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28 

 

Finding  the  Tuning  Frequency  of  a  Ported 
Subwoofer Box System  (f

B

) - continued

 

8.  Figure 5 below shows the impedance curve of a 12 inch 

woofer  in  a  ported  box  tuned  to  30  Hz.    The  minimum 
impedance  between  the  two  peaks  is  f

B

,  the  tuning 

frequency.  This is the frequency  at which the port has 
the  highest  air  velocity,  and  the  subwoofer  has  the 
lowest excursion. 

 

 

Figure 5 

To prevent excessive cone excursion and potential damage 
to  the  subwoofer  is  it  advisable  to  use  a  infrasonic 
(subsonic)  filter  set  approximately 

 

 

 

  octave  below  f

B

.    To 

calculate  this,  multiply  f

B

  by  .707    Thus  in  this  example  30 

Hz X .707 = 21.2 Hz.   

Содержание SMD IM-SG

Страница 1: ...IM SG Impedance Meter Signal Generator Owner s Manual ...

Страница 2: ...Impedance of Speaker Cables or Signal Cables 13 Checking the Final Speaker Load on an Amplifier 15 Finding the Resonant Frequency of a Driver fS 16 The Significance of fS 18 Finding the Resonant Freq of a Sealed Box System fC 21 Finding the Q of a Sealed Box System QTC 23 Finding the Volume of a Subwoofer Enclosure Vb 25 Finding the Tuning Freq of a Ported Box System fB 27 Finding the Tuning Freq ...

Страница 3: ...IM SG Impedance test harness Y adapter This manual Pride of ownership About the Design The IM SG combines 4 powerful analog blocks in one package A continuously variable audio frequency sine wave generator you can t do that in digital pink noise generator analog audio amplifier and a precision balanced instrumentation amplifier to measure voltage and current Combine all of this with a powerful dig...

Страница 4: ...e greater than 8Hz 20kHz in two ranges Frequency Measurement Accuracy greater than 99 5 Impedance Measurement Accuracy greater than 99 from 0 ohms 30 ohms greater than 96 from 30 ohms 99 ohms Pink Noise Generation Flatness 20 Hz 20 kHz 2 0dB Sine Wave Generation Frequency Response 8 Hz 15 kHz 0 8dB 8 Hz 20 kHz 1 5dB Battery Voltage Low Warning 7 2 Volts Generator Output Voltage unloaded 725 mV RMS...

Страница 5: ...compartment cover off towards bottom end of unit to reveal the 9V battery compartment Insert a fresh 9V battery make sure the wires are routed around the battery If they are under or on top of the battery the door may not close properly You are ready to go Do not run this unit off of any power source other than a 9V battery Wall adapters are electrically noisy as are vehicle electrical systems ...

Страница 6: ...y Bad Replace Now The battery s voltage is below 6 4 Volts and accuracy cannot be guaranteed The accuracy of the IM SG is not affected by the condition of the battery unless the battery voltage is below 6 4 Volts In this case it will not proceed to boot up but instruct the user to replace the battery instead This insures accuracy at all times Range button Press to cycle through the modes Low Range...

Страница 7: ...On button for 1 second 2 Adjust frequency to approximately 500 Hz 3 Connect the test harness to the IM SG and short the red and black test leads together by clipping them together 4 Wait for Impedance reading to settle it should settle to less than 0 50 ohms if not you may have a defective harness The reading must settle to below 0 50 ohms to continue 5 Press and hold the Range button for 3 second...

Страница 8: ... the speaker magnet is vented If the magnet vent is blocked or if the woofer has boundaries such as sitting on a table it can cause inaccurate readings One accurate method for testing woofers is to suspend them between two wood studs that are suspended parallel to the ground between two tables If this is just not practical you can set the woofer on a sturdy table and place something under two side...

Страница 9: ...button for 1 second 3 Power on the audio system 4 Increase volume to a moderate level 5 Starting at around 20 Hz slowly turn the frequency control up Sweep between 20 Hz 80 Hz while listening carefully 6 There will be one or more frequencies that play louder than the others Note these frequencies 7 This allows the SPL competitor to select tones or music that has frequencies that match the frequenc...

