AmpSpeakerMatch

Amplifier to Speaker Matching

The articles that I have found on power amplifier sizing keep speaking of this “Rule-of-Thumb” and I find differences of opinions on this “Rule-of-Thumb”. I found the three “Rule-of-Thumb” to recommend:

  • 80 to 125% RMS
  • 100 to 150% of RMS
    • If the specification show a difference between the Continuous (RMS) and Peak in only 3dB (2x RMS) then a maximum of 150% of the RMS should be used.
  • 150 to 250% continuous.
    • In case of the specification show 250 Continuous (RMS), 500 Program and 1000 Peak then we are looking at using 200% of the RMS.

These “Rule-of-Thumb” to recommendations are totally ridiculous and is telling me the lack of knowledge these people have in the sizing of an amplifier to a speaker.

The goal here is to get as much as you can of your speaker while still address in the protection from thermal or mechanical failure. The use of this “Rule-of-Thumb” is far from doing this. All we have here is a “Wild Ass Guess” and this is not acceptable.

The two primary causes for speaker failure:

  • Excessive power to speakers.  Continuous power output from the amplifier is greater than the speaker is designed to handle.  This will exceed the thermal limit of the voice coil in the woofers.
  • Exceeding the power supply limits of the amplifier will cause clipping and result in a mechanical failure by exceeding the Xmax of the coil in the speaker.
  • Excessive distortion caused by trying to get more volume (power) from the amplifier than it is capable of delivering.  This causes the amplifier to go into clipping, which produces excessive high frequency energy and can burns up the tweeters or horns.

By conducting my own study and research, the two major parameter that is not considered is the sensitivity and impedance of a given speaker. This makes a big difference in the size of amplifier can be used for a speaker. The first thing is to have a clear understanding of the speaker power specifications.

Do not rely on the vendor or the website data to be expressed correctly. I have found the program or music power defined as the RMS of the speaker which has no RMS as well as the program or music power defined as the maximum. Use the manufacturer’s specifications to confirm all parameters. If the manufacturer does not have the complete specifications published do not but from that manufacturer. ‘s

This is a subject of speaker power specifications can be misleading or misunderstood by many because of the lack or minimum standards provided by Audio Engineering Society (ASE). The following is an attempt to explain.

AES Power Handling, AES Continuous,
Nominal power handling (ASE), Power handling capacity

A speaker tested in free air for two hours using a continuous, band-limited pink noise signal as per AES standard. This continuous pink noise signal is provided with a 12 dB crest factor in the range of Fs-10Fs. The band-limiting filtering uses a 24 dB/octave. The voltage is monitored to the speaker and recorded. Then power is calculated as true RMS voltage squared divided by the nominal impedance of the loudspeaker.

Program Power, Music Power

This level of power is defined as 3 dB greater than nominal AES power. A 3 dB increase equal two times the AES rating. If this is available on the speaker specification, this will be the rating to match the amplifier power.

Continuous Power Handling

Power handling specifications refer to normal music program material, reproduced by an amplifier producing no more than 5% distortion. This voltage is monitored to the speaker and recorded. Then power is calculated as true RMS voltage squared divided by the nominal impedance of the loudspeaker.

Note: Some manufactures define Continuous as Program power which is defined as 3 dB greater than the AES rating. Please review footnotes on spec sheet carefully.

Peak Power

Refers to maximum instantaneous, short term power that a loudspeaker can handle. Peak power relates to the Xmax parameter of the speaker voice coil where this amount of power will extend the voice coil to the extreme limits. The rating will be used for setting limiter parameters for speaker protection.

The other parameters associated with the sizing of the amplifier are:

Sensitivity

This measurement is based on a 200-2 kHz pink noise signal with a 2.83 volt output into an 8 ohms load for a 1 watt reference. The Sound Pressure Level (SPL) is measured at a distance of 1 meter in an anechoic environment. The ideal value would be as close 100 dB SPL as possible. The best values for this parameters would be 95 to 100 dB SPL. An increase of 1 dB sensitivity using the same power level from the amplifier provides the same desired SPL 3 meters further.

Impedence

Measured in ohms (Ω), this is the DC resistance (DCR) of the voice coil and this is to be matched to the amplifier output. The typical pro amplifier provide for a 8, 4 and sometimes 2 ohm load. The means that an 8 Ohm speaker shall be connected to a 8 Ohm load amplifier. This also means that two 8 Ohm speakers wired in parallel can be connected for a 4 Ohm load amplifier.

