How to read the frequency response graphs of Jensen guitar speakers?
- Difficulties in frequency response measurements
- Analysing the IEC baffle measurement method, how edge diffraction alters the true acoustical response of the speaker.
- Converting frequency response graphs of some 12 inch Jensen guitar speakers to more meaningful infinite baffle responses.
Last updated: Nov. 16, 2016
Major problem with frequency response measurements of speakers is how to get rid off the masking effect of room reflections and cabinet edge diffraction. The ideal measurement environment would be if the speaker was mounted in an infinite baffle and could radiate into an ideal half space. Of course, nobody has an infinite baffle with an ideal half space, but fortunately we don't need any.
Problems with cabinets that edge diffraction effects may alter the true acoustical response of the speaker. Real baffles and cabinets roll-off at lower frequencies because they are finite. Open baffles suffer from phase cancellation at lower frequencies and their response is even more rugged.
Another problem comes from the allowed freedom in the choice of measurement methods (IEC, JIS): it is often difficult to compare results between different manufacturers (especially below 500 Hz). Some of the speaker manufacturers publish IEC baffle measurements, and some speaker manufacturers publish responses that are closer to an infinite wall response and (nearly) free from the distortions of the baffle. The best way to get the most accurate low end response of a speaker is to measure the Thiele-Small parameters. By Thiele-Small parameters and a box designer software we can simulate the half-space response of a 12" speaker up to 500 Hz.
Frequency response graphs found in Jensen guitar speaker's datasheets are measured on an IEC baffle (in an anechoic room). The graphs contain the response of the test baffle too so if we want to see the true acoustical output of the speakers we have to remove the effect of the baffle.
While I was surfing on the internet and reading guitar-tech forums I found that people compare the Jensen datasheet graphs directly with graphs from other speaker manufacturers (mainly with Celestion and Eminence). This is completely wrong because the measurement methods are completely different. As shown below the frequency response of Jensen guitar speakers are much flatter than one might expect from the factory measurements. The dip at 400Hz is the result of the IEC baffle measurement and not property of the speakers.
Simulated transfer function of an IEC baffle (1350x1650)
microphone distance: 1m, speaker diameter: 240mm
I simulated the response of the IEC baffle (1350x1650) by Edge diffraction simulator. I set the microphone distance to 1m and the speaker diameter to 240mm. Finally, I simulated the responses in box designers (Unibox, BassBox) too using the Thiele-Small parameters. The differences between the two types of simulations are less then 1 dB. Now the graphs are comparable to graphs from other manufacturers.
Jensen P12Q frequency response measured on IEC baffle (green, Jensen datasheet) and infinite baffle response (blue)
Jensen P12R frequency response measured on IEC baffle (green, Jensen datasheet) and infinite baffle response (blue)
Jensen P12N frequency response measured on IEC baffle (green, Jensen datasheet) and infinite baffle response (blue)