Страница 10: ...und they can be made quiet by tightening something that is loose or by applying a damping material such as the popular adhesive mats 1 Connect IM SG to any auxiliary input of a source unit or amplifier to use the IM SG as a signal generator Turn the volume of the source unit or amplifier gain down 2 Turn on the IM SG by holding the On button for 1 second 3 Power on the audio system 4 Increase volu...

Страница 11: ...ink noise generator Turn the volume of the source unit or amplifier gain down 2 Turn on the IM SG by holding the On button for 1 second 3 Power on the audio system 4 Press the Range button twice to activate Pink Noise mode 5 Increase volume of source unit to a moderate level 6 Place the RTA s microphone in the listening position 7 View the system s frequency response on the RTA 8 Look for holes or...

Страница 12: ...on for 1 second 3 Power on the audio system 4 Increase volume of source unit to a moderate level 5 Place the SPL meter s microphone in the listening position 6 Slowly sweep the frequency on the IM SG while viewing the SPL meter Look for holes or peaks in the frequency response of the system Many times just swapping the phase of the rear channels or subwoofer channel will clear up some of these Adj...

Страница 13: ...d inside the small alligator clips on the test harness Just make a good connection to a number of strands that comfortably fit the alligator clip 3 Set the frequency to around 50 Hz 4 If the wire gauge is 4 AWG or larger it might be hard to measure unless measuring a complete spool of at least 100 feet because the resistance is so small 5 It is a good way to compare two different types of wire as ...

Страница 14: ...ion as the harness will not be used for this test 2 Connect one end of the RCA to be tested to the IM SG directly Short the other end of the RCA cable by using a shorting plug 3 Set the frequency to around 1 kHz 4 Wait for the impedance reading to settle before reading it 5 This is a good way to compare two different types of wire as long as types are the same length 6 Vary the frequency see how i...

Страница 15: ...e voice coils they all need to be connected A parallel connection will work fine Make sure there is no amplifier connected or damage to the IM SG could occur 3 Turn on the IM SG by holding the On button for 1 second 4 After the unit is operating turn the frequency down as far as it goes Then slowly sweep the frequency all the way through the audio range while watching the impedance 5 This is a goo...

Страница 16: ...e IM SG by holding the On button for 1 second 4 After the unit is operating adjust the frequency to the low end of the fS range in the chart below Then slowly raise the frequency until the peak in impedance is found If you go too far and pass the peak just back down the frequency until the peak in impedance is found Figure 1 shows the impedance curve for a 15 inch woofer we tested We measured the ...

Страница 17: ...17 Figure 1 Impedance curve of a single 4 ohm voice coil 15 inch subwoofer Notice the fS at 24 Hz ...

Страница 18: ... Hz This speaker should be crossed over high pass filtered higher than 60 Hz to protect the driver from over excursion and damage We suggest setting the high pass filter on midbass drivers octave to 1 octave higher than the driver s fS for best power handling Use octave higher if using a 24dB Octave crossover and 1 octave higher if using a 12dB Octave crossover This is not law by any means just a ...

Страница 19: ...hows the impedance peak to occur at 1 59 kHz This is the fS for this tweeter This information is useful for determining a safe crossover frequency for this tweeter We suggest setting the high pass filter on tweeters 1 octave to 2 octaves higher than the driver s fS for best power handling Use 1 octave higher if using a 24dB Octave crossover and 2 octaves higher if using a 12dB Octave crossover Thi...

Страница 20: ...20 Figure 2 Impedance curve of a typical 4 ohm 1 tweeter Notice the fS at 1 59 kHz ...

Страница 21: ...o be connected A parallel connection will work fine Make sure there is no amplifier connected or damage to the IM SG could occur 4 Turn on the IM SG by holding the On button for 1 second 5 After the unit is operating turn the frequency down as far as it goes Then slowly raise the frequency until the peak in impedance is found If you go too far and pass the peak just back down the frequency until t...

Страница 22: ...22 Figure 3 fC of a single 4 ohm 15 inch subwoofer in a 2 cubic feet sealed box ...