Headroom

This is the amount of audio signal available that exceeds the nominal level (RMS). This area allows audio peaks to exceed the nominal level before the maximum amplifier power is exceeded. When this amplifier power level is exceeded, the waveform is cut off which is referred to as clipping and provide distortion and a heating effect. Minimum headroom recommendation is 3dB which is equivalent to 2 timer RMS. Some suggest 6dB or 4 times RMS.

I was considering create this document from scratch and write it all but I think I have finally found a document that addresses all of the parameters required to match an amplifier to a speaker or speaker cabinet. This document is located at

https://www.themasterswitch.com/how-to-match-speakers-and-amps

There are two technical terms we now must come to grips with. RMS (Root Mean Square) and Crest factor (Peak Power divided by RMS power). RMS is the AC heating equivalent of invariant DC power. Since music is an alternating signal (representing the alternating compression and rarefaction of air we perceive as sound), we must reproduce it with a signal whose voltage varies quickly with time. A single frequency, which sounds completely constant in amplitude, actually varies from zero power to its peak value from tens to tens of thousands of times per second, depending on the frequencies of the signal. RMS is an expression that comes from the mathematical equation used to find the square root of the mean of the squared values.

For those of us not so inclined we can think of it as the average value, but in fact it is not the average value as the peaks add more power than the lower than average values, so we would find our intuition on this point fails us. (Which is why we need to do the math!) Without the math, think of this RMS is 70.7% of the peak values.


 

Evaluation

With the use of the Crown Audio system design tools for Amplifier Power Requirement at http://www.crownaudio.com/en/tools/calculators, I am able to provide the detailed requirements need for the evaluation of a speaker and provide the good selection of the amplifier for the defined application.

 This calculation provides the required electrical power or power output to produce a desired Sound Pressure Level (SPL) at a given distance, along with an amount of headroom (with lol a minimum of 3 dB) to keep the amplifier(s) from clipping.Equations used to calculate the data:
dBW = Lreq – Lsens + 20 * Log (D2/Dref) + HR
W = 10 to the power of (dBW / 10)
Where:
Lreq = required SPL at listener
Lsens = loudspeaker sensitivity (1W/1M)
D2 = loudspeaker-to-listener distance
Dref = reference distance
HR = desired amplifier headroom
dBW = ratio of power referenced to 1 watt
W = power required

 


Examples

Three speakers are provided below to show the effects of the different value in sensitivity and the difference results that are provided. Provided is the model of the speaker, the speaker sensitivity, the calculated requires at 3dB headroom, program wattage rating which is the maximum RMS power that should be applied to this speaker and the calculated headroom using an amplifier with 311 watts RMS rating.

With this calculation, I can provide the requirements of power for a listening distance of 60 feet / 19 meters with a required sound pressure level of 90 dB at that distance.

Speaker                                     Speaker              Required      Program            3000DSP
Model                                        Sensitivity           Watts           Watts               Headroom
Peavey Pro12                              94.3 dB               261              400                  3.8 dB
Peavey Sheffield Pro1200             96.3 dB               250              500                   5.8 dB
Eminence Delta12A                     98.3 dB               104              500                  7.8 dB

My selection of amplifier is the Beringer iNuke 3000DSP that is rated at 311W RMS/440W Peak 2×8Ω and the above shows the available headroom for the amplifier application.

Notice the results of the Peavey Pro12 and the Eminence Delta12A. There is a difference of 4 dB in sensitivity between speakers. This 4 dBs reduces the power requirement by 157 Watts. The point is that an increase of 3 dBs in the sensitivity will reduce the power requirement by half. Currently the 94.3 dB sensitivity of the Peavey Pro12 requires 261 watts and if you were comparing to a speaker of 97.3 dBs would require half of the 261 or 130 watts.


NU3000DSP_bannerThe 3000DSP features high-density lightweight Class-D technology; XLR & ¼ ” TRS input / Speakon twist lock speaker connections outputs; Independent DC, LF and thermal overload protection on each channel automatically protects amplifier and speakers; USB accessed DSP features include delay, crossover, PEQ (8); DEQ (2) and “Zero-Attack” limiters for maximum output level with reliable overload protection.
315W RMS / 440W Peak x2@8Ω – 680W RMS / 880W Peak x2@4Ω
1250W RMS / 1520W Peak x1@8Ω – 2050W RMS / /3000W Peak x1@4Ω


Picture3PerkAudio – Live Sound Consulting
Houston, Texas – 713.714.6780 – live@perkaudio.net
perkaudio.net – ProSound / Sound System Engineering
perkhorns.net – Custom Speaker Design & Builder

Advertisement
%d bloggers like this: