string of speakers that normally connected to the Jukebox in a bar. For whatever reason the impedance at the connection point we were given was about 500 ohms, and the amplifier output was 8 ohms. Without the variac the volume was way too low for those who wanted band sound coming from the speakers. So the full winding went across the 500 ohm speaker line and the amplifier 8 ohm output went between the low end of the variac and the wiper. We started at half way up and reduced the input connection until the sound was what they wanted. The variac was finally mounted in the back of the music amplifier speaker enclosure and it worked well for the six week gig the band had at that bar. And my reputation as a problem solver was made.
 

ci139

Joined Jul 11, 2016
1,677
The variac was finally mounted in the back of the music amplifier speaker enclosure
? mono . . . from certain distance anything surround eventually comes such so there's no much difference -- i've noticed that the GSM codec mixes the L R better than anything else i've tried (wonder what magic they do there)
 

Audioguru again

Joined Oct 21, 2019
1,768
Stores and schools PA systems use a 70V transformer on the amplifier output and tapped 70V transformers on each speaker. Then MANY speaker transformers can be connected in parallel (not in series) and the tap on each one sets the volume.

Many speakers in series sound boomy due to the lack of resonance damping from the 0.04 ohms or less output impedance of an amplifier.
 
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