Страница 23: ... find the QTC of a sealed box subwoofer we need to know fS and the QTS of the raw driver see page 33 to measure these or use manufacturer s specs We also need to know the fC of the sealed subwoofer box system page 14 Once the fS and QTS of the driver is known and fC of the sealed subwoofer system is known it is easy to find the QTC of the sealed enclosure using the formula below Figure 4 shows the...

Страница 24: ...f the enclosure The larger the enclosure the lower the QTC value will be The box can be stuffed with polyester stuffing or lined with fiberglass insulation to make adjustments to the enclosure s QTC value Stuffing usually lowers the QTC value as well as flattening the impedance curve slightly Experiment and have fun ...

Страница 25: ... subwoofer is quiet and free from moving air 3 Connect the test leads to the speaker system if it has multiple voice coils they all need to be connected A parallel connection will work fine Make sure there is no amplifier connected or damage to the IM SG could occur 4 Turn on the IM SG by holding the On button for 1 second 5 After the unit is operating turn the frequency down as far as it goes The...

Страница 26: ... We also measured the VAS to be 1 98 cubic feet 8 Plug the VAS fS and fC into the equation above Vb 9 Solving for Vb we get 1 68 cubic feet This is the exact volume of the enclosure Try adding polyester box fill material or similar and remeasuring the fC Chances are adding some fill material will lower the fC and thus increase the apparent box volume In this same test box we added 1 lb of fill and...

Страница 27: ...connection will work fine Make sure there is no amplifier connected or damage to the IM SG could occur 3 Turn on the IM SG by holding the On button for 1 second 4 After the unit is operating turn the frequency down as far as it goes Then slowly raise the frequency until a first peak in impedance is found If you go too far and pass the peak just back down the frequency until the peak in impedance i...

Страница 28: ...n the two peaks is fB the tuning frequency This is the frequency at which the port has the highest air velocity and the subwoofer has the lowest excursion Figure 5 To prevent excessive cone excursion and potential damage to the subwoofer is it advisable to use a infrasonic subsonic filter set approximately octave below fB To calculate this multiply fB by 707 Thus in this example 30 Hz X 707 21 2 H...

Страница 29: ...stem on page 18 Make sure there is no amplifier connected or damage to the IM SG could occur Referring to Figure 5 fB represents the tuning frequency of the system This is the frequency at which the passive radiator has the highest excursion and the active driver has the lowest excursion To prevent excessive cone excursion and potential damage to the subwoofer is it advisable to use a infrasonic s...

Страница 30: ...second 4 Power on the amplifier 5 Increase amplifier gain or volume to a moderate level 6 Hold the loose not in box passive radiator very near the cone of the subwoofer 7 Sweep the frequency of the IM SG between 8 Hz and 60Hz until the passive radiator cone starts to vibrate from the sound the subwoofer is producing 8 Adjust the frequency on the IM SG up or down until the frequency is found that c...

Страница 31: ... Turn on the IM SG by holding the On button for 1 second 5 After the unit is operating turn the frequency down as far as it goes Then slowly raise the frequency until a first peak in impedance is found If you go too far and pass the peak just back down the frequency until the peak in impedance is found Let us call this frequency peak fL See figure 6 6 Continue increasing the frequency slowly until...

Страница 32: ...e usable bandwidth of the system Output of the subwoofer will start to decrease at frequencies below fL and subwoofer output will decrease at frequencies higher than fH Figure 6 the impedance curve of a single 4 ohm voice coil subwoofer in a single reflex bandpass enclosure 4th order Bandwidth is approx 30 90 Hz ...

Страница 33: ...aker Drivers page 8 1 Using a digital voltmeter zero the probes and then measure the DC resistance of the driver Record this value We will call this RE On our 12 inch woofer example we measured RE 3 4 ohms 2 Calibrate the IM SG to compensate for the harness Page 7 3 Connect the IM SG test leads to the speaker if it has multiple voice coils they all need to be connected Connecting the coils in para...

Страница 34: ...in the range above This Frequency is the fS 7 Record the impedance at fS We will be calling this RE RES We measured 38 3 ohms on our sample 12 inch woofer 8 Now we need to calculate RC with the following equation On our sample we measured RES RE to be 38 3 ohms and we measured RE to be 3 4 ohms Plug that into the equation above and it looks like this 11 26 9 Now we need to calculate RO with the fo...

