Nick asks:
I have a pair of KEF reference speakers (towers)4ft tall by 1.5x1.5feet box and I’d like to use them in my home theater. Can I do this? Is this a good idea and how can I get the best sonic result with this ?

Hi Nick,
  You could definitely use the KEF Reference speaker in a Home Theater use. What is the Model number for the speakers?

Thanks,
KEF

[size=3]Thanks for getting back to me.  Sadly, I miss spoke on the model of speaker.  I’d inherited it from my father and thought it was KEF.  In fact the speaker was made by a company called, IMF.  The IMF speaker is a TLS 50 II.  I went searching for them and it turns out its a custom made reference speaker.  The details are noted below.  How can I best use these in my home theater or will I need to dedicate them for Audio Performance only.

[/size]TLS 50 (II)
Big speaker, 3 way, Transmission Line.

Description
A ‘transmission line’ speaker is a purely professional concept in that performance, rather than price, size or operating principle is the primary criterion of merit. What is a transmission line? Why is it not more common among loudspeakers? To answer the first query is to answer the second - it is a complex and potentially expensive way of providing near perfect conditions for modern drive systems to perform at optimum and is not suitable for ‘mass production’.

The IMF ‘TLS’ are advanced versions of the transmission line reproducer which was developed some years ago to aid in laboratory research. They incorporate the latest and most refined drive systems enclosed in new and radically re-designed transmission lines. These speakers arefor use with the best equipment. A ‘TLS’ is a sophisticated speaker; its virtues will not be apparent to the novitate who will want something more obvious. ltwill be most appreciated bythe cognoscenti.

The transmission line independently provides for its driver a near ideal acoustic environment. The bass line absorbs the back wave, progressively rising with frequency, and is open ended. By careful graduation of the density of the absorbents, and inverse tapering of the line, the open end extends very low frequency generation over a wide band. At the low frequencies the line adds acoustic mass to the diaphragm, doubling its effective area and creating more ideal conditions for proper conception of bass in the room.

The crossover points have been carefully chosen to preserve homegeniety and to maintain smooth impedance curves. The ‘left’ speakers have their mid and high units grouped at the right side; the ‘right’ speakers have them on the left side. The purpose is to provide a carefully controlled dispersion pattern inward and away from room boundaries so that the maximum of direct information and the minimum of reflected reaches the listener.

No cost cutting or production compromises have been applied to the TLS; the entire concept has been preserved as one of performance at any price rather than to a price. Were there better drive systems, enclosure techniques or construction methods, they would have been incorporated.

THE STUDIO TLS 5O LOUDSPEAKER

A free standing loudspeaker system measuring approximately 36” x 14” x 15’ wide. Bass unit S’ foam surround impregnated diaphragm, mid - range 5” impregnated cone contained in separate line, tweeter 1 1/8” soft dome and 3/4” chemical dome super-tweeter. Crossover electrical four way at approximately 375Hz, 3kHz and 13kHz. Frequency range 23Hz to beyond audibility. Calibrated level controls - 2dB for mid and high frequencies. Efficiency measured via pink noise 1 metre on axis : 50 watts produces 100dB. Nominal impedance 8 ohms. Driving power requirement 25 to 60 watts. Supplied in matched ‘mirrorimage’ pairs in teak or walnut

Conditions of test. Measurements of samples taken under anechoic conditions with reflection coefficient better than 0 1. Equipment employed B & K pen recorder, noise and signal generator, third octave filters and polar turntable.

Subject to alteration without notice.

Distortion ref. 90 dB at 1 kHz
Studio TLS 50
Harmonic   2nd   3rd
40 Hz   0.25 %  0.03 %
100 Hz   1.0 %  0.15 %
500 Hz   0.15 %  0.06 %
1 kHz   0.15 %  0.05 %
5 kHz   0.12 %  0.04 %
10 kHz   0.8 %  0.03 %
Graphs

Hi,

The answer is “It depends”.

It depends on the condition of the speakers and their ability to deliver the performance you are looking for.

As old as they are, their drivers may have deteriorated. So first each driver should be physically inspected to make sure they are completely intact, particularly where the surrounds meet the cone and where they meet the baffle. If they pass this first test, they should be carefully tested with a signal such as swept sine wave or pink noise to make sure all drivers function without distortion.

Unlike music for which these speakers were designed, movies have a reference level of performance. That reference maximum volume level is 105 decibels at the listening position. This is the target a speaker and system must achieve to be able to playback a film soundtrack as the directer intended. A look at IMF specs shows sensitivity measured differently from how it is done these days but we can make some assumptions and approximations.  They show 50 watts of pink noise producing 100dB at 1 meter distance and the speaker has power handling to 60 watts (not much more than 50). Combined the two speakers should reach approx 100dB at 6.5 feet and 94dB at 13 feet. Even if we give them the benefit of the doubt and add 3dB (the equivalent of doubling the amp power), they are still 8dB lower than reference level at 13 feet. This can be significant. If you can live with this maximum level, then you can use them. If you try to get more out of them, they will likely break.

Not knowing what their weakest driver is, one can’t tell if you can extract more output from them by crossing them over to a subwoofer. Regardless, home theaters virtually always benefit from a sub (or 4).

Hope this helps. I remember them. They were nice sounding speakers.

Thanks for the feedback.  I’ll perform the due diligence as you suggested.  Fortunately, my father cared for his speakers, but with that said, I’ll be watchful.

If they work for my needs, which set of speakers would they be best suited for in a 5.1 surround sound application?

Nick