Страница 35: ...the fS frequency to find fL and above the fS frequency to find fH 10 Slowly sweep the frequency down to find fL It will be lower in frequency than fS was and will occur at RO ohms See figure 8 Record the frequency you found as fL We measured fL to be 15 3 Hz on our 12 inch woofer 11 Now slowly sweep the frequency up higher than the fS frequency We are looking for the frequency that is at RO ohms o...

Страница 36: ...Thiele Small T S parameters of a driver continued Figure 8 finding fL and fH 12 To find QMS plug in the recorded values for FS RC FH and FL into the equation below Using numbers from our 12 inch woofer example So 3 21 ...

Страница 37: ...record QES using this formula Using the measurements from our test woofer we get So QES 0 31 14 Now calculate and record QTS with this formula Using the measurements from our test woofer we get So QTS 0 28 We are almost done now we are going to find VAS Get out your scale and play dough ...

Страница 38: ... measure because the impedance reading on the IM SG won t be stable Select an appropriate mass from the chart below Use the postage scale to measure the mass in grams Speaker Diameter Approx Mass to Add to Cone 5 25 inches 30 grams 6 5 inches 50 grams 8 inches 80 grams 10 inches 150 grams 12 inches 200 grams 15 inches 300 grams 18 inches 500 grams 16 Find and record fMA Once the mass is firmly on ...

Страница 39: ...tio with by plugging in FS and Fratio into the equation below Record Fratio For our example we get So Fratio 1 62 19 Calculate and record MMD For our example we get So MMD 218 grams 20 Now measure the diameter of the cone from center of the surround on one side to center of the surround on the other side Our sample 12 inch woofer measured 10 0 inches See photo 1 Record this in inches as D ...

Страница 40: ...ter D we can calculate the cone area with this formula So for our woofer which measured 10 inches from center of surround to center of surround we get So we get Sd 78 54 square inches Record this value for Sd Note this is the formula for a round driver If the driver is not round compute the cone area as needed ...

Страница 41: ...le Small T S parameters of a driver continued 21 Calculate and record MMR For our example we get So MMR 6 6 grams 22 Calculate and record MMS For our example we get So MMS 224 6 grams this is the moving mass of the driver ...

Страница 42: ...inding the Thiele Small T S parameters of a driver continued 23 Calculate and record CMS For our example we get So CMS 0 000154 24 Calculate and record VAS For our example we get Finally VAS 1 98 cubic feet ...

Страница 43: ...le of parameters found in this process in order from 12 inch woofer used in example RE 3 4 Ohms fS 27 1 Hz RE RES 38 3 Ohms RO 11 4 Ohms QMS 3 21 unitless QES 0 31 unitless QTS 0 28 unitless MMS 224 6 grams CMS 0 000154 unitless Sd 78 5 Square Inches VAS 1 98 Cubic Feet ...

Страница 44: ...44 Notes ...

Страница 45: ...45 Notes ...

Страница 46: ...tor clips together if harness is good impedance should be less than 0 50 ohms If harness is good then reading was accurate Problem The IM SG keeps turning off Solution The IM SG has an auto turn off timer it will operate for approximately 8 minutes after the On button is pressed This is to prevent it from eating your battery when you forget to turn it off If you are in the middle of using it and n...

Страница 47: ...rranty accident misuse abuse product modification or neglect unauthorized repair attempts misrepresentations by the seller This warranty does not cover incidental or consequential damages Cosmetic damage due to accident or normal wear and tear is not covered under warranty Warranty is void if the product s serial number has been removed or defaced Any applicable implied warranties are limited in d...

Страница 48: ...ll be determined from the manufacturing date code Non defective items received will be returned COD Customer is responsible for shipping charges and insurance in sending the product to D Amore Engineering Shipping damage on returns is not covered under warranty To obtain service worldwide please e mail D Amore Engineering at Warranty DAmoreEngineering com D Amore Engineering IM SG Owner s Manual R...

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